WO2016115514A1

WO2016115514A1 - Current controlled resonant induction power supply

Info

Publication number: WO2016115514A1
Application number: PCT/US2016/013693
Authority: WO
Inventors: Oleg Fishman
Original assignee: Oleg Fishman
Priority date: 2015-01-16
Filing date: 2016-01-15
Publication date: 2016-07-21

Abstract

A resonance power supply for generating variable AC power to apply to an induction coil magnetically coupled to a work piece comprising: at least one rectifier (55) with power factor correction (PFC), at least one step down voltage regulator (68), at least one resonant inverter (53), at least one induction coil magnetically coupled with at least one work piece (13), and at least one control circuit (52) configured to adjust at least one parameter associated with at least one of the rectifier and the resonant inverter responsive to the output of the resonant inverter.

Description

- -

CURRENT CONTROLLED RESONANT INDUCTION POWER SUPPLY Field of the Invention

[0001] The present invention relates generally to devices for induction heating and melting of magnetically coupled electrically conductive material inserted into an induction coil, and more particularly to current control series resonant power supplies and precise measurement of the power delivered into a magnetically coupled electrically conductive load.

Background

[0002] Induction heating and melting of metals is a widely used process in metal parts production and treatment processes. In these processes, including but not limited to forging, extrusion, welding, brazing, hardening, galvanizing, welding, and melting, the AC electric current in the induction coil induces eddy currents on the surface of a metal body placed inside the coil. The power for heating is supplied from a variable frequency induction generator. The coil reactance is compensated by a capacitor.

[0003] Two types of prior art induction power generators include series resonant voltage source inverters and parallel resonant current source inverters.

[0004] A prior art series resonant voltage source generator is shown in Figure la. An implementation of the voltage source generator is shown in Figure 3. Referring to Figure la in conjunction with Figure 3, fixed voltage DC source 17 (Figure la) is implemented as a diode rectifier 31 (Figure 3), the stiff DC voltage supported by the DC capacitor 18 (Figure la) or 33 (Figure 3). The DC voltage is converted into AC via the inverter 16 (Figure la), implemented as a four semiconductor bridge circuit 34 (Figure 3). The semiconductors may be of bipolar, IGBT, MOSFET, SiC- MOSFET type transistors or thyristors with anti-parallel diodes. The AC output from the inverter is applied through an optional transformer 35 to a resonance loop 10 (Figure 3) containing inductor 14 and capacitor 11 (Figure 1) connected in series. The resistance 12 represents the resistivity of the induction coil and resistance 15 represents the introduced resistivity of the heated load 13 as it is inserted into the coil. When the heated load is removed from inside the coil the resistance 15 has zero value and when the heated load is fully inserted into the coil, the resistance 15 has its maximum value.

[0005] The power delivered to the load may be given by: [0006] Pload =Rload*Iac² =Rl5*Iac² (1) - -

[0007] The output power from the inverter may be given by:

[0008] P_out =( Rload + i?coi/ )*N²* Iac²= (Ri5 + Ri2)*N²*I_ac ² (2)

[0009] Where I_ac is the inverter current in the coil. When optional transformer 35 is present, the coil current I is equal to N - the transformer windings ratio. Otherwise N=l.

[0010] The series resonant circuit controls the output power by varying the frequency of the inverter. The impedance of the resonant loop 10 is given by:

[0011] Z = J(wL - -^ + (Rload + Rcoil)² (3) [0012] The output power P_out is determined as:

JV²* (wL-^) +(Rload+i?coii)²

[0014] where w is the inverter frequency and V_out is the series the resonant voltage source inverter output voltage.

[0015] The output power also can be measured as a vector product of the output voltage Vout and output current lout:

[0016] Pout = Vout * lac * cos ^? (5) [0017] were cos φ = (6)

[0018] The output power of the series resonant voltage source inverter is varied by changing the inverter operating frequency The frequency is changed around the resonance point or condition. The operating frequency is changed by varying the frequency of control pulses applied to the gate terminals of the semiconductors.

