Drive circuit

Abstract

A drive circuit for driving an induction heating system has a full-bridge inverter having a first leg and a second leg. Each of the first leg and the second leg has a high-side switch and a low-side switch. The drive circuit has a coil to be driven by the full-bridge inverter to generate a magnetic field, a plurality of first resonant networks for enabling zero-voltage switching of the high-side switches and the low-side switches, and a plurality of second resonant networks each for providing a constant current output to the coil.

Description

Technical Field

[0001] This invention relates to a drive circuit for driving an induction heating system. Background Technology

[0002] Induction heating systems operate by inducing eddy currents in an electrical conductor, whereby the eddy currents are induced by changing the magnetic field applied to the conductor. This changing magnetic field can be achieved by using a switch to change the current applied to the coil used to generate the magnetic field.

[0003] Ideally, the switches in induction heating systems should be operated at high switching frequencies, but the switching losses in known circuit topologies demonstrate that achieving high switching frequencies is impractical. Summary of the Invention

[0004] According to a first aspect of the invention, a drive circuit for driving an induction heating system is provided, the drive circuit comprising a full-bridge inverter having a first branch and a second branch, each of the first and second branches including a high-side switch and a low-side switch, a coil driven by the full-bridge inverter to generate a magnetic field, a plurality of first resonant networks for realizing zero-voltage switching of the high-side switch and the low-side switch, and a plurality of second resonant networks each providing a constant current output to the coil.

[0005] The drive circuit according to the first aspect of the invention can be advantageous because the drive circuit includes a plurality of first resonant networks and a plurality of second resonant networks, the plurality of first resonant networks being used to realize zero-voltage switching of the high-side switch and the low-side switch, and each of the second resonant networks being used to provide a constant current output to the coil.

[0006] In particular, by utilizing multiple first resonant networks to achieve zero-voltage switching, switching losses associated with the high-side and low-side switches can be reduced, allowing the switches to operate at higher frequencies than switches operating under hard-switching conditions. Operating the induction heating system at higher frequencies allows for a reduction in the required coil size, resulting in a smaller form factor for products incorporated into the induction heating system.

[0007] However, during induction heating, the magnetic field transmitted by the coil may vary depending on the load carried by the inductor. This varying load can affect the effective resistive load, thus altering the current flowing through the coil. This current variation can be undesirable and may result in reduced power delivered to the load. Furthermore, a short circuit may occur if there is no load. Such problems can be mitigated by utilizing multiple secondary resonant networks to provide a constant current output to the coil (i.e., a constant current independent of the resistive load).

[0008] Each of the first resonant networks may include an inductor and a capacitor connected in series. Each of the first resonant networks may include an inductor connected to a common node between the high-side and low-side switches of a corresponding one of the first and second branches, and a capacitor connected in series with the inductor, the capacitor being grounded. This arrangement minimizes the number of inductors required compared to, for example, providing a resonant network for each high-side and low-side switch. Each of the first resonant networks may be connected in parallel with a corresponding one of the low-side switches.

[0009] Each of the first resonant networks can be connected in parallel with a corresponding high-side or low-side switch, for example, such that there is one first resonant network associated with each high-side or low-side switch. This can reduce the rated voltage required for each inductor in the first resonant network and can reduce switching losses during high-voltage switching.

[0010] Each of the plurality of second resonant networks may include an LCL resonant circuit. Each of the plurality of second resonant networks may define an impedance transformation network. Using such an LCL resonant circuit, a constant current output can be provided to the coil. Each LCL resonant circuit may include a first inductor and a second inductor connected in series, with a capacitor connected in parallel at the point between the first inductor and the second inductor. The first inductor and the second inductor of each LCL resonant circuit may be connected in series with the coil, and the capacitor of each LCL resonant circuit may be connected in parallel with the coil. The first inductor and the second inductor of each LCL resonant circuit may be connected in series with an effective resistive load, and the capacitor of each LCL resonant circuit may be connected in parallel with the effective resistive load.

[0011] The coil can form part of each LCL resonant circuit in an LCL resonant circuit. For example, each LCL resonant circuit may include a first inductor connected in series with the coil, and a capacitor connected in parallel at the point between the first inductor and the coil. Here, the coil can act as a second inductor in each LCL resonant circuit. Compared to a drive circuit that, for example, uses a separate inductor in addition to the coil for the LCL resonant circuit, this reduces the number of components required for the drive circuit, thereby reducing complexity and cost.

