Switch Mode Power Converter

Abstract

A switched mode power converter includes an inverter, a transformer having a primary winding and a secondary winding, and a first inductor in series with the primary winding, and a second inductor magnetically coupled to the first inductor, the voltage at one end of which is used as a feedback signal to indirectly measure (i.e., approximate) the voltage on the secondary side from the measurement on the primary side.

Description

[Technical Field]

[0001] The present invention relates to a switched mode power converter, for example for use in an LED driver. [Background technology]

[0002] Switch-mode power converters are well known for use in both AC / DC and DC / DC conversion. Resonant converters have a resonant circuit that can be, for example, a series or parallel resonant circuit. Resonant converters with an LLC resonant circuit having two inductances and one capacitance, or an LCC resonant circuit having two capacitances and one inductance, are well known for use in LED drivers.

[0003] The switch-mode power converter can be configured or operated as a constant current source or a constant voltage source. A constant current source can be used to directly drive an LED arrangement, thus enabling a single-stage driver. A constant voltage source can be used, for example, for an LED module that also has driver electronics to ensure a corresponding power supply to the LEDs. In that case, a predetermined current is drawn from the output voltage provided by the constant voltage source.

[0004] Galvanic isolation is typically provided between the input and any output. A transformer is used to provide this isolation between the input power source (typically a high-voltage mains supply) and the load. The requirement for isolation between the primary and secondary sides typically means that the primary and secondary windings of the transformer must be physically separated, or must be arranged so that there is imperfect magnetic coupling between the primary and secondary windings. This imperfection in magnetic coupling manifests itself as leakage inductance in the output transformer.

[0005] In LED driver applications, current regulation is typically used, by which a switch-mode power converter is controlled to provide a desired LED current on the secondary side. The regulation function can also be used to limit or control the output voltage generated by the LED driver, for example, in situations when the LED load is (or becomes) disconnected from the driver.

[0006] In that case, it is highly preferred that the circuitry for measuring and limiting or controlling the driver output voltage be located on the primary side of the driver rather than on the isolated secondary side of the driver, in which case the signal must be returned from the isolated secondary side to the primary side via a suitably isolating path, such as via an opto-isolator.

[0007] Therefore, primary-side current and / or voltage regulation is more cost-effective and robust than secondary-side regulation. First, high-voltage or common-mode isolation of the control circuit is not required in a primary-side control approach. Second, the auxiliary circuitry powering the control unit is simplified. Furthermore, the regulation circuit located on the primary side can process all information from the mains supply in a very simple and effective way.

[0008] The voltage on the primary side of the output transformer is related to the output voltage of the driver, but the signal on the primary side is distorted due to the voltage drop across the leakage inductance of the output transformer.

[0009] In some cases, a separate winding for output voltage detection is arranged on the output transformer so that it has magnetic coupling with the secondary winding, in fact much better magnetic coupling than the typical primary-to-secondary winding. In that case, the separate winding is electrically connected to the primary circuit. This is feasible as long as no significant power needs to be transferred through this detection winding, so no significant current needs to flow in the detection winding and therefore no significant voltage drop across the leakage inductance of the detection winding occurs.

[0010] However, it is difficult to locate such a sensing winding with sufficient magnetic coupling to the secondary winding while maintaining the creepage and clearance distances associated with the insulation requirements between the primary and secondary sides of the output transformer. Summary of the Invention [Problem to be solved by the invention]

[0011] Therefore, there is a need for an improved primary side sensing technique that generates a signal representative of the driver's output voltage and compensates, among other things, for the effects of the leakage inductance of the output transformer. [Means for solving the problem]

[0012] The invention is defined by the claims.

[0013] According to an example according to an aspect of the present invention, an inverter having a high-side switch and a low-side switch, the high-side switch and the low-side switch being connected in series with a first node between the high-side switch and the low-side switch; a transformer having a primary winding and a secondary winding; a first inductor in series with the primary winding, the first inductor connected to the primary winding at a second node; a second inductor magnetically coupled to the first inductor, the second inductor defining an output terminal at one end; and A switched mode power converter is provided having a controller for controlling the inverter, the controller having an input for receiving a signal derived from the voltage at the output terminals.

