Metal base cookware induction heating apparatus having improved power supply and gating control circuit using infra-red temperature sensor and improved induction heating coil arrangement

Claim 2

An improved induction heating unit power supply and control circuit comprising an induction heating coil, inverter circuit means including a gate controlled power thyristor coupled to and supplying the induction heating coil with periodic energization currents, rectifier means for supplying a rectified unfiltered unidirectional high voltage excitation potential for said inverter circuit means and gating circuit means for periodically gating on said inverter circuit means, said gating circuit means including means for deriving a low voltage unidirectional self excitation potential from the high voltage rectified unfiltered unidirectional excitation potential supplied from said rectifier means, timing circuit charging means supplied by said low voltage excitation potential, voltage responsive switching means connected to and controlled by said timing circuit charging means, said voltage responsive switching means being rendered conductive upon the voltage of said timing circuit charging means attaining a preset value, a semiconductor switching device having a relatively flat current vs voltage conducting characteristic and having its load terminals connected to the output from the rectifier means, the control gate of said constant current semiconductor switching device being connected to and supplied by the voltage responsive switching means, and gating signal output circuit means connected in series circuit relationship with the constant current high voltage switching device and adapted for connection to the control gate of the gate controlled power thyristor comprising a part of the inverter circuit means.

Claim 3

An improved induction heating unit power supply circuit and control according to claim 1 further including soft-starting zero point energization switching means connected intermediate the output of the rectifier means and the inverter circuit means, and zero point sensing and turn-on control circuit means coupled to and controlling said soft starting zero point energization switching means for rendering the same conductive only near the beginning of a half cycle of the rectified unfiltered unidirectional excitation potential.

Claim 4

An improved induction heating unit power supply circuit and control according to claim 1 further including inhibit control means for inhibiting operation of said zero point sensing and turn-on circuit means, and load condition sensing means for sensing the operating condition of a load supplied by the inverter circuit means and deriving an on-off control signal in response to the load condition for controlling operation of said inhibit control means.

Claim 5

An improved induction heating unit power supply circuit and control according to claim 1 wherein the inverter circuit means is a chopper inverter comprised by a filter inductor having an inductance L2, a bidirectional conducting gate control semiconductor thyristor switching device connected in series circuit relationship with the filter inductor across the output from said rectifier means with the filter inductor interposed between the thyristor switching device and the rectifier means, commutating inductor means having an inductance L1 and commutating capacitor means having a capacitance C1 connected in series circuit relationship across the thyristor switching device and tuned to series resonance at a desired commutating frequency that provides a combined thyristor conduction and commutating period t1 during each cycle of operation, said commutating capacitor and inductor means including as a part thereof the induction heating coil, the gating circuit means being coupled to the control gate of the thyristor switching device for rendering the device conductive at a controlled frequency of operation that provides an operation period T for the chopper inverter including a quiescent charging period t2 in each cycle of operation where T t1 + t2 such that the value omega 2t2 equals substantially pi /2 radians or greater and where omega 2 1/ Square Root L2C1 whereby the reapplied forward voltage across the semiconductor thyristor switching device following each conduction interval is maintained substantially independent of load.

Claim 6

An improved induction heating unit power supply circuit and control according to claim 1 wherein the inverter circuit means comprises a chopper inverter circuit that produces periodic pulsed currents having radio frequency interference emission effects and the circuit further includes a smoothing inductor and a smoothing capacitor connected in circuit relationship across the chopper inverter circuit which serve to shape the output current supplied by the chopper inverter circuit to the load to substantially a sinusoidal waveshape having little or no radio frequency interference effects, and wherein the smoothing inductor comprises the induction heating coil.

Claim 7

An improved induction heating coil power supply circuit and control according to claim 1 further including soft starting zero point energization switching means connected between the rectifier means and the chopper inverter circuit means, zero point sensing and turn-on circuit means coupled to said soft start zero point switching means for switching said means-on only at or near the beginning of each half cycle of the rectified unfiltered unidirectional excitation potential, inhibit control means for inhibiting operation of said zero point sensing and turn-on circuit means, and temperature sensing means for sensing the temperature of an object being heated by the inductive heating coil and deriving an on-off control signal in response to the temperature rising above a preset level for controlling operation of said inhibit control means.

