INRUSH CURRENT LIMITING CIRCUIT CONFIGURED FOR LIMITING INRUSH CURRENT TO A CAPACITOR, AND CORRESPONDING POWER SUPPLY AND METHOD - Patent application

Abstract

An inrush current limiting circuit configured to limit an inrush current to a capacitor is provided, the inrush current limiting circuit comprising: a controllable current limiting device configured to be connected in series with the capacitor, the impedance of the current limiting device being controllable; and a control circuit configured to control the impedance of the controllable current limiting device based on a voltage across the capacitor and the inrush current to the capacitor.

Description

[Technical Field]

[0001] FIELD OF THE DISCLOSURE The present disclosure relates generally to the field of inrush currents, and more particularly to methods for effectively limiting inrush currents. [Background technology]

[0002] Inrush current may be defined as the current drawn by an electrical device when it is first turned on. Given that various types of capacitors or the like need to be charged, inrush current can rise to high levels. High inrush current is generally undesirable because it can gradually damage components and can blow fuses or trip circuit breakers.

[0003] This is the case, for example, with bus capacitors. A power supply may convert a mains voltage into a direct current (DC) voltage. The DC voltage is then supplied to a bus, to which capacitors are connected for various reasons. One of the reasons is to generate a stable bus voltage.

[0004] When a power supply is turned on, an inrush current may be generated to charge that particular bus capacitor. Initially, the bus capacitor is fully discharged so that a relatively high inrush current can be supplied to the bus capacitor.

[0005] Various types of inrush current limiting circuits are already known. One example involves the use of a resistor connected in series with the output of the power supply. This means that the inrush current must pass through the resistor, thereby effectively establishing an inrush current limiting element.

[0006] Such an approach is not very efficient, especially in high power devices, because the resistor has a voltage drop and dissipates some power, which reduces the efficiency of the overall power supply / electrical device.

[0007] Inrush current can also be reduced by negative temperature coefficient (NTC) thermistors. NTC thermistors are commonly used in switching power supplies, motor drives, and audio equipment to prevent damage caused by inrush current. Thermistors are heat-sensitive resistors whose resistance changes significantly and predictably as a result of temperature changes. The resistance of an NTC thermistor decreases as its temperature increases.

[0008] A disadvantage of known methods is that they do not effectively limit the inrush current to a desired amount. Summary of the Invention [Problem to be solved by the invention]

[0009] It is an object of the present disclosure to provide an inrush current limiting circuit configured to limit inrush current to a capacitor, e.g., a bus capacitor. It is a further object of the present disclosure to provide a corresponding method and a corresponding power supply. [Means for solving the problem]

[0010] In a first aspect of the present disclosure, there is provided an inrush current limiting circuit configured to limit an inrush current to a capacitor, the circuit comprising: - a controllable current limiting device configured to be connected in series with the capacitor, the impedance of the current limiting device being controllable; a control circuit configured to control the impedance of the controllable current limiting device based on the voltage across the capacitor and the inrush current into the capacitor.

[0011] The inventors have found that it is beneficial if the current limiting device is controlled based on the voltage across the capacitor in combination with the inrush current into the capacitor.

[0012] Typical prior art solutions control the inrush current in one dimension: they look at the amount of inrush current and want to prevent the inrush current from exceeding a certain threshold.

[0013] The inventors have found that this may not be the most beneficial way to limit the inrush current. The amount of allowable inrush current may depend on the voltage across the capacitor. Whenever the voltage across the capacitor is low, a relatively high inrush current is allowable, but whenever the voltage across the capacitor is high, a relatively low inrush current is desirable.

[0014] One advantage of the present disclosure is that the inrush current limiting circuit may control the inrush current based on the total amount of energy delivered to the capacitor, rather than regulating based only on the instantaneous amount of current delivered to the capacitor.

[0015] In an example, the control circuit is configured to control the impedance of the controllable current limiting device based on the voltage across the capacitor and the inrush current into the capacitor so that the total amount of energy supplied to the capacitor remains relatively constant over time.

