GB2197999A

GB2197999A - Improvements in or relating to induction heating circuits for cooking appliances

Info

Publication number: GB2197999A
Application number: GB08628100A
Authority: GB
Inventors: Julian Coppin; Francis Downham
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: GB8628100D0; GB2197999B; EP0269415A2; EP0269415A3

Abstract

A pan sensing circuit 7 responds to current flow through a pan heating coil L2 during periods when a GTO thyristor VT1, or a bipolar device, is non-conducting, the circuit 7 inhibiting a return of thyristor VT1 to conduction unless the sensed current indicates the presence of a suitable utensil. An interrogation circuit 11 allows turn on of thyristor VT1 at one second intervals until a suitable utensil is sensed. In normal operation thyristor VT1 is turned off when a current sensor 8 senses a rise in current to a predetermined level. Thyristor VT1 is turned on again when the current through the coil L2 reverses as sensed by a saturating current transformer T1 and a voltage detector 13. Pan sensor 7 detects the current through a freewheeling diode D1. A controller 9 sets the mark/space ratio of a generator 10 outside the limited range of power control obtainable by sensing current using circuit 8. Operation is inhibited if a protection circuit 12 senses an incorrect operating potential in any of circuits 6, 7, 8, 10, 11, 13. Circuit 12 also causes a predetermined powering-up sequence to be followed. A series ballast inductor L1 limits perk voltages, during resonance and also other optimisation of both the inductance and geometry of the pan coil L2. <IMAGE>

Description

2,97999 v Improve,ments in or relating-to Induction Heating Circuits for

Cooking Appliances This invention relates to induction heating circuits for cooking appliances. 5 Such circuits may comprise a rectifier for converting A.C. mains supply to direct current which is Uhen 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 ccil which induces currents in a suitable utensil placed over the coil.hereby heating the utensil and its contents.

Energisation of the coil must be prevented in the eve-It. that there is no utensil or an unsuitable utensil 1 L, ocated over 'he coil.

It is an object of the present invention to provide means whereby energisation in those circumstances is prevente-d.

According to the present invention, an induction heating circuit for a cooking appliance includes an inverter for powering an induction heating coil at values determined by. the duration of conduction of semi-conductor switching means and in which means are provided for responding to the current flow through the heating coil during periods when the switching device is non-conducting and for inhibiting a return of the switching device to conduction unless the current flow indica tes that a -suitable utensil is located on the heating coil.

The current responsive means may be connected to the junction of a diode and a -resistor in series connection with the heating coil.

Further means may be provided for allowing, in the event of inhibition of the return to conduction, a return to conduction of the switching means after the expiration of a preset time interval and at preset time intervals thereafter until the presence of a suitable urlensil is detected when 2 return to conduction is 1 0 al lowed.

I By way of example only, an embodiment of the invention will now be described in greater detail with reference to the accompanying drawing which is a circuit ciagram partly in block schematic form of the embodiment.

In the drawing, an A.C. mains input connected to input terminals 1, 2 is rectified and smoothed by full wave rectifier 3 and smoothing circuit 4 respectively. The output of the smoothing circuit is applied to a pan coil L2 in series connection with a semi-conductor switching device which could be a high voltage bipolar device and which is shown in the drawing as a gate turnoff thyristor VT1 and a resistor R2. Also in series connection with coil L2 are inductors Ll and L3 and the primary winding of a saturating transformer T1 which will be referred to again below.

In parallel connection across thyristor VT1 is the series connected combination of diode Di and resistor 1 f -3R1. Diode DI is the so-called commutating or "freewheeling" diode needed to divert load current through the inductive load when the thyristor is turned off as will be described in detail. below, thereby protecting thyristor M from damage by excessive voltages at the end of its non-conducting period.

In parallel connection across thyristor VT1 and resistor R2 is a series connected combination of diode D2 and capacitor CR. The capacitance of capacitor CR determines, with other circuit parameters including the inductance of pan coil 12 and of inductor L3, the resonant frequency of the circuit. Diode D2 also provides a conductive path for circulating currents during periods of resonance when thyristOr VT1 is turned-off as will be described below.

A third diode D3 is connected between the junction between the primary winding of transformer T1 and inductor L3 and that between diode D2 and capacitor CR. 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.

Connected to the gate-electrodes of thyristor VT1 is a drive circuit shown as block 5 and of conventional form which supplies pulses of variable,width to control the switching on and off of the thyristor and hence to control the power input to the pan coil L2.

The drive circuit is controlled by a timing -Acircuit shown as block 6 which receives inputs from a pan which responds to L2 during thyristor detector circuit st current flow through the pan coil turn-off periods and which in the event that the current flow indicates that no utensil or an unsuitable utensil has been placed above pan coil L2, inhibits the action of the timing and drive circuits 6, 5 respectively. As shown, circuit 7 responds to the flow of current', through own as bloc-k 7 dic.de DI as monitored by resistor RI.

in the event assumes a particular produces an output w that ie current throu.E-h diode DI value, circu-Lt 7 responds and hich is applied to the timing circuit 6 to inhibit the 1--azer and thereby the operation of the drive circuit 5.

