Resonant inverter and conversion method

Abstract

A resonant inverter has a switching network from which a phase signal is provided representing a phase of a switching signal. A resonant tank circuit is coupled to a first switching network output and provides a feedback signal comprising a resonant voltage across circuit elements of the resonant tank circuit. A reference current is set to be drawn from an input node and a reference phase is set based on the reference current. The switching signal of the switching network is controlled based on a phase difference between the resonant voltage and the phase signal and based on the reference phase. The resonant inverter employs a phase modulation scheme as a control scheme for the switching network of the resonant inverter. The approach is suitable for high frequency and very high frequency operation of the resonant converter, e.g. up to tens of MHz.

Description

TECHNICAL FIELD

[0001] The present invention relates to the field of resonant inverters, in particular to a resonant inverter for use in a resonant converter. BACKGROUND

[0002] Resonant converters with series or parallel resonant circuits are known. For example, resonant LLC converters used in LED drivers are known. Such converters have the advantage that energy efficient operation with relatively low switching losses is possible.

[0003] Resonant converters can be configured or operated as constant current sources or constant voltage sources. Constant current sources can be used to directly drive LED devices, enabling a single stage driver. Constant voltage sources can be used for example in LED modules with additional driver electronics in order to ensure that a predetermined current derived from the output voltage provided by the constant voltage source provides a corresponding power supply to the LEDs.

[0004] LLC converters comprise switching devices (referred to as inverter switches) for controlling the conversion operation and use feedback or feedforward control to control the switches in order to produce a desired output.

[0005] Another function implemented within a mains (or other AC) power powered power converter is power factor correction (PFC). The power factor of an AC power system is defined as the ratio of the real power flowing to the load to the apparent power in the circuit. A power factor less than 1 means that the voltage and current waveforms are not in phase, reducing the instantaneous product of the two waveforms. Real power is the ability of the circuit to do work at a particular time. Apparent power is the product of the current and voltage of the circuit. Apparent power will be greater than real power due to energy being stored in the load and returned to the source, or due to non-linear loads distorting the waveform of the current drawn from the source.

[0006] If the power supply operates at a low power factor, for the same amount of useful power delivered, the load will draw more current than for a higher power factor.

[0007] Power factor correction can be used to increase the power factor. For linear loads, this can include passive networks using capacitors or inductors. Non-linear loads often require active power factor correction to counteract the distortion and improve the power factor.

[0008] Passive PFC makes the power factor of an AC supply circuit closer to 1 by providing reactive power of opposite sign, increasing the inductor or capacitor effect used to counteract the load.

[0009] Active PFC utilizes power electronics to alter the waveform of the current drawn by the load to improve the power factor. Active PFC circuits can for example be based on a buck, boost or buck-boost switching mode converter topology. Active power factor correction can be single or multi-stage.

[0010] In the case of a switched mode power supply, a PFC boost converter is for example inserted between the bridge rectifier and the mains storage capacitor. The boost converter tries to maintain a constant DC bus voltage at its output while drawing a current that is always in phase with and has the same frequency as the line voltage. Another switching mode converter within the power supply generates the desired output voltage or current from the DC bus.

[0011] Power factor correction can be implemented in a dedicated power factor correction circuit, called a pre-regulator, which is for example placed between the (mains) power supply and a switched mode power converter, which then drives the load. This forms a two-stage system, which is a typical configuration for high power LED applications, for example more than 25 W.

[0012] Alternatively, power factor correction can be integrated into the switched mode power converter, which then forms a single stage system. In this case, there is a single resonant tank and switching device, which then implements power factor correction as well as control of the conversion ratio between input and output in order to maintain the desired output (in the case of an LED driver, a current) delivered to the load.

[0013] Active power factor correction typically involves providing the input current and voltage waveforms to a controller so that their relative phase angle can be controlled by adjusting the load.

[0014] It has been proposed in US 2014 / 0091718 to use an LLC DC / DC converter after the rectifier as a PFC circuit. The LLC resonant converter is frequency controlled for which an oscillator is used. The control value of the feedback control system is the switching frequency of the inverter. Resonant power converters are in fact typically feedback controlled with the switching frequency used as the manipulated value.

