Patent GB-1439232-A (GB)
Title: Constant duty cycle control of induction cooking inverter
IPC: H02M7/523
Filed: 1973-06-26
Abstract: 1439232 Controlling inverters; control systems for inductively heated cookers GENERAL ELECTRIC CO 26 June 1973 [28 June 1972] 30197/73 Headings H2F and H2H [Also in Division H5] In an inductive cooker, an inverter comprises a solid state power device 33 arranged to operate with alternate oscillating and non- oscillating intervals and a constant duty cycle control circuit supplying turn-on signals to said device 33 so as continuously to vary the frequency as a function of load condition of the induction heating coil 15 under control of a sensor 41, responsive to the conduction and non-conduction intervals of device 33. As described, a series resonant circuit comprising the induction heating coil 15 and capacitor 32 is bridged by a thyristor 33 having an inverse diode 34 and connected to D.C. input terminals 30, 31 via a reset inductor 35. In order to provide a constant duty cycle following variations in load, e.g. due to the presence or absence of a pan 17, the anode voltage of the thyristor 33 is monitored by a circuit 41 which provides a pulse in response to each conduction period of the thyristor 33. The pulses are averaged at 43 and the resulting D.C. used to adjust the frequency of a voltage controlled oscillator 44 which triggers the thyristor 33 and so maintain constant duty cycle. A starting circuit is included to allow commutating capacitor 32 to charge fully via reset inductor 35. Each trigger pulse fed to thyristor 33 discharges capacitor 32 via the heating inductor 15 giving a half cycle sinusoid, the second half cycle being passed by diode 34. Figs. 4-6 (not shown) depict thyristor anode voltage and coil current waveforms. Thyristor 33 may be replaced by one or more transistors. Starting.-Capacitor 86 charges via resistor 85 and the base/emitter path of transistor 88 which clamps the thyristor anode voltage pulses from A. Transistor 89 inhibits operation of the voltage controlled oscillator comprising complementary unijunction transistor CUJT59 until capacitor 86 is nearly charged when transistors 88, 89 are turned off and capacitor 62 charges via resistor 80 and diode 83 sufficiently to allow capacitor 55 to charge via transistor 56 so that CUJT59 oscillates at 18 kHz. The resulting pulses are passed via transistors 63, 66, 67 to the gate of the main thyristor 33. Control of duty cycle.-If the inverter is unloaded (no pan present) the natural period tr of the series resonant circuit 15, 32 is long, hence long-duration pulses are present at A. These pulses are converted by transistors 72, 75 into D.C. across capacitor 62 which controls the charging rate of capacitor 55 and hence reduces the frequency of pulses fed to thyristor 33 so as to maintain the constant duty cycle of 50%. Maximum and minimum frequency limitation. -Zener diode 79 provides a maximum frequency of 40 kHz. Resistor 80 and diodes 81, 82 limit the minimum frequency to 18 kHz; the frequency between these ranges may be adjusted by resistor 58. Construction.-Figs. 1 and 2 (see Division H5) relate to a cool-top cooker and a portable defroster/heater.