[0001] This invention relates to induction heating circuits for cooking appliances.
[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 coil which
induces currents in a suitable utensil placed over the coil thereby heating the utensil
and its contents.
[0003] Operation of the inverter is controlled by a timing circuit which switches the inverter
on and off as required to maintain a required power output from the coil.
[0004] It is an object of the present invention to provide an arrangement for determining
accurately the instants at which switching is to take place.
[0005] According to the present invention, an induction heating circuit for a cooking appliance
includes an invertor for powering an induction heating coil at values determined by
state of conduction of a semi-conductor switching device, the instant of switching
on of the switching device is determined by the point in time at which the direction
of current flow through the coil reverses.
[0006] The point in time at which the flow direction reverses may be determined by means
coupled to a circuit including the heating coil by a saturable transformer.
[0007] By way of example only, an embodiment of the invention will now be described in greater
detail with reference to the accomapnying drawings of which:
Fig. 1 is a circuit diagram partly in block schematic form of the embodiment, and
Fig. 2 shows explanatory waveforms.
[0008] In Fig. 1, 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
Fig. 1 as a gate turn-off thyristor VT1 and a resistor R2. Also in series conection
with coil L2 are inductors L1 and L3 and the primary winding of a saturating transformer
T1 which will be referred to again below.
[0009] In parallel connection cross thyristor VT1 is the series connected combination of
diode D1 and resistor R1. Diode D1 is the so-called commutating or "free-wheeling"
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 VT1
from damage by excessive voltages at the end of its non-conducting period.
[0010] 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 L2 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.
[0011] 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 opoposite to that permitted by diode D2.
[0012] 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.
[0013] The drive circuit is controlled by a timing circuit shown as block 6 which receives
inputs from a pan detector circuit shown as block 7 which responds to current flow
through the pan coil L2 during thyristor 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 diode
D1 as monitored by resistor R1.
[0014] In the event that the current through diode D1 assumes a particular value, circuit
7 responds and produces an output which is applied to the timing circuit 6 to inhibit
the latter and thereby the operation of the drive circuit 5.
[0015] Operation of the timing circuit 6 is also controlled by a load current detector shown
as block 8 to ensure that the power input to the pan coil L2 is that set by a power
controller indicated by block 9 and which is set by a user in accordance with the
heating requirement.
[0016] Only a limited range of power control is obtainable by sensing thyristor current
by detector 8 and therefore the controller 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.
[0017] The output of the mark/space generator 10 is also applied to an interrogation circuit
shown as block 11. The interrogation circuit 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 in turn
prevents the subsequent turn-on of thyristor VT1. Subsequently, the interrogation
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.
[0018] 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.
[0019] The embodiment also includes protection circuits shown in Fig. 1 as block 12. Such
circuits monitor the function of the system and should a malfunction be detected,
the drive circuit 5 is prevented from operating and the thyristor VT1 is rendered
non-conducting.
[0020] 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 of the mains supply and following a voltage surge or "gliche"
therein.
[0021] The system as thus described operates in known manner. Turning on the thyristor VT1
produces a ramped increase in the current through the pan coil L2. When that current
reaches a value set by controller 9, and sensed by control current detector 8, the
thyristor is turned-off. Resonance now occurs in the circuit including the pan coil
L2, inductor L3 and capacitor CR. The resonant circuit is coupled via transformer
T1 to a voltage detector shown as block 13 and when the voltage drops to a minimum
level, detector 13 responds and inputs to the timing circuit 6 which, via drive circuits,
turns on the thyristor VT1. The interrogation function referred to above is inihibited
once the presence above the pan coil of a suitable utensil has been sensed.
[0022] The use of a saturating transformer is particularly advantageous as it enables a
precise indication to be obtained of the instant when the voltage in the resonant
circuit reaches its minimum value.
[0023] The system shown in Fig. 1 will normally be powered from the mains supply and it
is found that the peak voltages developed during resonance are very high but 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.
[0024] 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.
[0025] Without an additional inductor L1, 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 application.
[0026] 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 resonsant 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 VT1 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.
[0027] The precise instant of time at which thyristor VT1 is turned-on is determined, as
explained above, by the control voltage detector 13 that is coupled to the pan coil
circuit via saturating transformer T1. Ideally, thyristor VT1 must be turned on again
at the instant when the voltage across capacitor CR is zero in order to reduce the
amplitude of the discharge current through thyristor VT1. However, under light load
conditions, i.e. low power input to the pan coil L2, the voltage across capacitor
CR does not return to zero. Thus, when the thyristor is next turned-on, it will be
forward biassed resulting in an uncontrolled, potentially destructive short duration
current flow through the thyristor. The minimum voltage is coincident with the reversal
of current through pan coil L2 as can be seen from the waveforms shown in Figs. 2A-2E.
[0028] Fig. 2A shows the waveform of the current through the thyristor VT1, Fig. 2B shows
that of the voltage across the thryistor VT1 and that across capacitor CR at power
levels of 100% and 75% and it will be observed that the voltage across capacitor CR
does reach zero.
[0029] Fig. 2C shows the waveform of the voltage across the thyristor and across capacitor
CR at power levels less than 50% of maximum and it will be observed that the voltage
across capacitor CR reaches a minimum value that is not zero.
[0030] Fig. 2D shows the waveform of the current through the pan coil L2 and it will be
noted that a reversal of the direction of flow of the current occurs 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
VT1 is turned on at the instant of minimum voltage across capacitor CR.
[0031] Fig. 2E shows the waveform 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".
[0032] In the embodiment shown in Fig. 1, the inductor L3 is added in series connction with
the pan coil L1 as shown. The effect of inductor L3 is to limit the rate of current
rise through the thyristor VT1 and the inductor will, therefore, reduce the possibility
of damage to the thyristor during turn-on. The inductor does not, however, reduce
thyristor dissipation and thus it is necesary to extend the power range over which
variation is effected by control of mark-space ratio.
[0033] Since the inductor L3 is in series connection with the pan coil L2, energy stored
in it does not have to be separately dissipated.
[0034] 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.
[0035] The cooking appliance 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 burners
and/or electric heating units.
[0036] The invention may also be embodied in a cooker which may be free-standing. One or
more of the top heating units may be of the form described above, other top heating
units may be gas burners and/or electric heating units.