[0012] The coil may have a total inductance value, and the driving circuit may include a first capacitor connected in series with the coil on a first side and a second capacitor connected in parallel with the coil on a second side. The first and second capacitors may have capacitances tuned to resonate with an inductance corresponding to the total inductance minus the inductance forming a portion of each LCL resonant network. For example, the total inductance value of the coil may include a first inductance value and a second inductance value, each corresponding to the inductance value of a respective LCL resonant network, and the first and second capacitors may have capacitances tuned to resonate with inductances corresponding to the total inductance minus the first and second inductance values. The first and second capacitors can smooth the voltage waveform supplied to the coil, and having both capacitors can minimize EMI.

[0013] The drive circuit may include multiple filters, each connected between the coil and a common node between the high-side and low-side switches of a corresponding branch in the first and second branches. Each filter can smooth the voltage waveform from the corresponding high-side or low-side switch in the first and second branches, for example, converting a square voltage waveform into a substantially sinusoidal waveform.

[0014] Each filter may include an inductor and a capacitor connected in series. Each filter may include a corresponding third resonant network. Each filter may include an LC series filter. Each filter may include a resonant frequency tuned to the switching frequency of the full-bridge inverter.

[0015] In the case where each of the multiple second resonant networks includes an LCL resonant circuit, the inductor of each filter can form part of the corresponding LCL resonant circuit. Each LCL resonant circuit can include a first inductor and a second inductor connected in series, with a capacitor connected in parallel between the first and second inductors. The inductor of each filter can act as the first inductor of the corresponding LCL resonant circuit. This reduces the number of components, complexity, and cost compared to, for example, arrangements where the filters and LCL resonant circuits include separate inductors. With the inductor of each filter acting as the first inductor of the corresponding LCL resonant circuit, each filter can have a resonant frequency that is not equal to the switching frequency of the full-bridge inverter.

[0016] Each second resonant network may include an LCL resonant circuit defined by a first inductor that simultaneously forms part of a corresponding LC series filter, a second inductor connected in series with the first inductor and defined by a coil, and a capacitor connected in parallel with the first and second inductors at a point between the first and second inductors. Such an arrangement can minimize the number of components, cost, and complexity.

[0017] The switching frequency of a full-bridge inverter can be greater than or equal to 1 MHz. The high-side and low-side switches can operate at a switching frequency greater than or equal to 1 MHz. The high-side and low-side switches can include gallium nitride (GaN) or silicon carbide (SiC) switches.

[0018] The drive circuit may include a full-wave rectifier for converting an AC voltage source to a DC voltage, and the rectifier may output a waveform with at least 50% ripple. The rectifier may output a waveform with 100% ripple. This can minimize or eliminate the need for storage capacitors, which can reduce component cost, quantity, and size. The drive circuit can be used to receive power from an AC power source.

[0019] The drive circuit may include a power factor correction circuit connected between the full-wave rectifier and the full-bridge inverter. This can eliminate modulation by the mains frequency from the output current to the coil.

[0020] This drive circuit can be used to receive power from a DC power source. Attached Figure Description

[0021] Figure 1 This is a schematic diagram illustrating a first embodiment of the drive circuit according to the present invention;

[0022] Figure 2 The diagram shows the... Figure 1 The current output by the inverter of the drive circuit;

[0023] Figure 3 This is a schematic diagram illustrating a second embodiment of the drive circuit according to the present invention;

[0024] Figure 4 The diagram shows the... Figure 3 The current output by the inverter of the drive circuit;

[0025] Figure 5 This is a schematic diagram illustrating a third embodiment of the driving circuit according to the present invention;

[0026] Figure 6 This is a schematic diagram illustrating a fourth embodiment of the driving circuit according to the present invention;

[0027] Figure 7 This is a schematic diagram illustrating a fifth embodiment of the driving circuit according to the present invention;

[0028] Figure 8 This is a schematic diagram illustrating a sixth embodiment of the driving circuit according to the present invention; and

[0029] Figure 9 Yes Figure 8A diagram illustrating the driving signals, voltages, and currents used in the driving circuit. Detailed Implementation

[0030] Figure 1 A drive circuit for driving an induction heating system is shown, generally labeled 10.

[0031] The drive circuit 10 includes an AC power supply 12, a rectifier 14, a full-bridge inverter 16, an induction coil 18, a first pair of resonant networks 20 and 22, a second pair of resonant networks 24 and 26, and a third pair of resonant networks 28 and 30.