[0014] This power converter utilizes an additional (second) inductor coupled to a first inductor. The first inductor may be an already required part of the switch-mode power converter, for example, used for energy storage or forming part of a resonant tank, or the first inductor may be added for the purpose of enabling the output terminal to be provided. This first inductor is in series with the primary winding of the transformer. For example, the primary circuit already includes the first inductor, which carries essentially the same current as the current flowing through the primary winding of the converter. This may be the case, for example, in LCC and LLC converters.

[0015] The second inductor provides a simple method for measuring the secondary voltage at the primary side. The second inductor may be implemented by adding an additional winding to the existing first inductor, which is then tapped so that the two inductors are defined by a single arrangement. The voltage across the additional winding is identical in shape and phase to the voltage across the leakage inductance and is used to compensate for errors in the voltage signal at the primary winding of the output transformer, such as those caused by the leakage inductance. Thus, voltage detection of the secondary voltage is more accurate than voltage detection on the primary side.

[0016] The detected voltage may then be used as part of a control scheme for the converter. The detected voltage may for example be used to trigger protection when an overvoltage is detected at the output.

[0017] The converter may further comprise a series capacitor in series with the first inductor and a capacitor in parallel with the secondary winding, which defines an LCC structure. Multiple secondary windings are possible.

[0018] The second inductor may have an inductance approximately equal to a combined primary series leakage inductance of the transformer.

[0019] "Composite primary series leakage inductance" in this context means the sum of the primary leakage inductance and the reflected secondary leakage inductance. Among other things, this combination accounts for the effect of imperfect coupling between the primary and secondary sides of the transformer (in a model of the transformer's electrical characteristics). In the case of multiple secondary windings, there may be multiple secondary leakage inductances, which can still be referred to as the secondary leakage inductance reflected to the primary.

[0020] In this approach, the inductance of the second inductor is made equal to the leakage inductance it is intended to compensate for. The leakage inductance may vary, for example, with temperature and may not be known with high precision. Therefore, the second inductor is selected to have a value close to the leakage inductance.

[0021] Alternatively, the second inductor may have a turns ratio relative to the first inductor such that the second inductor has an inductance greater than a combined primary series leakage inductance of the transformer.

[0022] In this way, the inductance of the second inductor is made larger than the leakage inductance it is intended to compensate for, in which case the measurement voltage can be scaled down.

[0023] The controller may have an input connected directly to the output terminal.

[0024] In that case, the voltage at the end of the second inductor directly serves as a feedback signal for the controller.

[0025] In another example, the controller may include a combining circuit for combining the signal at the second node and the signal at the output terminal to obtain a detection signal that is provided to the controller.

[0026] In this case, the voltage at each end of the second inductor is processed, e.g., scaled, before being used as a feedback signal for the controller. This may be necessary, for example, because only ratios of integer numbers of turns in the second inductor to integer numbers of turns in the first inductor are possible. In that case, a coupling function is used to improve the accuracy with which the leakage inductance is simulated.

[0027] The combining circuit may include a resistor network for combining a voltage at the second node and a voltage at the output terminal to provide a sense signal voltage.

[0028] This resistor network may be used to provide a weighted combination of voltages, or to derive any other function between the voltages at each end of the second inductor.

[0029] The coupling circuit may instead include circuitry for generating a sense signal current. The feedback signal supplied to the controller may therefore be a voltage or a current. However, even if a current is used as the feedback signal, it still depends on the voltage at the output node and therefore on the voltage across the second inductor.

[0030] The second inductor may have a first end connected to the second node and may have the output terminal at a second end, In this manner, the second inductor is connected such that compensation (e.g., signal subtraction) of the combined leakage inductance (represented by the primary side) is achieved by component placement.

[0031] In another example, the second inductor may have a first end connected to ground and the output terminal at a second end, in which case the compensation (e.g., signal subtraction) may be performed by the controller.

[0032] The converter may comprise a resonant converter comprising a resonant tank connected to the first node, the resonant tank comprising the first inductor.

[0033] The converter may further include a rectifier connected to the secondary winding and a reservoir capacitor across the output of the rectifier.

[0034] The converter therefore provides a DC output for a DC load.

[0035] In one set of examples, the rectifier comprises a four-diode bridge connected across the secondary winding. In another set of examples, the transformer comprises first and second secondary windings in series with a node defined between the first and second secondary windings, and the rectifier (D1-D4) comprises a two diode arrangement.