Claim 8

An improved induction cooking unit power supply circuit and control according to claim 4 further including a smoothing inductor having an inductance L3 and smoothing capacitor having a capacitance C3 connected in series circuit relationship across at least one of the commutating components and having impedance values such that the combined impedance of the commutating capacitor, the smoothing inductor and the smoothing capacitor is capacitive in nature and series resonates with the commutating inductor to establish the commutating period t1, and wherein the smoothing inductor and capacitor shape the output current flowing through the smoothing inductor to substantially a sinusoidal waveshape having little or no radio frequency interference emission effects, and the load is coupled to and derives output power from the smoothing inductor, and wherein the smoothing inductor comprises the induction heating coil.

Claim 9

An improved induction cooking unit power supply circuit and control according to claim 7 further including soft starting zeRo point energization switching means connected between the output of the rectifier means and the chopper inverter circuit, zero point sensing and turn-on circuit means coupled to the soft starting zero point switching means for switching said last mentioned switching means-on- only at or near the beginning of each half cycle of the rectified unfiltered unidirectional excitation potential, inhibit control means for inhibiting operation of said zero point sensing and turn-on circuit means, and temperature sensing and control means for sensing the temperature of an object being heated by said induction heating coil and deriving an on-off control signal in response to the temperature of the object for controlling operating of said inhibit control means.

Claim 10

An improved induction heating unit power supply circuit and control according to claim 8 wherein said temperature sensing and control means comprises an infra-red heat sensor device arranged to view the object being heated by said inductive heating coil, a constant current amplifier connected in series circuit relationship with the infra-red heat sensor device across a source of low voltage direct current excitation potential, means for periodically interrupting the view of the infra-red heat sensor device to produce an output alternating current signal whose amplitude is indicative of the temperature of the object being heated, an alternating current amplifier, an adjustable temperature set impedance coupled to and controlling the gain of said alternating current amplifier, the setting of said adjustable temperature set impedance serving to establish the temperature level at which the output from the alternating current amplifier attains an operative level of magnitude, on-off control amplifier means coupled to and controlled by the output from said alternating current amplifier for producing an amplified on-off output signal representative of the temperature of the object being heated by said inductive heating coil relative to the temperature setting of the adjustable temperature set impedance, said on-off output control signal being supplied to the inhibit control means and means for supplying operating bias potentials from a common reference point on the source of low voltage excitation potential supplying the alternating current amplifier and the on-off control amplifier means whereby the effect of variations in the excitation potential is minimized.

Claim 11

An improved induction heating unit power supply circuit and control according to claim 9 wherein the infra-red heat sensor device comprises a lead sulfide semiconductor cell, the constant current amplifier comprises a field effect transistor having a feedback resistor connected in series with its drain-source electrodes and having a feedback connection from the feedback resistor back to the gate of the field effect transistor for maintaining constant current through the amplifier and the series connected lead sulfide cell, the alternating current amplifier comprises an integrated circuit operational amplifier having inverting and non-inverting input terminals and an output terminal with the output signal from the infra-red heat sensor device being AC coupled to the non-inverting input terminal, and the adjustable temperature set impedance comprises three variable resistors connected in series circuit relationship between the output terminal and the inverting input terminal with the outer most variable resistors serving as minimum and maximum temperature limit settings and the middle variable resistor serving to establish the desired operating temperature setting, and means for supplying the inverting and non-inverting input terminals with operating bias potentials from a common point on the source of low voltage excitation potential supplying the alternating current amplifier and infra-red sensor device, and the output on-off control and amplifier means comprises on-off bistable Schmitt trigger circuit means responsive to the output from said alternating current ampLifier means for producing an output electric potential having a first value under conditions where the temperature sensed by the lead sulfide cell is below the temperature level established by the setting of the adjustable temperature set impedance, and for producing an output potential of a second different value under conditions where the sensed temperature is above the temperature setting of the adjustable temperature set impedance, the output from said Schmitt trigger circuit means being coupled to and controlling operation of said inhibit control means.

