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(11) | EP 0 269 417 A1 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Induction heating circuits for cooking appliances |
| (57) An induction heating circuit for a cooking appliance includes an inverter for powering
a pan heating coil (L2) and a resonating capacitor (CR) at values determined by a
gate turn off thyristor (VT1). A further semi-conductor switching device (VTS) is
included to permit discharge of the resonating capacitor (CR) before the initation
of power input to the pan coil (L2). |
[0002] Such circuits may comprise a rectifier for converting A.C. mains supply to direct current which is then converted by an inverter to an alternating supply at a suitable frequency usually in the range of from 20-35 kHz. That supply energises a pan heating coil which induces currents in a suitable utensil placed over the coil thereby heating the utensil and its contents.
[0003] The inverter is switched on and off at intervals determined by the heat output required from the pan heating coil. During off periods, the inverter resonates at a frequency deteramined by the inductance of the pan heating coil and other circuit parameters including a circuit capacitor.
[0004] When the circuit is in operation, excessively high currents may be experienced through the inverter because, when the inverter is switched on, residual potential present across the circuit capacitors at switch on must be discharged.
[0005] The inverter may comprise a gate turn-off thyristor and when this is switched on, a current that increases in ramped fashion flows through the pan heating coil. When the current reaches a specified level, the thyristor is gated off and the circuit resonates. Ideally, the thyristor is switched on when the voltage across the capacitor is zero but that value is not reached under light load conditions, i.e. low power output from the pan heating coil. Consequently, the next time the thyristor is switched on, it will be forward biassed by the residual capacitor voltage and an uncontrolled, potentially destructive current flows momentarily through the thyristor.
[0006] It has been proposed to counteract the risk by a so-called "snubber" circuit but existing forms of such circuit are not suitable for use in induction heating circuits for cooking appliances.
[0007] According to the present invention, an induction heating circuit for a cooking appliance includes an inverter for powering an induction heating coil and a resonating capacitor at values determined by a first semi-conductor switching device and in which a further semi-conductor switching device is provided to permit discharge of the resonating capacitor before the initiation of power input to the induction heating coil.
[0008] The discharge may be effected via a resistor in series connection with the first switching device, and in which the initiation of power input is delayed by the second switching device until discharge is complete.
[0009] Alternatively, the discharge may be effected by the second switching device and is completed before the initiation of power input to the heating coil.
[0010] The amplitude of the current flowing during discharge of the capacitor may be controlled by resistive means.
[0011] The first switching means may be a gate turn off thyristor which is in series connection with a parallel connection combination of a further thyristor and resistor.
[0012] The first switching device may be a gate turn off thyristor which is in parallel connection with a series connected combination of resistor and further gate turn off thyristor.
[0013] By way of example only, embodiments of the invention will now be described in greater detail with reference to the accompanying drawings of which:-
Fig. 1 is a circuit diagram of a first embodiment,
Figs. 2-6 are explanatory waveforms,
Fig. 7 is a circuit diagram of a second embodiment, and,
Figs.8-11 are explanatory waveforms.
[0014] In Fig. 1, the pan heating coil is shown as inductance L2 and this is powered from source 1, 2 under the control of a gate turn off thyristor VT1. The resonant capacitor for the heating coil L2 is capacitor CR.
[0015] In series connection between coil L2 and thyristor VT1 is a parallel-connected combination of thyristor VTS, resistor R2 and diode D5.
[0016] Connected in parallel across capacitor CR is diode D1 which provides a conductive path for resonant currents during non-conductive periods of thyristor VT1.
[0017] Figs. 2-6 illustate the operation of the circuit of Fig. 1. Fig. 2 is a waveform of the turn-on pulse applied to the gate of thyristor VT1. At turn-on any residual voltage present on CR is discharged through RS which limits the current flow to a safe value until thyristor VTS is turned on after a delay of about 3 µsec following the initiation of the turn-on pulse. Turn-on of thyristor VTS allows current flow in ramped fashion through both VTS and VT1 as shown in the waveforms of Figs. 5 and 6.
[0018] At the end of the current ramp, thyristor VT1 is turned off. That deprives thyristor VTS of current hence commutating the thyristor.
[0019] Fig. 3 shows the waveform of the discharge current through resistor RS and thus current also flows through thyristor VT1 as shown by the waveform of Fig. 6.
[0020] Although a current spike S1 occurs when thyristor VT1 is turned-on, the amplitude is limited by resistor RS to an acceptable value.
[0021] It will be appreciated that thyristor VTS must be of a current rating similar to that of thyristor VT1 whilst the voltage rating depends upon the maximum initial capacitor voltage. In addition, thyristor VTS must commutate within the minimum time for which thyristor VT1 is non-conducting.
[0022] Co-pending Patent Application No.86.28098 (Case 218) provides further detail of the induction heating system including means for controlling the conduction of thyristor VT1. Control pulses for thyristor VTS can be derived from the drive circuit referred to in the co-pending Patent Application.
[0023] The snubber circuit described above with reference to Fig. 1 requires the use of a thyristor VTS with a fast recovery time and which operates at a relatively low power level. Such devices tend to be expensive.
[0025] In parallel connection with the gate turn off thyristor VT1 is the series connected combination of resistor RS and gate turn off thyristor VTS, a diode DS being in parallel connection with resistor RS.
[0026] Thyristor VTS is turned on in advance of thyristor VT1 as can be seen from a comparison of waveforms of Figs. 8 and 9. Turning on thyristor VTS allows capacitor CR to discharge through resistor RS as is indicated by the waveform shown in Fig. 10. Discharge is subtantially completed before thyristor VT1 is turned on.
[0027] As thyristor VT1 is turned on, thyristor VTS is turned off and the load current through the pan heating coil transfers to thyristor VT1.
[0028] The arrangement shown in Fig. 1 has the advantage that thyristor VTS does not carry the main load current but it must support the peak resonant voltage at capacitor CR and thus is of the same voltage rating as thyristor VT1.
[0029] The circuit arrangements described above with reference to Figs. 1 and 7 permit a substantial reduction in the dissipation of the thyristor VT1 and this allows the power output of the pan heating coil to be reduced to comparatively low levels by using the current sensing techniques described in Patent Application No. (Case 218) without recourse to the mark/space control also referred to in that Application.
1. An induction heating circuit for a cooking appliance including an inverter for
powering an induction heating coil and a resonating capacitor at values determined
by a first semi-conductor switching device and in which a further semi-conductor switching
device is provided to permit discharge of the resonating capacitor before the initiation
of power input to the induction heating coil.
2. A circuit as claimed in claim 1 in which discharge is effected via a resistor in
series connection with the first switching device, and in which the initiation of
power input is delayed by the second switching device until discharge is complete.
3. A circuit as claimed in claim 1 in which discharge is effected by the second switching
device and is completed before the initiation of power input to the heating coil.
4. A circuit as claimed in claim 2 or 3 in which the amplitude of the current flowing
during discharge of the capacitor is controlled by resistive means.
5. A circuit as claimed in claim 2 in which the first switching means is a gate turn
off thyristor which is in series connection with a parallel connection combination
of a further thyristor and resistor.
6. A circuit as claimed in claim 4 in which the first switching device is a gate turn
off thyristor which is in parallel connection with a series connected combination
of resistor and further gate turn off thyristor.
7. An induction heating circuit for a cooking appliance substantially as herein described
with reference to Figs. 1-6 or Figs. 7-11 of the accompanying drawings.