Switching mode power converter

Abstract

A switching mode power converter includes: an inverter; a transformer having a primary side winding and a secondary side winding; and a first inductor connected in series with the primary side winding. A second inductor is provided, which is magnetically coupled to the first inductor, and the voltage at one end of the second inductor is used as a feedback signal for indirectly measuring (i.e., approximating) the voltage on the secondary side, but the measurement is made on the primary side.

Description

Technical Field

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

[0002] It is well known that switched-mode power converters are used for both AC / DC conversion and DC / DC conversion. Resonant converters, for example, have a resonant circuit, which can be a series resonant circuit or a parallel resonant circuit. It is well known that resonant converters including an LLC resonant circuit having two inductors and one capacitor or an LCC resonant circuit having two capacitors and one inductor are used for LED drivers.

[0003] A switched-mode power converter can be configured as or used as a constant current source or a constant voltage source. A constant current source can be used to directly drive an LED arrangement, thereby implementing a single-stage driver. A constant voltage source can be used, for example, for an LED module that has other driver electronics to ensure the corresponding power supply for the LEDs. Then, a pre-determined 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 supply (usually a high-voltage mains supply) and the load. The isolation requirement between the primary side and the secondary side typically means that the primary side winding and the secondary side winding of the transformer need to be physically separated or otherwise arranged such that the magnetic coupling between the primary side winding and the secondary side winding is not perfect. This imperfection in the magnetic coupling itself manifests as the leakage inductance of the output transformer.

[0005] When applying an LED driver, current regulation is typically used, by which, at the secondary side, the switched-mode power converter is controlled to deliver a desired LED current. The regulation function can also be used, for example, to limit or control the output voltage being generated by the LED driver in the situation where the LED load is disconnected (or becomes disconnected) from the driver.

[0006] Then, it is highly preferred that the circuitry for measuring and limiting or controlling the driver output voltage should be provided at the primary side of the driver, rather than at the isolated secondary side of the driver. In the latter case, signals need to be sent back from the isolated secondary side to the primary side via a suitable isolation 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, there is no need for high-voltage or common-mode insulation of the control circuitry in a primary side control scheme. Second, the auxiliary circuitry for powering the control part is simplified. Moreover, having the regulation circuitry on the primary side enables any information from the mains to be processed in a very simple and effective manner.

[0008] Although the voltage at the primary side of the output transformer is related to the output voltage of the driver, the primary side signal 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 sensing is provided at the output transformer such that the separate winding has a magnetic coupling with the secondary side winding and is in fact much better magnetically coupled than the normal magnetic coupling between the primary side winding and the secondary side winding. The separate winding is then electrically connected to the primary side circuit. This is feasible as long as no substantial power is transferred via the sensing winding, so that no substantial current flows in the sensing winding and thus no substantial voltage drop appears across the leakage inductance of the sensing winding.

[0010] However, it is challenging to provide such a sensing winding having sufficient magnetic coupling with the secondary side winding while maintaining the creepage distance and the electrical clearance associated with the isolation requirements between the primary side and the secondary side of the output transformer.

[0011] Therefore, there is a need for an improved primary side sensing scheme that generates a signal representative of the output voltage of the driver and, in particular, compensates for the effect of the leakage inductance of the output transformer. SUMMARY OF THE INVENTION

[0012] The present invention is defined by the claims.

[0013] According to an example in accordance with one aspect of the present invention, there is provided a switched-mode power converter comprising:

[0014] an inverter comprising a high-side switch and a low-side switch connected in series with a first node therebetween;

[0015] a transformer having a primary side winding and a secondary side winding; and

[0016] a first inductor connected in series with the primary side winding and connected to the primary side winding at a second node;

[0017] a second inductor magnetically coupled to the first inductor, the second inductor defining an output terminal at one end; and

[0018] a controller for controlling the inverter, wherein the controller has an input for receiving a signal derived from the voltage at the output terminal.

[0019] The power converter utilizes an additional (second) inductor coupled to the first inductor. The first inductor can be, for example, a part already required in a switched-mode power converter (such as for energy storage or forming part of a resonant tank), or otherwise, a first inductor can be added for the purpose of enabling the provision of output terminals. The first inductor is in series with the primary side winding of the transformer. For example, the primary side circuit already includes a first inductor that carries a current substantially the same as the current flowing through the primary side winding of the transformer. For example, this can be the case in LCC converters and LLC converters.

