Power switch and method for operating a power switch using a switchable current source

Abstract

A power switch comprises a switch transistor, wherein a load path of the switch transistor is coupled between an input of the power switch and an output of the power switch, and wherein a source terminal of the switch transistor is coupled to the output of the power switch. The power switch comprises a control capacitor which is coupled between a control terminal of the switch transistor and a reference potential conductor. The power switch comprises a switchable current source configured to charge the control capacitor in a switched-on state. A method for operating a power switch is also disclosed.

Description

[0001] Power Switch and Method for Operating a Power Switch Using a Switchable Current Source

[0002] Description

[0003] Technical Field

[0004] Embodiments according to the invention are related to a power switch.

[0005] Further embodiments according to the invention are related to a method for operating a power switch.

[0006] Embodiments according to the invention are related to a power switch for capacitive load to connect boost-DC / DC-converters to a supply rail.

[0007] Background of the Invention

[0008] In automated test equipment (ATE) test systems (but also in other technical applications), typically many different supply voltages are needed. Normally, several supply voltages on a test system module are generated by DC / DC-converters from a single power rail, e.g., 48V. For example, when a test system gets powered, these voltages may, for example, need to be turned on in a certain sequence not to overload or damage some circuitry parts, e.g., in the front end.

[0009] Different DC / DC converter topologies are relevant for such applications. In many cases, boost-converters are the best choice to generate higher voltages. However, it has been found that power-sequencing is, at least in some cases, complicated when using especially this type of DC / DC-converter. For example, its load is always connected to the input by a diode even if the converter is disabled. Therefore, in some cases, an additional switch may be needed in front of such a DC / DC-converter to enable proper power sequencing.

[0010] For example, a simple MOSFET-switch could be used to connect the capacitive load to the rail supply. However, it has been found that this is not a good solution since the current charging the load capacitors can be huge (see, for example, the simulation example in Figs.

[0011] 6 and 7). It has also been recognized that slowing down the switching behavior of a MOSFET (metal-oxide-semiconductor field effect transistor) is difficult since it turns on relatively abruptly if a certain VGsjHRESHOLD (e.g. a certain gate-source-threshold-voltage) is passed. It has also been recognized that, in some cases, such charging currents can cause power failures in the test system or even damage.

[0012] For some details, reference is made to Figs. 6 and 7 which show a LTSpice simulation example of a simple MOSFET-switch used to connect CAP load. In particular, it can be seen from Fig. 7 that a high current spike appears.

[0013] JP 2023131302 A describes an electronic circuit. To suppress rush current flowing in a field effect transistor when the field effect transistor is switched from off to on in an electric circuit including a capacitor provided between the field effect transistor and a load, an electric circuit is provided. The electronic circuit is configured to include a N-channel MOSFET. A drain terminal of the n-channel MOSFET is connected to a positive electrode terminal of a battery, and a source terminal is connected to the positive electrode terminal of a load. A capacitor is connected to the source terminal of the MOSFET. Detection parts for detecting voltages of the drain terminal and the source terminal of the MOSFET are also included. A switch is connected to the capacitor. A control part controls operation of the switch so that current flowing in the capacitor is limited to a prescribed value or less if the voltage of the drain terminal and the voltage of the source terminal are mutually matched when the MOSFET is switched from off to on.

[0014] The application report “Managing Inrush Current” published by Texas Instruments (SLVA 670 A - August 2014 - revised May 2015) describes that in most systems capacitors are placed throughout a design to ensure there are no voltage drops on the supply rails. It is also described that when power is initially applied to the system, charging these capacitors can result in an inrush current which can exceed the nominal load current. It is described that if this issue is left unaddressed, this can cause voltage rails to fall out of regulation, resulting in the system entering an undesired state. It is also described that the inrush current can exceed the current carrying capability of the board connectors as well as PCB traces, resulting in damaging the connectors and traces. It is further described that these problems can be mitigated by using Texas Instruments load switches. It is discussed that the load switches TPS229xx family are slew rate controlled to minimize inrush current. It is further noted that the article explores technical causes of inrush current, problems caused by inrush current, and solutions for inrush current featuring integrated load switches. The article “Active Inrush Current Limiting Using MOSFETs” published by Motorola as a semiconductor application note AN1542 describes an active inrush current limiting using MOSFETs. Different circuits are mentioned in said article.

[0015] US 2014 / 070831 A1 describes a system and method of protecting probes by using an intelligent current sensing switch. An apparatus and method for protecting probes used in automated testing is disclosed. The apparatus comprises a probe operable to provide power to a device under test from a device power source, wherein the probe is coupled to a contact pin of the DUT and a probe protector circuit is connected to the probe in series between the device power supply and the device under test. The probe protector circuit further comprises a current sense module operable to monitor a flow of a current from the device power supply to the device under test to determine if the current flow is below a predetermined threshold current level, and a switch for controlling the connection from the device power supply to the device under test. The switch is coupled to the current sense module and is operable to be used in conjunction with the current sense module that limits the current flow if it exceeds the predetermined threshold current level.

[0016] US 2022 / 0107247 A1 describes a material testing system. The material testing system comprises a material testing apparatus configured to receive an electrical supply, wherein the material testing apparatus comprises guide means, sample holding means configured for holding a sample, force means configured for applying force to the sample, and a crosshead arranged to support at least a portion of one or both of the sample holding means and the force means. The crosshead is movable about the guide means. The material testing system comprises a three phase switched-mode power supply unit arranged to provide the electrical supply for the material testing apparatus.

[0017] CN 108693457 A describes a chip test method and system for eliminating power-on overshoot. The method comprises the following steps of providing a direct current power supply used for power supplying, a chip to be tested and a chip test plate with a chip test socket; providing a load switch, wherein the direct current power supply provides power for the chip test socket via the load switch, wherein the input end of the load switch is connected to the direct current power supply; connecting the enabled end of the load switch and the grounding end of the chip to the chip test socket, wherein when the chip test socket is closed, the enabled end of the load switch and the grounding end of the chip generate electrical connection; connecting the enabled end of the load switch to the direct current power supply through a pull-up resistor; and inserting the chip into the chip test socket, tightly closing the chip test socket and testing after time delay. Said document also describes a chip test system for eliminating power-on overshoot. The chip is firstly contacted with the chip test plate and then is electrified, and an EOS damage rate during a chip test process can be effectively reduced.

[0018] JP 2020173197 A describes a semiconductor testing device and method for testing a semiconductor element. The semiconductor testing device includes a current control circuit which can superimpose a second constant current corresponding to a rush current on the first constant current made to float through the channel of the transistor. The time and the cycle of applying the first constant current through the transistor or the rate of cutting off a current through a transistor can be set. The semiconductor testing device includes an inductive reactance circuit which can change or set the value of an inductive reactance. It is possible to set the value of the inductive reactance of the testing circuit by controlling the inductive reactance circuit and to apply a surge voltage appropriate to an actual circuit to the transistor.

[0019] In view of the above, there is a desire to create a power switch that provides a good tradeoff between an implementation effort and a capability to avoid excessive current spikes.

[0020] Summary of the Invention

[0021] An embodiment according to the invention creates a power switch (e.g. for a capacitive load). The power switch comprises a switch transistor (e.g. a power MOSFET), wherein a load path of the switch transistor (e.g. a collector-emitter path of the switch transistor, e.g. in case of a bipolar switch transistor) or a drain-source path of the switch transistor, e.g. in the case of a field effect switch transistor) is coupled between an input of the power switch (e.g. SWJN; e.g. a conductor for applying a supply voltage that is to be forwarded, in a switchable manner, to the output of the power switch via the switch transistor) and an output of the power switch (e.g. SW_OUT). A source terminal of the switch transistor (e.g. an emitter terminal in the case of a bipolar switch transistor or a source terminal in the case of a field effect switch transistor) is coupled to the output of the power switch. The power switch comprises a control capacitor which is coupled between a control terminal of the switch transistor (e.g. a base terminal in the case of a bipolar switch transistor or a gate terminal in the case of a field effect switch transistor) and a reference potential conductor (e.g. having a capacitance larger than a transistor-internal capacitance between the control terminal of the transistor and the source terminal of the transistor; e.g. having a capacitance larger than a gate-source capacitance of the switch transistor or having a capacitance larger than or equal to two times the gate-source capacitance of the switch transistor or having a capacitance larger than or equal to five times the gate-source capacitance of the switch transistor or having a capacitance larger than or equal to ten times the gate-source capacitance of the switch transistor). The power switch comprises a switchable current source (e.g. switchable in dependence on a signal of a control line) configured to charge the control capacitor in a switched-on state (e.g. to charge the control capacitor in such a manner that a voltage of the control capacitor increases substantially linearly over time, e.g. with a deviation of less than 10 percent from a linear evolution, in a range from a capacitor voltage of 10 percent of the input voltage at the input of the power switch to a capacitor voltage of 90 percent of the input voltage of the power switch).

[0022] This embodiment is based on the idea that such a configuration allows for a good adjustment of a voltage slew rate at the output of the power switch, which in turn allows to control the current flowing through the power switch in the case of a capacitive load. By charging the control capacitor using a switchable current source, a well-defined evolution of the voltage at the control terminal of the switch transistor can be obtained, which in turn results in a well-defined temporal evolution of the voltage at the output of the power switch.

[0023] Thus, an excessive current peak of a current flowing through the power switch (or through the switch transistor) can be avoided (e.g. since the current is at least approximately proportional to the voltage slew rate in the presence of a capacitive load).

[0024] Moreover, it has been recognized that usage of a switchable current source for charging the control capacitor results in a simple implementation, wherein the at least approximately linear increase of the voltage at the control terminal of the switch transistor, a slope of which is defined predominantly determined by the current provided by the switchable current source and by the capacitance of the control capacitor, results in a well-controlled current flow through the switch transistor. For example, by dimensioning the power switch in such a manner that the slew rate of the voltage at the control terminal of the switch transistor does not exceed a desired value, a current flow through the switch transistor caused by a known load capacitance can be limited, which in turn helps to avoid a destruction of the switch transistor which also helps to avoid failure conditions (e.g. an excessive drop of the voltage at the input of the power switch). To conclude, the above-described embodiment allows for a relatively simple implementation and still is well suited for limiting the current flowing through the power switch and through the switch transistor.

[0025] In a preferred embodiment of the power switch, the switch transistor is configured to operate as a source follower (e.g. such that a voltage at the output of the power switch follows a voltage at the control terminal of the switch transistor with a substantially constant voltage shift over a wide range of voltages at the control terminal of the switch transistor).

[0026] Using such a design (or mode of operation) of the power switch, a slope of the voltage at the output of the power switch can be well defined. For example, if the voltage shift between the voltage at the control of the terminal of the switch transistor and the voltage at the source terminal of the switch transistor remains substantially constant (e.g., with a variation of no more than 0.5 volt or with a variation of more than 1 volt) over a range of at least 60% of the voltage between the input of the power switch and the reference potential conductor, the slope of the voltage at the output of the power switch can be well controlled, and can be limited to a value defined mostly by the current provided by the switchable current source and by the capacitance of the control capacitor. Consequently, the current flowing through the power switch can be limited. Moreover, by using such a design, a temporal evolution of the voltage at the output of the power switch is well predictable, which is sometimes helpful to avoid a destruction of circuitry or to ensure a reliable operation.

[0027] Moreover, it should be noted that using such a design, the slope of the voltage at the output of the power switch is mainly determined by quantities which are well controllable by design. Consequently, a well-defined behavior of the circuit can be achieved with moderate effort.

[0028] In a preferred embodiment of the power switch, the switchable current source is coupled between the input of the power switch (e.g. a conductor for applying a supply voltage that is to be forwarded, in a switchable manner, to the output of the power switch via the switch transistor) and the control terminal of the switch transistor (e.g. such that the switchable current source is supplied by the voltage at the input of the power switch). Alternatively or in addition, the switchable current source is coupled between the input of the power switch (e.g. a conductor for applying a supply voltage that is to be forwarded, in a switchable manner, to the output of the power switch via the switch transistor) and the control capacitor (e.g. such that the switchable current source is supplied by the voltage at the input of the power switch).