[0016] It has been found that rather than actually controlling or limiting the inrush current itself, it may be beneficial to control or limit the amount of energy supplied to the capacitor. The total amount of energy supplied to the capacitor depends on the voltage across the capacitor and the current with which the capacitor is charged. By controlling the current limiting device in both aspects, control of the total amount of energy can be achieved.

[0017] In an example, the control circuitry comprises: - a current sensing circuit configured to provide a measure of the inrush current; - a voltage measurement circuit configured to provide a measure of the voltage across the capacitor; The control circuit is configured to control the impedance of the controllable current limiting device based on the measure of the inrush current and the measure of the voltage across the capacitor.

[0018] The control circuit controls the current limiting device based on the inrush current and the voltage across the capacitor. This example provides an efficient implementation of this in practice. Typically, the measure of the inrush current is a voltage signal. The voltage drop across a resistor through which the inrush current flows represents the actual inrush current itself. This particular voltage drop can be used as the measure of the inrush current.

[0019] The measure of the voltage across the capacitor can be determined in several ways: According to the present disclosure, the bus capacitor can be connected in series with the current limiting device; The current limiting device can be connected to ground; The measure of the voltage across the capacitor can be determined by actually measuring the voltage across the capacitor, but also by measuring the voltage across the current limiting device. Both options are viable and in line with the present disclosure.

[0020] In a further example, the control circuitry comprises: - an operational amplifier circuit configured to receive the measure of the inrush current and the measure of the voltage across the capacitor, an output of the operational amplifier circuit controlling the impedance of the controllable current limiting device.

[0021] Preferably, the operational amplifier circuit includes an integrator circuit to provide the output proportional to the difference over time between the measure of the inrush current and the measure of the voltage across the capacitor.

[0022] The integrator circuit can be designed to integrate two currents or two voltages. Both options are feasible and covered by this disclosure. Integrating two voltages using an operational amplifier circuit is preferred, considering that this is a more efficient solution, especially considering that the output signal is preferably a voltage for controlling the current limiting device.

[0023] In a further example, the integrator circuit is configured to receive the measure of the inrush current at an inverting input of the integrator circuit and the measure of the voltage across the capacitor at a non-inverting input of the integrator circuit.

[0024] In another example, the controllable current-limiting device is a metal-oxide-semiconductor (MOS) field-effect transistor (FET) (MOSFET).

[0025] In a second aspect of the present disclosure, there is provided a power supply configured to provide a direct current (DC) voltage to a bus, the power supply comprising: an inrush current limiting circuit according to any one of claims 1 to 7, and a power supply is provided having said capacitor connected in series with said controllable current limiting device;

[0026] It should be noted that the advantages as described in relation to the first aspect of the present disclosure, which is the inrush current limiting circuit, also apply to the second aspect of the present invention, which is a power supply having such an inrush current limiting circuit.

[0027] In an example, the power source comprises: - a mains filter configured to be connected to a mains power supply; - a rectifier connected to the output of said mains filter; a power factor correction (PFC) circuit connected to an output of the bridge rectifier, the output of the PFC circuit being connected to the capacitor.

[0028] In a third aspect of the present disclosure, there is provided a method of controlling an inrush current limiting circuit according to any of the preceding examples, comprising: - controlling, by the control circuit, the impedance of the controllable current limiting device based on the voltage across the capacitor and the inrush current into the capacitor.

[0029] It should be noted that the advantages as described with respect to the first aspect of the present disclosure, which is the inrush current limiting circuit, also apply to the third aspect of the present invention, which is the method of controlling an inrush current limiting circuit.

[0030] In an example, the control circuit further comprises a current sensing circuit configured to provide a measure of the inrush current and a voltage measurement circuit configured to provide a measure of the voltage across the capacitor, and the method further comprises: - controlling, by the control circuit, the impedance of the controllable current limiting device based on the measure of the inrush current and the measure of the voltage across the capacitor.

