GB2197995A

GB2197995A - Induction heating circuits for cooking appliances

Info

Publication number: GB2197995A
Application number: GB08628099A
Authority: GB
Inventors: Peter William Lee
Original assignee: TI CREDA Ltd
Current assignee: TI CREDA Ltd
Priority date: 1986-11-25
Filing date: 1986-11-25
Publication date: 1988-06-02
Anticipated expiration: 2006-11-25
Also published as: EP0269414A3; GB8628099D0; EP0269414A2; GB2197995B

Abstract

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 included 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. <IMAGE>

Description

2197995 t, 1 1 Improvements in or relating to induction Heating Circuits

for Cooking Appliances This invention relates to induction heating circuits for cooking appliances.

Such circuits comprise a rectifier for converting A.C. mains supply to direct current which is then supply converted by an inverter to an alternating currenL. at, a suitable frequency usually in the range of from 20-35 kHz. That supply energises a coil which induces currents in a suitable utensil placed over the coil thereby heating the utensil and its contents.

The inver"Ler comprises a semi-conductor switching device for example a thyristor or a high voltage bipolar device and it is found that when opera.ting 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 11he phase of the powering cycle of coil can be excessive.

Accordingly, it is an object of the present invention to provide means for reducing the developed peak voltage.

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 further inductance is included in series connection with the heating coil of such value as in peak voltages developed during tc ei,'Lec a reduction resonance in the heating circuit.

According to another aspect, the present', invention alsc provides an induction heating circuit for mprisLng an inverter for powering an 1 a, values determined by the periods cf conduc--icn cf a switching device, in which a further incuctance is included in series connection between the hea--ing ccil and the switching device and in which a -Jonal current pa,h including the further Drovided for turn-on curren a cooking appliance c J r, -Jjc, ion he a ti n E cc i tance is device.

The swit-ching device ma ristor.

invention will reference to t Fig. 1 i Fig. 2 IS 2 In the embodimen are supplied with a D.C.

from the full-wave rectif current may b .,rough the v be a gate turn-on By way of example only, embodiments of the now be described in greater detail with he accompanying drawings of which: s a circuit diagram of one embodiment, it diagram o t of Fig. voltage obtained, for example, ication of an A.C. voltage. The e supplied from amny suitable source, for the co-pending Patent Applicat example that described in t Lion f a second embodiment.

1 innut terminals 1 2 - -R - 9 V k N o S c S? (Case 218) which also gIves further details of the overall system.

In series connection an inducto.r L2, which forms a cooking utensil is placed when -s induced 'I bhe current herein bv ' 1 across the terminals 1, 2 is pan coil over which a it is to be heated by coil, a second inductor Ll and a switch S1. In parallel connection across switch S1 is capacitor CR which is the resonance capacitor for I L2.

Ihe pan co-i- Switch S1 is a schematic representation of the device which would actually be used in practice P and which may be a semi-ccnduc-,cr switching device for example a thyristor which may be a gate turn off thyristor, or a high voltage bipolar device.

A diode D1 is connected as shown and is the so-called "free wheeling" diode.

The inductance of the pan coil L2 is resonated by capacitor CR whilst capacitor CS is the line filter and resonant reve.rsed commutator capacitor.

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 repeat W it h o U t coil L2 is define current, frequenc However, once tho established, it m inauctance anci cc 'he optimum for the application. the additional induct, or LI u c t an c e simil a r t alse -, f l e x i b. i z y i n. b o t h t In e i n d u c t a n c e o f i--s gen-mery.

s as long as the system is in use. inductor L1, the inductance of the pan d by the circuit oper2ting voltage V, y of resonance and power t se operating conditions hav -hat the result ay be found t il geometry of the pan coil may not be hroughput.

The cl he total : a r, 5. W r. -L 1 s t wen switch 5-, --c cicsed, i 1 5 circuit in this allows se.,ected o allow the voltag reduce during currenz flow i of closure of the switch S current during the resonant circuit. That implies tha inductances must be reduced. in the capacitance of capaci resonant frequency.

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 into two c be split ral-e of current rise rovides design pan coil L2 and permit ha-,, the capacitances c.f the the capacitor CE must- be so VCS across capacitor CS to Lhe pan coil during periods and to allow absorption of the periods of operation of the L.he value of the pan coil There must be some increase or CR to maintain the op 1 the voltage stress across switch S1. As is explained in Pa.tent (Case 218), the precise instant at closed is determined by the voltage across CR. Ideally L,he 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 tially desctructive resulting in an uncontrolled, potent short duration current Plow through the switch S1 when the 0 latter is embodied as a semi-conductor switch. To reduce the amplitude of the current Llow, some form of protection is required and such protection is commonly referred to as a "snubber". However, existing forms of snubber do not isfactor-ily Llined above.

meet sa Lhe situation ou' an improved form Fig. 2 is a circuit diagram ol snubber. The references used in Fig. 2 are the same as ol -s are the same. In those used in Fig. I where component t of Fig. 2, 1 the circuit Lhe 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.

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.

Diode D2 provides a conducting path for A p p 1 i c a t i o n N o. S16 '7-5^cck 17 which switch 51 is circulating currents during pericds of resonance when t ff as is cescribed in the Paten, 1hyristor VT1 is turned-o-I Ap!-Jcation referred to above. _Tn addition, D2 blocks turn on current through thyristor V--7i.

Diode D3 is poled to provide a furl'-her route for current flow during resonant periods in a direction opposite to that permitted by diode D2.

diode D3 provides a route for thvristcr VT1 but ',his route limits the rate cf increase In addition, turn-on current for includes inductor L3 which of current and peak current at turn-cr..:nductor '_3 does not, however, reduce thyristor -end the range dissipa,lon and zhus it is necessary -o ext over which variation cf power input to the pan coil Ll is effected by control cf mark-space ratio as is described in 'he Paten, Application referred tc, above.

The cooking applLance may be a hob unit in which case one or more of the pan heating units may be of the form described above. Other pan heating units may be gas 20 burners and/or electric heating units.

The invention may also be embodied in a cooker which may be free-standing. One or more of the top or pan heating units may be of the form described above. Other top heating units may be gas burners and/or electric 25 heating units.

1 Q, 1 Cl aims:

1. An induction hea_ting 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.

1 0 2. An induction heating circuit for a cooking appliance comprising an inverter for powering an induction heating coil a. values determined by the periods of conducLion 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- C) directional current path including the further inductance is provided for turn-on current through the switching d e v i c e.

3. A circuit as claimed in claim 1 or 2 in which the switching device is a gate turn-on thyristor.

4. An induction heating circuit for a cooking appliance substantially as herein described with reference to and as illustrated by the accompanying drawings.