[0015] Self-oscillating resonant converter circuits are also known, which utilize internal components to form a resonant tank. Recently, threshold based control schemes have been proposed to overcome control stability issues related to high gain ratios, for example required for a resonant LLC converter working as a PFC front end. Then, a signal value, for example a voltage level occurring in the circuit, is used to implement the switching operation. For example, US 8729830 discloses controlling a resonant DC / DC converter in a self-oscillating manner by using threshold detection of the state in the resonant tank to determine the inverter switching times, instead of employing an oscillator and frequency control.

[0016] However, these threshold-based methods become impractical at higher frequencies, e.g. above 0.5 MHz, mainly due to the effort required to compensate for the delay and inaccuracy caused by threshold sensing and noise.

[0017] It is therefore currently desirable to improve the operation of resonant converters, in particular the power factor of resonant converters when used as power factor correction (PFC) circuits. SUMMARY

[0018] The invention is defined by the claims.

[0019] According to an example of an aspect of the invention, there is provided a resonant inverter, comprising:

[0020] an input node for receiving an input for conversion;

[0021] a switching network connected to the input node, comprising at least a first switch and a second switch, wherein the switching network is controlled by a switching signal, and wherein a switching network output is defined at a node located between the first and second switch, wherein the switching network is adapted to provide a feedback signal comprising a phase signal representing a phase of the switching signal;

[0022] a resonant tank circuit coupled to the switching network output, wherein the resonant tank circuit is adapted to provide a feedback signal comprising a resonant voltage across a circuit element of the resonant tank circuit;

[0023] a phase setting unit for setting a reference phase based on the reference current; and

[0024] a phase control circuit for generating the switching signal for the switching network based on a phase difference between the resonant voltage and the phase signal and based on the reference phase.

[0025] The resonant inverter employs a phase modulation scheme as a control scheme for the switching network of the resonant inverter. The method is suitable for operation at all frequencies, including high and very high frequency operation of the resonant converter, e.g. up to tens of MHz, for example. The phase difference is measured, the phase between the inverter voltage and the resonant tank signal, e.g. the resonant capacitor voltage, is controlled to follow a phase reference. The phase signal to be measured is much less sensitive to noise than a threshold signal. The inverter can be implemented with low cost ICs (e.g. clock buffers, frequency modulator-demodulator circuits), with only small requirements to additional external circuitry.

[0026] The inverter can be used as part of an AC / DC converter with power factor correction, or as part of a DC / DC converter.

[0027] The phase control circuit comprises for example a phase locked loop. This provides a simple and low cost phase control method. The phase control circuit comprises for example a phase detector for detecting a phase difference between the resonant voltage and the phase signal.

[0028] The phase control circuit can comprise:

[0029] a loop filter for filtering a difference between the phase difference signal and the reference phase; and

[0030] a voltage controlled oscillator driven by an output from the loop filter.

[0031] The loop filter can for example be a PID filter.

[0032] In one example, the resonant tank comprises an LLC circuit. However, other resonant converters can be implemented, for example an LCC or other resonant converter.

[0033] For the example of an LLC circuit, the resonant voltage can be the voltage across the capacitor of the LLC circuit. The feedback signal will depend on the type of resonant converter. For example, for an LCC converter, the voltage across the series resonant capacitor can be used in the same way as for an LLC converter.

[0034] The phase signal is for example the voltage across the first switch or the second switch.

[0035] As mentioned above, the resonant inverter of the present invention is particularly useful for high frequency operation. For example, the frequency of the switching signal can be at least 0.5 MHz.

[0036] The resonant tank circuit is for example adapted to provide a further feedback signal comprising the output voltage, and the current setting unit is for setting the reference current based on at least the output voltage. This enables feedback control of the output voltage (for example the output voltage of a dc / dc converter), or it enables power factor correction by taking the shape of the output voltage into account.

[0037] The resonant inverter can also be adapted to provide a further feedback signal comprising the input current drawn from the input node, and the phase setting unit is for setting the reference phase based on the input current and the reference current. This enables feedback control of the output current (for example the output current of a dc / dc converter), or it can also form part of a power factor correction function.

[0038] The first switch and the second switch form for example a half bridge inverter.