[0036] "Two-diode arrangement" means that the rectifier function is performed with only two unidirectional conduction paths. Each path may have a single diode, although of course the same function would be achieved with each path having multiple diodes in series.

[0037] The series connection of the first secondary winding and the second secondary winding may include a component therebetween, for example a diode of the rectifier.

[0038] Therefore, different rectifier designs are possible depending on the design of the secondary side of the transformer.

[0039] The present invention provides a converter as defined above, and A lighting circuit is also provided having a lighting load in parallel with the reservoir capacitor.

[0040] These and other aspects of the invention will be elucidated and elucidated with reference to the following embodiments. [Brief explanation of the drawings]

[0041] For a better understanding of the present invention, and to show more clearly how the same may be carried into effect, reference will now be made, by way of example only, to the accompanying drawings, in which: [Figure 1] An example of an LCC resonant switch-mode power supply in an LED driver is shown. [Figure 2] The power supply of FIG. 1 is shown with leakage inductance indicated. [Figure 3] The power supply of Figure 2 is shown with the leakage inductance transformed to the primary side. [Figure 4] 1 shows a first example of a power supply according to the present invention. [Figure 5] 2 shows a second example of a power supply according to the present invention. [Figure 6] 1 illustrates different examples of input capacitor, output capacitor, transformer winding, and rectifier configurations that may be employed. [Figure 7] 1 illustrates different examples of input capacitor, output capacitor, transformer winding, and rectifier configurations that may be employed. [Figure 8] 1 illustrates different examples of input capacitor, output capacitor, transformer winding, and rectifier configurations that may be employed. [Figure 9] 1 illustrates different examples of input capacitor, output capacitor, transformer winding, and rectifier configurations that may be employed. [Figure 10A] Two examples of coupling circuits that can be used to obtain a more accurate approximation to the leakage inductance are shown. [Figure 10B] Two examples of coupling circuits that can be used to obtain a more accurate approximation to the leakage inductance are shown. DETAILED DESCRIPTION OF THE INVENTION

[0042] The present invention will be described with reference to the drawings.

[0043] It should be understood that the detailed description and specific examples, while indicating exemplary embodiments of the devices, systems, and methods, are for purposes of illustration only and are not intended to limit the scope of the invention. These and other features, aspects, and advantages of the devices, systems, and methods of the present invention will become better understood from the following description, appended claims, and accompanying drawings. It should be understood that the figures are merely schematic and are not drawn to scale. It should also be understood that the same reference numerals are used throughout the figures to indicate the same or similar parts.

[0044] The present invention provides a switched mode power converter having an inverter, a transformer having a primary winding and a secondary winding, and a first inductor in series with the primary winding, wherein a second inductor is provided magnetically coupled to the first inductor, and the voltage at one end of the second inductor is used as a feedback signal to indirectly measure (i.e., approximate) the voltage on the secondary side by measuring on the primary side.

[0045] Although the invention is described in terms of an LCC resonant switched mode power supply, the invention is more broadly applicable to other resonant tank designs and to switched mode power supplies more generally.

[0046] Figure 1 shows an example of an LCC resonant switched-mode power supply in an LED driver with isolated outputs.

[0047] The rectified mains input (or DC input of a DC / DC power converter) is fed to a half-bridge inverter formed by a high-side MOSFET Mhs and a low-side MOSFET Mls. This inverter uses feedback or feedforward control to control the switching and generate the required output. Each switch in the inverter has its operation timing controlled by its gate voltage.

[0048] A resonant tank is formed by the series capacitor Cs and series inductor Lres on the primary side of the output transformer 10 (having a primary winding Lprim and a secondary winding Lsec) and the parallel capacitor Cp on the secondary side of the output transformer 10. There may be multiple parallel capacitors, for example, if there are multiple secondary windings. Note that Lprim and Lsec represent physical windings (to which circuit components may be connected).

[0049] The combined leakage inductance effectively multiplies the series inductance. By placing the parallel capacitor Cp on the output side, the system still behaves as a third order system (with the inductor of the LCC tank being the sum of the leakage inductance and Lres, as further described below).

[0050] The output is supplied to LED loads Led1 and Led2 via a diode bridge rectifier D1 to D4 and a smoothing output capacitor Cout.

[0051] During converter operation, the controller (first shown in FIG. 4) controls the inverter switches at a particular frequency in an essentially complementary manner (essentially complementary in that there may be non-overlapping periods): a high gate drive signal turns one switch on and the other switch off, and a low gate drive signal turns one switch off and the other switch on.