Claim 12

An improved induction heating unit power supply circuit according to claim 10 wherein the inductive heating coil comprises a planar helically shaped inductive heating coil, a pyroceramic insulating heat transparent support for supporting metal base cooking vessels capable of being inductively heated, the helically shaped inductive heating coil having a central opening therein and being disposed under the pyroceramic heat transparent support, the lead sulfide infra-red heat detecting cell being disposed beneath the central opening so as to view the bottom of a metal base cooking vessel supported on the pyroceramic heat transparent insulating supporting member, the lead sulfide cell being supported within a heat reflector for concentrating infra-red heat rays from the bottom of the cooking vessel onto the lead sulfide cell, and an infra-red light chopper interposed in the infra-red light path between the bottom of the cooking vessel through the pyroceramic heat transparent pan supporting member and the central opening and the lead sulfide cell for periodically interrupting the view of the lead sulfide cell.

Claim 13

An improved induction heating unit power supply circuit and control according to claim 11 wherein the infra-red light chopper also includes fan means for producing a flow of cooling air through the ambient atmosphere surrounding the control and wherein the planar helically shaped inductive heating coil comprises a twisted multi-conductor insulated cable helically wound in a flat plane and having inner and outer terminal points, said multi-conductor cable being fabricated from multiple strands of insulated conductors interwoven together to form the insulated cable with the outer terminal points of certain respective strands being connected back to the inner terminal points of adjacent strands to provide a desired amount of inductive coupling to the metal base cooking vessel heated by the coil, and means for connecting the inner and outer terminal points of at least one of the multiple strands to the chopper inverter circuit as the smoothing inductor, and further including a pan-shaped ferrite core member having a central opening supporting the inductive heating coil below the surface of the heat transparent supporting member for concentrating the lines of magnetic flux up into the inductively heated cooking vessel.

Claim 14

An induction heating unit chopper inverter power supply circuit including in combination supply terminal means adapted to be connected to a source of electric energy for energizing the chopper inverter, a filter inductor having an inductance L2, a bidirectional conducting gate controlled semiconductor thyristor switching device connected in series circuit relationship with the filter inductor across said supply terminal means with the filter inductor interposed between the thyristor switching device and the supply terminal means, commutating inductor means having an inductance L1 and commutating capacitor means having a capacitance C1 connected in series circuit relationship across the thyristor switching device and tuned to series resonance at a desired commutating frequency that provides a combined thyristor conduction and commutating period t1 during each cycle of operation, said commutating inductor and capacitor means including as a part thereof an induction heating coil that serves as a load, and gating circuit means coupled to the control gAte of said thyristor switching device for rendering the device conductive at a controlled frequency of operation that provides an operating period T for the chopper inverter including a quiescent charging period t2 in each cycle of operation where T t1 + t2 such that the value omega 2t2 equals substantially pi /2 radians or greater and where omega 2 1/ Square Root L2C1 whereby the reapplied forward voltage across the semiconductor thyristor switching device following each conduction interval is substantially independent of load.

Claim 15

An induction heating unit chopper inverter power supply circuit including in combination supply terminal means adapted to be connected to a source of electric energy for energizing the chopper inverter, a filter inductor having an inductance L2, a bidirectional conducting gate controlled semiconductor thyristor switching device connected in series circuit relationship with the filter inductor across said supply terminal means with the filter inductor interposed between the thyristor switching device and the supply terminal means, commutating inductor means having an inductance L1 and commutating capacitor means having a capacitance C1 connected in series circuit relationship across the thyristor switching device and tuned to series resonance at a desired commutating frequency that provides a combined thyristor conduction and commutating period t1 during each cycle of operation, said commutating inductor and capacitor means including as a part thereof an induction heating coil that serves as a load, and gating circuit means coupled to the control gAte of said thyristor switching device for rendering the device conductive at a controlled frequency of operation that provides an operating period T for the chopper inverter including a quiescent charging period t2 in each cycle of operation where T t1 + t2 such that the value omega 2t2 equals substantially pi /2 radians or greater and where omega 2 1/ Square Root L2C1 whereby the reapplied forward voltage across the semiconductor thyristor switching device following each conduction interval is substantially independent of load.