[0020] The second inductor provides a simple method for measuring the secondary side voltage at the primary side. The second inductor can be implemented by providing an additional winding to the existing first inductor, which thus has a tap such that the two inductors are defined by a single arrangement. The voltage across the additional winding is the same in shape and phase as the voltage across the leakage inductance, and is used to compensate for the error of the voltage signal caused by the leakage inductance at the primary side winding of the output transformer. Thus, the voltage sensing of the secondary side voltage is made more accurate based on voltage sensing at the primary side.

[0021] Then, the sensed voltage can be used as part of the control scheme of the converter. For example, when an overvoltage is detected at the output, the sensed voltage can be used to trigger protection.

[0022] The converter can also include a series capacitor in series with the first inductor and a capacitor in parallel with the secondary side winding. This defines an LCC structure. Multiple secondary side windings are possible.

[0023] The inductance of the second inductor may be approximately equal to the combined primary side series leakage inductance of the transformer.

[0024] In this context, "combined primary side series leakage inductance" means the sum of the primary side leakage inductance and the reflected secondary side leakage inductance. Specifically, this combination (in a model of the transformer electrical characteristics) represents the effect of the imperfect coupling between the primary side and the secondary side of the transformer. In the case of multiple secondary side windings, there may be multiple secondary side leakage inductances, but they can still be transformed into secondary side leakage inductances reflected to the primary side.

[0025] In this way, the inductance of the second inductor is made equal to the leakage inductance to be compensated. The leakage inductance may vary, for example, with temperature, and it may not be known highly accurately. Therefore, the second inductor is selected to have a value approximating the leakage inductance.

[0026] Alternatively, the second inductor can have a turns ratio relative to the first inductor such that the inductance of the second inductor is greater than the combined primary side series leakage inductance of the transformer.

[0027] In this way, the inductance of the second inductor is made greater than the leakage inductance to be compensated. Then, the measured voltage can be reduced.

[0028] The input of the controller can be directly connected to the output terminal.

[0029] Then, the voltage at the ends of the second inductor is directly used as the feedback signal for the controller.

[0030] Alternatively, the controller can have a combining circuit that combines the signal at the second node and the signal at the output terminal to derive a sensing signal provided to the controller.

[0031] In this case, before being used as the feedback signal for the controller, the voltage at each end of the second inductor is processed, for example, scaled. This may be necessary, for example, because only the ratio between the integer turns of the second inductor and the first inductor is possible. Then, a combining function is used to improve the accuracy of simulating the leakage inductance.

[0032] The combining circuit can include a resistor network that combines the voltage at the second node and the voltage at the output terminal to provide a sensing signal voltage.

[0033] The resistor network can be used to provide a weighted combination of the voltages or to derive any other function between the voltages at each end of the second inductor.

[0034] The combining circuit can alternatively include a circuit for generating a sensing signal current. Thus, the feedback signal provided to the controller can be a voltage or a current. However, even when the current is used as the feedback signal, it still depends on the voltage at the output node and thus on the voltage across the second inductor.

[0035] The first end of the second inductor can be connected to the second node and the second end can be connected to the output terminal. In this way, the second inductor is connected such that compensation (e.g., signal subtraction) of the combined leakage inductance (represented at the primary side) is achieved by component placement.

[0036] Alternatively, the first end of the second inductor can be connected to ground and the second end can be connected to the output terminal. Then, the compensation (e.g., signal subtraction) can be performed by the controller.

[0037] The converter can include a resonant converter having a resonant tank connected to the first node, the resonant tank including at least a first inductor.

[0038] The converter can also include a rectifier connected to the secondary side winding and a storage capacitor across the rectifier output.

[0039] Accordingly, the converter delivers a DC output to the DC load.

[0040] In one set of examples, the rectifier includes a four-diode bridge connected across the secondary side winding. In another set of examples, the transformer has a first secondary side winding and a second secondary side winding in series, with a node defined therebetween, where the rectifier (D1-D4) includes a two-diode arrangement.

[0041] A "two-diode arrangement" means that the rectifier function is implemented using only two unidirectional conduction paths. Each path may have a single diode, but of course, the same functionality will be achieved if each path has multiple diodes in series.

[0042] The series connection of the first secondary side winding and the second secondary side winding may include components therebetween, e.g., the diodes of the rectifier.