[0029] Accordingly, it is easily possibly to reach a sufficiently large voltage at the control terminal of the switch transistor. For example, the relatively high supply voltage at the input of the power switch can be exploited to charge the control capacitor, which substantially helps to reduce the implementation effort.

[0030] In a preferred embodiment of the power switch, the power switch comprises a floating voltage source (e.g. a photo coupler), wherein the floating voltage source is coupled between the switchable current source and the control terminal of the switch transistor (e.g. between a current output of the switchable current source and the control terminal of the switch transistor), e.g. to allow for shifting a voltage at the control terminal of the switch transistor (e.g. to be above an input voltage at the input of the power switch in the case of a positive supply voltage, or to be below an input voltage at the input of the power switch in the case of a negative supply voltage) while maintaining a sufficient voltage drop across the switchable current source (e.g. even if the voltage at the control terminal of the switch transistor is higher than the input voltage at the input of the power switch in the case of a positive supply voltage or even if the voltage at the control terminal of the switch transistor is lower than the input voltage at the input of the power switch in the case of a negative supply voltage).

[0031] Alternatively or in addition, the floating voltage source may be coupled between the switchable current source and the control capacitor (e.g. between a current output of the switchable current source and the control capacitor), e.g. to allow for shifting a voltage at the control capacitor (and consequently a voltage at the control terminal of the switch transistor) (e.g. to be above an input voltage at the input of the power switch in the case of a positive supply voltage, or to be below an input voltage at the input of the power switch in the case of a negative supply voltage) while maintaining a sufficient voltage drop across the switchable current source (e.g. even if the voltage at the control terminal of the switch transistor is higher than the input voltage at the input of the power switch in the case of a positive supply voltage or even if the voltage at the control terminal of the switch transistor is lower than the input voltage at the input of the power switch in the case of a negative supply voltage). Using such a concept, it is possible to charge the control capacitor to a voltage which is even higher than the voltage at the input of the power switch (e.g., in the case of a positive voltage at the input of the power switch). Alternatively, it is possible to charge the control capacitor to a voltage which is more negative than a voltage at the input of the power switch (e.g., in the presence of a negative voltage at the input of the power switch).

[0032] For example, the usage of such a floating voltage source allows to have a substantially higher voltage at the control terminal of the switch transistor (e.g. higher than a voltage at the input of the power switch, and / or higher than a voltage at an output of the power switch) when compared to an implementation without a floating voltage source. This to allows for the switch transistor to be in a low-ohmic state when the maximum output voltage is reached, with low implementation effort.

[0033] In particular, the typically relatively high input voltage of the power switch can be exploited, such that the floating voltage source typically only needs to provide a relatively small voltage contribution and can therefore be implemented with moderate effort.

[0034] Moreover, by using such a concept, it can be ensured that there is a sufficient voltage drop across the switchable current source to allow for a proper operation of the switchable current source (such that the switchable current source can provide a specified current with good accuracy).

[0035] To conclude, the floating voltage source can provide a voltage that brings along a sufficient voltage drop across the switchable current source and that also brings along sufficiently high control voltage (e.g., gain source voltage) of the switch transistor even when the voltage at the output of the power switch has reached its maximum value.

[0036] However, by using both the relatively high input voltage at the input of the power switch and the floating voltage source (wherein the floating voltage source typically only needs to bring a comparatively small voltage contribution), a highly efficient circuited implementation is possible, which helps to reduce costs and space requirements.

[0037] In a preferred embodiment of the power switch, the power switch comprises a floating voltage source, wherein the floating voltage source is coupled between an input of the power switch and the switchable current source (e.g. between the input of the power switch and a supply conductor of the switchable current source) (e.g. to increase a voltage at a supply conductor of the switchable current source to be higher than a voltage at the input of the power switch in the case of a positive supply voltage, e.g. such that a voltage at a current output of the switchable current source can be higher than the voltage at the input of the power switch while maintaining a sufficient voltage drop across the switchable power source in the case of the positive supply voltage; e.g. to reduce a voltage at a supply conductor of the switchable current source to be smaller than a voltage at the input of the power switch in the case of a negative supply voltage, e.g. such that a voltage at a current output of the switchable current source can be lower than the voltage at the input of the power switch while maintaining a sufficient voltage drop across the switchable power source in the case of the negative supply voltage).

[0038] This embodiment is based on similar considerations like the embodiment described before. However, by coupling the floating voltage source between the input of the power switch and the switchable current source, it can be ensured that there is a sufficient voltage drop across the switchable current source and that a sufficient voltage at the control terminal of the switch transistors can be reached to put the switch transistor into a low-ohmic state.

[0039] Also, an implementation effort of such a circuit is comparatively small, since the typically relative high input voltage of the power switch is exploited, and since the floating voltage source therefore typically only need to provide a relatively small voltage contribution.

[0040] In a preferred embodiment of the power switch, the floating voltage source comprises a photo coupler (e.g. a photo coupler comprising a combination of a light source, e.g. LED, and a photocell; e.g. a photo coupler capable to (or configured to) provide (e.g. at its output port) an output current in a range between 5pA and 50pa).

[0041] For example, an output port of the photo coupler is coupled (e.g. directly coupled; e.g. without any further voltage converting circuitry) between a current output of the switchable current source and the control terminal of the switch transistor. Alternatively, an output port of the photo coupler is, for example, coupled (e.g. directly coupled; e.g. without any further voltage converting circuitry) between a current output of the switchable current source and the control capacitor. Alternatively, an output port of the photo coupler is, for example, coupled between the input of the power switch and a supply conductor of the switchable current source. It has been found that a photo coupler is a very efficient means for implementing the floating voltage source, and that a photo coupler is well suited for the specific application.

[0042] In particular, it has been recognized that a voltage which can easily be provided by such a photo coupler is typically sufficient to ensure a sufficient voltage drop across the switchable current source and also a sufficient (e.g. sufficiently high) voltage at the control terminal of the switch transistor. Moreover, it has also been recognized that a current that can typically be provided by such a photo coupler is typically sufficient in the power switch.

[0043] Moreover, the photo coupler also brings along the advantage that there is a galvanic separation between a voltage at the input of the photo coupler and a voltage at the output of the photo coupler (wherein the output of the photo coupler may therefore be considered to provide a floating voltage). Thus, by using a photo coupler, the power switch can be impemented in a very efficient manner, with a very small space demand. In particular, by using a photo coupler as the floating voltage source it is not necessary to use any transformers, which are typically relatively expensive and comprise a relatively high space consumption.

[0044] To conclude, it has been found that relying on the input voltage of the power switch and generating an additional voltage contribution using a photo coupler, provides for a very efficient and reliable solution for charging the control capacitor, wherein the presence of the relatively high input voltage of the power switch is exploited.

[0045] In a preferred embodiment of the power switch, the floating voltage source comprises a buffer capacitor which is coupled in parallel with an output port of the photo coupler. For example, a capacitance of the buffer capacitor is larger than a capacitance of the control capacitor. Alternatively, a capacitance of the buffer capacitor may be larger, at least by a factor of 2, than a capacitance of the control capacitor. Alternatively, a capacitance of the buffer capacitor may be larger, at least by a factor of 5, than a capacitance of the control capacitor. Alternatively, a capacitance of the buffer capacitor is larger, at least by a factor of 8, than a capacitance of the control capacitor.

[0046] By using such a design, the circuit provides for a high reliability even if a current provided by the photo coupler is comparatively small. For example the implementation can even be used if the current provided by the photo coupler is smaller than the current provided by the switchable current source. For example, when using such an arrangement, the buffer capacitor may be pre-charged by the output current of the photo coupler before the switchable current source is activated. Accordingly, provided that the capacitance of the buffer capacitor is chosen to be sufficiently large, the voltage across the buffer capacitor may remain sufficiently constant (or even substantially constant) (e.g., with a voltage drop of no more than 20% or no more than 30% or no more than 50%) over the time during which the control capacitor is charged by the switchable current source.

[0047] Accordingly, it is possible to use a current for the charging of the control capacitor that is larger than a current provided by the photo coupler at its output. In this case, the buffer capacitor helps to maintain a voltage (or voltage drop) at the output of the photo coupler (e.g. a voltage between the output terminals of the photo coupler) within a tolerable range, even if the current provided by the switchable current source is larger than a current provided by the photo coupler at its output. Thus, the control capacitor can be charged with the current provided by the switchable time source, even if the photo coupler can only provide a significantly smaller current at the desired output voltage of the photo coupler.

[0048] Consequently, this arrangement allows to use a comparatively small and inexpensive photo coupler while still maintaining the possibility to charge the control capacitor with a sufficiently large current (provided by the switchable current source), and while having a sufficiently large voltage across the output of the photo coupler (wherein this voltage is maintained by the buffer capacitor).

[0049] In a preferred embodiment of the power switch, the power switch or a control circuitry coupled to the power switch is configured to activate the photo coupler before the switchable current source is switched on (e.g. such that there is a predetermined minimum duration between an activation of the photo coupler and a switching on of the switchable current source; e.g. such that a voltage at an output port of the photo coupler reaches a sufficiently large value, e.g. at least 90% of its steady state value, before the switchable current source is switched on, e.g. even in the presence of the buffer capacitor).

[0050] For example, an input of the photo coupler is supplied from a supply voltage which is smaller than a voltage present at an input of the power switch. For example, an input of the photo coupler is supplied from a logic supply voltage which supplies a control circuitry that controls a switching of the switchable current source.

[0051] By activating the photo coupler before the switchable current source is switched on, the buffer capacitor may be (pre-) charged to a sufficiently high voltage before the switchable current source is turned on. Accordingly, a reliable operation of the circuit can be achieved.

[0052] For example, the input of the photo coupler can be powered from a relatively low supply voltage, wherein it should be noted that the input of the photo coupler typically comprises a light source, like, for example, one or more light emitting diodes. Furthermore, by ensuring (e.g., by an appropriate timing of the operation) that the buffer capacitor at the output of the photo coupler is charged to a sufficiently large voltage before the switchable current source is turned on, it can be achieved that the charge on the buffer capacitor is sufficiently large to fully charge the control capacitor while still having a sufficiently high voltage across the buffer capacitor to ensure a sufficient voltage drop across the switchable current source and to reach a sufficiently high control voltage of the switch transistor (e.g., to fully turn on the switch transistor, e.g to bring the switch transistor into a low-ohmic state) when the output voltage of the power switch has reached its final value (which may be similar to the input voltage of the power switch).

[0053] However, it should be noted that such an appropriate timing, which activates the input of the photo coupler well before the activation of the switchable current source, can easily be achieved using an appropriate control circuitry which can, for example, be operated using a supply voltage that is substantially smaller than the input voltage of the power switch. For example, the input of the photo coupler may be activated as soon as there is a supply voltage for a control circuitry available, or the input of the photo coupler may be activated as soon as a control circuitry is initialized.

[0054] In a preferred embodiment of the power switch, an output voltage of the floating voltage source (e.g. of the photo coupler) is larger than or equal to 1 .5 times a threshold voltage of the switch transistor. Alternatively or in addition, an output voltage of the floating voltage source (e.g. of the photo coupler) is larger than or equal to 6V, or an output voltage of the floating voltage source (e.g. of the photo coupler) is larger than or equal to 8V, or an output voltage of the floating voltage source (e.g. of the photo coupler) is larger than or equal to 10V. By using such a choice of the output voltage of the floating voltage source, it can be achieved that a voltage drop across the switchable current source is sufficiently large and that a sufficient control voltage (e.g. gate-source voltage) of the switch transistor can be reached. For example, it has been recognized that choosing the output voltage of the floating voltage source to be larger than or equal to 1.5 times a threshold voltage of the switch transistor allows to turn the switch transistor fully on while still having a sufficient voltage drop across the switchable current source. Similarly, it has been found that choosing the output voltage of the floating voltage source to be larger than or equal to 6 volts, or to be larger than or equal to a 8 volt, or to be larger than or equal to 10 volts, typically also allows to fully turn on the switch transistor while having a sufficient voltage drop across the switchable current source.