[0039] The present invention also provides an AC / DC PFC converter comprising:

[0040] an AC input;

[0041] a rectifier, wherein the AC input is coupled to an input of the rectifier; and a converter as defined above having an output of the rectifier as its input.

[0042] The invention also provides an apparatus comprising:

[0043] an inverter as defined above; and

[0044] a load downstream of the inverter, such as an LED arrangement having one or more LEDs.

[0045] For example, the LED arrangement can be provided after a further output stage for adapting the output of the converter to the LED arrangement.

[0046] The invention also provides a conversion method comprising:

[0047] receiving an input for conversion;

[0048] controlling a switching network using a switching signal, the switching network comprising at least a first switch and a second switch, wherein an output of the switching network is defined at a node located between the first switch and the second switch;

[0049] providing a feedback signal from the switching network, the feedback signal comprising a phase signal representing a phase of the switching signal;

[0050] providing the output of the switching network to a resonant tank circuit;

[0051] providing a feedback signal from the resonant tank circuit, the feedback signal comprising a resonant voltage across an element of the resonant tank circuit;

[0052] setting a reference current drawn from the input node;

[0053] setting a reference phase based on the reference current; and

[0054] generating the switching signal for the switching network based on a phase difference between the resonant voltage and the phase signal, and based on the reference phase.

[0055] The method can further comprise:

[0056] providing a further feedback signal comprising an input current drawn from the input node, and wherein setting the reference phase is based on the input current and the reference current; and / or

[0057] providing a further feedback signal comprising an output voltage, and wherein setting the reference current is based on at least the output voltage.

[0058] The present application also provides a method of driving an LED, comprising rectifying an AC input and providing conversion using the above method to achieve power factor correction, and driving an LED load based on the converted DC voltage.

[0059] These and other aspects of the present application will become apparent from the following description and the appended claims. BRIEF DESCRIPTION OF DRAWINGS

[0060] For a better understanding of the present application, and to show how it can be implemented, reference will now be made, purely by way of example, to the accompanying drawings in which:

[0061] Figure 1 An example of a resonant AC / DC converter is shown;

[0062] Figure 2 A known example of oscillator frequency control is shown;

[0063] Figure 3 A known example of threshold control is shown;

[0064] Figure 4 A first example of a circuit according to the present application is shown;

[0065] Figure 5 A first modification to Figure 4 is shown;

[0066] Figure 6 A known relationship between phase difference and current is shown;

[0067] Figure 7 A second modification to Figure 4 is shown; and

[0068] Figure 8 A third modification to Figure 4 for a dc-dc converter is shown. DETAILED DESCRIPTION

[0069] The present application will be described with reference to the accompanying drawings.

[0070] It should be understood that the detailed description and specific examples, while indicating exemplary embodiments of apparatuses, systems and methods, are intended for purposes of illustration only and are not intended to limit the scope of the present application. These and other features, aspects, and advantages of the apparatuses, systems and methods of the present application will become better understood from the following description, appended claims, and accompanying drawings. It should be understood that the drawings are diagrammatic and schematic only and are not drawn to scale. It should also be understood that the same reference numerals will be used throughout the drawings to refer to same or like parts.

[0071] The invention provides a resonant inverter having a switching network from which a phase signal indicative of the phase of the switching signal is provided. A resonant tank circuit is coupled to a first switching network output and provides a feedback signal of the resonant voltage across a circuit element of the resonant tank circuit. A reference current is set to be drawn from an input node and a reference phase is set based on the reference current. The switching signal of the switching network is controlled based on the phase difference between the resonant voltage and the phase signal and based on the reference phase. Thus, the resonant inverter employs a phase modulation scheme as a control scheme for the switching network of the resonant inverter. This approach is suitable for high frequency and very high frequency operation of the resonant converter, e.g. up to several tens of MHz.

[0072] Figure 1 An example of a resonant AC / DC converter is shown in Fig. 1. The LLC resonant circuit forms a PFC stage and thus can be used as a PFC pre-regulator by having a controlled output voltage. It can also be used as a single stage LED driver by having a controlled output current.

[0073] The circuit comprises a mains input 10, which is followed by a rectifier bridge 12 (e.g. with a smoothing capacitor at its output).