1 5 Operation of the controlled by a load current ensure that the power input by a power controller indicate set by a user in accordance Only a limited rang ing circuit 6 is also detector shown as block 8 to to the pan coil L2 is that set d by block 9 and which is w'h t iL, Ihe heating requirement. e of power control is obtainable by sensing thyristor current by detector 8 and therefore the controll.er 9 operates, outside that range, to set the mark-space ratio of a mark/space generator shown as block 10 whose output is applied directly to the timing circuit 6.

The output of the mark/space generator 10 is also applied to an interrogation circul shown as block 11.

cl L -5The interrogation circu-it 11 operates, in conjunction with the pan detector 7 in the following manner. In the absence of a suitable pan above the pan coil L2, the pan detection circuit inhibits the timing circuit and this inturn prevents the subsequent turn-on of thyristor VT1. Subsequently, the interrogation circuit 11 switches the t 6 back into operation at, typically, Uiming circuit one-second intervals until the presence of a suitable utensil above the pan coil 12 is detected by detector 7.

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, typically, a one-second delay before it produces an interrogation output. During that one-second delay, the complete system settles to a stable condition before any attempt is made to switch on thyristor VT1.

The embodiment also includes protection circuits shown in Fig. I as block 12. Such circuits monitor the 20- function of the system and should a malfunction be detected, the drive circuit 5 is prevented from operating and the thyristor M is rendered non-conducting.

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 -E- Lhe correct resonant' ci protection Ilpowering-upll sequence is followed at the commencement of a cooking of the main 11glichell therein.

The system value before power is connected to the rcuit and associated components. The circuits also ensure that a predetermined operation and following a temporary interruption s supply and following a voltage surge or es a ramped 10 increase in the current through the pan coil L2. When ler 9, and nat curren- reaches a value set bv control sensed bv control curren-, detector 8, the thyristor is turned- cif. Fesonance now occurs in the circuit including the pan ccil L2, inductor L3 and capacitor CR.

The resonant circuit is coupled via transformer TI to a vcltage detectcr drops to a minimum to the timing circ on the thyristor VTI to above is inhibite of a suitable utensi The use of larly advantageous a be obtained of the resonant circuit re The system powered from the ma E or, the thyristor VT1 produ shown as '--lo level, det uit 6 wh-L ch, via 3 and when tine voltage 13 responds and inputs drive circuits, turns The interrogation function referred d once the presence above the pan coil 1 has been sensed. a saturating transformer is particus it enables a precise indication to instant when the voltage in the aches its minimum value. shown in Fig. 1 will normally be ins supply and it is found that the 1 -7 ii A peak voltages developed during resonance are very high can be limited by the inclusion in series with the pan coil L2 of a ballast inductor L1. This also reduces the effective-supply voltage during periods when the thyristor is turned on.

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 coil L2.

WiLthout the additional inductor L1, the inductance of the pan coil L2 is defined by the circuit operaL,ing voltage and current, the frequency of resonance and the power throughput. The resultant inductance value and coil geometry of the pan coil L2 maynot be the optimum for the application.

The arran,gerrient- allows a rate of current rise through, the pan coil th.at 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' must be of a value that is a compromise to allow the voltage across capacitor CS to reduce during periods when ' thyristor M 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. which thyristor Ine precise instant of time at above, by ed to the T1 The VT1 is turned-on the control voltage detector 13 tha pan coil circuit 1ransformer T1 is Ideally, thyristo Jnstant when the 0 order to reduce 'nrcugh conditions, I.e.

resulting in duration the voltag reduce initiate a turn-or instant of on at the is determined, as explained t is coup via saturating transformer a saturating current transformer.

r VT11 must be voltage acros the amplitude o V-1. However low power JLnpu-1 c the pan voltage across capacitor CR does not return --nus, when the thyrist-cr isc next turned-on, forward biassed destructive short t hvr-,Js'.or. instant at which minimum in order to 20duration current' flow. with the reversal of current through pan coil L2.

Thus by allowing the control voltage detector to pulse from the drive circuit 5 at the he thyristor VT1 is turned current reversal, L instant of minimum voltage across capacitor CR.

The cooking appliance may be a hob unit in which e or more of the pan heating units may be of the Lurned on again at the capacitor CR is zero in the discharge current under light load coil L2, the zero. u will be an uncontrolled, potentially urrent flow through the he thyristor must be turned on again at the e across capacitor CR is at a Lhe amplitude of the short The minimum voltage is coincident form described above. Other pan burners and/or electric heating The invention may also be which may be free-standing. One or burners may be of the form described heating units may be gas burners units.

heating units may be gas units.

embodied in a cooker more of the top or pan. above. Other top and/or electric heating -1 0-

Claims

C 1 a i m s:

1. An induction heating circuit for a cooking appliance including an inverter for powering an induction heating coil at values determined by the duration of conduction of semi-conductor switching means and in which means are provided for responding to the current flow through the heating coil during periods when the switching device is non-conductling and for inhibiting a return of the switching device to conduction unless the current flow indicates that a suitable utensil is located on the heating coil.

2. A circuit as claimed in claim I in which the current responsive means is connected to the junction of a diode and a resistor in series connection with the heating coil.

r

3. A circuit as claimed in claim 1 or 2 which further means are provided for allowing, in the event of in-ihibition of the return to conduction, a return to conduction of the switching means after the expiration cf a preset', time interval and at preset time intervals thereafter until the presence of a suitable utensil is detected when a return to conduction is allowed.

4. An induction heating circuit for a cooking appliance substantially as herein described with reference 25 to and as illustrated by the accompanying drawing.

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