[0055] Moreover, it has been recognized that such a choice of the output voltage of the floating voltage source allows for a relatively inexpensive implementation, e.g., using available inexpensive photo couplers.

[0056] In other words, it has been recognized that the above mentioned choice of the output voltage of the floating voltage source provides for a good compromise between implementation effort and circuit characteristics (e.g. in terms of having a sufficient voltage drop across the switchable current source and in terms of having a sufficient control voltage of the switch transistor).

[0057] In a preferred embodiment of the power switch, an output current of the photo coupler is smaller (e.g. at least by a factor of 2, or at least by factor of 10, or at least by a factor of 20) than a current provided by the switchable current source. Alternatively, an output current of the photo coupler may be larger than a current provided by the photo coupler.

[0058] By choosing the output current of the photo coupler to be smaller than the current provided by the switchable current source, a cheap implementation of the photo coupler can be achieved. However, a sufficient voltage at the output of the photo coupler can still be ensured, e.g., using a buffer capacitor, as outlined above. However, by chosing the photo coupler such that an output current of the photo coupler is larger than a current provided by the switchable current source, usage of a buffer capacitor may be unnecessary, e.g., at the cost of using a more expensive photo coupler. In a preferred embodiment of the power switch, the switch transistor is a field effect transistor (e.g. a N-channel MOSFET; e.g. a N-channel power MOSFET; e.g. a P-channel MOSFET; e.g. a P-channel power MOSFET).

[0059] It has been recognized that usage of a field effect transistor as the switch transistor is particularly advantageous since, in a static or quality-static case, the field effect transistor does not require a control current (e.g., a gate current). Accordingly, the control voltage (e.g., the voltage at the gate terminal of the field effect transistor) can be well controlled by the switchable current source and by the control capacitor. For example, it has been recognized that by using a field effect transistor as the switch transistor, a well-defined slope of the voltage at the gate terminal of the field effect transistor (which serves at the switch transistor) can be achieved, wherein the slope of this voltage at the gate terminal (control terminal) of the switch transistor is mainly determined by the current provided by the switchable current source and by the capacitance of the control capacitor.

[0060] Moreover, since the field effect transistor typically requires a relatively small control current (or even no control current in a stationary or quasi-stationary case), the floating voltage source can be implemented easily, with moderate effort. Thus, it has been recognized that usage of a field effect transistor as the switch transistor brings along a number of advantages, e.g., when compared to the usage of a bipolar transistor as a switch transistor.

[0061] In a preferred embodiment of the power switch, the power switch comprises a switch-off circuit configured to at least partially discharge the control capacitor (e.g. to selectively discharge (at least partially) the control capacitor in case the switchable current source is switched off), e.g. to discharge the control capacitor in such a manner that a voltage at the control terminal of the switch transistor approximates a voltage at a source terminal of the switch transistor, or in such a manner that a voltage at the control terminal of the switch transistor approximates a voltage at an output of the power switch. For example, the switchoff circuit may be coupled in between the control terminal (e.g. gate terminal) of the switch transistor and the source terminal of the switch transistor, or in between the control terminal of the switch transistor and the output of the power switch. It has been recognized that usage of a switch off circuit (also designated as a discharge circuit herein) is advantageous since such a switch off circuit allows to quickly switch off the power switch. However, it has been found that it is advantageous if the switch off circuit “automatically" (e.g., without a further control signal) discharges the control capacitor in order to turn off the power switch, since this significantly reduces the control effort.

[0062] For example, the switch off circuit may be designed in such a manner that the activation of the switchable current source results in a deactivation of a switch off component (e.g., of a switch off transistor or of a discharge transistor) which at least partially discharges the control capacitor.

[0063] Similarly, the switch off circuit may be designed in such a manner that the deactivation of the switchable current source results in an activation of a switch off component (e.g., of a switch off transistor or of a discharge transistor) which at least partially discharges the control capacitor.

[0064] However, it should be noted that it is not necessary to fully discharge the control capacitor in order to deactivate (e.g. turn off or switch off) the switch transistor. Rather, in some cases it may be sufficient to discharge the control capacitor to such a degree that a control voltage of the switch transistor (e.g., a gate-source voltage of the switch transistor) is below a threshold voltage. However, a further discharge of the control capacitor may take place when the voltage at the output of the power switch reduces (e.g., to continuously keep the control voltage of the switch transistor below the threshold voltage).

[0065] To conclude, by providing a switch-off circuit (or discharge circuit) configured to at least partially discharge the control capacitor, e.g., when the switchable current source is switched off, a good overall switch off characteristic of the power switch can be achieved.

[0066] In a preferred embodiment of the power switch, the switch-off circuit is configured to be disabled (e.g. deactivated) by a current provided by the switchable current source which charges the control capacitor (wherein, for example, the current provided by the switchable current source causes a bias voltage that turns off a transistor of the switch-off circuit), and the switch-off circuit is configured to be enabled, to at least partially discharge the control capacitor, if the switchable current source is switched off (wherein, for example, the absence of a current provided by the switchable current source results in a bias voltage that turns on the transistor of the switch-off circuit).

[0067] By using this concept, a control mechanism can be significantly simplified. For example, it can be achieved that the switch off circuit (or discharge circuit) does not discharge the control capacitor as long as the switchable current source is switched on, since the current provided by the switchable current source may (automatically) bring the switch-off circuit (or discharge circuit) into a disabled state. For example, the current provided by the switchable current source may cause a voltage drop over a circuit component (e.g., over a diode or over a resistor), wherein this voltage drop may have the effect to turn off a transistor that at least partially discharges the control capacitor in an on-state. However, the switch-off circuit may also be configured in such a manner that the switch-off circuit is automatically activated as soon as the switchable current source is deactivated (e.g., provided that the control voltage of the switch transistor is sufficiently large). Accordingly, a simple mechanism is created which is suited to rapidly turn off the power switch without having a requirement for additional control lines.

[0068] In a preferred embodiment of the power switch, a first terminal of the control capacitor is connected (e.g. directly connected, or connected via a protection resistor) with a control terminal (e.g. gate terminal) of the switch transistor. A second terminal of the control capacitor is connected with the reference potential conductor. A first output terminal of the floating voltage source (e.g. an anode output terminal of the photo coupler) is coupled with the first terminal of the control capacitor via a diode (e.g. D2). A second output terminal of the floating voltage source is coupled with a current output of the switchable current source. A buffer capacitor is coupled between the first output terminal of the floating voltage source and the second output terminal of the floating voltage source. A source terminal (e.g. an emitter terminal or a source terminal) of a discharge transistor is coupled (e.g. directly coupled) to the first terminal of the control capacitor, a sink terminal (e.g. a collector terminal or a drain terminal) of the discharge transistor is coupled (e.g. directly coupled) with the output of the power switch and / or with the source terminal of the switch transistor, and a control terminal (e.g. a base terminal) of the discharge transistor is coupled with the first output terminal of the floating voltage source (and / or with the first terminal of the diode).

[0069] A resistor (e.g. R7) is coupled between the control terminal (e.g. base terminal) of the discharge transistor and the sink terminal (e.g. collector terminal) of the discharge transistor (wherein, for example, the resistor is designed such that a current flowing through the resistor is not larger than 50 percent of a current provided by the switchable current source when charging the control capacitor, or wherein, for example, the resistor is designed such that a current flowing through the resistor is not larger than 25 percent of a current provided by the switchable current source when charging the control capacitor).

[0070] For example, a sink terminal (e.g. a collector terminal or a drain terminal) of a current source output transistor is coupled with the second output terminal of the floating voltage source. For example, a source terminal of the switchable current source output transistor is coupled (e.g. directly or with one or more circuit components in between; e.g. with a resistor in between) with the input of the power switch, e.g. such that the switchable current source is supplied by the input voltage of the power switch. For example, one or more diodes (e.g. two diodes circuited in series) are coupled between the input of the power switch and the control terminal of the current source transistor. For example, a series connection of a resistor and of a load path (e.g. a collector-emitter-path) of a control transistor are circuited between the control terminal of the current source transistor and the reference potential conductor.

[0071] This implementation has been found to be particularly efficient. For example, a voltage drop across the diode may disable a discharge transistor when the switchable current source is switched on. On the other hand, when the switchable current source is switched off, the discharge transistor is automatically activated by the resistor which is coupled between the control terminal (e.g. base terminal) of the discharge transistor and the sink terminal (e.g. collector terminal) of the discharge transistor, such that the control capacitor is quickly discharged to a level such that the switch transistor is disabled (switched off).

[0072] Moreover, the usage of the photo coupler allows for a provision of a sufficiently high voltage for charging the control capacitor with moderate effort, wherein the typically comparatively high input voltage of the power switch is also used. Moreover, the usage of the buffer capacitor allows for the usage of a relatively small photo coupler, an output current of which is smaller than a current provided by the switchable current source.

[0073] To conclude, the above-described arrangement can be implemented with moderate effort but provides a very good functionality. Nevertheless, it should be noted that modifications can be made. For example, the order of the switchable current source and of the photo coupler could be changed. Also, the details regarding the switch-off circuitry could be modified. Moreover, a complimentary implementation could be used.

[0074] An embodiment according to the invention creates a method for operating a power switch, the power switch comprising a switch transistor (e.g. a power MOSFET), wherein a load path of the switch transistor (e.g. a collector-emitter path of the switch transistor (e.g. in case of a bipolar switch transistor) or a drain-source path of the switch transistor (e.g. in the case of a field effect switch transistor)) is coupled between an input of the power switch (e.g. SWJN) (e.g. a conductor for applying a supply voltage that is to be forwarded, in a switchable manner, to the output of the power switch via the switch transistor) and an output of the power switch (e.g. SW_OUT), wherein a source terminal of the switch transistor (e.g. an emitter terminal in the case of a bipolar switch transistor or a source terminal in the case of a field effect switch transistor) is coupled to the output of the power switch, and wherein the power switch comprises a control capacitor which is coupled between a control terminal of the switch transistor (e.g. a base terminal in the case of a bipolar switch transistor or a gate terminal in the case of a field effect switch transistor) and a reference potential conductor.

[0075] For example, the control capacitor may have a capacitance larger than a transistor-internal capacitance between the control terminal of the transistor and the source terminal of the transistor. For example, the control capacitor may have a capacitance larger than a gatesource capacitance of the switch transistor or may have a capacitance larger than or equal to two times the gate-source capacitance of the switch transistor or may have a capacitance larger than or equal to five times the gate-source capacitance of the switch transistor or may have a capacitance larger than or equal to ten times the gate-source capacitance of the switch transistor.

[0076] The method comprises charging the control capacitor using a current source (e.g. a constant current source), to thereby obtain a slope of a voltage at an output of the power switch which is substantially linear over time (e.g. with a deviation from a linear evolution of no more than 10 percent) for a switch-on operation at least in a range of the voltage at the output of the power switch between 10 percent of a maximum voltage and 90 percent of a maximum voltage. This method is based on the same considerations like the above discussed power switch. Moreover, the method may optionally be supplemented using any of the features, functionalities and details disclosed herein, also with respect to the power switch.