[0032] AC power supply 12 provides AC voltage to rectifier 14.

[0033] Rectifier 14 is a full-wave rectifier, comprising a full bridge with diodes D1-D4. Rectifier 14 converts the AC voltage received from AC power supply 12 into a DC rectified voltage to supply the full-bridge inverter 16. Rectifier 14 is grounded.

[0034] The full-bridge inverter 16 receives DC rectified voltage from the rectifier 14. The full-bridge inverter 16 includes an H-bridge with switches Q1-Q4, the H-bridge including a first branch 32 and a second branch 34. The first branch 32 includes switch Q1 as a high-side switch and switch Q2 as a low-side switch, while the second branch 34 includes switch Q3 as a high-side switch and switch Q4 as a low-side switch.

[0035] The full-bridge inverter 16 converts the DC rectified voltage received from the rectifier 14 into AC current via appropriate switching of switches Q1-Q4, and supplies the AC current to the induction coil 18. The induction coil 18 is described as an inductor L9 and is shown connected in series with the reflected load R1.

[0036] The function of an induction heating system is to induce eddy currents in an electrical conductor by changing the magnetic field applied to it. With the conductor fixed, the applied magnetic field must be varied to induce the desired eddy currents. In this embodiment, the varied magnetic field is achieved by appropriately switching switches Q1-Q4 to drive alternating current through the induction coil 18. Operating the induction heating system at higher frequencies allows for a reduction in the size of the induction coil, which enables products using this system to have smaller form factors and also allows for efficient heating of certain materials.

[0037] To achieve high-frequency switching, the switches Q1-Q4 discussed in this paper are gallium nitride (GaN) or silicon carbide (SiC) switches. While such switches are advantageous for high-frequency switching, i.e., high-level switching at kHz or MHz, the switching losses in practice may preclude operation at such frequencies.

[0038] For this purpose, the drive circuit 10 has a first pair of resonant networks 20, 22, each enabling zero-voltage switching of the high-side switch and low-side switch of a corresponding branch in the first branch 32 and the second branch 34 of the full-bridge inverter 16.

[0039] The first resonant network 20 in the first pair of resonant networks 20, 22 includes an inductor L1 connected to a common node between the high-side switch Q1 and the low-side switch Q2 of the first branch 32. The first resonant network 20 in the first pair of resonant networks 20, 22 also includes a capacitor C1 connected in series with the inductor L1 and grounded. The first resonant network 20 in the first pair of resonant networks 20, 22 can be considered to be connected in parallel with the low-side switch Q2 of the first branch 32.

[0040] The second resonant network 22 in the first pair of resonant networks 20, 22 includes an inductor L2 connected to a common node between the high-side switch Q3 of the second branch 30 and the low-side switch Q4 of the second branch 28. The second resonant network 20 in the first pair of resonant networks 20, 22 includes a capacitor C2 connected in series with the inductor L2 and grounded. The second resonant network 22 in the first pair of resonant networks 20, 22 can be considered to be connected in parallel with the low-side switch Q4 of the second branch 30.

[0041] The first pair of resonant networks 20 and 22 enable zero-voltage switching of the high-side and low-side switches of a corresponding branch in the first branch 28 and the second branch 30 of the full-bridge inverter 16.

[0042] Specifically, inductor L1 drives the voltage across high-side switch Q1 and low-side switch Q2 to zero by discharging the output capacitor of the switch. Inductor L2 drives the voltage across high-side switch Q3 and low-side switch Q4 to zero by discharging the output capacitor of the switch. Capacitors C1 and C2 in the first pair of resonant networks 20 and 22 are DC blocking capacitors.

[0043] By utilizing zero-voltage switching in the manner described above, switching losses associated with the high-side and low-side switches can be reduced, allowing these switches to operate at higher frequencies than switches operating under hard-switching conditions. Operating the induction heating system at higher frequencies allows for a reduction in the required coil size, resulting in a smaller form factor for products incorporating the induction heating system.

[0044] Another problem encountered during induction heating is that the magnetic field transmitted by the induction coil 18 may vary depending on the load carried by the sensor interacting with the magnetic field, and the varying load may affect the effective resistive load, thereby altering the current flowing through the induction coil 18. This current variation may be undesirable and could lead to a reduction in the power delivered to the load. Moreover, in the absence of a load, a short circuit may occur.