[0074] The converter comprises a primary side circuit 16 and a secondary side circuit 18. There can be electrical isolation between the primary side circuit 16 and the secondary side circuit 18. A transformer comprising a primary winding 20 and a secondary winding 22 is provided for isolation. The primary winding 20 has a magnetizing inductance, which also serves as one of the inductances of the series LLC resonant circuit. The LLC resonant circuit has e.g. a second inductance (so that winding 20 represents two inductors) and a capacitance (formed in this example by two capacitors 26 and 27).

[0075] In the LLC circuit, the inductances and the capacitances can be in any series order. The inductors can comprise discrete components, or can be implemented as leakage inductances of a transformer.

[0076] The primary side circuit 16 comprises a half bridge having a first power switch 28 and a second power switch 30. The first and second switches can be identical, and the half bridge can be in the form of a symmetrical half bridge (with a symmetrical duty cycle). However, the invention is not limited to a symmetrical duty cycle. The switches can be in the form of field effect transistors. The resonant LLC circuit is connected to a node between the two switches.

[0077] The operation timing of each switch is controlled by a respective gate voltage GS0 and GS1 provided by a controller 32. Feedback is used to determine the control timing of the switches 28, 30.

[0078] During operation of the converter, the controller 32 controls the switches at a certain frequency and in a complementary manner. The two gate voltages can be derived from a single gate control signal GS.

[0079] In summary, Figure 1 The illustrated circuit is thus an AC / DC PFC single stage converter, comprising an AC input 10, a rectifier 12, a half bridge inverter comprising a high side switch (first power switch 28) and a low side switch (second power switch 30), wherein an output is defined from a node between the switches. A self-oscillating LLC circuit is coupled to the output. A controller is used to generate gate drive signals GS that control the switching of the high side and low side switches. The high gate drive signal turns on one switch and turns off the other, while the low gate drive signal turns off the one switch and turns on the other.

[0080] In a known approach, the primary side circuit 16 detects a variable indicative of the average over time of the current flowing in the circuit, e.g. through the first or second switch. Information about the load is derived based on the measured current in the primary side circuit. The measured current can have a direct relationship with the load.

[0081] The secondary side 18 has a rectifier 34 connected downstream of the secondary winding 22. The rectifier can be a full bridge rectifier (e.g. a diode bridge), and a single secondary winding can be used, which is coupled at its ends to the rectifier circuit. As an alternative, the center of the secondary winding 22 can be coupled to the output of the secondary side circuit. The ends of the secondary winding 22 can then be coupled to the output via a half bridge rectifier with only two diodes.

[0082] A storage capacitor 36 is connected between the outputs of the rectifier, through which the output voltage vo is delivered. An LED load or other output stage is connected to the output, directly or through another output circuit. The LED load can comprise one LED or multiple LEDs or one laser diode or multiple laser diodes.

[0083] A control scheme is needed to drive the switches 28, 30 to their on and off states such that the output voltage or current is regulated to a certain desired value or value range, and also such that the PFC circuit achieves power factor correction.

[0084] To best utilize the power train and achieve maximum efficiency, it is desirable to operate the converter symmetrically (at least at full load) and to equally load the transformer and the rectifier on the secondary side. In the case of a transformer with a center tapped output winding that is symmetrical in terms of turns ratio and leakage, it can be ensured that the secondary side is symmetrical if the duty cycle of the half bridge (i.e. its switch node) is kept at 50%.

[0085] The control of the converter aims to maintain a given output voltage vo and to make the mains current im proportional to the mains voltage vm. Various ways have been described for this approach.

[0086] Direct frequency control

[0087] The standard approach to control of a resonant converter uses the switching frequency (i.e. the oscillator frequency) as the immediate manipulation variable of a feedback system that controls, for example, the converter input current.

[0088] Figure 2 An example of oscillator frequency control is shown, in which Figure 1 the resonant converter of Fig. 1 is represented as a single unit 40, and the figure shows the circuitry used to generate the gate signal GS.

[0089] The output voltage vo is provided to a current setting unit 42, which converts the output voltage to a reference input current im_ref. The reference input current is also based on the mains input voltage Figure 1 in vm), the mains input voltage giving the shape that the current must follow to provide unity power factor. The reference input current is compared to the measured input current im, and this difference is filtered by a loop filter 44. The output of the loop filter controls a voltage controlled oscillator VCO 46, which in turn produces the gate signal GS.