[0052] In one known approach, the primary circuit senses a variable indicative of the average value over time of the current flowing in the circuit, for example via a first or second switch, and information about the load is derived based on the measured current in the primary circuit, which may have a direct relationship to the load.

[0053] Figure 1 shows a secondary full-bridge rectifier with a single secondary winding Lsec, which couples to the rectifier circuit at its end. Examples of circuits with multiple secondary windings are shown further below. Such circuits may alternatively use a rectifier with only two diodes.

[0054] This is an example of an LCC resonant tank circuit, but LLC circuits and other resonant circuits are also possible, as well as non-resonant converters.

[0055] The general operation of switch mode power converters, including resonant converters, will be familiar to those skilled in the art.

[0056] The input to the resonant tank is node N1, where a generally square-wave high-voltage AC signal is present. It is a "generally" square-wave signal in that there may be ramp transitions. If there are ramp transitions, both transistors must be off during such ramp transitions. The frequency of this AC signal matches the switching frequency of transistors Mhs and Mls and is typically in the kHz range, e.g., 10 kHz to 300 kHz. The amplitude of the signal at this location may vary over time, for example, especially if the input is a rectified mains signal. In that case, there is a low-frequency envelope of the high-frequency signal at node N1.

[0057] The circuit of FIG. 1 includes a magnetic element Lres on the primary side that carries essentially the same current as that flowing through the primary winding Lprim of the transformer 10.

[0058] The present invention preferably involves providing an additional second inductor by providing an additional winding on the existing series inductor, in this example Lres. Thus, in that case, there is a first inductor and a second inductor in series. The second inductor acts as a sensing element.

[0059] The present invention is of particular interest to switched mode power conversion circuits comprising such series inductors. However, a series inductor (forming a first inductor) may also be added to a circuit not already having such a component, again with an additional winding defining an additional second inductor.

[0060] As a result, the voltage across the second inductor will have the same shape and phase as the voltage dropped across the transformer's leakage inductance. If a dedicated magnetic element is added to function as the first inductor, an inductance can be chosen that has minimal effect on circuit function.

[0061] By subtracting the voltage across the second inductor from the voltage that can be measured on the primary side of the output transformer, a better and more accurate representation of the driver output voltage on the primary side is obtained. This subtraction may be performed by the controller or the circuit arrangement itself may perform the subtraction.

[0062] 1 shows a first node N1 between the transistors and a second node N2 where the primary winding connects to the first inductor Lres. This node N2 is accessible, meaning that the voltage at this location can be monitored.

[0063] If the other side of the primary winding is grounded, the voltage at node N2 with respect to ground will be the voltage across the primary winding. Other series components, such as a capacitor, may be present, for example, between the other end of the primary winding Lprim and ground (or between the primary winding and the bus voltage supplying the transistor inverter). The techniques of the present invention may be applied to all of these possibilities.

[0064] 2, the non-ideal magnetic coupling between the primary and secondary sides of the transformer 10 can be depicted as a leakage inductance LsPrim on the primary side and a leakage inductance LsSec on the secondary side. The physical windings are then represented by the combination of leakage inductances LsPrim, LsSec and ideal windings Lprim" and Lsec".

[0065] The voltage between node N2 and ground is no longer a good representation of the secondary AC voltage between nodes N3 and N4. A conceptual node N2" is defined between the leakage inductance LsPrim in the primary and the (ideal) primary winding Lprim", but this is not an accessible location in the circuit.

[0066] A voltage will be developed across the leakage inductances LsPrim and LsSec due to the current flowing through these leakage inductances.

[0067] In Figure 3, the secondary leakage inductance LsSec is further transformed to the primary side by scaling the impedance by the square of the transformation ratio. Combining the transformed secondary leakage inductance LsSec and the primary leakage inductance LsPrim results in a single total leakage inductance Ls. Therefore, the secondary leakage inductance is removed from the secondary side because it is instead represented on the primary side.

[0068] This combined primary leakage inductance Ls can be considered to be the effective primary and secondary leakage inductance of the transformer, represented entirely at the primary side of the transformer.