[0077] Brief Description of the Figures

[0078] Embodiments according to the present invention will subsequently be described taking reference to the enclosed figures, wherein

[0079] Fig. 1 shows a block schematic diagram of a power switch, according to an embodiment of the present invention;

[0080] Fig. 2 shows a schematic of a power switch, according to an embodiment of the present invention;

[0081] Fig. 3 shows a schematic of a power switch, according to an embodiment of the present invention;

[0082] Fig. 4 shows a graphic representation of voltages and currents in a power switch according to Fig. 3;

[0083] Fig. 5 shows a flowchart of a method, according to an embodiment of the present invention;

[0084] Fig. 6 shows a schematic of a conventional power switch; and

[0085] Fig. 7 shows a graphic representation of voltages and currents in the conventional power switch according to Fig. 6.

[0086] Detailed Description of the Embodiments

[0087] 1. Power Switch according to Fig, 1 Fig. 1 shows a block schematic diagram of a power switch 100, according to an embodiment of the invention. The power switch 100 comprises an input 110 and an output 112. Moreover, the power switch 100 comprises a switch transistor 120, wherein a load path 120a of the switch transistor (e.g., a drain-source path of the switch transistor in the case of field effect switch transistor, or, alternatively, a collector-emitter path of the switch transistor in the case of a bipolar switch transistor) is coupled between the input 110 of the power switch 100 and the output 112 of the power switch 100. For example, the input 110 of the power switch is a conductor for applying a supply voltage to be forwarded, in a switchable manner, to the output 112 of the power switch via the switch transistor 120. A source terminal 120b of the switch transistor (e.g., a source terminal in the case of a field effect switch transistor or, alternatively, an emitter terminal in the case of a bipolar switch transistor) is coupled to the output 112 of the power switch (e.g., directly or via one or more circuit components in between). For example, a sink terminal 120c of the switch transistor 120 (e.g., a drain terminal in the case of a field effect switch transistor, or a collector terminal in the case of a bipolar switch transistor) may be coupled to the input 110 of the power switch (e.g., directly, or with one or more circuit component in-between). Preferably, but not necessarily, the source terminal 120b of the switch transistor 120 may be coupled to the output 112 of the power switch using a lower-ohmic connection (e.g. directly or with a shunt resistor for a current measurement in between). Similarly, the sink terminal 120c of the switch transistor 120 may be coupled with the input 110 of the power switch via a low-ohmic connection (e.g., directly or with a shunt resistor for a current measurement in-between).

[0088] Moreover, it should be noted that the power switch 120 comprises a control capacitor 130 which is coupled between a control terminal 120d of the switch transistor 120 (e.g., a gate terminal in the case of a field effect switch transistor, or, alternatively, a base terminal in the case of a bipolar switch transistor) and reference potential conductor GND. For example, the reference potential conductor GND may be a ground conductor. For example, the control capacitor 130 may have a capacitance which is larger than a transistor-internal capacitance between the control terminal 120d of the switch transistor 120 and the source terminal 120b of the switch transistor. For example, the control capacitor may have a capacitance larger than a gate-source capacitance of the switch transistor (e.g., in the case of a field effect switch transistor), or the control capacitor may have a capacitance larger than or equal to two times the gate source capacitance of the switch transistor, or the control capacitor may have a capacitance larger than or equal to 5 times the gate source capacitance of the switch transistor, or the control capacitor may have a capacitance larger than or equal to 10 times the gate source capacitance of the switch transistor. Moreover, the power switch 100 comprises a switchable current source 140 (e.g. switchable in dependence on a signal of a control line) configured to charge the control capacitor 130 in a switched-on state (e.g. to charge the control capacitor in such a manner that a voltage of the control capacitor increases substantially linearly over time, e.g., with a deviation of less than 10 percent from a linear evolution, e.g. in a range from a capacitor voltage of 10 percent of the input voltage at the input of the power switch to a capacitor voltage of 90 percent of the input voltage of the power switch or in a range from a capacitor voltage of 20 percent of the input voltage at the input of the power switch to a capacitor voltage of 80 percent of the input voltage of the power switch).

[0089] For example, a current output of the switchable current source 140 may be directly coupled to a terminal (e.g. a first terminal) of the control capacitor 130. However there may be one or more circuit components in between the current output of the switchable current source and a terminal (e.g a first terminal) of the control capacitor 130.

[0090] Regarding the functionality of the power switch 100, it should be noted that the voltage at the output 112 of the power switch may, substantially (e.g. over a wide voltage range) , follow the voltage at the control terminal 120d of the switch transistor 120, at least over a wide range of the voltage at the control terminal 120d of the switch transistor 120. For example, the switch transistor 120 may operate as a source follower or as an emitter follower, wherein, for example, a voltage at the output 112 of the power switch may follow a voltage at a control terminal 120d of the switch transistor 120 with a voltage shift that may be determined by a threshold voltage of the switch transistor (e.g., in the case of a field effect switch transistor). The voltage shift may also be determined by a forward voltage of a base-emitter diode, e.g., in the case of a bipolar switch transistor. Consequently, the voltage at the output 112 of the power switch is well-controllable by the voltage at the control terminal 120d of the switch transistor 120 and follows the voltage at the control terminal 120d of the switch transistor 120 over a wide voltage range.

[0091] Moreover, the presence of the control capacitor 130 allows to control and stabilize the voltage at the control terminal 120d of the switch transistor 120. The control capacitor 130 allows to generate a well-defined voltage ramp at the control terminal 120d of the switch transistor 120 using the switchable current source 140, since the switchable current source 140 may charge the control capacitor 130 with a substantially constant current when the switchable current source 140 is switched on (at least up to a certain voltage).

[0092] Accordingly, by switching on the switchable current source 140, a substantially linear voltage ramp can be generated at the control terminal 120d of the switch transistor 120, which in turn results in a substantially linear voltage ramp at the output 112 of the power switch. Consequently, a voltage slope at the output 112 of the power switch can be well- defined by a proper choice of the capacitance of the control capacitor 130 and also by a proper choice of the current provided by the switchable current source. Therefore, the slope of the voltage at the output 112 of the power switch can be reliably adjusted in such a manner that a current flowing through the switch transistor does not exceed a maximum allowable current even in the presence of a capacitive load. Accordingly, damage of circuit components can be avoided, and an excessive load (or stress) of a power supply providing the voltage at the input 110 of the power switch can be avoided. Accordingly, an excessive drop of a voltage at the input 110 of the power switch 100 can be reliably prevented.

[0093] To conclude, the power switch 100 brings along very advantageous characteristics, wherein the voltage at the output 112 of the power switch can be ramped in a very smooth and well- controlled manner, while avoiding excessive current peaks.

[0094] As outlined above, the switch transistor 120 may be configured to operate as a source follower (e.g., such that a voltage at the output 112 of the power switch 100 follows a voltage at the control terminal 120d of the switch transistor with a substantially constant voltage shift over a wide range of voltages at the control terminal 120d of the switch transistor).

[0095] For example, the switchable current source 140 may be coupled between the input 110 of the power switch 100 (e.g., a conductor for applying a supply voltage that is to be forwarded, in a switchable manner, to the output 112 of the power switch via the switch transistor 120) and the control terminal 120d of the switch transistor 120 (e.g. such that the switchable current source 140 is supplied by the input voltage at the input 110 of the power switch 100). Alternatively (or in addition), the switchable current source 140 may be coupled between the input 110 of the power switch (e.g. a conductor for applying a supply voltage that is to be forwarded, in a switchable manner, to the output 112 of the power switch via the switch transistor 120) and the control capacitor 130 (e.g. such that the switchable current source 140 is supplied by the voltage at the input 110 of the power switch). Optionally, the power switch may comprise a floating voltage source (e.g. a photo coupler) not shown in Fig. 1 , wherein the floating voltage source may, for example, be coupled between the switchable current source 140 and the control terminal 120d of the switch transistor, and / or wherein the floating voltage source may be coupled between the switchable current source 140 and the control capacitor 130. As another (optional) alternative, the power switch may comprise a floating voltage source, wherein the floating voltage source is coupled between the input 110 of the power switch and the switchable current source 140.

[0096] In both cases, the floating voltage source helps to ensure sufficient voltage drop across the switchable current source 140, since a “real world” switchable current source typically requires a certain minimum voltage drop to operate as intended. Moreover, the floating voltage source also provides the possibility to achieve a potential at the control terminal 120d of the switch transistor 120 that fully turns on the switch transistor 120. For example, in some cases (e.g. in case the switch transistor 120 is a N-channel MOSFET having a positive threshold voltage, e.g. an N-channel MOSFET of enhancement type), it is desirable to have a potential at the control terminal 120d of the switch transistor 120 which is above the potential at the input 110 of the power switch and which is also above the potential at the output 112 of the power switch. For example, to have the switch transistor 120 fully switched on (e.g. when the voltage at the output 112 has reached its maximum value), the potential at the control terminal 120d should, in some cases, exceed the potential at the source terminal 120b and the potential at the sink terminal 120c by more than the threshold voltage (to have the current path between the sink terminal 120c and the source terminal 120b in a low-ohmic state, or, equivalently, to have the switch transistor 120 fully turned on). Thus, the optional floating voltage source allows for an efficient implementation with good characteristics.

[0097] In an example, the floating voltage may comprise a photo coupler. For example, such a photo coupler may provide for a sufficient voltage, sufficient to ensure a sufficient voltage drop across the switchable current source and sufficient to bring the potential at the control terminal 120d to an appropriate potential for fully switching on the switch transistor 120. Moreover, a photo coupler may inherently provide for a sufficient galvanic separation, such that the output voltage provided at the output of the photo coupler can be considered as freely floating. Moreover, it has been recognized that the current capability of a photo coupler (at least when taken in combination with a buffer capacitor) is typically sufficient in order to charge the control capacitor 130 with an appropriate voltage slope. Moreover, it has been recognized that the photo coupler is typically smaller and more cost efficient than alternative solutions for providing a floating voltage source.

[0098] The floating voltage source may, in some implementations, optionally comprise a buffer capacitor which is coupled in parallel with and output port of the photo coupler. Such a buffer capacitor may buffer the voltage across the output of the photo coupler, wherein, as a consequence, the photo coupler is useable even if the output current of the (core) photo coupler is smaller than the current provided by the switchable current source (and, consequently, smaller than the current using which the control capacitor 130 is charged). Preferably, the capacitance of the buffer capacitor may be larger than a capacitance of the control capacitor, or even larger, at least by a factor of two, than the capacitance of the control capacitor. In some cases, the capacitance of the buffer capacitor may, for example, even be larger, at least by a factor of 5, or at least by a factor of 8, than a capacitance of the control capacitor 130. This allows for the usage of a photo coupler with relatively small output current capabilities, which in turn helps to save cost.

[0099] In some embodiments, the power switch 100 or a control circuitry coupled to the power switch 100 may for example, be configured to activate the photo coupler before the switchable current source is switched on. This may, for example, help to (pre-) charge the buffer capacitor, that is coupled to the output of the photo coupler, to a sufficient voltage before the switchable current source is activated. Accordingly, a charging of the control capacitor 130 can be performed with good reliability even if the output current capability of the (core) photo coupler is comparatively small.

[0100] In some cases, for example, an output voltage of the floating voltage source is greater than or equal to 1.5 the threshold voltage of the switch transistor 120. Alternatively, in some cases, the output voltage of the floating voltage source may, for example, be greater than or equal to 6 volt, or the output voltage of the floating voltage source may, for example, be greater than or equal to 8 volt, or the output voltage of the floating voltage source may, for example, be greater than or equal to 10 volt. By having such an “additional” voltage contribution, it can be ensured that there is sufficient voltage drop across the switchable current source 140, which allows for a reliable operation of the switchable current source and which allows the switchable current source to provide the desired current. Such a choice of the output voltage of the floating voltage source also allows to bring the potential at the control terminal 120d of the switch transistor 120 to an appropriate value to fully turn on the switch transistor 120 when the switch-on process is completed. In particular, a voltage of 1.5 times the threshold voltage of the switch transistor 120, or a voltage of at least 6 volt, or of at least 8 volt, or of at least 10 volt is typically sufficient to fully turn on the switch transistor 120 and to have a sufficient voltage drop across the switchable current source 140.