[0045] To maintain zero-voltage switching operation, the second pair of resonant networks 24 and 26 will provide a constant current output to the induction coil 18.

[0046] The first resonant network 24 in the second pair of resonant networks 24, 26 includes a first inductor L3 connected in series with the first resonant network 28 of the third pair of resonant networks 28, 30, a second inductor L4 connected in series with the first inductor L3, and a capacitor C3 connected in parallel with the first inductor L3 and the second inductor L4 at a point between the first inductor L3 and the second inductor L4. The first resonant network 24 in the second pair of resonant networks 24, 26 can be considered an LCL resonant circuit, which can also be called a T-network circuit. The first resonant network 26 in the second pair of resonant networks 24, 26 acts as an impedance transformation network to transform the constant voltage across Q2 into a constant current. By using such an impedance transformation network, the current output from the first resonant network 24 in the second pair of resonant networks 24, 26 to the coil 18 can be independent of the reflected load R1, and therefore a constant current amplitude can be output to the coil 18.

[0047] The second resonant network 26 in the second pair of resonant networks 24, 26 includes a first inductor L5 connected in series with the second resonant network 30 in the third pair of resonant networks 28, 30, a second inductor L6 connected in series with the first inductor L5, and a capacitor C4 connected in parallel with the first inductor L5 and the second inductor L6 at a point between the first inductor L5 and the second inductor L6. The second resonant network 26 in the second pair of resonant networks 24, 26 can be considered an LCL resonant circuit, which can also be called a T-network circuit. The second resonant network 26 in the second pair of resonant networks 24, 26 acts as an impedance transformation network to transform the constant voltage across Q4 into a constant current. By using such an impedance transformation network, the current output from the second resonant network 26 in the second pair of resonant networks 24, 26 to the coil 18 can be independent of the reflected load R1, and therefore a constant current amplitude can be output to the coil 18.

[0048] The third pair of resonant networks 28 and 30 are used to smooth the voltage waveform supplied from the full-bridge inverter 16 to the coil 18.

[0049] One of the third pair of resonant networks 28, 30, resonant network 28 includes a capacitor C5 and an inductor L7. The capacitor C5 is connected in series with the common node between the high-side switch Q1 and the low-side switch Q2 of the first branch 32, and the inductor L5 is connected in series with the capacitor C5. This smooths the square wave voltage provided by the full-bridge inverter 16 (e.g., the square wave voltage provided when switches Q1 and Q4 are closed) to one of the second pair of resonant networks 24, 26, such that the one resonant network 24 in the second pair of resonant networks 24, 26 receives a substantially sinusoidal voltage.

[0050] Another resonant network 30 in the third pair of resonant networks 28, 30 includes a capacitor C6 and an inductor L8. The capacitor C6 is connected in series with the common node between the high-side switch Q3 and the low-side switch Q4 of the second branch 34, and the inductor L8 is connected in series with the capacitor C6. This smooths the square wave voltage provided by the full-bridge inverter 16 (e.g., the square wave voltage provided when switches Q3 and Q2 are closed) to the other resonant network 26 in the second pair of resonant networks 24, 26, so that the other resonant network 26 in the second pair of resonant networks 24, 26 receives a substantially sinusoidal voltage.

[0051] The drive circuit 10 includes a first resonant capacitor C7 connected in series between inductors L3 and L4 of one of the second pair of resonant networks 24, 26, and a second resonant capacitor C8 connected in series between inductors L5 and L6 of the other resonant network 26 in the second pair of resonant networks 24, 26. The first resonant capacitor C7 and the second resonator C8 are tuned to resonate with the inductance of the coil 18 and can smooth the voltage across the corresponding capacitors C3 and C4 in the second pair of resonant networks 24, 26.

[0052] Another capacitor C9 is shown connected in parallel across the two ends of coil 18 and represents the capacitance of coil 18.

[0053] exist Figure 1 In the drive circuit 10 of the embodiment, it can be seen that there is no DC link capacitor between the rectifier 14 and the full-bridge inverter 16. This means that the current supplied by the rectifier 14 to the full-bridge inverter 16 has 100% ripple. Therefore, the output current supplied to the induction coil 18 is modulated by the frequency of the AC power supply 12, typically 50Hz. Figure 2 As shown.

[0054] Figure 3 A second embodiment 100 of the driving circuit according to the present invention is shown.