[0043] Accordingly, different rectifier designs are possible depending on the design of the secondary side of the transformer.

[0044] The present invention also provides an illumination circuit, comprising:

[0045] a converter as defined above; and

[0046] an illumination load, in parallel with a storage capacitor.

[0047] These and other aspects of the present invention will become apparent and be elucidated with reference to the embodiments described hereinafter. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] For a better understanding of the present invention and to show more clearly how it may be carried into effect, reference will now be made, by way of example only, to the accompanying drawings, in which

[0049] Figure 1 an example of an LCC resonant switched-mode power supply within an LED driver is shown;

[0050] Figure 2 a power supply in which leakage inductance is shown is shown; Figure 1 of the power supply;

[0051] Figure 3 a power supply in which the leakage inductance is transformed to the primary side is shown; Figure 2 of the power supply;

[0052] Figure 4 a first example of a power supply according to the present invention is shown; and

[0053] Figure 5 a second example of a power supply according to the present invention is shown;

[0054] Figures 6 to 9Shows different examples of input capacitors, output capacitors, transformer windings, and rectifier configurations that can be employed; and

[0055] Figure 10A and Figure 10B Shows two examples of a combined circuit that can be used to more precisely approximate the leakage inductance. Detailed Description

[0056] The present invention will be described with reference to the accompanying drawings.

[0057] It should be understood that although the detailed description and specific examples indicate exemplary embodiments of the apparatus, system, and method, they are for illustrative purposes only and are not intended to limit the scope of the present invention. These and other features, aspects, and advantages of the apparatus, system, and method of the present invention will be better understood from the following description, the appended claims, and the drawings. It should be understood that the drawings are merely schematic and are not drawn to scale. It should also be understood that the same reference numerals are used in all the drawings to indicate the same or similar parts.

[0058] The present invention provides a switched-mode power converter, comprising an inverter; a transformer having a primary-side winding and a secondary-side winding; and a first inductor connected in series with the primary-side winding. A second inductor is provided, which is magnetically coupled to the first inductor, and the voltage at one end of the second inductor is used as a feedback signal for indirectly measuring (i.e., approximating) the secondary-side voltage, and the measurement is performed on the primary side.

[0059] The present invention will be described with reference to an LCC resonant switched-mode power supply, but the present invention is more generally applicable to other resonant tank designs and is also more generally applicable to switched-mode power supplies.

[0060] Figure 1 Shows an example of an LCC resonant switched-mode power supply within an LED driver and having an isolated output.

[0061] The rectified mains input (or the DC input of a DC / DC power converter) is provided to a half-bridge inverter formed by a high-side MOSFET Mhs and a low-side MOSFET Mls. The inverter controls the conversion operation, and the switches are controlled using feedback control or feed-forward control in order to generate the desired output. The operation timing of each switch of the inverter is controlled by its gate voltage.

[0062] The resonant tank circuit is formed by a series capacitor Cs and a series inductor Lres at the primary side of an output transformer 10 (including a primary side winding Lprim and a secondary side winding Lsec), and a parallel capacitor Cp at the secondary side of the output transformer 10. For example, if there are multiple secondary side windings, there can be multiple parallel capacitors. Note that Lprim and Lsec represent physical windings (to which circuit components can be connected).

[0063] The combined leakage inductance is effectively added to the series inductor Lres. By placing the parallel capacitor Cp at the output side, the system still behaves as a third-order system (where the inductor of the LCC tank circuit is the sum of Lres and the leakage inductance, which will be further discussed below).

[0064] The output is provided to the LED loads Led1, Led2 through a diode bridge rectifier D1 to D4 and a smoothing output capacitor Cout.

[0065] During operation of the converter, the controller (first shown in Figure 4 ) controls the switches of the inverter at a specific frequency and in a substantially complementary manner ("substantially" complementary because there may be non-overlapping time periods). A high gate drive signal turns on one switch and turns off the other switch; while a low gate drive signal turns off one switch and turns on the other switch.

[0066] In a known scheme, the primary side circuit detects a variable that indicates, for example, the average value of the current flowing in the circuit over time through the first switch or the second switch. Information about the load is derived based on the current measured in the primary side circuit. The measured current can have a direct relationship with the load.

[0067] Figure 1 A full-bridge rectifier at the secondary side and a single secondary side winding Lsec whose ends are coupled to the rectifier circuit are shown. Examples of circuits with multiple secondary side windings are further described below. Such a circuit can instead use a rectifier with only two diodes.