[0101] In some implementations, the output current of the photo coupler may, for example, be chosen to be smaller than a current provided by the switchable current source. Such a design may be advantageous, since a relatively cheap photo coupler may be used. Nevertheless, the voltage at the output of the photo coupler may be maintained at a sufficiently high level by virtue of a buffer capacitor coupled in parallel with the output of the photo coupler. Thus, such an implementation brings along a significant advantage.

[0102] Preferably, but not necessary, the switch transistor 120 may be a field effect transistor. In particular, it has been found the field effect transistors are particularly well-suited for the present application, since they have a substantially negligible control current (gate current) in a static or quasi-static case and typically also a relatively low on-resistance. Thus, it advantageous to use a field effect transistor (e.g. a MOSFET) as a switch transistor 120, but this is not absolutely necessary.

[0103] Moreover, optionally, the power switch 100 may comprise a switch off circuit (which is also designated as a discharge circuit herein) configured to at least partially discharge the control capacitor 130 when a switch-off of the power switch 100 (or, equivalently, of the switch transistor 120) as desired. However, details regarding this optional feature will be described below.

[0104] Moreover, it should be noted that the power switch 100 may optionally be supplemented by any of the features, functionalities and details disclosed herein, both individually and taken in combination.

[0105] 2. Power Switch according to 2 Fig. 2 shows a block schematic diagram of a power switch 200, according to an embodiment of the present invention. In particular, Fig. 2 shows a block diagram of a power switch for a capacitive load.

[0106] For example, Fig. 2 shows, at reference numeral 200, the power switch, and Fig. 2 shows, at reference numeral 280 an example of a capacitive load, or of a mixed load.

[0107] As can be seen in Fig. 2, an input 210 of the power switch 200 is coupled to a power rail, which is illustrated in Fig. 2 as a voltage source 270. For example, the voltage source 270 may provide a voltage of 48 volt at the input 210 of the power switch 200, and the power rail (or, equivalently, the voltage source 270 modeling the power rail) may a provide charging a current 272 to the input 210 of the power switch 200.

[0108] However, it should be noted that different types of power sources may be coupled to the input 210 of the power switch, where it is desirable (but not absolutely necessary) that the power source provides a substantially constant voltage.

[0109] Regarding the load 280, it should be noted that the load 280 shown in Fig. 2 should be considered as an example only, and that different types of loads may be coupled to the output 212 of the power switch 200.

[0110] In the example shown in Fig. 2, the load 280 may comprise a boost converter 282, wherein input 282a of the boost converter is coupled with the output 212 of the power switch. An output 282b of the boost converter 282 may, for example, be coupled to one or more decoupling capacitors 284a, 284b and to a load circuitry 286 (wherein the one or more decoupling capacitors 284a, 284b may be considered as optional, and wherein the one or more decoupling capacitors 284a, 284b may, for example, be circuited in parallel to the load circuitry 286).

[0111] However, it should be noted that, for example, the voltage source 270, which provides the input voltage at the input 210 of the power switch 200 may be coupled between a reference potential (or a reference potential conductor) GND and the input 210 of the power switch. Moreover, the DC / DC converter (or boost converter) 282 may also be coupled to the reference potential conductor or reference potential GND. Moreover, the optional one or more decoupling capacitors 284a, 284b and the load circuitry 286 may, for example, be coupled between the output 282b of the DC / DC converter (e.g., boost converter) 282 and the reference potential conductor or reference potential GND.

[0112] In the following, some (optional) details of the DC / DC converter will be described as an example. For example, the DC / DC converter (e.g. boost converter) 282 may comprise an input capacitor 282c, which is coupled between the input 282a of the DC / DC converter 282 and the reference potential conductor or reference potential GND. Moreover, an inductor 282d is coupled between the input 282a of the DC / DC converter 282 and an internal node 282e of the DC / DC converter 282. Moreover, a switch 282f (e.g. a load path of a switch transistor) is coupled between the internal node 282e of the DC / DC converter 282 and the reference potential conductor or reference potential GND. In addition, a diode 282g is coupled between the internal node 282e and the output 282b of the DC / DC converter. In addition, an (optional) output capacitor is coupled between the output 282b of the DC / DC converter 282 and the reference potential conductor or reference potential GND.

[0113] Regarding the DC / DC converter 282, it should be noted that the DC / DC converter 282 comprises an input capacitor 282c, which is permanently coupled between the input 282a of the DC / DC converter 282 and the reference potential conductor or reference potential GND. The input capacitor 282c is active to avoid excessive fluctuations of an input current of the DC / DC converter 282. The DC / DC converter 282 typically comprises a control which repeatedly (e.g. periodically; e.g. in a controlled manner) opens and closes the switch 282f, such that a current flow is built up through the inductor 282d when the switch 282f is closed. Once the switch 282f is opened, the current that was built up in the inductor 282d continues flowing, and flows through the diode 282g, to thereby charge the output capacitor 282h of the DC / DC converter 282 (e.g. to a voltage which is higher than the voltage at the input 282a of DC / DC converter). Accordingly, it can be achieved that the output voltage at the output 282b of the DC / DC converter 282 is larger than the input voltage at the input 282a of the DC / DC converter 282. Also, it should be noted that there is always a current flow through the load circuitry 286 as soon as there is a voltage present at the input 282a of the DC / DC converter, since there is permanently a conductive path from the input 282a of the DC / DC converter to the output 282b of the DC / DC converter through the inductor 282d and the diode 282g.

[0114] However, it should be noted that the DC / DC converter 282 should be considered as an example only. However, it should also be noted that the power switch 200 is well-suited for usage in combination with such an (up converting) type of DC / DC converter, since the power switch can properly deal with the fact that there is an input capacitor 282c comprising a comparatively large capacitance which is always active, and also with the fact that there is always a current path to the load circuitry 286.

[0115] However, it should be noted that the power switch 200 is naturally also usable with different types of load, and that the power switch 200 does not require the presence of a capacitive load.

[0116] Turning now to the power switch 200, it should be noted that the power switch 200 comprises, as a switch transistor, a MOSFET (metal-oxide-semiconductor-field-effect- transistor) 220. For example, the MOSFET 220 is a N-channel MOSFET, but it should be noted that complementary implementations (wherein a P-channel MOSFET is used instead of a N-channel MOSFET, and wherein a NPN-bipolar transistor is used instead of a PNP bipolar transistor, and wherein polarities of diodes and voltage sources are swapped (reversed)) are naturally also possible. In the example of Fig. 2, a source terminal 220b of the MOSFET 220 is coupled to the output 212 of the power switch, and a drain terminal 220c of the MOSFET 220 is coupled to the input 210 of the power switch. As an example, the source terminal 220b is directly coupled to the output 212 of the power switch, and the drain terminal 220c is coupled directly to the input 210 of the power switch. However, low- ohmic components, like, for example, a shunt resistor, could optionally be coupled in between the respective terminals of the MOSFET 220 and the input 210 and / or the output 212, respectively.

[0117] The power switch 200 comprises a control capacitor 230, which is coupled between the control terminal (gate terminal) 220d of the MOSFET 220 and the reference potential conductor or reference potential GND.

[0118] Moreover, the power switch 200 comprises a switchable current source 240, wherein a first terminal 240a of the switchable current source 240 is coupled to the input 210 of the power switch and wherein a second terminal 240b of the switchable current source is coupled to a second terminal 250b of a floating voltage source 250(also designated as a floating auxiliary supply 250). A first terminal 250a of the floating voltage source (or of the floating auxiliary supply) 250 is coupled to the control terminal 220d of the MOSFET 220 via a bootstrap circuit (also designated as discharge circuit or switch-off circuit) 260. Equivalently, it can be said that the first terminal of the floating voltage source 250 is coupled with the first terminal of the controller capacitor 230 via the bootstrap circuitry 260. However, it should be noted that the bootstrap circuitry or discharge circuitry 260 may be considered as being optional, such that the first terminal 250a of the floating voltage source 250 might also be coupled directly to the control terminal 220d of the MOSFET 220 or to the first terminal 230a of the control capacitor 230.

[0119] Regarding the switchable current source 240, it should be noted that the switchable current source 240 may generally be considered as a current source that can be activated and deactivated via a control signal or a control line 242, wherein the control signal or control line 242 may, for example, be driven by a control circuitry like, for example, a field programmable gate array, or a microprocessor, or a microcontroller, or the like. However, it should be noted that, since the switchable current source 240 is a “real-world” switchable current source, the switchable current source 240 typically requires some voltage drop across its first terminal 240a and its second terminal 240b to operate properly. Also, it should be noted that the switchable current source 240 is illustrated in Fig. 2 as a series connection of a fixed current source 240c and a switch 240d, but it should be noted that different implementations of the switchable current source are naturally also possible. Typically, the switchable current source is implemented using one or more transistors (e.g., field effect transistors and / or bipolar transistors), and that it may be desirable to have a voltage drop between the first terminal 240a and the second terminal 240b of the switchable current source 240 of at least one volt or at least two volt. Moreover, it should be noted that, in some cases, a current provided by the switchable current source may be adjustable. However, this functionality is not essential.

[0120] Taking reference now to the floating voltage source (or floating auxiliary supply) 250, it should be noted that a floating voltage source 250 may, for example, comprise a polarization such that a potential at the first terminal 250a of the floating voltage source is higher than a potential at the second terminal 250b of the floating voltage source 250 (wherein it is assumed that the voltage at the input 210 of the power switch is positive with respect to the reference potential at the reference potential conductor GND). Accordingly, the floating voltage source 250 may, for example compensate (or, preferably, even over compensate) the voltage drop which is typically unavoidable between the first terminal 240a of the switchable current source 240 and the second terminal 240b of the switchable current source 240. Accordingly, the switchable voltage source 250 may bring along the effect that the potential at the first terminal 250a of the floating voltage source is higher than the potential at the input 210 of the power switch, while there is still a sufficient voltage drop between the first terminal 240a of the switchable current source 240 and the second terminal 240b of the switchable current source (sufficient for ensuring a proper operation of the switchable current source).

[0121] Taking reference now to the bootstrap circuitry or discharge circuitry or switch-off circuitry 260, it should be noted that the bootstrap circuitry or discharge circuitry or switch-off circuitry is coupled between the first terminal 250a of the floating voltage source and the control terminal 220d of the switch transistor 220 (or, equivalently, the first terminal of the control capacitor 230), and that the bootstrap circuitry or discharge circuit or switch-off circuitry 260 is also coupled between the control terminal 220d of the MOSFET 220 and the source terminal 220b of the MOSFET 220.

[0122] For example, the discharge circuitry may comprise a diode 260a, which is coupled between the first terminal 250a of the floating voltage source 250 and the control terminal 220d of the MOSFET 220. For example, an anode of the diode 260a is coupled to the first terminal 250a of the floating voltage source 250, and a cathode of the diode 260a is coupled to the control terminal 220d of the MOSFET 220. Moreover, the discharge circuit 260 also comprises a transistor (also designated as discharge transistor) 260b which, in the example of Fig. 2, is a bipolar transistor (e.g. a PNP bipolar transistor). However, it should be noted that a field detect transistor could be used as well (wherein, in the case of the field detect transistor, a source terminal takes the role of the emitter terminal, a drain terminal takes the role of the collector terminal, and a gate terminal takes the role of a base terminal). For example, an emitter terminal of the discharge transistor 260b is coupled with a cathode terminal of the diode 260a and with the control terminal 220d of the MOSFET 220 (or, equivalently, with the first terminal 230a of the control capacitor 230, or, equivalently, with a node between the control terminal 220d of the MOSFET 220 and the first terminal 230a of the control capacitor 230).

