Induction heating apparatus

Claim 2

An induction heating apparatus having an inverter circuit including an induction heating coil coupled to a DC power supply for energizing the heating coil, the inverter circuit including a gate controlled rectifier for conducting current pulses of a first polarity through the heating coil, an oppositely poled diode coupled in parallel with the gate controlled rectifier for conducting current pulses of opposite polarity through the heating coil, a resistive-capacitive impedance connected in series with the diode which develops a voltage during conduction of the diode that is maintained for a time after conduction of the diode, means coupling said voltage to the gate of the gate controlled rectifier to provide gate reverse biasing for the rectifier during conduction of the diode and for said time after conduction.

Claim 3

The induction heating apparatus of claim 1 in which the resistive-capacitive impedance comprises a capacitor coupled in parallel with a resistor.

Claim 4

The induction heating apparatus of claim 2 in which there is a junction between the resistor-capacitor parallel combination and the anode of the diode, this junction being coupled to the gate of the gate controlled rectifier.

Claim 5

The induction heating apparatus of claim 3 in which there is a second resistor in series with the capacitor and this series combination is connected in parallel with said first resistor.

Claim 6

The induction heating apparatus of claim 3 in which said biasing voltage is coupled to the gate of the gate controlled rectifier through a transistor.

Claim 7

An induction heating apparatus having an inverter circuit including an induction heating coil coupled to a DC power supply for energizing the heating coil, the inverter circuit including a gate controlled rectifier for conducting current pulses of a first polarity through the heating coil, an oppositely poled diode coupled in parallel with the gate controlled rectifier for conducting current pulses of opposite polarity through the heating coil, an impedance having an at least partially reactive component connected in series with the diode which develops a voltage during conduction of the diode that is maintained for a time after conduction of the diode, means coupling said voltage to the gate of the gate controlled rectifier to provide gate reverse biasing for the rectifier during conduction of the diode and for said time after conduction.

Claim 8

An induction heating apparatus having an inverter circuit including an induction heating coil coupled to a DC power supply for energizing the heating coil, the inverter circuit including a gate controlled rectifier for conducting current pulses of a first polarity through the heating coil, an oppositely poled diode coupled in parallel with the gate controlled rectifier for conducting current pulses of opposite polarity through the heating coil, an impedance having an at least partially reactive component connected in series with the diode which develops a voltage during conduction of the diode that is maintained for a time after conduction of the diode, means coupling said voltage to the gate of the gate controlled rectifier to provide gate reverse biasing for the rectifier during conduction of the diode and for said time after conduction.

Claim 9

An induction heating apparatus having an inverter circuit including an induction heating coil coupled to a DC power supply for energizing the heating coil, in which the DC power supply is unregulated and produces a varying voltage output having peaks and valleys, the inverter circuit includes a gate controlled thyristor cyclically gated to produce a conductive first period followed by a nonconductive second period in order to energize the heating coil, and a protection circuit for the inverter circuit comprising: (a) circuit means coupled to the inverter for producing a signal having a positive excursion at a time t1 which is the start of said second period and a negative excursion at a time t2 which is the end of said second period, the magnitude of said excursions being related to the energy in the inverter at the respective times t1 and t2; (b) a first peak charged capacitor connected to said circuit means for receiving energy therefrom over substantially the entire range of voltage variation of the DC power supply to produce a first output signal related to the energy in the inverter at time t1; (c) a second peak charged capacitor connected to said circuit means for receiving energy therefrom over substantially the entire range of voltage variation of the DC power supply to produce a second output signal of opposite polarity to the first output signal and related to the energy in the inverter at time t2; and (d) means for taking a ratio of said first output signal and said second output signal and comparing the ratio to a reference to produce an inverter circuit protection signal as a result of said comparison if said ratio differs from the reference ratio by a predetermined amount.

Claim 10

An induction heating apparatus having an inverter circuit including an induction heating coil coupled to a DC power supply for energizing the heating coil, in which the DC power supply is unregulated and produces a varying voltage output having peaks and valleys, the inverter circuit includes a gate controlled thyristor cyclically gated to produce a conductive first period followed by a nonconductive second period in order to energize the heating coil, and a protection circuit for the inverter circuit comprising: (a) circuit means coupled to the inverter for producing a signal having a positive excursion at a time t1 which is the start of said second period and a negative excursion at a time t2 which is the end of said second period, the magnitude of said excursions being related to the energy in the inverter at the respective times t1 and t2; (b) a first peak charged capacitor connected to said circuit means for receiving energy therefrom over substantially the entire range of voltage variation of the DC power supply to produce a first output signal related to the energy in the inverter at time t1; (c) a second peak charged capacitor connected to said circuit means for receiving energy therefrom over substantially the entire range of voltage variation of the DC power supply to produce a second output signal of opposite polarity to the first output signal and related to the energy in the inverter at time t2; and (d) means for taking a ratio of said first output signal and said second output signal and comparing the ratio to a reference to produce an inverter circuit protection signal as a result of said comparison if said ratio differs from the reference ratio by a predetermined amount.