[0069] A notional node N2' is now defined between this composite primary leakage inductance Ls and the (representative of) primary winding Lprim'. The voltage between this node N2' and ground is a better representation of the voltage across the primary winding Lprim', but again this is not an accessible location in the circuit.

[0070] Note that this is based on representing a model of the transformer as an ideal transformer combined with additional leakage components (i.e., parasitics). One such model is known as a cantilever model, although other models may be used. Any suitable model may be used to represent all leakage inductances on the primary side. Note that the cantilever model yields a turns ratio for an ideal transformer, but the turns ratio may differ from the actual physical turns ratio. Note also that, as a result of the different representation of the transformer, Lprim", Lsec", and N2" in FIG. 2 differ from Lprim', Lsec', and N2' in FIG. 3.

[0071] Figure 4 shows a first example of a circuit according to the present invention, based on the transformer representation of Figure 3. A first inductor is designated LresA. A second inductor LresB is provided, having a first end connected to a second node N2 and an output terminal, node N5, at a second end. The first and second inductors are magnetically coupled, and thus the first inductor is designated LresA and the second inductor is designated LresB. They may be combined into a single inductor with a tap defining a junction between them. Separate inductors sharing a common magnetic core are also possible.

[0072] In FIG. 3, the current through the first inductor Lres and the current through the leakage inductance Ls are the same since they are in series, and therefore the shape and phase of the voltage across Lres and Ls are the same.

[0073] If a second inductor is added, resulting in a main first inductor LresA (formed from the main first winding) and a second inductor LresB (formed from an additional winding), as shown in Figure 4, the magnetic coupling between LresA and LresB is considered to be good, and this is quite feasible since no significant insulation between the two windings is required. However, since no significant current flows in the second inductor LresB, some leakage inductance will not be an issue.

[0074] The end of the second inductor opposite node N2 forms an output terminal, node N5, which connects to a controller 40. The controller 40 has an input 42 for receiving a voltage (or, in other examples, a current) derived from the voltage at output terminal N5. In the example of Figure 4, the controller input draws minimal current so that substantially no current flows through the second inductor LresB.

[0075] By subtracting the voltage across the second inductor from the voltage that can be measured on the primary side of the output transformer (measured at node N2), a better and more accurate representation of the driver output voltage on the primary side is obtained.

[0076] The voltage across the second inductor LresB has the same shape and phase as the voltage across the first inductor LresA, and therefore also has the same shape and phase as the voltage across the leakage inductor Ls.

[0077] With an appropriate turns ratio between inductors LresB and LresA, the voltage at node N5 can be adjusted to be essentially the same as the voltage at node N2', thus providing a physically accessible node that carries a desirably accurate representation of the AC output voltage between nodes N3 and N4, thereby enabling primary side measurement to limit or control the driver output voltage.

[0078] The turns ratio between LresB and LresA can only be adjusted to compensate for the nominal value of the primary leakage inductance Ls, and therefore the voltage at node N5 (referenced to ground) will only be essentially the same as the voltage at node N2' in Figure 3 (referenced to ground) for the nominal value of the leakage inductance Ls.

[0079] Any deviation in leakage inductance from its nominal value will result in an imperfection in the voltage at node N5 compared to the voltage at node N2' in Figure 3. However, using the signal at node N5 is still substantially better than using the signal at node N2.

[0080] In Figure 4, one end of the second inductor LresB connects to node N2. This automatically generates at node N5 the sum of the voltage at node N2 plus the voltage across the second inductor LresB. Thus, the circuit provides the required addition / subtraction. However, this is not required.

[0081] 5 shows a modification in which one end of the second inductor LresB is grounded and the other end forms node N5. In this manner, an isolated auxiliary winding can be used to compensate the primary voltage signal for the effects of the output transformer leakage inductance Ls. The addition / subtraction can be performed by a separate circuit or by the controller.

[0082] The fact that the second inductor LresB is magnetically coupled to the first inductor LresA is sufficient to allow us to represent the leakage inductance Ls, which provides another way to generate the weighted sum of the voltage at node N2 and the voltage across the second inductor LresB.

[0083] Although the above example uses a single secondary winding for the transformer, other possibilities exist.

[0084] 6-9 show other configurations to which the present invention may be applied, without showing the second inductor of the present invention, which may be added in the manner shown in FIGS.

[0085] Figure 6 shows the transformer secondary as two series secondary windings LsecA and LsecB, the junction between them providing the first output terminal, and the two end terminals connecting to the second output terminal via a two-diode rectifier.

