EP2384083A1 - Switching assembly for an induction cooker, method for operating the switching assembly and induction cooker - Google Patents

Abstract

The arrangement has an oscillating circuit with an induction coil (1) for inductively heating an induction cooking tableware, where a switching frequency for recharging the oscillating circuit is constant. The oscillating circuit includes a controllable throttle (7) i.e. transductor-throttle, for varying an inductor and/or capacitors (2) of the oscillating circuit. The throttle is connected in series to the coil. Frequency change of the oscillating circuit is balanced due to change in the inductance of the coil by variation of the throttle. Independent claims are also included for the following: (1) a method for controlling a switching arrangement (2) an induction cooker comprising a switching arrangement.

Description

The invention is in the field of induction devices for the catering industry and, in particular, relates to an improved control of induction cooking appliances, which takes into account different inductances of cookware materials and is especially suitable for multi-hob appliances.

In cooking devices with multiple fields, the problem is that the magnetic fields of the individual, operated at different frequencies, induction coils of cooktops, influence or interfere with each other. This influence, in particular unwanted noise, can be prevented by operating all hobs at the same frequency. However, this in turn has the disadvantage that only one frequency is available. Depending on the inductance of a pan material used, the inductance resonant circuit no longer operates at a desired or optimum frequency (resonant frequency). In addition, you can not easily work with independent power level. The variation of the resonant frequency may cause, depending on the switching frequency, individual switches of the power stage to be switched on under voltage, and / or that a short circuit over the power stage occurs for a short time until a corresponding freewheeling diode blocks. This leads to circuit losses.

A solution to this problem is that an external protection circuit of the power stage switches is made (snubber circuit). Although such an external circuit can not prevent unsuitable operating points, at least higher inrush currents can be absorbed. However, this still leads to switching losses. Such a circuit is also quite complex and correspondingly expensive.

It is therefore an object of the invention to provide a circuit arrangement for an induction device, a method for driving this circuit, as well as an induction device with such a circuit arrangement, which overcomes the disadvantages of the known circuits for induction devices. It is a particular object of the invention to provide a circuit arrangement and a method for driving such a circuit arrangement, which enables safe operation and with less switching losses operating an induction device and preferably simultaneously solves the problem of interacting magnetic fields and oscillations of adjacent induction cooktops ,

The object is achieved by the circuit arrangement, the method for driving the circuit arrangement and the induction cooking appliance, as described in the independent claims.

The inventive circuit arrangement for an induction cooking appliance has a resonant circuit with at least one induction coil for inductive heating of an induction cookware and at least one capacity. In addition, the resonant circuit includes a variable circuit element for varying the inductance and / or the capacitance of the resonant circuit. The variable circuit element is preferably by at least one controllable throttle formed, wherein this at least one controllable throttle, for example, controlled by DC (transductor).

In order to operate the circuit in a safe area, a switching frequency which corresponds to a drive frequency of the power stage of the circuit arrangement should always be greater than a resonance frequency of the resonant circuit. If a switching frequency is less than or equal to the resonance frequency, resonance operation is practically uncontrollable. In addition, smaller frequencies cause unwanted audible noise.

In the method according to the invention for driving the circuit arrangement, starting from an output power of an induction cooking appliance, a reduction in the output power is achieved by reducing the resonant frequency of the oscillatory circuit by means of variation of the variable circuit element. Preferably, a further reduction of the output power is achieved by a reduction of a modulation level of a feeding alternating voltage.

By varying the variable circuit element, it is ensured that a switch of a bridge circuit for supplying the resonant circuit is in each case switched on without current. In particular, a transistor is turned on without power, preferably only when an associated freewheeling diode is energized. Thus, no corresponding protective circuit is required, and their switching losses omitted. Furthermore, short circuits can be prevented via the power supply.

The circuit arrangement is preferably constructed in such a way that when a current-conducting transistor of a bridge branch is switched off when driving the circuit, the current is then applied to a freewheeling diode of an opposite one Bridge branches commuted. A transistor of this opposite bridge branch is turned on, during which the freewheeling diode is still in the conducting state. A transistor is thus switched on only in de-energized state.