[0068] This is an example of an LCC resonant tank circuit, but LLC circuits and other resonant circuits as well as non-resonant converters are also possible.

[0069] For those skilled in the art, the general operation of a switched-mode power converter including a resonant converter will be well known.

[0070] The input of the resonant tank is node N1, at which there is typically a high-voltage square-wave AC signal. It is a "roughly" square-wave signal because there may be a sloping transition. If there is a sloping transition, both transistors must be turned off during such a sloping transition. The frequency of this AC signal matches the switching frequencies of transistors Mhs and Mls and is typically in the kHz range, for example, from 10 kHz to 300 kHz. The amplitude of the signal at this point may vary over time, particularly in the case where the input is a rectified mains signal. There is a low-frequency envelope of the high-frequency signal at node N1.

[0071] Figure 1 The circuit of Figure 1 includes a magnetic element Lres at the primary side, and the current carried by this magnetic element is substantially the same as the current flowing through the primary-side winding Lprim of transformer 10.

[0072] The present invention includes providing an additional second inductor, preferably, in this example, by providing an additional winding to the existing series inductor Lres. Thus, there are a first inductor and a second inductor in series. The second inductor serves as a sensing element.

[0073] The present invention is of particular interest for a switched-mode power converter circuit having such series inductors. However, a series inductor (forming the first inductor) can be added to a circuit that does not yet have such a component, and similarly, an additional winding is provided to define an additional second inductor.

[0074] The result is that the voltage across the second inductor will be the same in shape and phase as the voltage that drops across the leakage inductance of the transformer. When a dedicated magnetic element is added to serve as the first inductor, an inductance that has the least impact on the circuit function can be selected.

[0075] By subtracting the voltage across the second inductor from the voltage that can be measured at the primary side of the output transformer, a better and more accurate primary-side representation of the output voltage of the driver is obtained. This subtraction can be performed by a controller, or otherwise, the circuit arrangement itself can also implement the subtraction.

[0076] Figure 1 The first node N1 between the transistors and the second node N2 to which the primary-side winding is connected to the first inductor Lres are shown. This node N2 is accessible, which means that the voltage at this point can be monitored.

[0077] If the other side of the primary-side winding is connected to ground, the voltage at node N2 with respect to ground will be the voltage across the primary-side winding. There may be other series components, such as capacitors, between the other end of the primary-side winding Lprim and ground (or between the primary-side winding and the bus voltage that powers the transistor inverter). The solution of the present invention can be applied to all these possibilities.

[0078] As Figure 2 shown, the non-ideal magnetic coupling between the primary and secondary sides of transformer 10 can be depicted as leakage inductance LsPrim at the primary side and leakage inductance LsSec at the secondary side. The physical windings are then represented by the combination of leakage inductances LsPrim, LsSec and ideal windings Lprim", Lsec".

[0079] The voltage between node N2 and ground no longer represents well the secondary side AC voltage between nodes N3 and N4. A fictitious node N2" is defined between the leakage inductance LsPrim at the primary side and the (idealized) primary side winding Lprim", but this is not an accessible point in the circuit.

[0080] Due to the current flowing through these leakage inductances, voltages will be generated across leakage inductances LsPrim and LsSec.

[0081] In Figure 3 , the secondary side leakage inductance LsSec can also be transformed to the primary side by scaling the impedance by the square of the turns ratio. By combining the transformed secondary side leakage inductance LsSec with the primary side leakage inductance LsPrim, a single total leakage inductance Ls is produced. Thus, the secondary side leakage inductance is removed from the secondary side as it is already represented on the primary side.

[0082] This combined primary side leakage inductance Ls can be considered as the effective primary side leakage inductance and secondary side leakage inductance of the transformer, all represented on the primary side of the transformer.

[0083] Now, a fictitious node N2' is defined between this combined primary side leakage inductance Ls and the representation of the primary side winding Lprim'. The voltage between this node N2' and ground is then a better representation of the voltage across the primary side winding Lprim', but this is also not an accessible point in the circuit.