[0086] There may be a single parallel secondary capacitor Cp as shown, or there may be one capacitor per winding, i.e., capacitor CpA in parallel with LsecA and capacitor CpB in parallel with LsecB.

[0087] Figure 7 shows a variation of Figure 6, which has advantages in terms of isolation requirements between LsecA and LsecB and in terms of EMI performance. In this design, there is a parallel capacitor across each secondary winding, and one of the diodes of the two-diode rectifier is between the secondary windings.

[0088] The transformer in Figure 7 again has first and second secondary windings LsecA and LsecB in series, with a node (diode D2, the anode of one of the diodes of the two-diode rectifier) ​​defined between the first and second secondary windings LsecA and LsecB, which defines a first output node. Diode D1, the cathode of the other diode of the two-diode rectifier, connects to a second output node.

[0089] In this case, it is not possible to place a single parallel capacitor Cp across the two secondary windings.

[0090] Figure 8 shows another example operating as a voltage doubler, where the output capacitor Cout is split into two capacitors CoutA and CoutB placed in series. There may be an additional capacitor Cout across the LED.

[0091] The transformer in Figure 8 has a single secondary winding Lsec. The rectifier again has two diodes defining both output nodes in the same manner as shown in Figure 6.

[0092] Figure 9 shows a configuration with different positions of the series capacitor Cs on the primary side. The capacitor Cs is formed as two series capacitors CsB and CsA between the bus voltage and ground. One end of the primary winding Lprim connects to the junction between the two capacitors, and the other end connects to the first inductor Lres. The series capacitor may actually be placed at CsA, CsB, or both positions (as shown), all being substantially equivalent. This equivalence occurs because the bus voltage (which supplies the inverter) is usually decoupled from ground via a large capacitance.

[0093] Typically, Cs is large, so here the bottom of the primary winding Lprim refers to the (nearly) DC voltage at the node between CsA and CsB. Therefore, only the AC component of the voltage at node N2 represents the output voltage. When combining the voltage at node N2 with the voltage across LresB in accordance with the present invention, only the AC component needs to be considered.

[0094] When the first inductor (LresA in FIGS. 4 and 5) is the main component of the switch-mode power converter (i.e., the first inductor is not simply added to allow the LresB inductor to be provided), the power converter task of the inductor LresA drives the design of the inductor LresA (core, air gap, number of turns, wire, etc.). In that case, the inductance of the main first inductor LresA is generally large compared to the leakage inductance and therefore also large compared to the second inductor LresB. Therefore, the second inductor LresB has relatively few windings. Because the number of turns in the winding is an integer, it may not be possible to adjust the inductance of the second inductor LresB to exactly match the nominal leakage inductance Ls.

[0095] One option is to use the next higher integer number of turns, or indeed even a larger number of turns, for the second inductor LresB. This causes the voltage at node N5 to be overcompensated for the effects of leakage inductance Ls. This overcompensation may be taken into account with the recognition that node N2 is not compensated for the effects of leakage inductance Ls.

[0096] In particular, a combining circuit may be used to combine the signal at the second node N2 with the signal at the output terminal (node ​​N5) to obtain a sense signal that is supplied to the controller. In this manner, the amount of compensation can be adjusted to more closely match the effect of the leakage inductance Ls. For example, a weighted sum of the voltages at nodes N2 and N5, or other combination function, may be used as a representation of the driver's output voltage. This representation is then used as a feedback signal to limit or control the output voltage.

[0097] FIG. 10A shows an example of a combining circuit 10 in the form of a resistive network for combining the voltage at the second node N2 and the voltage at the node N5 to provide the sense signal voltage Vsense.

[0098] This circuit has a first resistor divider circuit formed by resistors Ra and Rc, and a second resistor divider circuit formed by resistors R1 and R2. Vsense=(V N2 *Rc+V N5 *Ra) / (Ra+Rc)*R2 / (Ra / / Rc+R1+R2) where V N2 is the voltage at node N2, and V N5 is the voltage at node N5, and Ra / / Rc denotes the parallel combination of resistors Ra and Rc.

[0099] This circuit therefore provides a weighted sum.Of course, many other passive circuit designs can be used.

[0100] Alternatively, R2 may be an open circuit and R1 may be a short circuit so that there is a simple voltage divider between N2 and N5.