[0019] The maximum power is achieved at the resonance frequency w₀ when: [0020] w₀L = ^-_c (7)

[0021] or w₀ = 1/v C (8)

[0022] Following formulas (7) and (6) at resonance condition - -

[0023] cos φ = 1 , or φ = 0° (9);

[0024] which means that the inverter voltage and current are in the same phase.

[0025] In Figure lb the bell curve 18 demonstrates the relationship of power and frequency for the case when the load is inserted inside the coil, while the curve 19 shows the function of power and frequency for an empty coil.

[0026] Similarly, Figure 2 illustrates a parallel resonant current source inverter 26 with fixed supply 28 while Figure 4 illustrates implementation of the parallel resonant current source inverter and supply. In the parallel resonant current source inverter the DC current is supported by the choke 27 (Figure 2). Fixed supply 28 (Figure 2) may be implemented as a diode rectifier 41 (Figure 4). The bridge inverter 43 (Figure 4) consists of four semiconductors that can be bipolar, IGBT, MOSFET, SiC MOSFET type transistors with in-line diodes. The AC output from the inverter is applied through an optional transformer 44 (Figure 4) to resonance loop 20 containing inductor 24 and capacitor 21 (Figure 2) connected in parallel. The resistance 22 represents the resistivity of induction coil and resistance 25 represents the introduced resistivity of the heated load 23 as it is moved into the coil. When the heated load is removed from inside the coil the resistance 25 has zero value and when the heated load is fully inserted into the coil the resistance 25 has its maximum value.

[0027] The output power of the parallel resonant current inverter is varied by changing the inverter operating frequency in a fashion similar to frequency variation of the series resonant inverter.

[0028] In order to control the power level delivered to the load in the prior art series resonant voltage source inverter and parallel resonant current source inverter the frequency is varied and thus do not operate at the resonant frequency, when inverter operation is most efficient.

[0029] Moreover, the magnitude of coil resistance 12 of the series resonant loop 10 (Figure la) or magnitude of coil resistance 22 of the parallel resonant loop 20 (Figure 2) are changing with frequency in the process of induction heating or melting. This change prevents accurate measurements or computation of the power lost in the coil and power delivered to the load in the induction heating process in prior art inverters.

[0030] It is an object of the present invention to provide an induction power inverter configured to operate in an efficient manner, when the switching between positive and negative branches of the bridge inverter occurs at zero current in the semiconductors, and therefore when the inverter - - switching losses are minimal. In an embodiment of this invention, this condition may require operating near or at resonance.

[0031] It is another object of the present invention to provide an induction power inverter configured to enable continuous and accurate measurement not only of the output power P_out but also the power delivered into the inductively coupled load Pi_oad.

[0032] It is another object of the present invention to operate a power supply with one common PFC rectifier and a number of inverters each supplying AC current connected to individual induction coils, or to a set of magnetically coupled coils heating one work piece.

Brief Description of the Drawings

[0033] For the purpose of illustrating aspects of the invention, there is shown in the drawings a form that is presently preferred; it being understood, however, that this invention is not limited to the precise arrangements and instrumentalities shown. The details of the invention, both as to its structure and operation, may be obtained by a review of the accompanying drawings, in which like reference numerals refer to like parts, and in which:

[0034] FIG. la is a simplified block diagram of one example of a prior art series resonant loop including a constant voltage AC power Source, heating coil, and conductive load.

[0035] FIG. lb is a chart of power vs. frequency series resonance in constant voltage series resonant inverter 16 of FIG. la.

[0036] FIG. 2 is a simplified block diagram of one example of a prior art parallel resonant loop with current source inverter, heating coil and conductive load.

[0037] FIG. 3 is a simplified schematic diagram of one example of a prior art constant voltage power source 30 with series resonant looplO.

[0038] FIG.4 is a simplified schematic diagram of one example of a prior art constant current source inverter 40 with parallel resonant loop 20.