In contrast to, for example, external circuits, the described solution does not subsequently influence inappropriate states for a resonant circuit or a power stage, but completely or almost completely prevents them. Thus, the sometimes massive circuit losses can be prevented.

The variable circuit element is preferably connected in series with the at least one induction coil. If now a cookware with a certain inductance and capacity brought into the sphere of action of the induction coil of the cooking appliance, this will affect the resonant frequency of the resonant circuit. With an increase in the inductance, a resonant frequency of the resonant circuit is reduced. A variation of the resonant frequency is also possible by means of the variable circuit element, wherein with increasing deviation of the resonant frequency from the switching frequency of the power stage, a power output to the resonant circuit and the cooking appliance decreases. For a stepless variation, in particular reduction of the power output is possible, for example, by reducing the resonant frequency at a constant switching frequency. If, in addition, a modulation level (or duty cycle) of the power stage is reduced, then a device with very low power can be operated. Due to the reduced resonant frequency can be ensured that - compared with a control which varies only the duty cycle - at high duty cycles, a switch or transistor turns off power.

In a varied method for driving the circuit arrangement, influences of different inductances or capacities of cookware materials on the inductance and possibly also the capacitance of the resonant circuit and thus its resonant frequency are compensated by the variable circuit element. A caused by an induction cookware frequency change of the resonant circuit can be compensated by a variation of the controllable circuit element again. A control is preferably carried out automatically, in which is set to a predetermined, preferably constant frequency. Preferably, a variable circuit element in an optimal state, which corresponds for example to a very good induction cookware, not driven.

With the inventive circuit arrangement, it is possible to use a wide variety of induction cookware with a wide variety of inductances and capacitances. An induction cooker equipped with such an improved circuit also has the advantage that the cooking appliance and cookware do not have to be matched or even offered together. With a simple and favorable change, it is also possible to improve existing induction devices and induction devices with multiple induction coils.

The circuit arrangement according to the invention is preferably operated at a constant switching frequency of a feeding power stage. This offers the additional advantage that several induction coils can be arranged next to one another in a device with a multi-hob, as is common in gastronomy and commercial kitchens. Typically, these devices have a contiguous cooking plate, such as a ceramic plate, which includes several cooktops. Each cooktop is preferably associated with one or at least one induction coil, eg, 2.3 or 4 induction coils. By operating all induction coils with the same constant frequency, mutual negative disturbances excluded. Due to the variable circuit element, a control parameter is now introduced directly in the resonant circuit itself. Preferably, the plurality of feeding power stages of the plurality of induction coils are fed by a common DC power supply.

It is also possible to use a plurality of variable circuit elements which, for example, can cover a wider range of induction / capacitance.

• Fig.1 einen Schaltkreis für ein Induktionsgerät gemäss Stand der Technik;
• Fig. 2 einen Schaltkreis mit gesteuerter Drossel.
• Fig. 3a-c simulierte Spulenströme bei unterschiedlichen Betriebsarten der Schaltungsanordnung.

In the following the invention is explained in more detail with reference to schematic figures. Showing:

• Fig.1 a circuit for an induction device according to the prior art;
• Fig. 2 a circuit with controlled throttle.
• Fig. 3a-c simulated coil currents in different modes of the circuit.

In FIG. 1 a circuit arrangement is shown, as it can be found in conventional induction devices. The arrangement has an induction coil 1, capacitances 2 and a power stage 3 connected to a control 4. Switches of the power stage are electronic circuit breakers like bsw. Power transistors of various types. Schematically drawn is also a cookware 5, which on a hotplate 6, z. B. ceramic plate, provided interacts with the inductance of the induction coil. Depending on the ferromagnetic properties of the cookware, the influence on the induction coil different, but manifests itself in a change in the inductance and / or capacity of the resonant circuit and thus its frequency.

The frequency or switching frequency used in the power stage control circuit 3 is typically varied between 20-40kHz. With a frequency variation of the switching frequency can be achieved that the resonant circuit is operated in the vicinity of its resonance frequency and an optimum operating point for the resonant circuit and / or the power stage 3 is produced.

The less optimal or matched to the resonant circuit of the induction device is the material of a cookware, the more energy is applied for the unwanted heating of other circuit components, in particular for circuit losses in the power stage 3. This can lead to a malfunction of an induction device.