[0123] Moreover, a base terminal of the discharge transistor 260b is coupled with an anode of the diode 260a (or, equivalently, with the first terminal 250a of the floating voltage source 250, or, equivalently, with a node between the first terminal 250a of the floating voltage source 250 and the anode of the diode 260a). A collector terminal of the discharge transistor 260b is coupled with the source terminal 220b of the MOSFET 220 (or, equivalently, with the output 212 of the power switch 200 or, equivalently, with a node between the source terminal 220b of the MOSFET 220 and the output 212 of the power switch 200). Moreover, a resistor 260c is coupled between the base terminal of the discharge transistor 260b and the collector terminal of the discharge transistor 260b. Regarding the functionality of the discharge circuit 260, it should be noted that the discharge transistor 260b will be switched off (e.g. automatically) when a charging current is flowing into the control capacitor 230a (via the diode 260a). However, when there is no charging current flowing into the control capacitor 230, the discharge transistor 260b will be turned on, thereby establishing a current path between the first terminal 230a of the control capacitor 230 and the source terminal 220b of the MOSFET (or the output 212 of the power switch 200). For example, when a (positive) current is flowing from the first terminal 250a of the floating voltage source 250 towards the first terminal 230a of the control capacitor 230, there will be a voltage drop across the diode 260a, and consequently a potential at the base terminal of the discharge transistor 260b will be more positive than a potential at the emitter terminal of the discharge transistor 260b. Accordingly, the discharge transistor 260b will be automatically turned off in this situation, since the discharge transistor 260b is a PNP transistor. On the other hand, if there is no current (more precisely, no forward current) flowing through the diode 260a, the resistor 260c pulls the potential at the base terminal of the discharge transistor 260b towards the potential at the collector terminal of the discharge transistor 260b, which turns the discharge transistor 260b on if the potential at the emitter terminal of the discharge transistor 260b (which is typically equal to the potential at the first terminal of the control capacitor 230) is higher than the potential at the base terminal of the discharge transistor 260b (e.g. provided that the voltage difference is larger than or equal to a threshold voltage of the base-emitter diode of discharge transistor 262b). Accordingly, assuming a proper dimensioning of the resistor 260c, the discharge transistor 260b will be turned on if there is no (forward) current flowing through the diode 260a and if the potential at the source terminal 220b of the MOSFET 220 is lower (or significantly lower) than the potential at the first terminal 230a of the control capacitor 230 (or the potential at the gate terminal 220d of the MOSFET 220). Accordingly, if here is no (forward) current flowing through the diode 260a (which is typically the case if the switchable current source is deactivated), the discharge capacitor 260b will be automatically activated if the potential at the gate terminal 260d is higher (or substantially higher) than the potential at the source terminal 220b of the MOSFET 220. When the discharge transistor 260b is turned on, discharge transistor 260b will allow for a current flow between the gate terminal 220d of the MOSFET 220 and the source terminal 220b of the MOSFET 220, to thereby approximate the potential at the gate terminal 220d of the MOSFET 220 to the potential at the source terminal 220b of the MOSFET 220. As a result, the gate source voltage of the MOSFET 220 will be reduced down to a relatively small value, which will turn the MOSFET 220 off. In other words, generally speaking, when there is no current flowing through the diode 260a (i.e. when the switchable current source is switched off), the discharge transistor 260b will discharge the control capacitor 230 so much that the MOSFET 220 is turned off.

[0124] In the following, the overall functionality of the power switch 200 will be briefly summarized. When the switchable current source 240 is switched on, the switchable current source 240 will provide a predetermined current (e.g. ICTRL), provided that there is a sufficient voltage drop across the switchable current source 240 (e.g. between the first terminal 240a and the second terminal 240b of the switchable current source 240).

[0125] However, the presence of a sufficient voltage drop across the switchable current source is typically ensured by the floating voltage source 250, wherein the floating voltage source has the effect that the potential at the first terminal 250a of the floating voltage source is elevated (generally speaking: shifted) with respect to the potential at the second terminal 250b of the floating voltage source 250. Accordingly, by properly dimensioning the floating voltage source 250, it can be achieved that the potential at the first terminal 250a of the floating voltage source 250 is even higher than the potential at the input 210 of the power switch 200, while still having a sufficient voltage drop across the switchable current source 240.

[0126] As a consequence, when the switchable current source 240 is switched on, a current will flow through the diode 260a towards the control capacitor 230a, thereby charging the control capacitor 230. Since, preferably, a capacitance value of the control capacitor 230 is significantly larger than a gate source capacitance of the MOSFET 220, a time constant of the charging process will be dominated by the capacitance of the control capacitor 230 and is therefore well-defined. A voltage slope of a voltage at the first terminal 230a of the control capacitor 230 is determined primarily by the current ICTRL and by the capacitance CSLP of the control capacitor 230.

[0127] Accordingly, when starting from an initial state in which the control capacitor 230 is discharged, the potential at the first terminal 230a of the control capacitor 230, and consequently the potential at the gate terminal 220d of the MOSFET 220, will steadily increase as soon as the switchable current source 240 is activated (at least until a saturation is reached). However, in the absence of significant parasitic effects, the temporal evolution of the potential at the gate terminal 220d of the MOSFET 220 will be approximately linear over time, until a saturation is reached (wherein this saturation may be determined, for example, by the potential at the input 210 of the power switch, by the voltage provided by the floating voltage source 250, and by the voltage drop required by the switchable current source 240).

[0128] As a consequence, since the MOSFET 220 is operated as a source follower, a potential at the source terminal 220b of the MOSFET 220 will follow the potential at the gate terminal 220d of the MOSFET 220, wherein a potential difference between the potential at the gate terminal 220d of the MOSFET and the potential at the source terminal 220b of the MOSFET 220 will be defined by the gate source voltage VGS of the MOSFET 220, and will be of the order of the threshold voltage (but typically somewhat larger than the threshold voltage) of the MOSFET 220 (at least as soon as the threshold voltage is reached, and until a saturation is reached). Accordingly, it can be expected that the potential at the source terminal 220b of the MOSFET 220 substantially follows the potential at the gate terminal 220d of the MOSFET 220 as soon as the gate source voltage of the MOSFET reaches the threshold voltage and until a saturation is reached. Accordingly, it can be assumed that the potential at the source terminal 220b of the MOSFET 220, and consequently the potential at the output 212 of the power switch, comprises a substantially linear increase over time until a maximum value is reached.

[0129] For example, when a sufficiently high gate source voltage is finally reached (e.g. when the voltage at the gate terminal 220d further increases, while the voltage at the source terminal has reached its maximum value), the MOSFET will finally be in a low-omic condition, such that there is only a small voltage drop between the input 210 of the power switch and the output 212 of the power switch in a fully-turned-on state of operation.

[0130] Accordingly, the voltage at the output 212 of the power switch will increase in a substantially linear temporal evolution when the switchable current source 240 is activated.

[0131] Moreover, as already explained in some detail above, when the switchable current source 240 is activated, the current is flowing through the diode 260a, charging the control capacitor 230, which in turn results in a deactivation (turn-off) of the discharge transistor 260b.

[0132] As a consequence, the control capacitor 230 is charged in a substantially linear manner over time (e.g., with a substantially constant current), at least until a saturation region is reached. Preferably, the MOSFET 220 is finally fully activated, wherein the gate source voltage is significantly higher than the threshold voltage of the MOSFET 220. Accordingly, the MOSFET 220 is finally in a low-ohmic mode of operation, such that there is only a small voltage drop between the input 210 and the output 212 of the power switch 200.

[0133] When the switchable current source 240 is deactivated, there is no longer a current flow through the diode 260a and the discharge transistor 260b is activated (as outlined above). In this case, the resistor 260c allows for the flow of a base current that turns on (activates) the discharge transistor 260b. Accordingly, the control capacitor 230 is discharged until the potential at the gate terminal 220d of the MOSFET 220 is only slightly above the potential at the source terminal 220b of the MOSFET 220. Accordingly, the MOSFET 220 is switched off in (put into high-ohmic state-of-operation). Consequently, the voltage at the output 212 of the power switch typically reduces, since no more current is supplied to the load 280. However, the discharge circuit 260 ensures that the MOSFET 220 remains switched off. In particular, the discharge circuit 260 ensures that the potential at the gate terminal 220d of the MOSFET 220 is not substantially above the potential at the source terminal 220b of the MOSFET 220 as long as there is no current flowing through the diode 260a (i.e., as long as the switchable current 240 is not switched on) by providing a conductive path between the gate terminal 220d of the MOSFET 220 and the source terminal 220b of the MOSFET 220.

[0134] However, it should be noted that the power switch 200 may optionally be supplemented by any of the features, functionalities and details disclosed herein.

[0135] Moreover, it should also be noted that a complementary implementation would also be possible, wherein the voltage at the input 210 of the power switch 200 is negative, wherein the N-channel MOSFET 220 is replaced by a P-channel MOSFET, wherein the PNP- transistor 260b is replaced by an NPN transistor, wherein a polarity of the diode 260a is reversed, and wherein polarities of any voltage sources are also reversed.

[0136] Moreover, it should be noted that the bipolar transistor 260b could optionally be replaced by a field effect transistor, and that the MOSFET 220 could optionally be replaced by a bipolar transistor of by any other type of field effect transistor.

[0137] Moreover, it should be noted that the input 210 of the power switch corresponds to the input 110 of the power switch 100, that the output 212 of the power switch 200 corresponds to the output 112 of the power switch 100, that the MOSFET 220 corresponds to the switch transistor 120, that the control capacitor 230 corresponds to the control capacitor 130 and that the switchable current source 240 corresponds to the switchable current source 140, Any of the explanations made with respect to the power switch 100 may also apply.

[0138] 3. Power Switch according to Fig. 3

[0139] Fig. 3 shows a schematic of a power switch 300, according to an embodiment of the invention. Fig. 3 also shows a (simplified) schematic of a load 380.

[0140] However, it should be noted that specific types of circuit elements and specific parameters of circuit elements shown in Fig. 3 should be considered as examples only. It should be noted that other types of circuit elements and other element values of the circuit elements could also be used. In particular, any of these details (part numbers, resistance values, capacitance values, and so on) should be considered as examples only.

[0141] As can be seen, the power switch 300 comprises an input 310, to which a voltage source 370 is connected. The power switch 300 also comprises an output 312 which is coupled to the load 380.

[0142] In the example of Fig. 3, the load is shown as a boost converter, having an input capacitor 382c and an output capacitor 382h. However, it should be noted that different types of loads could also be used, and that the specific load is only provided as an example to allow for a meaningful circuit simulation.

[0143] The power switch 300 comprises a MOSFET 320, which corresponds to the MOSFET 220 and to the switch transistor 120, and which takes the role of a switch transistor. The MOSFET 320 is also designated as “switching MOSFET” (even though the MOFET primarily operates as a source follower). A source terminal 320b of the MOSFET is coupled to the output 312, and a drain terminal 320c is coupled to the input 310. The power switch 300 also comprises a control capacitor 330, which corresponds to the control capacitor 230 and to the control capacitor 130. A first terminal 330a of the control capacitor 330 is coupled to the gate terminal 320d of the MOSFET 320.

[0144] The power switch 320 also comprises a switchable current source 340, which corresponds to the switchable current source 240 and to the switchable current source 140. A first terminal 340a of the switchable current source is coupled to the input 310, and the second terminal 340b of the switchable current source 340 is coupled to a second terminal (e.g., a second output terminal) 350b of a floating voltage source 350.

[0145] A first terminal (e.g., a first output terminal) 350a of the floating voltage source 350 is coupled with the gate terminal 320d of the MOSFET 320 (or, equivalently, with the first terminal 330a of the control capacitor 330) via a diode 360a of a discharge circuit 360 (which corresponds to the discharge circuit 260, and which can also be considered as a switch-off circuit or as a bootstrap circuit). However, it should be noted that the floating voltage source 350 corresponds to the floating voltage source 250, and that the discharge circuit 360 corresponds to the discharge circuit 260.