[0039] FIG. 5 is a simplified block diagram of an exemplary embodiment of the present invention showing a current control series resonant power supply 50 with series resonant loop 10 comprised of series resonant capacitor 11, heating coil 12, and conductive load 13.

[0040] FIG. 6 is a simplified schematic diagram of one example of a current controlled resonant power supply with series resonant loop utilized in the present invention and one phase supply. - -

[0041] FIG 7 is a simplified schematic diagram of one example of a current controlled resonant power supply with series resonant loop utilized in the present invention and three phase supply.

[0042] FIG 7a is a simplified schematic diagram of one example of a current controlled resonant power supply with parallel resonant loop utilized in the present invention and three phase supply. [0043] FIG. 8a is a simplified schematic diagram of one example of a resonance detection circuit configured in accordance with embodiments of the present invention.

[0044] FIG. 8b shows exemplary wave forms of the signals associated with the resonance detection circuit of FIG. 8a configured in accordance with embodiments of the present invention.

[0045] FIG. 9 shows exemplary input current waveforms and signals to control input current configured in accordance with embodiments of the present invention.

[0046] FIG. 10 is a simplified block diagram illustrating a power supply with dual AC output configured in accordance with embodiments of the present invention.

[0047] FIG. 11 is a simplified block diagram illustrating a power supply with multiple AC outputs and sectional coil configured in accordance with embodiments of the present invention. Summary of the Invention

[0048] In one aspect of the invention there is disclosed a system and method for induction heating and melting of magnetically coupled electrically conductive material inserted into an induction coil, and particularly to current controlled series resonant power supply and precise measurement of the power delivered into a magnetically coupled electrically conductive load. Additional embodiments of the present invention include concurrent operation of a number of inverters with multiple induction furnaces or with a sectional induction coil distributed along a single work piece to produce dispersed heating. Each of the coils can deliver to the work piece different power and achieve uneven heating according to a set pattern. In one aspect of the present invention there is provided a current controlled resonant power supply with series resonant loop, for example, as shown in Figure 5. The power supply may comprise a variable voltage rectifier with line power factor correction (PFC) topology, step down voltage regulator, and voltage fed inverter. The output of the inverter is connected through an optional transformer to a series resonant loop containing an induction coil and serially connected capacitor. The electromagnetic field produced by the induction coil couples with the conductive load inserted into the coil inducing eddy current onto the load surface. This eddy current subsequently heats the load. - -

[0049] In another aspect of the present invention the amount of energy delivered to the load is monitored and controlled by a control circuit. The control circuit is configured to a) adjust the inverter frequency to assure that the inverter always operates at resonance condition, b) adjusts the DC voltage across to the voltage fed inverter to assure that the delivered power to the inductive load is equal to a preset value. A user interface device such as a touch screen may be utilized for user input and feedback/display purposes.

[0050] Another aspect the present invention is to control the input power factor correction (PFC) rectifier- regulator in a way that input line current is always in phase with the input line voltage and has a sine wave shape with low line total harmonic distortions (THD). [0051] A resonance power supply for generating variable AC power to apply to an induction coil magnetically coupled to a work piece comprising: at least one rectifier (55) with power factor correction (PFC), at least one step down voltage regulator, at least one resonant inverter, at least one induction coil magnetically coupled with at least one work piece, and at least one control circuit configured to adjust at least one parameter associated with at least one of the rectifier and the resonant inverter responsive to the output of the resonant inverter.

Detailed Description of the Invention

[0052] Referring now to FIG. 5 there is shown a power supply 50 including variable voltage rectifier 55 with line power factor correction (PFC) topology, current controller labeled as step down voltage regulator 68, and voltage fed inverter 53. The output of the inverter is connected through optional transformer 56 to a series resonant loop 10 comprised of capacitor 11 and induction coil 12. The electromagnetic field produced by the induction coil 12 couples with a conductive load 13 inserted into the coil inducing eddy current onto the load surface. This eddy current subsequently heats the load.