[0101] 10B, the current signal Isense can alternatively be generated by shorting R1 and connecting the Isense node to a fixed voltage (e.g., an IC pin) instead of to ground via R2. In that case, the current into or drawn from that pin represents the driver's output voltage. Thus, the coupling circuit may include circuitry for generating the sense signal current.

[0102] If the constant voltage at the Isense node is zero (ground), Ca and Cc can be omitted. If the constant voltage is not equal to zero, at least Cc is required to support the DC voltage at the pin, but Ca can be omitted as long as the voltage at node N2 is large compared to the constant voltage.

[0103] The series capacitor allows for AC signal processing, for example as described above with reference to FIG.

[0104] As noted above, the present invention is particularly attractive in circuits where there is already a magnetic element (Lres) that carries the same current as the primary winding of the transformer, such as an LCC converter.

[0105] However, not all circuits with isolated outputs already have a magnetic component such as Lres that conducts the same current as (or a scaled version of) the leakage current of the output transformer. In such cases, an additional series inductance (i.e., forming LresA) can be added to the existing circuit, which may include an additional winding that provides the function of LresB to create the desired node N5. In that case, the first inductor LresA is kept small, as it is not a required component of the power supply circuit.

[0106] Inductors LresA and LresB are referred to as the first and second inductors. As explained above, they may be separate portions (i.e., windings) of a single inductor structure, or they may be separate inductors.

[0107] The present invention provides an improvement that can be applied to (LED) drivers having an output transformer with an isolated output. The invention is particularly attractive for use in LCC type resonant converter stages such as those often used in isolated LED drivers.

[0108] Those skilled in the art can understand and effect 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. A single processor or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain means are recited in mutually different dependent claims does not indicate that a combination of these means cannot be used to advantage. Any reference signs in the claims should not be construed as limiting the scope.

Claims

1. an inverter having a high-side switch and a low-side switch, the high-side switch and the low-side switch being connected in series with a first node between the high-side switch and the low-side switch; a transformer having a primary winding and a secondary winding; a first inductor in series with the primary winding, the first inductor connected to the primary winding at a second node; a second inductor magnetically coupled to the first inductor, the second inductor defining an output terminal at one end; and 1. A switched mode power converter comprising: a controller for controlling the inverter based on a signal representative of a secondary voltage of the transformer, the controller having an input for receiving a signal derived from a voltage at the output terminals; The second inductor has an inductance approximately equal to or greater than the combined primary-side series leakage inductance of the transformer, so that the effect of the combined primary-side series leakage inductance is compensated for and a signal representative of the secondary-side voltage is obtained based on the voltage at the output terminals.

2. 2. The converter of claim 1, further comprising a series capacitor in series with the first inductor and a capacitor in parallel with the secondary winding.

3. 3. A converter as claimed in any one of claims 1 to 2, wherein the controller has an input connected directly to the output terminal.

4. 3. A converter according to any one of claims 1 to 2, wherein the controller comprises a combining circuit for combining the signal at the second node and the signal at the output terminal to obtain a detection signal supplied to the controller.

5. 5. The converter of claim 4, wherein the combining circuit comprises a resistive network for combining a voltage at the second node and a voltage at the output terminal to provide a sense signal voltage.

6. 5. The converter of claim 4, wherein the coupling circuitry includes circuitry for generating a detection signal current.

7. 7. A converter as claimed in any preceding claim, wherein the second inductor has a first end connected to the second node and has the output terminal at a second end.

8. 7. A converter as claimed in any one of claims 1 to 6, wherein the second inductor has a first end connected to ground and has the output terminal at a second end.

9. 9. A converter as claimed in any preceding claim, comprising a resonant converter comprising a resonant tank connected to the first node, the resonant tank comprising the first inductor.

10. 10. A converter as claimed in any preceding claim, further comprising a rectifier connected to the secondary winding and a reservoir capacitor across the output of the rectifier.

11. 11. The converter of claim 10, wherein the rectifier comprises a four-diode bridge connected across the secondary winding.

12. 11. The converter of claim 10, wherein the transformer has first and second secondary windings in series with a node defined between the first and second secondary windings, and the rectifier has a two-diode arrangement.

13. A converter according to claim 10, 11 or 12, and A lighting circuit having a lighting load in parallel with the reservoir capacitor.

Images