Claim 16

An induction heating unit chopper inverter power supply circuit according to claim 13 further including a filter capacitor having a capacitance C2 connected across the series connected filter inductor and thyristor switching device.

Claim 17

An induction heating unit chopper inverter power supply circuit according to claim 13 further including rectifier means connected across said supply terminal means intermediate a source of alternating current supply and said thyristor switching device for supplying rectified unfiltered unidirectional excitation potentials to said thyristor switching device across said supply terminal means from said alternating current supply.

Claim 18

An induction heating unit chopper inverter power supply circuit according to claim 15 wherein the gate controlled semiconductor thyristor switching device is unidirectional conducting and the rectifier means is comprised by a fast recovery diode bridge that serves simultaneously as a full wave rectifier of supply alternating current potential and to provide feedback of reverse currents around the gate controlled power thyristor in the inverter circuit means and wherein the filter inductor is located between the supply of alternating current potential and the rectifier means.

Claim 19

An induction heating unit chopper inverter power supply circuit according to claim 13 wherein a single commutating inductor is provided which also comprises the induction heating coil.

Claim 20

An induction heating unit chopper inverter power supply circuit according to claim 13 further including on-off control circuit means for rendering said gating circuit means effective to control turn-on of the thyristor switching device in accordance with the power demands on the circuit.

Claim 21

An induction heating unit chopper inverter power circuit according to claim 18 further including temperature sensing and control means for sensing the temperature of an object being heated and controlling said on-off control circuit means in response to the sensed temperature, and wherein said on-off control circuit means includes soft starting zero point energization switching means coupled to the supply terminal means and controlling energization of said gating circuit means, zero point sensing turn-on and control circuit means coupled to said soft start zero point switching means for switching said soft start switching means on only at the beginning of a half cycle of the excitation potential appearing across the supply terminal means, inhibit control means for inhibiting operation of said zero point sensing and control circuit means, said temperature sensing means controlling operation of said inhibit control means, and a smoothing inductor having an inductance L3 and a smoothing capacitor having a capacitance C3 connected in series circuit relationship across at least one of said commutating components, the combined impedance of the commutating capacitor, the smoothing inductor and the smoothing capacitor being capacitive in nature and series resonant with the commutating inductor to establish the commutating period t1, and wherein the smoothing inductor and capacitor shape the output current flowing through the smoothing inductor to substantially a sinusoidal waveshape having little or no radio frequency interference emission effects and wherein the smoothing inductor comprises the induction heating coil.

Claim 22

An induction heating unit chopper inverter power supply circuit according to claim 13 further including soft starting zero point energization switching means coupled to the supply terminal means and controlling energization of said gating circuit means, and zero point sensing and turn-on control circuit means coupled to said soft start zero point switching means for switching said soft start switching means on only at the beginning of a half cycle of the excitation potential appearing across the supply terminal means.

Claim 23

An induction heating unit chopper inverter power supply circuit according to claim 13 further including a smoothing inductor having an impedance L3 and a smoothing capacitor having a capacitance C3 connected in series circuit relationship across at least on of said commutating components, the combined impedance of the commutating capacitor, the smoothing inductor and the smoothing capacitor being capacitive in nature and series resonant with the commutating inductor to establish the commutating period t1, and wherein the smoothing inductor and capacitor shape the output current flowing through the smoothing inductor to substantially a sinusoidal waveshape having little or no radio frequency interference emission effects, the load being coupled to and deriving output power from across the smoothing inductor, and the smoothing inductor comprising an inductive heating coil.

Claim 24

An induction heating unit chopper inverter power supply circuit according to claim 13 wherein said commutating capacitor comprises a plurality of parallel connected capacitors and power rate controlling switching means for switching selected ones of said parallel connected capacitors into effective circuit relationship in accordance with a desired rate of power production.

