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(11) | EP 0 269 414 A2 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Induction heating circuits for cooking appliances |
| (57) An induction heating circuit for a cooking appliance comprises an inverter for powering
a pan coil (L2) at values determined by the periods of conduction of a gate turn-off
thyristor (S1, VT1). A further inductance (L1) is inclined in series connection with
the pan coil (L2) to reduce peak voltages developed during resonance in a resonant
circuit including the pan coil (L2). To effect a reduction in the current flow when
the thyristor (S1, VT1) is turned on, a snubber comprising a diode (D3) and another
inductor (L3) is included. |
[0002] Such circuits comprise a rectifier for converting A.C. mains supply to direct current which is then converted by an inverter to an alternating current supply at a suitable frequency usually in the range of from 20-34 kHz. That supply energises a coil which induces currents in a suitable utensil placed over the coil thereby heating the utensil and its contents.
[0003] The inverter comprises a semi-conductor switching device for example a thyristor or a high voltage bipolar device and it is found that when operating from mains supply commonly available in the U.K. and in Europe, high peak voltages are developed during resonant periods of the operating cycle of the circuit and this is undesirable. It is also found that the current rise during the initial phase of the powering cycle of the coil can be excessive.
[0004] Accordingly, it is an object of the present invention to provide means for reducing the developed peak voltage.
[0005] According to the present invention, an induction heating circuit for a cooking appliance comprises an inverter for powering an induction heating coil at values determined by the periods of conduction of a switching device and with a further inductance is included in series connection with the heating coil of such value as to effect a reduction in peak voltage developed during resonance in the heating circuit.
[0006] According to another aspect, the present invention also provides an induction heating circuit for a cooking appliance comprising an inverter for powering an induction heating coil at values determined by the periods of conduction of a switching device, in which a further inductance is included in series connection between the heating coil and the switching device and in which a uni-directional current path including the further inductance is provided for turn-on current through the switching device.
[0008] By way of example only, embodiments of the invention will now be described in greater detail with reference to the accompanying drawings in which:
Fig. 1 is a circuit diagram of one embodiment, and,
Fig. 2 is a circuit diagram of a second embodiment.
[0009] In the embodiment of Fig. 1, input terminals 1, 2 are supplied with a D.C. voltage obtained, for example, from the full-wave rectification of an A.C. voltage. The current may be supplied from amny suitable source, for example that described in the co-pending Patent Application No. 86.28098 (Case 218) which also gives further details of the overall system.
[0010] In series connection across the terminals 1, 2 is an inductor L2, which forms a pan coil over which a cooking utensil is placed when it is to be heated by currents induced therein by the coil, a second inductor L1 and a switch S1. In parallel connection across switch S1 is capacitor CR which is the resonance capacitor for the pan coil L2. Switch S1 is a schematic representation of the device which would actually be used in practice and which may be a semi-conductor switching device for example a thyristor which may be a gate turn off thyristor, or a high voltage bipolar device.
[0012] The inductance of the pan coil L2 is resonated by capacitor CR whilst capacitor CS is the line filter and resonant reversed commutator capacitor.
[0013] The induction heating system operates in the manner described in the Application referred to above. Briefly, the inverter is switched on and the current through the pan coil L2 increases until a set and sensed level is detected at which point the inverter is switched off. The inverter now resonates at a frequency determined by the inductance of pan coil L2 and other circuit constants. When the voltage in the resonant circuit reaches a minimum value, the inverter is turned on again. That cycle repeats as long as the system is in use.
[0014] Without inductor L1, the inductance of the pan coil L2 is defined by the circuit operating voltage V, current, frequency of resonance and power throughput. However, once those operating conditions have been established, it may be found that the resultant inductance and coil geometry of the pan coil may not be the optimum for the application.
[0015] The introduction of the additional inductor L1 enables the total circuit inductance to be split into two parts. Whilst this allows a similar rate of current rise when switch S1 is closed, it also provides design flexibility in both the inductance of the pan coil L2 and its geometry.
[0016] However, to permit that, the capacitances of the filter capacitor CS and of the capacitor CR must be so selected to allow the voltage VCS across capacitor CS to reduce during current flow in the pan coil during periods of closure of the switch S1 and to allow absorption of the current during the resonant periods of operation of the circuit. That implies that the value of the pan coil inductances must be reduced. There must be some increase in the capacitance of capacitor CR to maintain the optimum resonant frequency.
[0017] The reduction in pan coil inductance and increase in the capacitance of capacitor CR reduce the voltage excursion across capacitor CR during resonance and hence the voltage stress across switch S1.
[0018] As is explained in Patent Application No. (Case 218), the precise instant at which switch S1 is closed is determined by the voltage across CR. Ideally the closure of the switch S1 should be effected when the voltage is zero. However, under low load conditions the voltage across CR does not return to zero. Thus, when the switch S1 is next closed, there will be a forward bias resulting in an uncontrolled, potentially desctructive short duration current flow through the switch S1 when the latter is embodied as a semi-conductor switch. To reduce the amplitude of the current flow, some form of protection is required and such protection is commonly referred to as a "snubber". However, existing forms of snubber do not meet satisfactorily the situation outlined above.
[0019] Fig. 2 is a circuit diagram of an improved form of snubber. The reference used in Fig. 2 are the same as those used in Fig. 1 where components are the same. In the circuit of Fig. 2, the switch S1 is shown as a gate turn-on thyristor but it could also be some other form of high voltage bipolar semi-conductor device.
[0020] In Fig. 2, a further inductor L3 is in series connection between the pan coil L2 and thyristor VT1 across the capacitor CS. Across inductor L3 are series connected diodes D2 and D3, the resonant capacitor CR being connected to the junction between the diodes.
[0021] Diode D2 provides a conducting path for circulating currents during periods of resonance when thyristor VT1 is turned-off as is described in the Patent Application referred to above. In addition, D2 blocks turn on current through thyristor VT1.
[0022] Diode D3 is poled to provide a further route for current flow during resonant periods in a direction opposite to that permitted by diode D2. In addition, diode D3 provides a route for turn-on current for thyristor VT1 but this route includes inductor L3 which limits the rate of increase of current and peak current at turn-on.
[0023] Inductor L3 does not, however, reduce thyristor dissipation and thus it is necessary to extend the range over which variation of power input to the pan coil L1 is effected by control of mark-space ratio as is described in the Patent Application referred to above.
1. An induction heating circuit for a cooking appliance comprising an inverter for
powering an induction heating coil at values determined by the periods of conduction
of a switching device and in which a further inductance is included in series connection
with the heating coil of such value as to effect a reduction in peak voltages developed
during resonance in the heating circuit.
2. An induction heating circuit for a cooking appliance comprising an inverter for
powering an induction heating coil at values determined by the periods of conduction
of a switching device, in which a further inductance is included in series connection
between the heating coil and the switching device and in which a uni-directional
current path including the further inductance is provided for turn-on circuit through
the switching device.
3. A circuit as claimed in claim 1 or 2 in which the switching device is a gate turn-on
thyristor.
4. An inductive heating circuit for a cooking appliance substantially as herein described
with reference to and as illustrated by the accompanying drawings.