[0090] Thus, current is used as the feedback control parameter, with the target current being set based on the desired output voltage.

[0091] The problem with this approach is that if the converter has to cope with a relatively large gain ratio (i.e. a large variation in the output to input voltage ratio), it is difficult to avoid control instabilities. These instabilities are caused by the steepness of the large variation in the voltage gain versus frequency characteristic, which is typical for resonant converters.

[0092] As an example, for an LLC converter in a PFC application, the problem is more pronounced the closer to zero mains the converter is operated, however, this is required to provide a high power factor in terms of low total harmonic distortion.

[0093] Threshold control

[0094] There are various threshold control schemes that all exploit the fact that the converter state variable at the instant of inverter switching (e.g. the resonant tank capacitor voltage vC) is linearly related to the conversion energy per switching cycle.

[0095] Figure 3 An example of threshold control is shown, in which again Figure 1 the resonant converter of Fig. 1 is represented as a single unit 40, and the figure shows the circuitry used to generate the gate signal GS.

[0096] The converter state variable is the capacitor voltage vC, and it is provided to a control unit 50.

[0097] The output voltage vo is again converted to a target current im_ref in unit 42, and this in turn to a target value of a converter state variable, in this example the capacitor voltage. This target is shown as vCTH_ref. This happens in unit 52.

[0098] There can be a current feedback path for the current im, as shown in dashed lines, or not.

[0099] Direct threshold control

[0100] In this case, the inverter commutates directly in response to threshold detection. This scheme does not require an oscillator and is referred to as "self-oscillating". US 8729830 provides one example.

[0101] This scheme can overcome the instability of frequency control, as it directly controls the conversion energy. However, threshold detection is susceptible to noise, which can cause sudden disruption of the (self-) oscillation.

[0102] Cascaded threshold control

[0103] Threshold control can be cascaded by adding an additional inner loop. In this case, the inverter is again actuated by an oscillator, which in turn is steered by threshold control.

[0104] The noise problem is overcome due to the (re-) introduction of an oscillator, but the direct power control approach associated with threshold control is maintained. However, reliable threshold sensing requires a considerable amount of circuitry effort (in terms of cost, size and complexity), and is no longer practical at higher frequencies above about 0.5 MHz.

[0105] Figure 4 A first example of a circuit according to the invention is shown. Figure 1 The resonant converter of Fig. 1 is again shown as a single unit 40, and this figure shows the circuitry for generating the gate signal GS.

[0106] The control variable is the phase-lag signal φ.

[0107] The output voltage vo is again used (in combination with the input voltage) to produce a target current im_ref in a current setting unit 42, and this in turn is converted to a target value of a phase difference, namely a phase-lag φ_ref. This happens in a phase setting unit 60.

[0108] The phase control circuit 62 produces a feedback phase difference, namely a phase-lag signal φ.

[0109] The switching network within the resonant converter 40 provides a feedback signal vy, which is a phase signal representing the phase of the switching signal.

[0110] The resonant tank of the resonant converter 40 provides a further feedback signal comprising a resonant voltage vC across circuit elements of the resonant tank circuit. Furthermore, in this example, the output voltage v0 is provided as a further feedback signal.

[0111] In this example, the current setting unit 42 sets a reference current to be drawn from the input node based on the output voltage vo and the input voltage vm. The phase setting unit 60 sets a reference phase difference (ie, reference phase lag) φ_ref based on the reference current im_ref.

[0112] The phase control circuit 62 has a phase detector 64 that detects the phase difference between the resonant voltage vC and the phase signal vy. The phase difference is compared with a reference phase difference φ_ref, and a phase error φ_err is derived from the difference.

[0113] This phase error is applied to a loop filter 66, and the output of the loop filter drives a VCO 68 to derive a gate signal GS.

[0114] Therefore, resonant inverters use phase modulation as the control scheme for their switching networks. This method is suitable for high- and very-high-frequency operation of resonant converters, such as up to tens of MHz. The phase difference signal to be measured is much less sensitive to noise than the threshold signal.