Claim 25

An induction heating unit chopper inverter power supply circuit according to claim 21 further including rectifier means connected across said supply terminal means intermediate a source of alternating current supply and said thyristor switching device for supplying rectified unfiltered unidirection excitation potentials to said thyristor switching device across said supply terminal means from said alternating current supply.

Claim 26

An induction heating unit chopper inverter power supply circuit according to claim 23 further including on-off control circuit means for rendering said gating circuit means effective to control turn-on of the thyristor switching device in accordance with the power demands on the circuit.

Claim 27

An induction heating unit chopper inverter power supply circuit according to claim 24 further including soft starting zero point energization switching means coupled to the supply terminal means and controlling energization of said gating circuit means, and zero point sensing and turn-on control circuit means coupled to said soft start zero point switching means for switching said soft start switching means on only at the beginning of a half cycle of the excitation potential appearing across the supply terminal means.

Claim 28

An induction heating unit chopper inverter power supply circuit according to claim 25 wherein the gate controlled semiconductor thyristor switching device is unidirectional conducting and the rectifier means is comprised by a fast recovery diode bridge that serves simultaneously as a full wave rectifier of supply alternating current potential and to provide feedback of reverse currents around the gate controlled power thyristor in the inverter circuit means and wherein the filter inductor is located between the supply of alternating current potential and the rectifier means.

Claim 29

An induction heating unit chopper inverter power supply circuit according to claim 23 wherein the gate controlled semiconductor thyristor switching device is unidirectional conducting and the rectifier means is comprised by a fast recovery diode bridge that serves simultaneously as a full wave rectifier of supply alternating Current potential and to provide feedback of reverse currents around the gate controlled power thyristor in the inverter circuit means and wherein the filter inductor is located between the supply of alternating current potential and the rectifier means.

Claim 30

An induction heating unit chopper inverter power supply circuit according to claim 27 further including a filter capacitor having a capacitance C2 connected across the series connected filter inductor and thyristor switching device.

Claim 31

An induction heating unit power supply according to claim 21 further including selectively operable inhibit control means coupled to said gating circuit means for selectively inhibiting operation of said gating circuit means, and delay means for introducing a delay period intermediate selective operation of the inhibit means in a direction to remove the inhibit and enable operation of the gating circuit means after initial start-up.

Claim 32

An induction heating unit power supply according to claim 13 further including selectively operable inhibit control means coupled to said gating circuit means for selectively inhibiting operation of said gating circuit means, and delay means for introducing a delay period intermediate selective operation of the inhibit means in a direction to remove the inhibit and enable operation of the gating circuit means after initial start-up.

Claim 33

A temperature sensing and control signal deriving arrangement for an inductively heated cooking vessel comprising a heat transparent supporting member for a cooking vessel, an infra-red heat detecting device viewing the cooking vessel directly through the heat transparent supporting member for deriving an output electric control signal that is directly related to the temperature of the cooking vessel, and a planar, spirally wound inductive heating coil having a central opening disposed under the insulating heat transparent supporting member for use with cooking vessels that are metal based and capable of being inductively heated, the infra-red heat detecting device viewing the bottom of the cooking vessel through the central opening in the heating coil.

Claim 34

A temperature sensing and control signal deriving arrangement for an inductively heated cooking vessel comprising a heat transparent supporting member for a cooking vessel, an infra-red heat detecting device viewing the cooking vessel directly through the heat transparent supporting member for deriving an output electric control signal that is directly related to the temperature of the cooking vessel, and a planar, spirally wound inductive heating coil having a central opening disposed under the insulating heat transparent supporting member for use with cooking vessels that are metal based and capable of being inductively heated, the infra-red heat detecting device viewing the bottom of the cooking vessel through the central opening in the heating coil.

Claim 35

A temperature sensing and control signal deriving arrangement according to claim 31 further including an infra-red light chopping device disposed intermediate the heat transparent supporting member and the infra-red heat detecting device to thereby produce an alternating current signal component in the output of the infra-red heat sensor device whose amplitude is indicative of the temperature of the cooking vessel.

Claim 36

A temperature sensing and control signal deriving arrangement according to claim 32 wherein the light chopping device also includes fan means for producing a flow of cooling medium through the ambient atmosphere surrounding the arrangement.