Claim 11

The induction heating apparatus of claim 7 which further comprises (e) means for preventing the gating of the gate controlled thyristor in response to said inverter circuit protection signal.

Claim 12

The induction heating apparatus of claim 8 which further comprises (f) means for rechecking said ratio after the expiration of a period of time of non-gating of the gate controlled thyristor.

Claim 13

The induction heating apparatus of claim 7 further including (g) means activating said means (d) to make a comparison of said ratios near times at which the DC power supply is nearest to zero.

Claim 14

The induction heating apparatus of claim 10 in which said circuit means includes a snubber capacitor connected in series with a snubber resistor, with said series capacitor and resistor being coupled substantially in parallel with the gate controlled thyristor, the means (b) and (c) being connected to the junction between said snubber capacitor and said snubber resistor.

Claim 15

In an induction heating apparatus having an inverter circuit including an induction heating coil coupled to a DC power supply for energizing the heating coil, in which the DC power supply is unregulated and produces a varying voltage output having peaks and valleys, the inverter circuit includes a gate controlled thyristor cyclically gated to produce a conductive first period followed by a nonconductive second period in order to energize the heating coil, a protection circuit for the inverter circuit comprising: (a) a snubber capacitor connected in series with a snubber resistor at a junction point, the series combination being connected in parallel with the gate controlled thyristor to comprise means for producing a signal having a positive excursion at a time t1 which is the start of said second period and a negative excursion at a time t2 which is the end of said second period, the magnitude of said excursions being related to the energy in the inverter at the respective times t1 and t2; (b) means coupled to said junction for receiving energy therefrom over substantially the entire range of voltage variation of the DC power supply to produce a first voltage supply related to the energy in the inverter at time t1; (c) means coupled to said junction for receiving energy therefrom over substantially the entire range of voltage variation of the DC power supply to produce a second voltage supply related to the energy in the inverter at time t1; (d) means for taking a ratio of the first voltage supply and the second voltage supply; (e) means for preventing gating of the gate controlled thyristor if said ratio deviates beyond a reference value.

Claim 16

In an induction heating apparatus having an inverter circuit including an induction heating coil coupled to a DC power supply for energizing the heating coil, in which the DC power supply is unregulated and produces a varying voltage output having peaks and valleys, the inverter circuit includes a gate controlled thyristor cyclically gated to produce a conductive first period followed by a nonconductive second period in order to energize the heating coil, a protection circuit for the inverter circuit comprising: (a) a snubber capacitor connected in series with a snubber resistor at a junction point, the series combination being connected in parallel with the gate controlled thyristor to comprise means for producing a signal having a positive excursion at a time t1 which is the start of said second period and a negative excursion at a time t2 which is the end of said second period, the magnitude of said excursions being related to the energy in the inverter at the respective times t1 and t2; (b) means coupled to said junction for receiving energy therefrom over substantially the entire range of voltage variation of the DC power supply to produce a first voltage supply related to the energy in the inverter at time t1; (c) means coupled to said junction for receiving energy therefrom over substantially the entire range of voltage variation of the DC power supply to produce a second voltage supply related to the energy in the inverter at time t1; (d) means for taking a ratio of the first voltage supply and the second voltage supply; (e) means for preventing gating of the gate controlled thyristor if said ratio deviates beyond a reference value.

Claim 17

In an induction heating apparatus having an inverter circuit, including an induction heating coil operable at operator selected power levels to be loaded by various metallic loads, coupled to a DC power supply for energizing the heating coil, in which the inverter circuit includes a gate controlled thyristor, a heating coil power control comprising: (a) means at a given selected power level for producing a gating signal for the gate controlled thyristor after a period of time during which the inverter circuit has been nonconductive; and (b1) means for producing a signal representative of the length of conduction time of the inverter circuit; and (b2) means at said selected power level for adjusting the nonconduction time substantially inversely proportional to said signal so as to deliver substantially constant power to the various metallic loads for said selected power level.