[0146] In the following, some details regarding the individual components will be described.

[0147] The switchable current source 340 comprises an output transistor 340c, which is, for example, a PNP bipolar transistor. However, different transistor types could also be used. For example, an emitter terminal of the output transistor 340c is coupled with the input 310 of the power switch via a resistor 340d. For example, two diodes 340e and 340f are circuited in series between the input 310 of the power switch and the base terminal of the transistor 340c (e.g. to provide a relatively stable voltage across the base-emitter path of the output transistor 340c and the resister 340d). Moreover, the switchable current source 340 comprises another transistor 340g which, for example, is a NPN bipolar transistor. For example, a collector terminal of the further transistor 340g is coupled with the base terminal of the transistor 340c via a resistor 340h. Moreover, a base terminal of the transistor 340g is coupled with a control voltage source 344 via a resistor 340i. Moreover, an emitter terminal of the transistor 340g is coupled with a reference potential conductor GND.

[0148] Accordingly, it can be seen that the switchable current source 340 is supplied by the input voltage of the power switch 300 at the input 310 of the power switch 300. When the transistor 340g is turned on by an appropriate control signal provided by the control signal source 344, an appropriate (and preferably well-defined) bias voltage is applied at the base terminal of the transistor 340c, and a typically well-defined current is flowing through the collector emitter path of the transistor 340c, provided that there is a sufficient voltage drop across the collector emitter path of the transistor 340c (and also across the resistor 340d). In contrast, if the transistor 340g is switched off, the transistor 340c is also deactivated and there will be no current flowing through the collector emitter path of the transistor 340c. Accordingly, the switchable current source 340 can be switched on and off by the control signal provided by the control signal provider 344. Moreover, it should be noted that, for an operation of the switchable current source 340, it is desirable to have a certain minimum voltage drop across the resistor 340b and across the current emitter path of the transistor 340c which may, for example, be of the order of 1 V or of the order of 2V. However, it should be noted that the switchable current source can also be implemented in a different manner, e.g. using a current mirror circuit structure.

[0149] The floating voltage source 350 is implemented, for example, using a photo coupler 351. For example, an input side of the photo coupler may comprise one or more light-emitting diodes (or other types of light source). For example, a first input terminal 351a of the photo coupler is coupled with a voltage source 354 via a resistor 353, which serves to limit a current. A second input terminal 351 b of the photo coupler is coupled, for example, with a reference potential conductor GND. However, it should be noted that it is not relevant for the present invention how the input side of the photo coupler is powered. However, it is advantageous to use a power supply which is available before the power switch is to be turned on.

[0150] An output side of the photo coupler 351 may, for example, comprise one or more photo diodes or other photo elements which provide an output voltage on the basis of an illumination generated on the basis of an input voltage of the photo coupler 351. For example, a first output terminal 351 c of the photo coupler 351 is coupled with the first output terminal 350a of the floating voltage source, and the second output terminal 351 d of the photo coupler 351 is coupled with the second output terminal 350b of the floating voltage source 350. In other words, the second output terminal 351 d of the photo coupler is coupled with the second terminal 340b of the switchable current source.

[0151] Moreover, a buffer capacitor 355 is coupled between the first output terminal 351c of the photo coupler 351 and the second output terminal 351 d of the photo coupler 351 (or, equivalently, between the first output terminal 350a of the floating voltage source and the second output terminal 350b of the floating voltage source 350). For example, the buffer capacitor 355 may be charged by a current provided by the photo coupler as soon as a sufficient voltage is applied at the input terminals 351 a, 351 b of the photo coupler 351 , e.g. even before the switchable current source 340 is activated. The buffer capacitor 355 may, for example, be charged up to a voltage which is close to the maximum output voltage of the photo coupler 351. For example, the capacitor 355 may be charged before the switchable current source is switched on. For example, an output current of the photo coupler (e.g. at a desired output voltage of the photo coupler) may be smaller than a current provided by the switchable current source 340.

[0152] Accordingly, when the switchable current source 340 is activated, a voltage between the first output terminal 350a and the second output terminal 350b of the floating voltage source may be kept at least substantially constant by the buffer capacitor 355, even though an (instantaneous) current provided at the output of the photo coupler 351 is smaller than the current provided by the switchable current source 340. Accordingly, the combination of the photo coupler 351 and of the buffer capacitor 355 acts at as a floating voltage source, a voltage of which is at least approximately constant during a time when the control capacitor 330 is charged.

[0153] Thus, the current provided by the switchable current source 340 is forwarded to charge the control capacitor 330, with only a small fraction of the current being “lost” via the resistor 360c of the discharge circuit 360. As a consequence, the control capacitor 330 is charged with an approximately constant current as soon as the switchable current source 340 is switched on, e.g. until a saturation region is reached.

[0154] Regarding the functionality of the discharge circuit 360, the above explanations with respect to discharge circuit 260 also apply.

[0155] Consequently, the voltage at the output 312 substantially follows the voltage at the gate terminal 320d of the MOSFET 320, until a saturation is reached (wherein it is clear that the voltage at the output 312 cannot be higher than the voltage at the input 310).

[0156] When turning off the switchable current source 340, there will be no longer a current flowing through the diode 360a of the discharge circuit. Accordingly, the discharge circuit 360 will be activated, as explained above, and the MOSFET 320 will be turned off.

[0157] In the following, a temporal evolution of signals in the circuit 300 will be described taking reference to Fig. 4.

[0158] For example, in Fig. 4, an abscissa 410 describes a time. A first ordinate 412 describes a voltage, a second ordinate 414 describes a voltage and a third ordinate 316 describes a current. A first curve 420 describes a temporal evolution of a control voltage used to turn on and off the switchable current source. A second curve 422 describes a temporal evolution of a gate source voltage of the MOSFET 320. A third curve 424 describes a temporal evolution of a voltage at the input 310 of the power switch 300. A fourth curve 426 describes a temporal evolution of a gate voltage (between the gate terminal 420d and the reference potential conductor GND) of the MOSFET 320, a fifth curve 428 describes a temporal evolution of a voltage at an output 312 of the power switch 300, a sixth curve 430 describes a sum of the currents in capacitors C1 (382h) and C2 (382c). A seventh curve 432 describes a temporal evolution of a current flowing through a resistor 390, which is a load resistor.

[0159] For example, at a time t1 the control signal for the switchable current source is activated. As a consequence, a voltage at the gate terminal 320d of the MOSFET 320 (as shown in curve 426) starts to increase. The increase is at least approximately linear over time, with a non-linearity in a starting region (e.g. between times t1 and t2) and in a saturation region (e.g. between times t3 and t4). It can also be seen that the gate source voltage increases between times t1 and t2. However, at time t2, the gate source voltage reaches the threshold voltage and remains substantially constant between times t2 and t3 (with only a minor increase over time). Also starting at time t2, the voltage at the output 312 of the power switch 300 starts to raise, since, starting from time t2, the MOSFET 320 operates as a source-follower. The substantially linear increase of the output voltage at the output 312 over time lasts until the time t3. At time t3, the output voltage at the output 312 of the power switch 300 reaches the input voltage at the input 310 of the power switch, and therefore cannot increase further.

[0160] However, between times t3 and t4, the gate source voltage increases further, e.g., from approximately 4 volt up to approximately 8 volt. Between times t3 and t4, an increase of the voltage at the gate terminal 320d is slowed down, but the increase of the voltage at the gate terminal 320d continues up to time t4 (and possibly even after time t4). However, the output voltage at the output 312 of the power switch remains substantially constant between times t3 and t5, wherein the further increase of the gate source voltage between times t3 and t4 may help to reduce an ON-resistance of the MOSFET 320.

[0161] At time t5, the control signal for the switchable current source is deactivated. As a consequence, the gate source voltage reduces quickly (e.g. between times t5 and t6), due to the functioning of the discharge circuit 360. Thus, the MOSFET 320 is switched off. However, the decay of the voltage at the output 312 of the power switch is mainly caused by the load 380 (including the resistor 390). In other words, after time t5, capacitors 382c and 382h are discharged substantially by resistor 390, which results in a decay of the voltage at the output 312 of the power switch 300. However, it can be seen that the voltage at the gate terminal 320d of the MOSFET 320 is closely following the voltage at the output 312 of the power switch 300 after time t6, which is caused by the functionality of the discharge circuit 360.

[0162] Moreover, it can be seen that the sum of the charge currents of capacitors 382c and 382h (C2 and C1 ) is substantially constant between times t2 and t3, with some reduction towards t3. This substantially constant charging current, which does not exhibit an excessive peak, is caused by the substantially linear increase of the voltage at the output 312 of the power switch over time between times t2 and t3. The current flow through the capacitors 382c and 382h after time t5 is substantially caused by the discharge process through resistor 390.

[0163] Moreover, the current flowing through the resistor 390 is substantially proportional to (or at least linearly related to, when considering a voltage drop across a diode of the DC / DC converter) the voltage at the output of the boost converter 380.

[0164] To conclude, it becomes apparent that the power switch 300 helps to avoid excessive current peaks, but still reaches a low ON-resistance and a good switch-off behavior.

[0165] Moreover, it should be noted that the power switch 300 may optionally be supplemented by any of the features, functionalities and details, both individually and taken in combination.

[0166] Method according to Fig. 5

[0167] Fig. 5 shows a flowchart of a method for operating a power switch.

[0168] The power switch comprises a switch transistor, wherein a load part of the switch transistor is coupled between an input of the power switch and an output of the power switch. A source terminal of the switch transistor is coupled to the output of the power switch. The power switch comprises a control capacitor which is coupled between a control terminal of the switch transistor and a reference potential conductor. The method 500 comprises charging 510 the control capacitor using a current source, to thereby obtain a slope of a voltage at an output of the power switch which is substantially linear over time for a switch-on operation at least in a range of the voltage at the output of the power switch between 10% of a maximum voltage and 90% of a maximum voltage.

[0169] However, the method 500 may optionally be supplemented by any of the features, functionalities and details disclosed herein, also with respect to the power switch, both individually and taken in combination.

[0170] Conclusions and Further Remarks

[0171] To conclude, embodiments according to the invention create a solid state switch which can connect capacitive loads to a supply rail while big in rush currents are suppressed. For example, a typical application for a such a switch is to connect a boost-DC / DC converter including its load circuitry to a rail supply.

[0172] It has been recognized that embodiments according to the invention enable connecting capacitive load in a well-controlled way. The power switch for capacitive load generates a defined voltage ramp at its output which enables to limit the in rush current during turning on to an allowable maximum value.

[0173] A block diagram has been discussed with reference to Fig. 2, wherein Fig. 2 shows a block diagram of a power switch for capacitive load.

[0174] In the following, a short functional description will be provided, which may be applicable to any of the embodiments disclosed herein. For example, the block diagram of Fig. 2 gives an overview of the operation principle of the power switch for capacitive load:

[0175] For example, before connecting the load circuitry (boost-converter-DC / DC) (e.g. the load circuitry 280) to the 48 volt power rail (e.g. to the power rail 270), the capacitor CSLP (e.g. the control capacitor 230) in the switch (e.g. in the power switch 200) and the load-caps CBOOST.IN, CBOOST.OUT (e.g. capacitors 282c, 282h) are discharged (VSLP = 0 volt, VSW.OUT = 0 volt). The Switchable Current Source (e.g. the switchable current source 240) is turned on when the Control Line from FPGA (e.g. the control line 242) turns to HIGH. Then the capacitor CSLP (e.g. the control capacitor 230) begins to charge. As soon as the VGS, THRESHOLD is applied to the Switching MOSFET (e.g. to the gate terminal 220d of the MOSFET 220), CSLP gets further charged by an almost constant current. This generates a ramp with constant slew rate at the MOSFET Gate (e.g. at the gate terminal 220d) and at the Source (e.g. at the source terminal 220b) (switch output) accordingly.