Claim 37

A temperature set and control signal deriving arrangement according to claim 31 further including a pan-shaped ferrite core member having a central opening supporting the inductive heating coil below the surface of the heat transparent supporting member for concentrating the lines of magnetic flux up into the inductively heated cooking vessel.

Claim 38

A temperature sensing and control signal deriving arrangement according to claim 31 further including a heat reflector with the infra-red heat detecting device being supported within the heat reflector for concentrating the infra-red heat rays emitted from the cooking vessel upon the infra-red heat detecting device.

Claim 39

A temperature sensing and control signal deriving arrangement according to claim 31 wherein the heat transparent insulating supporting member is pyroceramic, the spirally-wound planar inductive heating coil is disposed under the pyroceramic heat transparent member within pan-shaped ferrite core member with both the heating coil and ferrite core member having central apertures formed therein for allowing the infra-red heat detecting device to directly view the bottom of a cooking vessel supported on the pyroceramic heat transparent member, the infra-red heat detecting device comprises a lead sulfide semiconductor cell and is disposed within a heat reflector for concentrating infra-red rays emitted from the cooking vessel onto the lead sulfide cell, and a light chopper is interposed between the lead sulfide semiconductor cell and the light path from the bottom of the heated cooking vessel for periodically interrupting the view of the lead sulfide infra-red heat detecting cell and includes fan means for circulating cooling air through the arrangement.

Claim 40

A temperature sensing and control signal deriving arrangement according to claim 36 wherein the spirally wound planar inductive coil comprises a twisted multi-conductor insulated cable helically wound in a flat plane defining a central opening and having inner and outer terminal points, said multi-conductor cable being fabricated from multiple strands of insulated conductors interwoven together to form the insulated cable with the outer terminal points of the certain respective strands being connected back to the inner terminal points of adjacent strands to provide a desired amount of inductive coupling to an object to be heated by the coil, and means for connecting a source of electric energy across an inner and outer terminal point of at least one of the multiple strands.

Claim 41

An improved induction heating unit power supply and control circuit comprising an induction heating coil, inverter circuit means including a gate controlled power thyristor coupled to and supplying the induction heating coil with periodic energization currents, rectifier means for supplying a rectified unfiltered unidirectional high voltage excitation potential for said inverter circuit means and gating circuit mean for periodically gating on said inverter circuit means, said gating circuit means including means for deriving a low voltage unidirectional self excitation potential from the high voltage rectified unfiltered unidirectional excitation potential supplied from said rectifier means, timing circuit charging means supplied by said low voltage excitation potential, voltage responsive switching means connected to and controlled by said timing circuit charging means, said voltage responsive switching means being rendered conductive upon the voltage of said timing circuit charging means attaining a preset value, a semiconductor switching device having a relatively flat current vs voltage conducting characteristic and having its load terminals connected to the output from the rectifier means, the control gate of said constant current semiconductor switching device being connected to and supplied by the voltage responsive switching means and gating signal output circuit means connected in series circuit relationship with the constant current high voltage switching device and adapted for connection to the control gate of the controlled power thyristor comprising a part of inverter circuit means, selectively operable inhibit control means coupled to said gating circuit means for selectively inhibiting operation of said gating circuit means, and delay means for introducing a delay period intermediate selective operation of the inhibit means in a direction to remove the inhibit and enable operation of the gating circuit means.

Claim 42

An improved induction heating unit power supply and control circuit comprising an induction heating coil, inverter circuit means including a gate controlled power thyristor coupled to and supplying the induction heating coil with periodic energization currents, rectifier means for supplying a rectified unfiltered unidirectional high voltage excitation potential for said inverter circuit means and gating circuit mean for periodically gating on said inverter circuit means, said gating circuit means including means for deriving a low voltage unidirectional self excitation potential from the high voltage rectified unfiltered unidirectional excitation potential supplied from said rectifier means, timing circuit charging means supplied by said low voltage excitation potential, voltage responsive switching means connected to and controlled by said timing circuit charging means, said voltage responsive switching means being rendered conductive upon the voltage of said timing circuit charging means attaining a preset value, a semiconductor switching device having a relatively flat current vs voltage conducting characteristic and having its load terminals connected to the output from the rectifier means, the control gate of said constant current semiconductor switching device being connected to and supplied by the voltage responsive switching means and gating signal output circuit means connected in series circuit relationship with the constant current high voltage switching device and adapted for connection to the control gate of the controlled power thyristor comprising a part of inverter circuit means, selectively operable inhibit control means coupled to said gating circuit means for selectively inhibiting operation of said gating circuit means, and delay means for introducing a delay period intermediate selective operation of the inhibit means in a direction to remove the inhibit and enable operation of the gating circuit means.