Claim 18

In an induction heating apparatus having an inverter circuit, including an induction heating coil operable at operator selected power levels to be loaded by various metallic loads, coupled to a DC power supply for energizing the heating coil, in which the inverter circuit includes a gate controlled thyristor, a heating coil power control comprising: (a) means at a given selected power level for producing a gating signal for the gate controlled thyristor after a period of time during which the inverter circuit has been nonconductive; and (b1) means for producing a signal representative of the length of conduction time of the inverter circuit; and (b2) means at said selected power level for adjusting the nonconduction time substantially inversely proportional to said signal so as to deliver substantially constant power to the various metallic loads for said selected power level.

Claim 19

The power control of claim 13 in which the means (b1) includes a capacitor coupled to the voltage developed across the thyristor during the nonconductive time of the inverter circuit, the voltage on the capacitor being determined dependent upon a reference signal voltage level at the beginning of the nonconduction period, with the discharge time of the capacitor setting the time at which the nonconductive period ends with the production of a gating pulse for the thyristor.

Claim 20

The power control of claim 13 in which the means (b2) controls said nonconduction time such that it equals a constant value minus a fraction of the conduction time.

Claim 21

The power control of claim 15 in which said fraction is between 0.3 and 0.5.

Claim 22

An induction heating apparatus having an inverter circuit including an induction heating coil coupled to a DC power supply for energizing the heating coil, the DC power supply being unregulated to produce a varying voltage output having peaks and valleys, the inverter circuit including a gate controlled thyristor cyclically gated to produce a conductive first period followed by a nonconductive second period in order to energize the heating coil at operator selected power settings, an oppositely poled diode coupled in parallel with the gate controlled thyristor, a resistive-capacitive impedance connected in series with the diode which develops a voltage during conduction of the diode that is maintained for a time after conduction of the diode has ceased, said voltage being coupled to the gate of the gate controlled thyristor to provide gate reverse biasing for the thyristor during conduction of the diode and for said time after conduction, comprising a protection circuit for the inverter circuit including: (a) circuit means coupled to the inverter for producing a signal having a positive excursion at a time t1 which is the start of said second period and a negative excursion at a time t2 which is the end of said second period, the magnitude of said excursions being related to the enrgy in the inverter at the respective times t1 and t2; (b) a first peak charged capacitor connected to said circuit means for receiving energy therefrom over substantially the entire range of voltage variation of the DC power supply to produce a first output signal related to the energy in the inverter at time t1; (c) a second peak charged capacitor connected to said circuit means for receiving energy therefrom over substantially the entire range of voltage variation of the DC power supply to produce a second output signal of opposite polarity to the first output signal and related to the energy in the inverter at time t2; and (d) means for taking a ratio of said first output signal and said second output signal and comparing the ratio to a reference to produce an inverter circuit protection signal as a result of said comparison if said ratio differs from the reference ratio by a predetermined amount; and a heating coil power control including: (e) means for producing at a selected power setting a gating signal for the gate controlled thyristor after a period of time during which the inverter circuit has been nonconductive; and (f) means at said selected power setting for decreasing said nonconduction time when conduction time of the inverter circuit increases and for increasing said nonconduction time when the conduction time of the inverter circuit decreases thereby to maintain substantially constant power delivery at said selected power setting.

Claim 23

An induction heating apparatus having an inverter circuit including an induction heating coil coupled to a DC power supply for energizing the heating coil, the DC power supply being unregulated to produce a varying voltage output having peaks and valleys, the inverter circuit including a gate controlled thyristor cyclically gated to produce a conductive first period followed by a nonconductive second period in order to energize the heating coil at operator selected power settings, an oppositely poled diode coupled in parallel with the gate controlled thyristor, a resistive-capacitive impedance connected in series with the diode which develops a voltage during conduction of the diode that is maintained for a time after conduction of the diode has ceased, said voltage being coupled to the gate of the gate controlled thyristor to provide gate reverse biasing for the thyristor during conduction of the diode and for said time after conduction, comprising a protection circuit for the inverter circuit including: (a) circuit means coupled to the inverter for producing a signal having a positive excursion at a time t1 which is the start of said second period and a negative excursion at a time t2 which is the end of said second period, the magnitude of said excursions being related to the enrgy in the inverter at the respective times t1 and t2; (b) a first peak charged capacitor connected to said circuit means for receiving energy therefrom over substantially the entire range of voltage variation of the DC power supply to produce a first output signal related to the energy in the inverter at time t1; (c) a second peak charged capacitor connected to said circuit means for receiving energy therefrom over substantially the entire range of voltage variation of the DC power supply to produce a second output signal of opposite polarity to the first output signal and related to the energy in the inverter at time t2; and (d) means for taking a ratio of said first output signal and said second output signal and comparing the ratio to a reference to produce an inverter circuit protection signal as a result of said comparison if said ratio differs from the reference ratio by a predetermined amount; and a heating coil power control including: (e) means for producing at a selected power setting a gating signal for the gate controlled thyristor after a period of time during which the inverter circuit has been nonconductive; and (f) means at said selected power setting for decreasing said nonconduction time when conduction time of the inverter circuit increases and for increasing said nonconduction time when the conduction time of the inverter circuit decreases thereby to maintain substantially constant power delivery at said selected power setting.