[0053] Apart from the prior art, the frequency of the inverter is not changed to adjust the power per formula (4), but maintained relatively constant at resonant frequency w₀ per formula (6). The output power P_out is changed by varying the inverter voltage V_out per formula (4)..

[0054] In another aspect the present invention the amount of energy delivered to the load is carefully monitored and controlled by control circuit 52. This circuit a) -assures that the inverter always operate at resonance condition according to formulas (7), (8), (9); b) adjusts the DC voltage supplied to the voltage fed inverter to assure that the delivered power to the inductive load is equal to a preset value. The touch screen 51 serves as a user interface device. Utilizing the user interface the operator may monitor the process parameters such as but not limited to - - inverter output power P_out, power delivered to the load Pi_oad , inverter frequency w, inverter current I_aCi inverter output voltage V_{out ,} line voltage Vii_ne, the DC voltage applied to inverter in_v_DC, as well as calculated value of inverter efficiency Pi_oad pout _. Using the touch screen panel the operator may set process parameters for power delivered to the load ( work-piece) Po. [0055] When the induction loop 10 operates at the resonance frequency condition (formula (8)), the inductive impedance 14 (shown e.g. in FIG. 6) of the induction coil is compensated by the capacitive impedance of the capacitor 11 as indicated by formula (7) . Therefore the loop current (I) will generate only active power on the coil resistance 12 (FIG. 6) and induced load resistance 15. The circuit control is configured such that the inerter voltage is adjusted to generate a preset value of power into inductive load 13.

[0056] The frequency of the inverter 53 (in FIG. 5; 70 in FIG. 6) current does not significantly change in the process of induction heating and therefore the coil resistance does not change. The system and method of the present invention leverages this to enable separate computation of the power losses in the induction coil and power delivered to the work-piece. [0057] In another aspect the present invention the system is configured to control the input power factor correction (PFC) rectifier- regulator in a way that input line current is always in phase with the input line voltage and has a sine wave shape with low line total harmonic distortions (THD).

[0058] FIG. 6 is a simplified one line of a single phase step down voltage regulator resonant power supply 60 according to an embodiment of the invention. The circuit comprises a PFC rectifier shown in Figure 5 as element 55 and labeled in Figure 6 as 65 with step-up voltage regulation, step-down voltage regulator 68, series resonant inverter 70 (labeled as 53 in Figure 5), and resonance induction loop 10 with optional transformer 35. The full bridge inverter 70 consists of four semiconductor switches 73 that could be bipolar, IGBT, MOSFET, SiC

MOSFET transistors 71 or thyristor type with anti-parallel diodes, by way of example. Pairs of switches diagonally across the inverter bridge A-B and C-D are turning on/off periodically with frequency( w₀). When this frequency is equal to the resonance frequency the inverter voltage V_out will be in phase with inverter current I_ac per formula (9).

[0059] Figure 8a shows non-limiting elements of the control circuit 52 (FIG. 5) that can be used for detection of the resonance. Referring to Figure 8a in conjunction with Figure 8b, the sine wave signal representing inverter current I_ac passed through the comparator 75 produces pulses 77(Figure 8b) synchronous with the inverter current. Another set of pulses 78 is synchronous with the inverter voltage. Both strings of pulses are compared on a XOR circuit 76 which generates the difference pulses 79.

[0060] Figure 8b shows, when the inverter operates below resonance (w< w₀) the inverter current signal pulses 77 are leading the inverter voltage signal pulses 78. Conversely when the inverter operates below resonance (w> w₀) the inverter current signal pulses 77 are lagging the inverter voltage signal pulses 78. In resonance condition the current pulses 77 are totally in phase with the voltage pulses 79 and the XOR pulses' duration are near zero. The objective of the inverter frequency control is to minimize the duration of the XOR pulses.

[0061] Referring to the drawings, the step-down voltage regulator such as shown by element 68 in Figure 6 regulates the DC voltage across series resonant inverter 70, in the range below the PFC regulator voltage.