[0176] This induced voltage ramp which appears at the load capacitors (CBOOSUN CBOOST.OUT ... ) (e.g. at the load capacitors 282c, 282h, but, for example, also at the decoupling capacitors 284a, 284b) leads to a constant (or at least approximately constant) charging current and not to a high current spike.

[0177] The Floating Auxiliary Supply (e.g. the floating voltage source 250) enables to provide a VGS (e.g. a gate source voltage between the gate terminal 220d and the source terminal 220b) high enough to turn the MOSFET (e.g. the MOSFET 220) on completely even if the voltage at the switch output (VSW.OUT) (e.g. at the output 212) is close to the input voltage (VSW.IN) (e.g. at the input 210).

[0178] For example, a photocoupler (e.g. a photo coupler 351 ) is sufficient for such purpose, like e.g. Toshiba TLP3914 which can provide a current of 20uA and a voltage of 7V min. A photocoupler can, for example, be supplied by a small digital supply voltage, e.g. 3.0V, which is typically available before the real power sequence gets started (e.g. before the switchable current source is switched on).

[0179] When turning the Switch off (e.g. Control Line from FPGA turns to LOW), discharging of the Load-Caps (e.g. of the load capacitors 282c, 282h) is done by the load circuitry and is basically no critical procedure where uncontrollable high currents appear. The function of the Bootstrap Circuitry (e.g. of the switch-off circuit or discharge circuit 260) is to reset the switch circuitry itself to Off-state: Turning off Switching MOSFET (e.g. MOSFET 220) (VGS ~ 0) and discharging of CSLP (e.g. control capacitor 230) (VSLP = 0).

[0180] In the following, an implementation example will be described.

[0181] For example, a calculation of switch parameters will be explained as an example. This is an example, how to adjust the switch circuitry (e.g. the power switch as disclosed herein, e.g. as described taking reference to Fis. 1 , 2 and 3) to get a certain, uncritical Cap-Load-Charging-Current during Turning-On:

[0182] • Determine a target charge current for load caps: for example, lcHARGE,max=0.2A

[0183] • Calculate corresponding Slew Rate (SR): for example, SR = ICHARGE,,™ / CLOAD = 0.2A I 200uF= 1V / ms

[0184] • Determine adequate current to charge CSLP during turning-on: for example, ISLP = 10UA

[0185] • Calculate CSLP: for example, CSLP = SR / lsLP =(1V / ms) / 10uA=10nF

[0186] • Calculate lcTRL(Can, for example, be adjusted by circuitry change), assumed, for example, IB=0, IC=0, VGS,THRESHOLD=4V: for example, IBS = VGS,THRESHOLD / RBS=4V / 2MQ=2UA for example, ICTRL = ISLP+IBS = 10uA + 2uA = 12uA

[0187] In the following, an example of a simulation of a switching sequence will be described.

[0188] The LTSpice-Model shown in Fig. 3 contains the complete circuitry for the Power Switch 300 for Capacitive Load.

[0189] In other words, Fig. 3 shows a LTSpice Model of a power switch for capacitive load.

[0190] Fig. 4 shows simulation results of the switch for capacitive load.

[0191] For example, the sequence in the simulation is as follows:

[0192] 1. V(CTRL) (e.g. the voltage controlling the switchable current source 340) changes from 0V to 3V Initiates Turn-ON, V(GATE,SW_OUT) (e.g. the gate-source voltage of the MOSFET 320) starts to increase 2. V(GATE,SW_OUT) (e.g. the gate-source voltage of the MOSFET 320) has passed VGS.THRESHOLD (e.g. the threshold voltage of the MOSFET 320), Voltage ramp at switch output V(SW_OUT) (e.g. at switch output 312) begins, Charging current l(C1)+l(C2) (e.g. a charging current of capacitors 382c, 382h) is reasonably constant, for example, at 0.2A (like calculated)

[0193] 3. Voltage at switch output (e.g. at switch output 312) reaches V(SWJN) (e.g. voltage at switch input 310), voltage ramp ends, Charging current l(C1)+l(C2) decreases to 0, gate voltage V(GATE) (e.g. a voltage at the gate terminal 320d) increases furtherly

[0194] 4. MOSFET (e.g. MOSFET 320) is turned on completely, V(GATE) (e.g. a voltage at the gate terminal 320d) is constant now

[0195] 5. V(CTRL) (e.g. the voltage controlling the switchable current source 340) changes from 3V to 0V —> Initiates Turn-OFF,

[0196] V(GATE,SW_OUT) (e.g. the gate-source voltage of the MOSFET 320) starts to decrease,

[0197] Voltage at switch output V(SW_OUT) (e.g. at switch output 212) starts to decrease, Discharging of load caps (e.g. of capacitors 382c, 382h) is done by resistive load current l(R1 )

[0198] 6. V(GATE,SW_OUT) (e.g. the gate-source voltage of the MOSFET 320) is quite stable at low voltage, MOSFET is turned off

[0199] 7. Load caps (e.g. of capacitors 382c, 382h) are nearly discharged V(SW_OUT)=0

[0200] In the following, some conclusive remarks will be provided.

[0201] According to an aspect, embodiments according to the invention enable connecting capacitive load in a well-controlled way.

[0202] For example, the power switch for capacitive load generates a defined voltage ramp at its output which enables to limit the inrush current during turning on to an allowable maximum value. Embodiments according to the invention are usable in the context of, or in combination with, an active power filter for higher voltages.

[0203] 5

Claims

Claims1. A power switch (100;200;300), wherein the power switch comprises a switch transistor (120;220;320), wherein a load path of the switch transistor is coupled between an input (110;210;310) of the power switch and an output (112;212;312) of the power switch, wherein a source terminal (120b;220b;320b) of the switch transistor is coupled to the output (112;212;312) of the power switch, wherein the power switch comprises a control capacitor (130;230;330) which is coupled between a control terminal (120d;220d;320d) of the switch transistor (120;220;320) and a reference potential conductor (GND), wherein the power switch comprises a switchable current source (140;240;340) configured to charge the control capacitor (130;230;330) in a switched-on state.

2. Power switch (100;200;300) according to claim 1 , wherein the switch transistor (120;220;320) is configured to operate as a source follower.

3. Power switch (100;200;300) according to claim 1 or 2, wherein the switchable current source (140;240;340) is coupled between the input (110;210;310) of the power switch and the control terminal (120d;220d;320d) of the switch transistor (120;220;320), and / or wherein the switchable current source (140;240;340) is coupled between the input (110;210;310) of the power switch and the control capacitor (130;230;330).

4. Power switch (100;200;300) according to one of claims 1 to 3, wherein the power switch comprises a floating voltage source (250;350); wherein the floating voltage source (250;350) is coupled between the switchable current source (140;240;340) and the control terminal (120d;220d;320d) of the switch transistor (120;220;320), and / or wherein the floating voltage source (250;350) is coupled between the switchable current source (140;240;340) and the control capacitor (130;230;330).

5. Power switch (100;200;300) according to one of claims 1 to 3, wherein the power switch (100;200;300) comprises a floating voltage source (250;350); wherein the floating voltage source (250;350) is coupled between an input (110;210;310) of the power switch and the switchable current source (140; 240;340).

6. Power switch (100;200;300) according to claim 4 or claim 5, wherein the floating voltage source (250;350) comprises a photo coupler (352).

7. Power switch (100;200;300) according to claim 6, wherein the floating voltage source (250;350) comprises a buffer capacitor (355) which is coupled in parallel with an output port of the photo coupler (351 ); wherein a capacitance of the buffer capacitor (355) is larger than a capacitance of the control capacitor (130;230;330), or wherein a capacitance of the buffer capacitor (355) is larger, at least by a factor of 2, than a capacitance of the control capacitor (130;230;330), orwherein a capacitance of the buffer capacitor (355) is larger, at least by a factor of 5, than a capacitance of the control capacitor (130;230;330), or wherein a capacitance of the buffer capacitor (355) is larger, at least by a factor of 8, than a capacitance of the control capacitor (130;230;330).

8. Power switch (100;200;300) according to one of claims 6 or 7, wherein the power switch or a control circuitry coupled to the power switch is configured to activate the photo coupler (351) before the switchable current source (140;240;340) is switched on.

9. Power switch (100;200;300) according to one of claims 5 to 8, wherein an output voltage of the floating voltage source (250;350) is larger than or equal to 1.5 times a threshold voltage of the switch transistor (120;220;320); and / or wherein an output voltage of the floating voltage source (250;350) is larger than or equal to 6V, or wherein an output voltage of the floating voltage source (250;350) is larger than or equal to 8V, or wherein an output voltage of the floating voltage source (250;350) is larger than or equal to 10 V.

10. Power switch (100;200;300) according to one of claims 5 to 9, wherein an output current of the photo coupler (351) is smaller than a current provided by the switchable current source (140;240;340).

11. Power switch (100;200;300) according to one of claims 1 to 10,wherein the switch transistor (120;220;320) is a field effect transistor.

12. Power switch (100; 200;300) according to one of claims 1 to 11, wherein the power switch comprises a switch-off circuit (260;360) configured to at least partially discharge the control capacitor (130;230;330).

13. Power switch (100;200;300) according to claim 12, wherein the switch-off circuit (260;360) is configured to be disabled by a current provided by the switchable current source (140;240;340) which charges the control capacitor (130;230;330), and wherein the switch-off circuit (260;360) is configured to be enabled, to at least partially discharge the control capacitor, if the switchable current source (140;240;340) is switched off.

14. Power switch (100;200;300) according to one of claims 1 to 13, wherein a first terminal (130a;230a;330a) of the control capacitor (130;230;330) is connected with a control terminal (120d;220d;320d) of the switch transistor (120;220;320), wherein a second terminal of the control capacitor (130;230;330) is connected with the reference potential conductor (GND), wherein a first output terminal (250a;350a) of the floating voltage source (250;350) is coupled with the first terminal (130a;230a;330a) of the control capacitor (130;230;330) via a diode (260a;360a); wherein a second output terminal (250b;350b) of the floating voltage source (250;350) is coupled with a current output (240b;340b) of the switchable current source (140;240;340);wherein a buffer capacitor (355) is coupled between the first output terminal (250a;350a) of the floating voltage source (250;350) and the second output terminal (250b;350b) of the floating voltage source (250;350); wherein a source terminal of a discharge transistor (260b;360b) is coupled to the first terminal (130a;230a;330a) of the control capacitor (130;230;330), wherein a sink terminal of the discharge transistor (260b; 360b) is coupled with the output (112;212;312) of the power switch and / or with the source terminal (120b;220b;320b) of the switch transistor; wherein a control terminal of the discharge transistor (260b;360b) is coupled with the first output terminal (250a;350a) of the floating voltage source; wherein a resistor (260c;360c) is coupled between the control terminal of the discharge transistor (260b;360b) and the sink terminal of the discharge transistor (260b;360b).

15. A method (500) for operating a power switch (100;200;300), the power switch comprising a switch transistor (120;220;320), wherein a load path of the switch transistor is coupled between an input (110;210;310) of the power switch and an output (112;212;312) of the power switch, wherein a source terminal (120b;220b;320b) of the switch transistor (120;220;320) is coupled to the output (112;212;312) of the power switch, and wherein the power switch comprises a control capacitor (130;230;330) which is coupled between a control terminal (120d;220d;320d) of the switch transistor and a reference potential conductor (GND), wherein the method comprises charging (510) the control capacitor (130;230;330) using a current source (140;240;340), to thereby obtain a slope of a voltage at an output (112;212;312) of the power switch which is substantially linear over time for a switch-onoperation at least in a range of the voltage at the output of the power switch between 10 percent of a maximum voltage and 90 percent of a maximum voltage.

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