Claim 24

The induction heating apparatus of claim 17 which further comprises (g) means for rechecking said ratio of the means (d) after the expiration of a period of time of nongating of the gate controlled thyristor.

Claim 25

The induction heating apparatus of claim 18 in which the means (f) comprises: (f1) means for producing a signal representative of the length of conduction time of the inverter circuit; and (f2) means for setting the nonconduction time substantially inversely proportional to said voltage signal.

Claim 26

An induction heating apparatus having an inverter circuit including an induction heating coil coupled to a DC power supply for energizing the heating coil, the inverter circuit including a gate controlled thyristor cyclically gated to produce a conductive first period followed by a nonconductive second period in order to energize the heating coil at operator selected power settings, having timing and protection circuitry comprising: (a) an improper load detection means for detecting the absence of a proper metallic pan load coupled to the heating coil to interrupt the gating of the gate controlled thyristor; (b) gate trigger time means operative at a given power setting for decreasing said nonconduction time when the conduction time of the inverter circuit increases and for increasing said nonconduction time when the conduction time of the inverter circuit decreases with proper metallic pan loads, so that substantially constant power is coupled to a selected one of a plurality of proper metallic pan loads at the given power setting.

Claim 27

An induction heating apparatus having an inverter circuit including an induction heating coil coupled to a DC power supply for energizing the heating coil, the inverter circuit including a gate controlled thyristor cyclically gated to produce a conductive first period followed by a nonconductive second period in order to energize the heating coil at operator selected power settings, having timing and protection circuitry comprising: (a) an improper load detection means for detecting the absence of a proper metallic pan load coupled to the heating coil to interrupt the gating of the gate controlled thyristor; (b) gate trigger time means operative at a given power setting for decreasing said nonconduction time when the conduction time of the inverter circuit increases and for increasing said nonconduction time when the conduction time of the inverter circuit decreases with proper metallic pan loads, so that substantially constant power is coupled to a selected one of a plurality of proper metallic pan loads at the given power setting.

Claim 28

An induction heating apparatus having an inverter circuit including an induction heating coil coupled to a DC power supply for energizing the heating coil, in which the DC power supply is unregulated and produces a varying voltage output having peaks and valleys, the inverter circuit includes a gate controlled thyristor cyclically gated to produce a conductive first period followed by a nonconductive second period in order to energize the heating coil, and a protection circuit for the inverter circuit comprising: (a) first peak charged means connected to receive energy from the inverter over substantially the entire range of voltage variation of the DC power supply to produce a first output signal related to the energy in the inverter at a time t1 which is the start of said second period; (b) second peak charged means connected to receive energy from the inverter over substantially the entire range of voltage variation of the DC power supply to produce a second output signal related to the energy in the inverter at a time t2 which is the end of said second period; and (c) means for taking a ratio of said first output signal and said second output signal and comparing the ratio to a reference to produce an inverter circuit protection signal as a result of said comparison if said ratio differs from the reference ratio by a predetermined amount.

Claim 29

An induction heating apparatus having an inverter circuit including an induction heating coil coupled to a DC power supply for energizing the heating coil, in which the DC power supply is unregulated and produces a varying voltage output having peaks and valleys, the inverter circuit includes a gate controlled thyristor cyclically gated to produce a conductive first period followed by a nonconductive second period in order to energize the heating coil, and a protection circuit for the inverter circuit comprising: (a) first peak charged means connected to receive energy from the inverter over substantially the entire range of voltage variation of the DC power supply to produce a first output signal related to the energy in the inverter at a time t1 which is the start of said second period; (b) second peak charged means connected to receive energy from the inverter over substantially the entire range of voltage variation of the DC power supply to produce a second output signal related to the energy in the inverter at a time t2 which is the end of said second period; and (c) means for taking a ratio of said first output signal and said second output signal and comparing the ratio to a reference to produce an inverter circuit protection signal as a result of said comparison if said ratio differs from the reference ratio by a predetermined amount.