[0062] The step down voltage regulator 68 includes capacitor 72, inductor 66, semiconductor switch 69 , and diode 67. When the voltage on the input into step down voltage regulator 68 exceeds the voltage required to satisfy condition supplying a set power to the induction load, the switch 64 is periodically tumed off for duration t each interval T. During these "off periods the current is not supplied from the PFC rectifier 65 and circulates via diode 67. As a result the voltage on the capacitor 72 voltage is reduced to:

[0063] V_inv__Dc = V_pfc * t /T (10)

[0064] Where V_PfC is the DC voltage on the output of the PFC rectifier. [0065] The voltage Vi_nv_DC in eq. (10) has about the same value as to Vi_nv__out in eq. (4).

[0066] The step-down voltage regulator is activated only when the DC voltage across the inverter(s) must be reduced below the minimum voltage achievable by the PFC rectifier 65 equal to .When the voltage Vi_nv_DC is required to be higher than minimum voltage achievable by the PFC rectifier 65, the transistor switch 69 is conducting continuously and the voltage on capacitor 72 is regulated by the PFC rectifier 65. With the help of PFC rectifier and the step- down voltage regulator of value of Vi_nv_DC may be regulated between 0 and 1000 volt DC at nominal line voltage Vii_ne= 480 V AC.

[0067] Referring still to Figure 6, the PFC rectifier with step up voltage regulation capability 65 comprises line inductors 61, diode rectifier bridge 62, decoupling diode 63, and DC capacitor 75. The PFC rectifier serves a double purpose: a) when required, to generate a voltage higher than - -

Λ/2 Vii_ne on capacitors 75 and 72; and b) ensure the input line current exhibits a near sinusoidal form in phase with the input line voltage.

[0068] The elevated voltage is produced by periodically turning on transistor switch 64 for duration t_var each interval T_var. Both the values of t_var and T_var change during the input line voltage period Τ_11ιιε according to a PFC control algorithm and illustrated in figure 9.

[0069] When the switch 64 is closed the line voltage is applied to the line inductors 61, the diode bridge 62, and switch 64. The current builds-up in the inductors 61 and the reactive energy stored in the inductor increases. When switch 64 opens, the current in the inductor continues to flow via diode 63 into capacitor 75 and the stored energy flows into the capacitor, charging it to the voltage which magnitude may be larger than the input line voltage.

[0070] The principle of the PFC control algorithm is shown in Figure 9 which is referred to as "bang-bang" control The value of the desired or target line sine wave current 91 is computed based on the current consumption of the resonant loop. The signal 95 is proportional to the rectified line voltage. Controller circuitry implemented in one or more hardware and/or software modules such as voltage and/or current transformers for monitoring and or adjusting one or more of current, voltage, and power, may be utilized for performing the functionality described herein.

[0071] S_ref = (k_pe + ki / e dt + k_d + de/dt) y/2 Vi_ine sin (w _mt) (11)

[0072] Where wu_m is the line voltage frequency, S_ref= reference signal 95;

e = Pi_oad - P_o - error between the actual measured load power in work piecel3 and set P₀, the desired value of load power; K_p, ¾,¾ -are the coefficients of the control loop.

[0073] Another pair of reference sine wave signals: Si= S_ref +A signal 96, and

S_ref -Δ (where Δ is the control precision) is produced from of the reference signal 95.

[0074] The "bang- bang" algorithm operates as follows: when the current in inductors 61 reaches the signal SI the transistor 64 is turned on, and the instantaneous current increases as illustrated in Figure 9. When the current in inductors 61 reaches the signal S2 the transistor 64 is turned off, and the instantaneous current decreases. In this way the current maintains sine wave form with a small saw-tooth ripple (93, 94). The string of pulses 97 is applied to the control gate of transistor 64 to cause the saw tooth current in the inductors 61.

