GB2199453A

GB2199453A - Inverter for induction cooker

Info

Publication number: GB2199453A
Application number: GB08628098A
Authority: GB
Inventors: Robert Ian Bullough
Original assignee: TI CREDA Ltd
Current assignee: TI CREDA Ltd
Priority date: 1986-11-25
Filing date: 1986-11-25
Publication date: 1988-07-06
Anticipated expiration: 2006-11-25
Also published as: GB2199453B; EP0269413A3; EP0269413A2; GB8628098D0

Abstract

In an induction heating circuit for a cooking appliance, a pan heating coil (L2) is powered at values determined by the state of conduction of a gate turn-off thyristor (VT1). To reduce the amplitude of current flow through the thyristor at turn-on, a circuit responsive to the voltage across a capacitor (CR) that resonates the pan coil (L2) is employed. The circuit is coupled to the resonant circuit comprising pan coil (L2) and capacitor (CR) by a saturating transformer (T1) and senses the instant when the current flow reverses, which point indicates the capacitor voitage 15 reaching a minimum. This information is used to turn-on the thyristor (VT1). <IMAGE>

Description

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I 0 O 0 O 0 0 0 O " ", -b I" (I) " 0 c.- O c't- I I Subsequently, the nerrogation circuit 11 switches the timing circuit 6 back into operation at, typically, one- second intervals until the presence of a suitable utensil above the pan coil 12 is detected by detector 7.

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It will be appreciated that the interrogation circuit 11 will also operate on normal start-up with a suitable utensil over the pan coil. The circuit provides, on normal start-up, typicaiiy, a one-second delay before it produces an interrogation output. During that i one-second delay, the complete system settles to a stable I condition before any attempt is made to switch on thyristor VTI.

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The embodiment also includes protection circuits shown in Fig. I as block 12. uch circuits monitor the 15 funclon of the system and shouId a malfunction be detected, the drive circuit 5 is prevented from operating and the thyristor VTI is rendered non-conducting.

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For example, the protection circuits may monitor operating potentials for the various control functions, i.e. the generator 10, interrogation circuit 11, timing circuit 6, pan detector 7, control voltage detector and control current detector to ensure those potentials are of the correct value before power is connected to the resonant circuit and associated components. The protection circuits also ensure that a predetermined "powering,up" sequence is followed at the commencement of a cooking oPeration following a temporary interruption it.

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O 0q O 0q O I O I The inclusion of a separate ballast inductor also enables the value of the inductance of the pan coil L2 to be chosen to allow optimisation of both inductance value and geometry of the pan col L2.

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Without an additional inductor LI, the inductance of the pan coil L2 is defined by the circuit operating voltage and current, the frequency of resonance and the power throughput. The resultant inductance value and coil geometry of the pan coil L2 may not be the optimum for the 0 application.

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t The arrangement allows a rate of current rise through the pan coil that is similar to that obtained with the pan coil only. TO obtain that rate of rise, capacitor CS, the filter and resonant reversed commutation capacitor 15 must be of a value that is a compromise to allow the voltage across capacitor CS to reduce during periods when thyristor VTI is turned-on and to allow absorption of the circulating current during resonance. That means a reduction in the inductance of the pan coil L2. It also follows that it is necessary to increase the capacitance of capacitor CR to maintain the optimum resonant frequency. The reduction in pan coil inductance and increase in CR capacitance proportionally reduces the voltage excursions across capacitor CR during resonance.

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The precise instant of time at which thyristor VTI is turned-on is determined, as explained above, by the control voltage detector 13 that is coupled to the r,o o c l) I-a' c " 0 - C LO I-'- 0 < (n i--, u o o o c- -' 0 0 cf ct- m 0 < (D 0 I- p. YR H. 0 (-] ": C (n t C (9 C c" <: (9 C) O. (I) 0 0 0 C 6o c o" C: 1- I-', :::3 Z3- Z3::3" o'- (9 0 ('D.. (D 0 I "K C 0 " D. C') w. 0 -' 0 " 0 m 0 0 F-'. t,,1 C C (D 0:5" 0 (D 0 < " Q- ::3I--'.:"

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0 O O - -% 0 - (1) (D ct (D < I GO I when the voltage across CR is a minimum. Thus by allowing the control voltage detector to initiate a turn-on pulse from the drive circuit 5 at the instant of current reversal, the thyristor VTI is turned on at the instant of minimum voltage across capacitor CR.

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Fig. 2E shows the wa@eform of current flow through the thyristor at turn-on showing the large magnitude "spike" S. To reduce the amplitude of the spike S, some form of protection is required and such protection is commonly referred to as a "snubber".

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In the embodiment shown in Fig. I, the inductor L3 is added in series connction with the pan coil LI as shown. The effect of inductor L3 is to limit the rate of current rise through the thristor VTI and the inductor 15 will, therefore, reduce the possibility of damage to the thyristor during turn-on. The inductor does not, however, r@duce thyristor dissipation and thus it is necesary to extend the power range over which variation, is effected by control of mark-space ratio.

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Since the inductor L3 is in series connection with the pan coil L2, energy stored in it does not have to be separately dissipated.

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It will be appreciated that the minimum value of the voltage in the resonant circuit can be determined in other ways not involving a saturating transformer. For. example, detector 13 may respond to voltage changes across a potentiometer in the resonant circuit.

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Claims

Claims:

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I. An induction heating circuit for a cooking appliance including an invertor for powering an induction heating coil at values determined by state of conduction of a semi-conductor switching device and in which the instant of switching on of the switching device is determined by thepoint in time at which the direction of current flow through the coil reverses.

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2. A circuit as claimed in claim I in which the point in time at which the flow direction reverses is t determined by means coupled to a circuit including the heating coil by a saturating transformer.

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3. An induction heeting circuit for a cooking appliance substantially as herein described with reference 15 to and as illustrated by the accompanying drawings.

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Published 1988 at The Patent Office, State House, 66,"i High Holborn, London WCIR 4TP. Further copies may be obtmned from The P&tent Oce, Sales Branch, St Ma.y Cray, 0rpington, Kent BR8 3RD. Prlnted by Mu]tlplex techrques ltd, St Mary Cray, Kent. Con. 1/87.

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