For 'intercepting' of high currents and corresponding circuit losses in the power stage exist external circuits (not shown) on the power level 3. As already briefly described above, these only prevent that flow too high currents, but circuit losses are still present and it is so no active intervention in the resonant circuit of the induction circuit itself possible.

In FIG. 2 an embodiment of the inventive circuit arrangement is shown. The basic arrangement corresponds to that FIG. 1 , wherein like elements are provided with the same reference numerals.

In series with the induction coil 1, a controllable throttle 7 is arranged, which is preferably driven by direct current. By controlling the throttle deliberately caused frequency changes, or even frequency changes due to an induction change in the induction coil, which is caused by a cookware, are balanced. The device is preferably set so that the default value of the operation of the device is set with a 'good', so ideal, good ferromagnetic cookware.

Two bridge branches 8.9 of a bridge circuit form the power stage 3. A bridge branch in each case includes a transistor T1, T2 and a freewheeling diode D1, D2 associated therewith. Not shown is a DC voltage source, typically a rectifier circuit, which feeds the bridge circuit.

Depending on the operation of the circuit, the throttle is not or driven. If now a, worse ', so less ferromagnetic cookware used, so the inductance of the induction coil will decrease. In accordance with this reduction, the throttle can be controlled, preferably automatically, and driven partially or completely into saturation until the total inductance of the oscillatory circuit has compensated again. In one example of a circuit, a choke has an inductance range of typically 0-200μH, e.g. 0-20μH, 0-50μH or 0-100μH.

A throttle has the advantage that it is a relatively cheap and less error-prone circuit element with limited space requirements. In addition, a very wide induction / capacitance range can be covered by one or more chokes.

The circuit arrangement shown can be operated with a variable frequency. Then, an optimization of the resonant circuit via the two control parameters frequency and variable switching element can be made. In a preferred embodiment, the circuit is operated at a constant frequency of, for example, 20kHz. Any frequency change in the resonant circuit caused by the operation of an induction device can then be compensated (to a degree) solely by control of the choke. Alternatively, a power output to the resonant circuit or the cooking appliance can be controlled by means of the throttle.

It can also be used multiple chokes, which are then preferably connected in series.

There are also other variable circuit elements conceivable with which directly the inductive and / or capacitive properties of the resonant circuit can be controlled. For example, instead of a DC-controlled throttle, a throttle with variably insertable core can be used. Since an influence of a cookware for the most part causes inductance changes, a controllable switching element which directly effects a compensation in the inductance is preferred. However, for example, there are also controlled capacities which can be introduced into the circuit in the form of stepless controllable capacitors or as connected capacitor stages.

In the FIGS. 3a to 3c are the voltage applied at the bridge center and thus at the resonant circuit voltage V1, V2, V3 and the coil current I1, I2, I3, according to the circuit according to FIG. 2 , represented as it is in different modes. The switching frequency is the same for all three operating modes. The arrows with the labels T1, T2, D1, D2 describe through which component the current flows at the appropriate time. The example values of the circuit and the operating mode used are: 300V AC voltage in different modulation (V1-V3), inductance 160μH or 180 μH, capacitance 470nF, resistance R = 3.75ohm.

In FIG. 3a a maximum power output is achieved in which the switching frequency is in the vicinity of the resonant frequency of the resonant circuit and a complete modulation (maximum duty cycle) is present. The resonance frequency is given by the formula f _R = 1 / 2II √ (LC). In this mode, the control throttle 7 is deactivated. The current curve I1 is on the whole symmetrical. While the upper transistor T1 is conducting, the current flows through the upper bridge branch 8. In the falling phase of the current (about 876μs), the upper transistor T1 is turned off. This is done with a certain safety margin to the zero crossing of the current, since the zero crossing is not known precisely due to the variability of the resonant circuit, and since later the lower transistor T2 is to be turned off. Because of the turning off of the upper transistor T1, the current commutates to the lower freewheeling diode D2. The switching point S1 at which the transistor T2 of the second, lower bridge branch 9 is turned on, is at 0ampere or shortly before, the transistor is thus turned off and de-energized. In principle, during the entire time in which the lower freewheeling diode D2 conducts, the transistor T2 can be switched on substantially without voltage and current. After the lower transistor T2 has taken over the current, the current can also flow in the opposite direction to the lower freewheeling diode D2 through the lower bridge branch 9. After a certain time, the current will swing back again, and the current will be commutated to the upper freewheeling diode D1 of the upper bridge branch 8 by switching off the lower transistor T2 and then, as long as D1 is conducting, the upper transistor T1 will be switched on.