[0075] FIG. 7 is a simplified diagram of a three phase current controlled resonant power supply 70 with series resonant loop 10. This circuit is very similar to the circuit shown in Figure 6, - - however, the PFC rectifier/regulator contains three inductors connected to line phases A, B, and C. It also contains three diode bridges 62, three semiconductor switches 54 and three sets of decoupling diodes 63. It also includes three PFC controls, each synchronized with voltages A, B, and C and operating as shown on Figure 9. [0076] Embodiments of the present invention allow for accurate monitoring of the power delivered into the work-piece inserted into induction coil. When the coil is empty the control can register the power dissipated on the coil:

[0077] P_∞1i = N²*I_ref ² R₁₂ ;

that allows to compute the value of R = P_COii N²* I_ref² (12) [0078] Where I_ref² is the reference current at which the calibration was carried out.

[0079] Once the load work piece is inserted into the coil the control may register the total output power P_out in accordance with expression (5). The power delivered into the work piece is equal to:

[0080] Pioad= Pout - N²*I_ac ²* R₁₂ (13) where Pi_oad is used as a control variable in the control circuit.

[0081] FIG. 7a is a simplified one line of a three phase current controlled resonant power supply 70 with parallel resonant loop 20. This circuit is very similar to the circuit shown in Figure 7, however, the resonant capacitor 11 is connected in parallel with inductor 14 rather than in series as shown in FIG.7. [0082] In another embodiment of the invention, the power supply shown in Figure 10 presents a non-limiting block diagram of a power supply 100, comprising one of PFC rectifier/regulator 55, and a pair of inverters 53A and 53B each controlled by a step-down voltage regulator 68A and 68B. Each inverter generates induction current in coils 14A and 14B, heating induction loads 13A and 13B, respectively. The power supply is with a pair of induction melting furnaces for sequential processing. This power supply may be comprised of more than two inverters and step- down voltage regulators.

[0083] Yet another embodiment of the invention includes the power supply shown in Figure 11, which presents a non-limiting block diagram of a power supply 110, comprising a PFC rectifier/regulator coupled to a number of step-down voltage regulators 68A-68D and inverters 53A- 53D, each connected to a section (14A-14D) of a multi-section coil. All the inverters operate at the same frequency, however, at different set power levels. Due to the equal frequency - - operation there is very little magnetic interruption between the coils, thereby ensuring a smooth transition between heating zones.

[0084] Thus, there is disclosed a resonance power supply for generating variable AC power to apply to an induction coil magnetically coupled to a work piece. The supply includes at least one rectifier (55/65) with power factor correction (PFC); at least one step down voltage regulator (68) having an input coupled to the at least one rectifier, at least one resonant inverter (70) having an input coupled to the at least one step down voltage regulator, at least one induction coil (14) magnetically coupled to at least one work piece (13), and at least one control circuit (52; FIG. 8) configured to adjust at least one operating parameter associated with at least one of the rectifier with PFC and the resonant inverter, responsive to the output of the resonant inverter, to cause the resonance inverter to operate at resonance frequency. The at least one step down voltage regulator is operative to regulate the DC voltage across resonance inverter when the voltage at the input of the current regulator exceeds a voltage threshold for supplying a set power level to the at least one induction coil. The at least one rectifier with PFC converts AC line current into DC current, such that the shape and phase of the line current follows the shape of the line voltage. In one configuration, the rectifier with PFC is a one phase rectifier. In another configuration, the rectifier with PFC is a three phase rectifier. In an embodiment, the at least one control circuit includes controller circuitry for bang-bang control of the rectifier with PFC. In an embodiment, the at least one control circuit includes controller circuitry for voltage control across the resonant inverter according to an output of the step down voltage regulator. In an embodiment, the at least one control circuit includes controller circuitry for frequency control of at least one resonance inverter. In an embodiment, the at least one control circuit includes controller circuitry for monitoring power applied to the at least one induction coil and work piece, computing power losses in induction coil, and computing power applied to the work piece. [0085] There is further disclosed a method of generating variable AC power to apply to induction coil to heat a magnetically coupled work piece comprising: generating an input power factor correction (PFC) via at least one rectifier; generating AC power on an induction coil from at least one resonant inverter; controlling DC voltage across the at least one resonant inverter using at least one step down voltage regulator; using at least one induction coil to magnetically couple with at least one work piece, and controlling the inverter and the PFC of the at least one rectifier according to the output of the inverter to operate the at least one inverter at resonance frequency. In an embodiment, the method further comprises controlling via at least one step down voltage regulator, the amount of DC current to the at least one resonant inverter to set a power level in the at least one work piece. In an embodiment, the method further comprises - - converting in the at least one rectifier with power factor correction AC line current into DC current to cause shape and phase of the line current to follow the shape of line voltage.