[0031] In a further example, the control circuit includes an operational amplifier circuit configured to receive the measure of the inrush current and the measure of the voltage across the capacitor, an output of the operational amplifier circuit controlling the impedance of the controllable current limiting device.

[0032] In yet another example, the operational amplifier circuit includes an integrator circuit to provide the output proportional to the difference over time between the measure of the inrush current and the measure of the voltage across the capacitor.

[0033] In an example, the integrator circuit is configured to receive the measure of the inrush current at an inverting input of the integrator circuit, and to receive the measure of the voltage across the capacitor at a non-inverting input of the integrator circuit.

[0034] In a fourth aspect of the present disclosure, there is provided a computer program product including a computer readable medium having instructions stored on the computer readable medium that, when executed by control circuitry, cause the control circuitry to perform a method according to any of the examples provided above.

[0035] The present disclosure will be described in conjunction with the accompanying drawings. It is emphasized that, according to standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of various features may be arbitrarily increased or decreased for clarity of illustration.

[0036] In the accompanying figures, similar components and / or features may have the same reference label. Furthermore, various components of the same type may be distinguished by following the reference label with a dash and a second label that distinguishes among the similar components. When only a first label is used in this specification, the description applies to any one of the similar components having the same first label, regardless of the second label.

[0037] In a further example, a lighting system is provided, the lighting system comprising a power supply and a light source.

[0038] The power supply comprises an inrush current limiter and a capacitor. The lighting system also comprises a light source powered by the power supply.

[0039] These and other aspects of the present disclosure are illustrated and elucidated with reference to the following examples. [Brief explanation of the drawings]

[0040] [Figure 1] An example of an inrush current limiting circuit according to the present disclosure is disclosed. [Figure 2] An example implementation of an inrush current limiting circuit according to the present disclosure is disclosed. [Figure 3] Exemplary voltage, current and power diagrams for a controllable current limiting device are disclosed. DETAILED DESCRIPTION OF THE INVENTION

[0041] It should be noted that in the description of the figures, the same reference numerals refer to the same or similar components that perform the same or essentially similar functions.

[0042] In order that the features of the present disclosure may be more fully understood, a more detailed description will now be provided with reference to specific examples, some of which are illustrated in the accompanying drawings. It should be noted that the drawings merely illustrate typical examples and therefore should not be considered to limit the scope of the claimed subject matter. The drawings are included to facilitate understanding of the disclosure and, therefore, are not necessarily drawn to scale. Advantages of such claimed subject matter will become apparent to those skilled in the art upon reading the description in conjunction with the accompanying drawings.

[0043] FIG. 1 discloses an example 1 of an inrush current limiting circuit according to the present disclosure.

[0044] The inrush current limiting circuit comprises a controllable current limiting device 2 and a control circuit 3 configured to control the current limiting device 2. The principle of operation will be described later. The control circuit 3 comprises an input current measuring circuit 8, a reference set point module 9 and an integrator 10.

[0045] The inrush current limiting circuit is implemented in the power supply, which comprises a mains filter 4 , a bridge rectifier 5 , a power factor correction (PFC) module 6 and a bus capacitor 7 .

[0046] The mains filter 4 may for example comprise common mode filtering components to limit any common mode disturbances. Furthermore, the mains filter may comprise filtering components to attenuate any flicker on the mains supply lines or the like.

[0047] The bridge rectifier 5 is configured to convert an alternating current (AC) voltage into a direct current (DC) voltage. The bridge rectifier may be implemented using two diodes in the form of a diode rectifier.

[0048] The power factor correction (PFC) module 6 is configured to improve the efficiency of the power distribution system it is installed in. Power factor can be considered as the ratio of the real power absorbed by any load to the apparent power flowing in the circuit.