[0055] Figure 3 The driving circuit 100 of the embodiment and Figure 1The difference between the driver circuit 10 in the embodiment is that, Figure 3 The driver circuit 100 in the intermediate embodiment includes a power factor correction (PFC) circuit 102 located between the rectifier 14 and the full-bridge inverter 16. The exact shape of the PFC circuit 102 can be chosen based on other factors, such as cost or complexity, but the purpose of the PFC circuit 102 is to eliminate ripple in the current supplied from the rectifier 14 to the full-bridge inverter 16, so that the current output to the induction coil 18 is not modulated by the frequency of the AC power supply 12, such as... Figure 4 As shown.

[0056] Figure 5 Another embodiment 200 of the drive circuit according to the present invention is shown. Figure 5 The drive circuit 200 is powered by a DC power supply 202 (e.g., a battery), and the rectifier 14 is omitted. Figure 5 The rest of the drive circuit 200 and Figure 1 The driving circuit 200 is identical to that of the induction coil 18, including a first pair of resonant networks 20, 22 and a second pair of resonant networks 24, 26. The driving circuit 200 outputs current to the induction coil 18, similar to... Figure 4 The current shown.

[0057] Figure 6 Another embodiment 300 of the drive circuit according to the present invention is shown.

[0058] exist Figure 6 In the driving circuit 300 of the embodiment, Figure 1 The inductors L7 and L8 of the third pair of resonant networks 28 and 30 in the embodiment have been connected with Figure 1 The second pair of resonant networks 24, 26 in the embodiment combines the first inductors L3 and L5, and now in Figure 6 The diagram shows individual inductors L3 and L5.

[0059] Individual inductors L3 and L5 each form part of a corresponding second resonant network 24, 26 and a corresponding third resonant network 28, 30. Depending on the situation, inductors L3 and L5 are tuned to resonate as part of the second pair of resonant networks 24, 26 and the third pair of resonant networks 28, 30. Figure 6 In the embodiment, the resonant frequency of the third pair of resonant networks 28 and 30 is not equal to the switching frequency of the full-bridge inverter.

[0060] By utilizing inductors L3 and L5, which form part of the corresponding second resonant networks 24, 26 and the corresponding third resonant networks 28, 30, and with Figure 1 Compared to the driving circuit, Figure 6 The number of components, cost, and complexity of the drive circuit 300 can be reduced.

[0061] Figure 7 Another embodiment 400 of the drive circuit according to the present invention is shown.

[0062] Figure 7 The drive circuit 400 and Figure 6 The driving circuit 300 is basically the same, except that the second inductors L4 and L6 of the second pair of resonant networks 24, 26 are combined with coil 18 here into a single inductor L10, which in practice is coil 18. Here, coil 18 acts as an induction coil that generates a magnetic field to interact with the heater, and also as the second inductor of the second pair of resonant circuits 24, 26. Therefore, the inductance of coil 18 plays a role in impedance transformation to provide a constant current output. By utilizing a single inductor as an induction coil that generates a magnetic field to interact with the heater, and as the second inductor of the second pair of resonant circuits 24, 26, and... Figure 6 Compared to the driving circuit, Figure 7 The number of components, cost, and complexity of the drive circuit 400 can be reduced.

[0063] Understandable. Figure 1 The second inductors L4 and L6 of the third pair of resonant networks 24, 26 of the drive circuit 10 can be combined with the coil 18 (i.e., inductor L9) in a similar manner.

[0064] Figure 8 Another embodiment 500 of the drive circuit according to the present invention is shown.

[0065] and Figure 7 Compared to the first pair of resonant networks 20 and 22 of the driving circuit 400, except... Figure 8 In addition to the four first resonant networks 502, 504, 506, and 508, the driving circuit 500 has a driving circuit 500. Figure 8 The driver circuit 500 and Figure 7 The driving circuit is basically the same as that of the 400.

[0066] exist Figure 8 In the driving circuit 500, each of the first resonant networks 502, 504, 506, 508 includes inductors L1, L2, L11, and L12 connected in series with DC blocking capacitors C1, C2, C10, and C11. Each of the resonant networks 502, 504, 506, and 508 is connected in parallel with respect to one of the corresponding switches Q4, Q2, Q1, and Q3.