Claim 43

An improved induction heating unit power supply circuit and control according to claim 38 wherein said rectifier means is comprised by a fast recovery diode bridge that serves simultaneously as a full wave rectifier of supply alternating current potential and to provide feedback of reverse currents around the gate controlled power thyristor in the inverter circuit means.

Claim 44

An improved induction heating unit power supply circuit and control according to claim 38 wherein the inverter circuit means is a chopper inverter comprised by a filter inductor having an inductance L2, said gate control semiconductor thyristor switching device being connected in series circuit relationship with the filter inductor across the output from said rectifier means with the filter inductor interposed between the thyristor switching device and the rectifier means, commutating inductor means having an inductance L1 and commutating capacitor means having a capacitance C1 connected in series circuit relationship across the thyristor switching device and tuned to series resonance at a desired commutating frequency that provides a combined thyristor conduction and commutating period t1 during each cycle of operation, said commutating capacitor and inductor means including as a part thereof the induction heating coil, the gating circuit means being coupled to the control gate of the thyristor switching device for rendering the device conductive at a controlled frequency of operation that provides an operating period T for the chopper inverter including a quiescent charging period t2 in each cycle of operation where T t1 +t2 such that the value 2t2 equals substantially pi /2 radians or greater where omega 2 1/ Square Root L2C1 whereby the reapplied forward voltage across the semiconductor thyristor switching device following each conduction interval is maintained substantially independent of load.

Claim 45

An improved induction heating unit power supply circuit and control according to claim 38 wherein the inverter circuit means comprises a chopper inverter circuit that produces periodic pulsed currents having radio frequency interference emission effects and the circuit further includes a smoothing inductor and a smoothing capacitor connected in circuit relationship across the chopper inverter circuit which serve to shape the output current supplied by the chopper inverter circuit to the load to substantially a sinusoidal wave shape having little or no radio frequency interference effects and the smoothing inductor comprises the induction heating coil.

Claim 46

An improved induction heating unit power supply circuit and control according to claim 41 wherein the smoothing inductor has an inductance L3 and the smoothing capacitor has a capacitance C3 and are connected in series circuit relationship across at least one of the commutating components and have impedance values such that the combined impedance of the commutating capacitor, the smoothing inductor and the smoothing capacitor is capacitive in nature and series resonates with the commutating inductor to establish the commutating period t1, and wherein the smoothing inductor and capacitor shape the output current flowing through the smoothing inductor to substantially a sinusoidal waveshape having little or no radio frequency interference emission effects.

Claim 47

An improved induction heating unit power supply circuit and control according to claim 42 further including on-off control circuit means for turning said gating circuit means on and off in accordance with the power demands on the circuit, said on-off control circuit means controlling said inhibit control means for selectively clamping-off the excitation potential to the gating circuit means to thereby inhibit operation thereof, zero point sensing and turn-on circuit means coupled to and controlling said inhibit control means for enabling the gating circuit means only at or near the beginning of a half cycle of the rectified, unfiltered unidirectional excitation potential, operating condition sensing means for sensing the operating condition of the chopper inverter and deriving on-off control signal in response to the operating condition for controlling operation of the inhibit control means.

Claim 48

An improved power supply circuit and control according to claim 43 wherein said rectifier means is comprised by a fast recovery diode bridge that serves simultaneously as a full wave rectifier of the supply alternating current potential and to provide feedback of reverse currents around the gate controlled power thyristor in the inverter circuit means.