[0084] Note that this is a model representation based on a transformer (as a combination of an ideal transformer and additional leakage (i.e., parasitic) components). One such model is called the cantilever model, although other models can be used. Any suitable model can be used to represent all the leakage inductances at the primary side. Note that the cantilever model may result in a turns ratio of the ideal transformer being different from the actual physical turns ratio. It should also be noted that due to the different representations of the transformer, Figure 2 the Lprim”, Lsec” and N2” of Figure 3 are different from the Lrpim', Lsac' and N2' of

[0085] Figure 4 is shown based on Figure 3A first example of a circuit according to the present invention represented by a transformer. The first inductor has been referred to as LresA. A second inductor LresB is provided, the first end of which is connected to the second node N2 and the second end of which is connected to the output terminal (node N5). The first inductor and the second inductor are magnetically coupled, so the first inductor is now referred to as LresA and the second inductor is referred to as LresB. Together, they can be a tapped inductor, the tap defining the node between them. Independent inductors sharing a magnetic core are also possible.

[0086] In Figure 3 , since the first inductor Lres and the leakage inductor Ls are in series, the current flowing through the first inductor Lres and the leakage inductor Ls is the same, so that the shapes and phases of the voltages across Lres and Ls are the same.

[0087] When as Figure 4 shown, a second inductor is added to produce a main first inductor LresA (formed by the main first winding) and a second inductor LresB (formed by the additional winding), assuming good magnetic coupling between LresA and LresB, this is very feasible because there is no need for significant isolation between the two windings. However, since there is no substantial current flow in the second inductor LresB, the leakage inductance will not be a problem.

[0088] The end of the second inductor opposite to the node N2 forms the node N5, which is the output terminal. It is connected to the controller 40. The controller 40 has an input 42 for receiving a voltage (or in an alternative example, a current) derived from the voltage at the output terminal N5. In Figure 4 the example, the controller input draws a minimum current such that substantially no current flows through the second inductor LresB.

[0089] 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 the node N2), a better and more accurate primary side representation of the output voltage of the driver is obtained.

[0090] Since the voltage across the second inductor LresB is the same in shape and phase as the voltage across the first inductor LresA, it is also the same in shape and phase as the voltage across the leakage inductor Ls.

[0091] By a suitable turns ratio between the inductors LresB and LresA, the voltage at the node N5 can be tuned to be substantially the same as the voltage at the node N2', thus providing a physically accessible node that carries the desired correct representation of the AC output voltage between the nodes N3 and N4. Therefore, primary side measurements can be made to limit or control the output voltage of the driver.

[0092] The turns ratio between LresB and LresA can only be tuned to compensate for the nominal value of the primary side leakage inductance Ls, and thus the voltage at node N5 (with respect to ground) is only the same as Figure 3 the voltage at node N2' (with respect to ground) for the nominal value of the leakage inductance Ls in

[0093] Compared with Figure 3 the voltage at node N2' in , any deviation of the leakage inductance from its nominal value will result in an imperfect voltage at node N5. However, using the signal at node N5 will still be substantially better than using the signal at node N2.

[0094] In Figure 4 , one end of the second inductor LresB is connected to node N2. This will automatically produce the sum of the voltage at N2 plus the voltage across the second inductor LresB at node N5. Thus, the circuit provides the required summing / subtraction. However, this is not essential.

[0095] Figure 5 A modification is shown in which one end of the second inductor LresB is connected to ground and the other end forms node N5. In this way, the isolated auxiliary winding can be used to compensate for the effect of the output transformer leakage inductance Ls on the primary side voltage signal. The addition / subtraction can be achieved by a separate circuit or by a controller.

[0096] The fact that the second inductor LresB is magnetically coupled to the first inductor LresA is sufficient to enable the representation of the leakage inductance Ls. This provides an alternative way to generate the weighted sum of the voltage at node N2 and the voltage across the second inductor LresB.

[0097] The above example utilizes a single secondary side winding of the transformer, but there are other possibilities.

[0098] Figures 6 to 9 An alternative arrangement where the invention can be applied but the second inductor of the invention is not shown is illustrated. The second inductor can be added in the manner shown in Figure 4 or Figure 5 .

[0099] Figure 6 The secondary side of the transformer is shown as two secondary side windings LsecA and LsecB in series. The node between them provides the first output terminal and the two end terminals are connected to the second output terminal via a two-diode rectifier.

[0100] As shown, there can be a single shunt secondary-side capacitor Cp, or there can be one capacitor per winding, i.e., capacitor CpA is in parallel with LsecA, and capacitor CpB is in parallel with LsecB.