FIG. 3b shows the coil current I2 at a medium power, but also full modulation. The choke is activated and the resonance frequency is thereby reduced and thus further away from the constant switching frequency of the power stage than at full power output as in FIG. 3a , This can be continuously reduced to a complete saturation of the throttle a power output. The control of the transistors T1, T2 is analogous to that at full power.

Now theoretically could be even further reduced by an even larger throttle the power output. However, if the resonant circuit is operated at a frequency which deviates too much from its resonant frequency, the circuit becomes inefficient. Thus, now, as in Figure 3c shown a modulation (duty cycle reduced, eg <50%) reduced. For example, the upper transistor T1 is turned off earlier. So that the average current remains zero, the lower transistor T2 must be turned off later. In order to prevent an undesired short circuit through the circuit, a level control may only be reduced so that a transistor is only turned on when current flows through the associated diode (T1, D1 or T2, D2). However, since a resonance frequency has already been reduced by the connection of the control throttle 7, this problem no longer applies to a wider range of modulation.

The switching point S3, in which the transistor of the second bridge branch is turned on, in contrast to the operation at full modulation time shifted forward, just before the zero crossing of the current. This avoids that current already flows in the 'wrong direction' and the transistor can not be switched on without power.

In a preferred embodiment of the method, a variable circuit element is thus used in series with an induction coil in combination with a variable duty cycle of a power stage, in particular an IGBT drive.

Claims (15)

Circuit arrangement for an induction cooking appliance comprising a resonant circuit with at least one induction coil (1) for inductive heating of an induction cookware (5) and at least one capacitor (2), characterized in that the resonant circuit is a variable circuit element for varying the inductance and / or the capacitance of the resonant circuit having. Circuit arrangement according to claim 1, wherein the variable circuit element is at least one controllable throttle (7). Circuit arrangement according to claim 2, wherein the at least one controllable throttle (7) is controlled by direct current. Circuit arrangement according to one of claims 1-3, wherein the variable circuit element is connected in series with the at least one induction coil (1). Circuit arrangement according to one of claims 1-4, wherein a switching frequency for feeding the resonant circuit is constant. Circuit arrangement according to one of claims 1-5, wherein a change in frequency of the resonant circuit due to a change in the inductance of the induction coil (1) is compensated by variation of the controllable circuit element. Method for driving a circuit arrangement according to one of claims 1-6, wherein starting from an output of an induction cooking appliance, a reduction of the output power by reducing the resonant frequency of the resonant circuit is achieved by means of variation of the variable circuit element. A method for driving a circuit arrangement according to claim 7, wherein a further reduction of the output power by reducing a modulation level of a feeding AC voltage (V1, V2, V3) is achieved. Method for driving a circuit arrangement according to claim 7 or 8, wherein switches of a bridge circuit for supplying the resonant circuit are each turned on without power. Method for driving a circuit arrangement according to claim 9, wherein a current-carrying transistor (T1, T2) of a bridge branch is switched off, the current commutated to a freewheeling diode (D1, D2) of an opposite bridge branch, and a transistor (T1, T2) of this opposite bridge branch turned on is during which the freewheeling diode (D1, D2) is still in the conductive state. Induction cooking appliance with a circuit arrangement according to one of claims 1-6. Induction cooking appliance according to claim 11 comprising a plurality of induction coils (1), wherein each induction coil is associated with a circuit arrangement according to one of claims 1-6. Induction cooking appliance according to claim 12, which is adapted to the
several induction coils (1) to feed with the same switching frequency. Induction cooking appliance according to one of claims 11-13, comprising a
contiguous cooking plate (6) comprising a plurality of cooktops, each cooktop is associated with at least one induction coil (1). Combination of an induction cooking appliance according to any of claims 11-14 and at least one induction cookware.

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