Claim 30

The induction heating apparatus of claim 21 which further comprises (d) means for preventing the gating of the gate controlled thyristor in response to said inverter circuit protection signal.

Claim 31

The induction heating apparatus of claim 22 which further comprises (e) means for rechecking said ratio after the expiration of a period of time of non-gating of the gate controlled thyristor.

Claim 32

The induction heating apparatus of claim 21 which further comprises (f) means for producing said inverter circuit protection signal for a period of time when power from the DC power supply is first applied to the inverter circuit.

Claim 33

The induction heating apparatus of claim 21 in which the DC power supply is derived from an AC power source and further comprising (g) means for producing a zero-crossing synchronization signal from the AC power source and (h) means for combining said synchronization signal with said comparison result before the means (c) referencing with said reference value, whereby the occurrence of said synchronization signal is necessary before an inverter circuit protection signal is produced.

Claim 34

In an induction heating apparatus having a loadable induction heating coil in an inverter circuit coupled to a DC power supply for energizing the heating coil at operator selected power settings, in which the inverter circuit includes a gate controlled thyristor, a heating coil power control comprising: (a) means operative at a selected power setting for producing a gating signal for the gate controlled thyristor after a period of time T2 during which the inverter circuit has been nonconductive; and (b) means operative at the selected power setting for varying said nonconduction time T2 in inverse proportion to conduction time T1 approximately according to the formula T2=aT1+b, where "b" is a non-zero constant and "a" is a negative constant, thereby to maintain power delivery substantially constant at the selected power setting.

Claim 35

In an induction heating apparatus having a loadable induction heating coil in an inverter circuit coupled to a DC power supply for energizing the heating coil at operator selected power settings, in which the inverter circuit includes a gate controlled thyristor, a heating coil power control comprising: (a) means operative at a selected power setting for producing a gating signal for the gate controlled thyristor after a period of time T2 during which the inverter circuit has been nonconductive; and (b) means operative at the selected power setting for varying said nonconduction time T2 in inverse proportion to conduction time T1 approximately according to the formula T2=aT1+b, where "b" is a non-zero constant and "a" is a negative constant, thereby to maintain power delivery substantially constant at the selected power setting.

Claim 36

The power control of claim 26 in which "a" is between -1 and 0 and "b" is greater than 20.

Claim 37

The power control of claim 26 in which the inverter circuit includes a commutating capacitor which charges from the DC supply and in which "a" is selected to end the nonconduction time T2 when the capacitor is appropriately charged, the charge on the commutating capacitor when the SCR is gated being indicative of the power supplied to the heating coil.

Claim 38

In an induction heating apparatus having an induction heating coil to which a variety of loads of different magnetic properties can be presented, the heating coil being connected in an inverter circuit coupled to a DC power supply for energizing the heating coil at a plurality of operator selected power settings, in which the inverter circuit includes a gate controlled thyristor, a heating coil power control comprising: (a) means operative at a selected power setting for producing a gating signal for the gate controlled thyristor after a period of time T2 during which the inverter circuit has been nonconductive; (a1) the inverter including means for responding to the gating signal for initiating a conductive cycle having a duration T1 dependent on the magnetic properties of the load presented to the induction heating coil; and (b) means operative at the selected power setting for adjusting said nonconduction time T2 dependent upon the conduction time T1 in such a way as to deliver power to each of said variety of loads at a level which is substantially independent of the magnetic properties of the loads.

Claim 39

In an induction heating apparatus having an induction heating coil to which a variety of loads of different magnetic properties can be presented, the heating coil being connected in an inverter circuit coupled to a DC power supply for energizing the heating coil at a plurality of operator selected power settings, in which the inverter circuit includes a gate controlled thyristor, a heating coil power control comprising: (a) means operative at a selected power setting for producing a gating signal for the gate controlled thyristor after a period of time T2 during which the inverter circuit has been nonconductive; (a1) the inverter including means for responding to the gating signal for initiating a conductive cycle having a duration T1 dependent on the magnetic properties of the load presented to the induction heating coil; and (b) means operative at the selected power setting for adjusting said nonconduction time T2 dependent upon the conduction time T1 in such a way as to deliver power to each of said variety of loads at a level which is substantially independent of the magnetic properties of the loads.

Claim 40

The power control of claim 29 wherein said power level varies no more than 15% from a nominal value.