[0101] Figure 7 is shown Figure 6 a variant that has advantages both in terms of the isolation requirements between LsecA and LsecB and in terms of EMI performance. In this design, there is a shunt capacitor across each secondary-side winding, and one of the diodes of the two-diode rectifier is located between the secondary-side windings.

[0102] Similarly, Figure 7 the transformer in has a first secondary-side winding LsecA and a second secondary-side winding LsecB in series, with a node defined therebetween (the anode of diode D2, which is one of the diodes of the two-diode rectifier). It defines the first output node. The cathode of diode D1 (i.e., the other diode of the two-diode rectifier) is connected to the second output node.

[0103] In this case, it is not possible to place a single shunt capacitor Cp across the two secondary-side windings.

[0104] Figure 8 shows another example used as a voltage multiplier. Now, the output capacitor Cout is divided into two capacitors CoutA and CoutB placed in series. There may also be another capacitor Cout across the LED.

[0105] Figure 8 the transformer in has a single secondary-side winding Lsec. Similarly, the rectifier includes two diodes and defines two output nodes in the same way as in Figure 6 does.

[0106] Figure 9 shows a configuration where the series capacitor Cs is at a different position 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-side winding Lprim is connected to the node between the two capacitors, and the other end is connected to the first inductor Lres. In fact, the series capacitor can be placed in the position of CsA, CsB, or both (as shown), all of which are actually equivalent. This equivalence is because the bus voltage (which is the supply of the inverter) is normally decoupled from ground via a large capacitor.

[0107] Under normal circumstances, Cs is very large, so the bottom side of the primary side winding Lprim now refers to the (almost) DC voltage at the node between CsA and CsB. Therefore, only the AC component of the voltage at node N2 indicates the output voltage. When, in accordance with the present invention, combining the voltage at node N2 with the voltage across LresB, only the AC components need to be considered.

[0108] If the first inductor ( Figure 4 and Figure 5 LresA in

[0109] is a main component of the switched-mode power converter (i.e., if it is not simply added to enable the provision of the LresB inductor), then the power converter task of the inductor LresA is primary for the design of the inductor LresA (core, air gap, turns, wire, etc.). Then, the inductance of the main first inductor LresA is generally large compared to the leakage inductance, and thus also large compared to the second inductor LresB. Therefore, the number of turns of the winding of the second inductor LresB is relatively small. Since the number of winding turns is an integer, it may not be possible to tune the inductance of the second inductor LresB to exactly match the nominal leakage inductance Ls.

[0110] One option is to use the next higher integer number of turns for the second inductor LresB, or actually use an even larger number of turns. This causes the voltage at node N5 to be overcompensated due to the influence of the leakage inductance Ls. This overcompensation can be considered based on the understanding that node N2 is not compensated for the influence of the Ls leakage inductance.

[0111] Figure 10A An example of a combining circuit 50 in the form of a resistor network for combining the voltage at the second node N2 and the voltage at node N5 to provide a sense signal voltage Vsense is shown.

[0112] The circuit includes a first resistor divider formed by resistors Ra, Rc and a second resistor divider formed by resistors R1, R2. The circuit produces:

[0113] Vsense = (V N2 *Rc + V N5 *Ra) / (Ra + Rc)*R2 / (Ra / / Rc + R1 + R2)

[0114] where V N2 is the voltage at node N2, V N5 is the voltage at node N5, and Ra / / Rc represents the parallel combination of resistors Ra and Rc.

[0115] Thus, the circuit provides a weighted sum. Of course, many other passive circuit designs can be used.

[0116] Alternatively, R2 can be open and R1 can be shorted, so there is a simple voltage divider between N2 and N5.

[0117] As Figure 10B shown, 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 connecting it to ground through R2. The current entering or drawn from this pin is then representative of the output voltage of the driver. Thus, the combinational circuit can include a circuit for generating the sense signal current.

[0118] If the constant voltage at the Isense node is zero (ground), then both Ca and Cc can be omitted; if the constant voltage is not zero, then at least Cc is required to support the DC voltage of the pin, and Ca can be omitted as long as the voltage at node N2 is greater than the constant voltage.

[0119] For example, series capacitors can implement the AC signal processing explained above with reference to Figure 9 the above.

[0120] As described above, the present invention is particularly attractive in circuits where a magnetic element (Lres) already exists that carries the same current as the primary side winding of a transformer (such as in an LCC converter).