[0115] Figure 4 The circuit provides high power factor without mains current measurement. The known relationship between mains current and phase can be exploited to avoid mains current measurement. The circuit controls the phase lag φ between the resonant tank state variable (vC in this example) and the voltage associated with the inverter switching state vy in a closed loop.

[0116] exist Figure 4 In the example of , the state variable is the resonant capacitor voltage vC, and the voltage vy is the voltage across the upper switch of the inverter. In the case of a full-bridge inverter (i.e., an inverter employing a second half-bridge), vy may be the voltage across the lower switch of the second half-bridge.

[0117] Alternatively, the gate drive signal GS0 may be used as a reference signal for processing the phase lag φ.

[0118] The voltage is measured via a capacitive voltage divider, and the phase detector preferably has a self-biased input, which helps it better cope with varying amplitudes of the two measured signals.

[0119] If the input current im is too high, this translates into a lower reference phase lag φ, which in turn will result in a higher VCO input voltage, which means lower conversion power and therefore lower current (and vice versa).

[0120] Figure 5 shows a modification of Figure 4 where the phase difference φ is converted by unit 70 into an input current value im*. Figure 4 The components repeated in

[0121] The feedback controlled subtraction step is then between the reference current im_ref and the input current value im*. An input current error im_err is then generated. This is converted into a phase difference error φ_err in unit 72.

[0122] In Figure 5 the relationship between mains current and phase is applied in reverse to generate a simulated (or observed) mains current im* instead of the actually measured mains current im*. The resulting current error is proportional to the phase error and controlled in the same way as in Figure 4 .

[0123] As mentioned above, Figure 4 and Figure 5 the example avoids the need for mains current measurement, relying on a known relationship between phase difference (phase lag) and current. Figure 6 This relationship is shown for an example resonant converter. The shown relative linearity between mains current im and phase lag φ still allows for achieving a high power factor of e.g. more than 0.9.

[0124] Figure 7 shows an alternative design shown as an alternative modification of Figure 4 where the mains current is measured.

[0125] Figure 4 The components repeated in

[0126] The mains current measurement signal im is subtracted from the reference current im at the output of the current setting unit 42. This results in a current error im_err provided to an additional control loop in the form of a loop filter 80. It generates a reference phase difference φ_ref which is then processed in the same way as in Figure 4 .

[0127] The phase difference remains an internal feedback control parameter, so there is an (internal) phase control loop.

[0128] Figure 8 shows a dc-dc converter (thus without PFC functionality) whose output current io is controlled. In contrast to Figure 7 the internal current control loop is not changed. The modulation of the reference current io_ref based on the output voltage is removed (i.e. Figure 7The circuit can for example be used as an isolated output stage for an LED driver. The same internal control loop can alternatively be used to control an output voltage.

[0129] For example, an HC 4046 voltage controlled oscillator circuit can be used as the phase control circuit 62 (comprising a phase detector and VCO, and a loop filter can be connected to the circuit). The circuit produces a signal proportional to the phase lag.

[0130] A phase detector IC of this type is for example designed to control the phase difference between the inputs to zero. However, this circuit can also be used to control the phase difference to assume any given reference value. The circuit comprises self-biased inputs (SIG_IN and COMP_IN) as well as a phase comparator and a VCO. It is designed to be able to add a loop filter as well as form a difference with a reference phase lag of the control error. This circuit can be used in various frequency ranges up to several tens of MHz.

[0131] The phase detector can alternatively be implemented for example by an EXOR detector, a positive edge triggered phase and frequency detector or a positive edge triggered sequential phase detector. The phase detector and VCO can also be implemented by discrete (non-integrated) circuits.

[0132] Delays in the control, for example related to sensing, signal conditioning or gate driving, are almost constant in time and can easily be compensated. These delays are for example in the order of magnitude of 1 to 10 ns. Figure 6 An offset is introduced in the shown relationship, i.e. the curve is then shifted upwards or downwards depending on the total delay of the two inputs of the phase detector.

[0133] Variations to the disclosed embodiments can become apparent to those of ordinary skill in the art upon reading the foregoing description, which is provided by way of example only. 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. The mere fact that measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

[0134] A single processor or other unit can implement the functions of several items recited in the claims.