[0049] The current limiting device 2 is connected in series with the bus capacitor 7. The impedance of the current limiting device 2 can be controlled, thereby effectively controlling the inrush current flowing into the bus capacitor 7.

[0050] According to the present disclosure, a control circuit may be configured to control the impedance of the controllable current limiting device based on the voltage across and the inrush current into the capacitor so that the total amount of energy delivered to the capacitor remains relatively constant over time.

[0051] An input current measurement circuit 8 , or current sensing circuit, is configured to provide a measure of the inrush current and provide the measure to an integrator 10 .

[0052] A reference set point module 9 , ie a voltage measurement circuit, is configured to provide a measure of the voltage across the capacitor and provide the measure to an integrator 10 .

[0053] An integrator 10 is configured to receive the measure of the inrush current at an inverting input of the integrator circuit and the measure of the voltage across the capacitor at a non-inverting input of the integrator circuit, the output of which is used to control a controllable current limiting device 2.

[0054] The current limiting device 2 may be, for example, a metal oxide semiconductor (MOS) field effect transistor (FET) (MOSFET).

[0055] The MOSFET is a voltage-driven component, making it more efficient to drive. The gate voltage may have a tolerance of about 20% to 40%, which is temperature dependent. The output voltage range of the integrator 10 may be about 0V to 10V, so as not to cause problems at high input currents. The losses in the switch are due to the R dson and when the bus capacitor 7 is discharged, by the body diode.

[0056] MOSFETs can be advantageous because they can operate as impedance controlled switches and have very low losses if desired.

[0057] Charging the bus capacitor 7 can take several milliseconds. This time depends on the value of the bus capacitor 7, the voltage level, and the inrush current. This current should be limited, but the maximum duration may be limited by the start-up time specifications of the power supply. While the bus capacitor 7 is charging, current is flowing through the MOSFET and there is a voltage across the MOSFET. This means that there may be a relatively "high" power loss in the MOSFET during the charging time.

[0058] The inventors have also found that the total amount of energy delivered to the bus capacitor 7 can exceed a certain threshold for a certain pulse length. Increasing the pulse length reduces the maximum total allowable exceedance of the threshold.

[0059] Therefore, the control circuit may further control the controllable current limiting device based on the duration of the total amount of energy delivered to the bus capacitor 7, which may be represented by the total amount of current delivered to the bus capacitor. This is achieved by using the integrator 10.

[0060] Essentially, shorter pulses allow higher power to be dissipated by the MOSFET.

[0061] FIG. 2 discloses an example implementation 21 of an inrush current limiting circuit according to the present disclosure.

[0062] The reference set point circuit 9 takes as input the voltage at the drain of the MOSFET and provides this to the non-inverting input of the integrator 10 via "out1".

[0063] The input current measurement circuit 8 has a sense resistor 22, the voltage across which is used as a measure of the inrush current flowing into the bus capacitor 7. The output of the measurement circuit 8 is fed to the inverting input of an integrator 10.

[0064] The output of the integrator 10 is connected to the gate of the MOSFET to control the impedance of the MOSFET.

[0065] FIG. 3 discloses a typical example 31 of voltage 32, current 33 and power 34 diagrams for a controllable current limiting device.

[0066] According to the present disclosure, the total amount of energy, i.e., power, supplied to the bus capacitor 7 can be controlled. The power can be controlled by taking into account the voltage across the bus capacitor 7 and the inrush current.

[0067] The inrush current is indicated at 33, the voltage across the MOSFET is indicated at 32, and the power is indicated at 34. As shown, due to the concept that the control circuit takes into account voltage as well as current, the total amount of energy, i.e., power, delivered to the bus capacitor 7 remains relatively constant. Preferably, the total amount of energy, i.e., power, delivered to the bus capacitor 7 remains relatively constant during at least a portion of the inrush event. Note that the voltage across the MOSFET is inversely proportional to the voltage across the bus capacitor 7.

[0068] In the above description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of embodiments of the disclosed technology. However, it will be apparent to one skilled in the art that embodiments of the disclosed technology may be practiced without some of these specific details.