[0121] However, not all circuits with isolated outputs already have a magnetic component such as Lres that conducts the same (or a scaled version) of the leakage current of the output transformer. In those cases, an additional series inductor (i.e., forming LresA) can be added to the existing circuit and equipped with an additional winding that provides LresB for the function of generating the desired node N5. The first inductor LresA then remains small because it is not a required component of the power circuit.

[0122] The inductors LresA and LresB are referred to as the first inductor and the second inductor. As explained above, they can be separate segments (i.e., windings) of a single inductor structure, or they can be separate inductors.

[0123] The present invention provides an improvement that can be applied to an (LED) driver having an output transformer with isolated output. The present invention is particularly attractive in terms of an LCC-type resonant converter stage as commonly used in isolated LED drivers.

[0124] By studying the drawings, the disclosure, and the appended claims, those skilled in the art can understand and realize variations of the disclosed embodiments when practicing the claimed invention. In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. A single processor or other unit may implement the functions of several items recited in the claims. 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 construed as limiting the scope.

Claims

1. A switching mode power converter, comprising: an inverter including a high side switch (Mhs) and a low side switch (Mls) connected in series therebetween with a first node (N1) therebetween; a transformer having a primary side winding (Lprim) and a secondary side winding (Lsec), the transformer having a combined primary side leakage inductance (Ls); and a first inductor (LresA) in series with the primary side winding and connected to the primary side winding (Lprim) at a second node (N2); a second inductor (LresB) magnetically coupled to the first inductor (LresA), the second inductor (LresB) defining an output terminal (N5) at one end, wherein both the first inductor (LresA) and the second inductor (LresB) are different from the combined primary side leakage inductance (Ls); and a controller (40) for controlling the inverter, wherein the controller has an input (42) for receiving a signal derived from the voltage at the output terminal (N5); wherein the inductance of the second inductor (LresB) is greater than or approximately equal to the combined primary side series leakage inductance of the transformer; and wherein the turns ratio between the second inductor (LresB) and the first inductor (LresA) is tuned to compensate for the nominal value of the combined primary side leakage inductance (Ls).

2. The converter according to claim 1, wherein the turns ratio between the second inductor (LresB) and the first inductor (LresA) is tuned in such a way that the voltage at the output terminal (N5) with respect to ground is substantially the same as the voltage of the node at the primary side winding (Lprim) with respect to ground for compensating the nominal value of the combined primary side leakage inductance (Ls).

3. The converter according to claim 1, further comprising a series capacitor (Cs) in series with the first inductor (LresA) and a capacitor (Cp) in parallel with the secondary side winding.

4. The converter according to claim 1, wherein the controller has an input (42) directly connected to the output terminal (N5).

5. The converter according to claim 1, wherein the controller has a combining circuit (50) for combining the signal at the second node (N2) and the signal at the output terminal (N5) to derive a sensing signal (Vsense, Isense), the sensing signal (Vsense, Isense) being provided to the controller.

6. The converter according to claim 5, wherein the combining circuit (50) includes a resistor network (Ra, Rb, R1, R2) for combining the voltage at the second node (N2) and the voltage at the output terminal (N5) to provide a sensing signal voltage (Vsense).

7. The converter according to claim 5, wherein the combinational circuit (50) includes a circuit for generating a sense signal current (Isense).

8. The converter according to any one of claims 1 to 7, wherein the second inductor has a first end connected to the second node (N2) and the output terminal (N5) at a second end.

9. The converter according to any one of claims 1 to 7, wherein the second inductor has a first end connected to ground and the output terminal (N5) at a second end.

10. The converter according to any one of claims 1 to 7, comprising a resonant converter having a resonant tank connected to the first node (N1), the resonant tank including the first inductor (LresA).

11. The converter according to any one of claims 1 to 7, further comprising a rectifier and a storage capacitor (Cout) across the output of the rectifier, the rectifier being connected to the secondary side winding (Lsec).

12. The converter according to claim 11, wherein the rectifier includes a four-diode bridge connected across the secondary side winding.

13. The converter according to claim 11, wherein the transformer has a first secondary side winding and a second secondary side winding in series, with a node defined therebetween, and wherein the rectifier includes a two-diode arrangement.

14. An illumination circuit, comprising: The converter according to claim 11, 12 or 13; and An illumination load (Led1, Led2), in parallel with the storage capacitor.

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