[0135] The mere fact that measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

[0136] If the term “adapted” is used in the claims or specification it should be noted that the term “adapted” is intended to be equivalent to the term “configured”.

[0137] Any reference signs in the claims should not be construed as limiting the scope.

Claims

1. A resonant inverter, comprising: an input node for receiving an input for conversion (Vm); a switching network connected to the input node, comprising at least a first switch and a second switch, wherein the switching network is controlled by a switching signal, and wherein a switching network output is defined at a node located between the first switch and the second switch, wherein the switching network is adapted to provide a feedback signal comprising a phase signal (vy) representing a phase of the switching signal; a resonant tank circuit coupled to the switching network output, wherein the resonant tank circuit is adapted to provide a feedback signal comprising a resonant voltage (vC) across a circuit element of the resonant tank circuit; a current setting unit (42) for setting a reference current (im_ref) to be drawn from the input node; a phase setting unit (44, 60, 70, 72, 80) for setting a reference phase (φ_ref) based on the reference current (im_ref); and a phase control circuit (66, 68) for generating the switching signal for the switching network based on a phase difference between the resonant voltage (vC) and the phase signal (vy) and based on the reference phase (φ_ref).

2. The inverter of claim 1, wherein the phase control circuit comprises a phase-locked loop.

3. The inverter of claim 1 or 2, wherein the phase control circuit comprises a phase detector (44) for detecting a phase difference between the resonant voltage (VC) and the phase signal (vy).

4. The inverter of any of claims 1 to 2, wherein the phase control circuit comprises: a loop filter (66) for filtering a difference between the phase difference signal and the reference phase; and a voltage-controlled oscillator (68) driven by an output from the loop filter.

5. The inverter of any of claims 1 to 2, wherein the resonant tank comprises an LLC circuit.

6. The inverter of claim 5, wherein the resonant voltage (vC) is a voltage across a capacitor of the LLC circuit.

7. The inverter of any of claims 1, 2 and 6, wherein the phase signal is a voltage across the first switch or the second switch.

8. The inverter of any of claims 1, 2 and 6, wherein the first switch and the second switch form a half-bridge inverter.

9. The inverter of any of claims 1, 2 and 6, further adapted to provide an input current (im) drawn from the input node as a further feedback signal, and wherein the phase setting unit is for setting the reference phase (φ_ref) based on the input current (im) and the reference current (im_ref). ​ 10. The resonant inverter of any of claims 1, 2 and 6, wherein the resonant tank circuit is adapted to provide a further feedback signal comprising an output voltage (vo), and wherein the current setting unit (42) is configured to set the reference current (im_ref) based on at least the output voltage (vo).

11. An AC / DC PFC converter comprising an AC input; a rectifier, wherein the AC input is coupled to an input of the rectifier; and the converter of claim 10 having an output of the rectifier as an input of the converter.

12. An electrical device comprising: the inverter of any of claims 1 to 10; and a load downstream of the inverter, such as an LED device having one or more LEDs.

13. A conversion method comprising: receiving an input (Vm) for conversion at an input node; controlling a switching network using a switching signal, the switching network comprising at least a first switch and a second switch, wherein a switching network output is defined at a node located between the first switch and the second switch; providing a feedback signal from the switching network, the feedback signal comprising a phase signal (vy) representing a phase of the switching signal; providing an output of the switching network to a resonant tank circuit; providing a feedback signal from the resonant tank circuit, the feedback signal comprising a resonant voltage (vC) across an element of the resonant tank circuit; and setting a reference current drawn from the input node; setting a reference phase (φ_ref) based on the reference current; and generating the switching signal for the switching network based on a phase difference between the resonant voltage (vC) and the phase signal (vy) and based on the reference phase (φ_ref).

14. The method of claim 13, further comprising: providing a further feedback signal comprising an input current (im) drawn from the input node, and wherein setting the reference phase (φ_ref) is based on the input current (im) and the reference current; and / or providing a further feedback signal comprising an output voltage (vo), and wherein setting the reference current (im_ref) is based on at least the output voltage (vo).

15. A method of driving an LED, comprising: rectifying an AC input and providing conversion using the method of claim 13 or 14 to achieve power factor correction, and driving an LED load based on a converted DC voltage.

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