[0086] The present invention has been described in terms of several embodiments solely for the purpose of illustration. Embodiments of the present invention may be applied to small heating systems generating power in single kilowatts or to large melting systems generating thousands of kilowatts. Persons skilled in the art will recognize from this description that the invention is not limited to the embodiments described, but may be practiced with modifications and alterations limited only by the spirit and scope of the appended claims.

Claims

1. A resonance power supply for generating variable AC power to apply to an induction coil magnetically coupled to a work piece comprising:

at least one rectifier (55/65) with power factor correction (PFC),

at least one step down voltage regulator (68) having an input coupled to the at least one rectifier,

at least one resonant inverter (70) having an input coupled to the at least one voltage regulator;

at least one induction coil (14) configured to receive an output of the at least one resonant inverter and magnetically coupled to at least one work piece (13), and

at least one control circuit (52) configured to adjust at least one operating parameter associated with at least one of the rectifier with PFC and the resonant inverter, responsive to the output of the resonant inverter, wherein the resonance inverter operates at resonance frequency.

2. The resonance power supply according to claim 1, wherein the at least one step down voltage regulator is operative to regulate the DC current supplied to the resonance inverter when the voltage at the input of the step down voltage regulator exceeds a voltage threshold for supplying a set power level to the at least one induction coil.

3. The resonance power supply according to claim 1, wherein the at least one rectifier with PFC converts AC line current into DC current, such that the shape and phase of the line current follows the shape of the line voltage.

4. The resonance power supply according to claim 1, wherein the rectifier with PFC is a one phase rectifier.

5. The resonance power supply according to claim 1, wherein the rectifier with PFC is a three phase rectifier.

6. The resonance power supply according to claim 1, wherein the at least one control circuit includes controller circuitry for bang-bang control of the rectifier with PFC.

7. The resonance power supply according to claim 6, wherein the at least one control circuit includes controller circuitry for voltage control across the resonant inverter according to an output of the step down voltage regulator.

8. The resonance power supply according to claim 7, wherein the at least one control circuit includes controller circuitry for frequency control of at least one resonance inverter.

9. The resonance power supply according to claim 7, wherein the at least one control circuit includes controller circuitry for monitoring power applied to the at least one induction coil and work piece, computing power losses in induction coil, and computing power applied to the work piece.

10. The resonance power supply according to claim 1, wherein the at least one inverter is a series resonant inverter.

11. The resonance power supply according to claim 1, wherein, the at least one inverter is a parallel inverter.

12. A method of generating variable AC power to apply to induction coil to heat a magnetically coupled work piece comprising:

generating an input power factor correction (PFC) via at least one rectifier;

generating AC power on an induction coil from at least one resonant inverter;

controlling DC voltage across at least one resonant inverter using at least one step down voltage regulator;

using at least one induction coil to magnetically couple with at least one work piece, and controlling the inverter and the PFC of the at least one rectifier according to the output of the inverter to operate the at least one inverter at resonance frequency.

13. The method of claim 12, further comprising controlling via at least one step down voltage regulator, the amount of DC current to the at least one resonant inverter to set a power level in the at least one work piece.

14. The method of claim 12, wherein the method further comprises converting in the at least one rectifier with power factor correction, AC line current into DC current to cause the shape and phase of the line current to follow the shape of line voltage.

15. The method of claim 12, wherein the at least one inverter operates as a series resonant inverter.