[0067] Inductors L1, L2, L11, and L12 are used to ensure that the voltage across each switch Q4, Q2, Q1, and Q3 is zero volts before they are switched on. Zero-voltage switching is achieved by adjusting the output capacitor C of the switch. oss It is achieved through discharge.

[0068] The values ​​of inductors L1, L2, L11, and L12 are determined using the following equation:

[0069]

[0070] Where Δt ZVS It is the transition time of the switch from the open state to the closed state, f is the switching frequency, and C is the switching frequency. oss It is the output capacitor of the switch specified in the manufacturer's datasheet.

[0071] Capacitors C1, C2, C10, and C11 are DC blocking capacitors with very high values, typically above 100nF.

[0072] Given the high voltage and resulting high current involved in the switching of switches Q1-Q4, utilizing a single inductor on the low side of the full-bridge inverter 16 for zero-voltage switching may be lossy. For example, in Figure 8 In the drive circuit 500, an inductor is used for each switch Q1-Q4. Figure 8 The rated current of inductors L1, L2, L11, and L12 relative to Figure 7 The rated current of inductors L1 and L2 can be halved, and losses can be reduced.

[0073] Understandable, it depends on the situation. Figure 8 The resonant networks 502, 504, 506, and 508 can be used in... Figure 1 , 3 Implemented in any of the drive circuits 10, 100, 200, 300, and 400 for 5, 6, and 7.

[0074] Figure 9 Schematic map shows Figure 8 The various components of the drive circuit 500 utilize drive signals and experience voltage and current.

[0075] The drive circuits 10, 200, 300, 400, and 500 disclosed herein can be driven at high frequency by means of a first resonant network capable of zero-voltage switching and a second resonant network providing a constant current output to the induction coil 18.

Claims

1. A drive circuit for driving an induction heating system, the drive circuit comprising a full-bridge inverter having a first leg and a second leg, each of the first leg and the second leg comprising a high-side switch and a low-side switch; a coil driven by the full-bridge inverter to generate a magnetic field; a plurality of first resonant networks for enabling zero voltage switching of the high side switches and the low side switches; and a plurality of second resonant networks each providing a constant current output to the coil, wherein each of the plurality of second resonant networks includes an LCL resonant circuit, the coil forms a portion of each of the LCL resonant circuits, the coil has a total inductance value, the drive circuit includes a first capacitor connected in series with the coil on a first side of the coil, and a second capacitor connected in series with the coil on a second side of the coil, the first and second capacitors have a capacitance tuned to resonate with an inductance corresponding to the total inductance minus an inductance forming a portion of each of the LCL resonant networks.

2. The drive circuit of claim 1, wherein, Each of the first resonant networks includes an inductor and a capacitor connected in series.

3. The drive circuit of claim 1 or claim 2, wherein, Each of the first resonant networks includes an inductor and a capacitor, the inductor connected to a common node between the high side switch and the low side switch of a respective one of the first and second legs, the capacitor connected in series with the inductor, the capacitor grounded.

4. The drive circuit of claim 1 or claim 2, wherein, Each of the first resonant networks is connected in parallel with a respective one of the low side switches.

5. The drive circuit according to any one of claims 1 or 2, wherein, Each of the first resonant networks is connected in parallel with a respective high side switch or low side switch.

6. The drive circuit of claim 1 or claim 2, wherein, The drive circuit includes a plurality of filters, each filter connected between the coil and a common node between the high side switch and the low side switch of a respective one of the first and second legs.

7. The drive circuit of claim 6, wherein, Each filter includes an inductor and a capacitor connected in series.

8. The drive circuit of claim 7, wherein, Each of the plurality of second resonant networks includes an LCL resonant circuit, and the inductor of each filter forms a portion of a respective LCL resonant circuit.

9. The drive circuit of claim 8, wherein, A resonant frequency of each filter is not equal to a switching frequency of the full bridge inverter.

10. The drive circuit of claim 1 or claim 2, wherein, The high side switches and the low side switches operate at a switching frequency greater than or equal to 1 MHz.

11. The drive circuit of claim 1 or claim 2, wherein, The drive circuit includes a full wave rectifier for converting an AC voltage source to a DC voltage, and the rectifier is for outputting a waveform having a ripple of at least 50%.

12. The drive circuit of claim 11, wherein, The drive circuit includes a power factor correction circuit connected between the full wave rectifier and the full bridge inverter.

13. The drive circuit of claim 1, wherein, The drive circuit receives power from a DC power source.

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