[0069] Those skilled in the art can understand and effect other variations to the disclosed embodiments in practicing the claimed invention, from a study of the drawings, the specification and the appended claims. In the claims, the word "comprises" does not exclude other elements or steps, and the singular does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be interpreted as limiting the scope of the claims.

Claims

1. an inrush current limiting circuit configured to limit an inrush current to a capacitor; a controllable current limiting device configured to be connected in series with the capacitor, the impedance of the current limiting device being controllable; a control circuit configured to control the impedance of the controllable current limiting device based on a voltage across the capacitor and the inrush current into the capacitor, an inrush current limiting circuit configured to control the impedance of the controllable current limiting device based on the voltage across the capacitor and the inrush current into the capacitor so that a total amount of energy delivered to the capacitor remains relatively constant during at least a portion of an inrush event.

2. The control circuit a current sensing circuit configured to provide a measure of the inrush current; a voltage measurement circuit configured to provide a measure of the voltage across the capacitor; 2. The inrush current limiting circuit of claim 1, wherein the control circuit is configured to control the impedance of the controllable current limiting device based on the measure of the inrush current and the measure of the voltage across the capacitor.

3. 3. The inrush current limiting circuit of claim 2, wherein the control circuit comprises an operational amplifier circuit configured to receive the measure of the inrush current and the measure of the voltage across the capacitor, an output of the operational amplifier circuit controlling the impedance of the controllable current limiting device.

4. 4. The inrush current limiting circuit of claim 3, wherein the operational amplifier circuit includes an integrator circuit to provide the output proportional to the difference over time between the measure of the inrush current and the measure of the voltage across the capacitor.

5. 5. The inrush current limiting circuit of claim 4, wherein the integrator circuit is configured to receive the measure of the inrush current at an inverting input of the integrator circuit and the measure of the voltage across the capacitor at a non-inverting input of the integrator circuit.

6. 6. An inrush current limiting circuit as claimed in any one of claims 1 to 5, wherein the controllable current limiting device is a metal oxide semiconductor field effect transistor.

7. a power supply configured to provide a DC voltage to the bus, An inrush current limiting circuit according to any one of claims 1 to 6, and a power supply having the capacitor connected in series with the controllable current limiting device;

8. a mains filter configured to connect to a mains power supply; a rectifier connected to an output of the mains filter; 8. The power supply of claim 7, further comprising a power factor correction circuit connected to an output of said bridge rectifier, said power factor correction circuit output connected to said capacitor.

9. 7. A method of controlling an inrush current limiting circuit according to any one of claims 1 to 6, comprising the step of controlling, by the control circuit, the impedance of the controllable current limiting device based on the voltage across the capacitor and the inrush current into the capacitor.

10. 10. The method of claim 9, wherein the controlling step includes controlling, by the control circuit, the impedance of the controllable current limiting device based on the voltage across and the inrush current into the capacitor so that a total amount of energy delivered to the capacitor remains relatively constant during at least a portion of an inrush event.

11. 11. The method of claim 9 or 10, wherein the control circuit further comprises a current sensing circuit configured to provide a measure of the inrush current and a voltage measurement circuit configured to provide a measure of the voltage across the capacitor, the method comprising controlling, by the control circuit, the impedance of the controllable current limiting device based on the measure of the inrush current and the measure of the voltage across the capacitor.

12. 12. The method of claim 11, wherein the control circuit comprises an operational amplifier circuit configured to receive the measure of the inrush current and the measure of the voltage across the capacitor, an output of the operational amplifier circuit controlling the impedance of the controllable current limiting device.

13. 13. The method of claim 12, wherein the operational amplifier circuit includes an integrator circuit to provide the output proportional to the difference over time between the measure of the inrush current and the measure of the voltage across the capacitor.

14. 9. A lighting system comprising a power supply according to claim 7 or 8 and a light source.

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