JP2007073341A - Induction heating cooker - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an induction heating cooker having a miniaturized resonance circuit in which heating is performed irrespective of a load type, and generation of heat is reduced. <P>SOLUTION: The induction heating cooker has the resonance circuit consisting of a heating coil and resonant capacitors, an inverter circuit comprising switching devices, and a load detection circuit which detects the load type. The resonance circuit is excited by the inverter circuit, and circuit connection of the resonance circuit is changed according to output of the load detection circuit. Plural capacitors that are connected in parallel mutually via relays respectively are prepared in the resonance circuit, and change action of the relays is carried out according to the output of the load detection circuit while stopping on/off action of the switching devices. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an induction heating cooker used in general households.

The induction heating cooker turns on and off the switching element and resonates the heating coil and the resonance capacitor, thereby generating a high-frequency current in the heating coil and heating the load.
However, since the inductance of the heating coil differs depending on the type of load, the heating coil and the resonant capacitor resonate differently, and depending on the type of load, a large amount of heat may be generated in the heating coil, resonant capacitor, switching element, etc. .
In such a case, heating to the load is limited, and heat generation from the heating coil, the resonance capacitor, the switching element, and the like is limited.
For this reason, the heating coil, the resonance capacitor, or both are switched according to the load so that heat is not generated in the heating coil, the resonance capacitor, the switching element, and the like.
For example, in Patent Document 1, in the case of a load (magnetic material) where the inductance of the heating coil is large, the number of turns of the heating coil is reduced, the inductance of the heating coil is reduced, and the load (nonmagnetic material) where the inductance of the heating coil is small Increases the number of turns of the heating coil and increases the inductance of the heating coil so that the inductance of the heating coil is the same for both magnetic and non-magnetic materials.

JP 2000-48945

In order to perform heating not depending on the load as described above, it is necessary to switch the resonance circuit according to the load, and a resonance circuit switching switch is necessary to switch the resonance circuit. A relay is mainly used as the resonance circuit changeover switch. However, since a large current flows through the resonance circuit, the resonance circuit changeover relay requires a large relay and generates a large amount of heat. In general, a relay used in a high-current power circuit has a larger volume as the effective value of current increases. For example, a volume that is twice or more is required to pass a current effective value that is twice as large.
In view of the above points, an object of the present invention is to reduce the size of a resonance circuit and reduce heat generation in an induction heating cooker regardless of the type of load.

  The induction heating cooker according to the present invention excites a resonance circuit formed by a heating coil and a resonance capacitor by an inverter circuit composed of switching elements, and the resonance circuit according to the output of a load detection circuit that detects the type of load. In the induction heating cooker for switching the circuit connection, the resonance circuit is provided with a plurality of resonance capacitors connected in parallel to each other via a relay, and the output of the load detection circuit is turned on and off with the switching element stopped. In response to this, the switching operation of the relay is performed.

    According to the induction heating cooker of the present invention, the resonance circuit is provided with a plurality of resonance capacitors connected in parallel to each other via relays, and the switching element is turned on and off according to the output of the load detection circuit. Since the switching operation of the relay is performed, it is possible to prevent a current from flowing locally through one relay, and a plurality of small relays can be used in place of the conventional large relay, and the resonance circuit can be downsized. At the same time, the heat generation of the relay can be reduced.

Embodiment 1 FIG.
FIG. 1 is a circuit diagram showing a first embodiment of the present invention, which is a half-bridge type that excites a resonance circuit 20 by a rectifier circuit 11 and a smoothing capacitor 12 connected to a commercial AC power source 10 and switching elements 31 and 31. An inverter circuit 30 is provided.
The resonance circuit 20 is formed by heating coils 21a and 21b, resonance capacitors 22a and 22b, and relays 23a and 23b connected in series to the resonance capacitors 22a and 22b, respectively. The resonance capacitors 22a and 22b are connected via the relays 23a and 23b. Are connected in parallel to each other to form a series resonance circuit in cooperation with the heating coils 21a and 21b.
Further, a current transformer 18 is provided at the outlet end of the resonance circuit 20, and the output of the current transformer 40 is supplied to the load detection circuit 50. The load detection circuit 50 detects the magnitude of the high-frequency current flowing through the resonance circuit 20 based on the output of the current transformer 40, thereby judging the impedance of the heating coil, and judging the material of the load 100 such as a pan based on the magnitude. The load determination result of the load detection circuit 50 is input to the control circuit 60. The control circuit 60 controls ON / OFF of the relays 23a and 23b and controls the inverter drive circuit 70.

In the above configuration, when the contacts of the relays 23a and 23b are switched to the A side, the series connection body of the relay 23a and the resonance capacitor 22a and the series connection body of the relay 23b and the resonance capacitor 22b are connected in parallel. A serial connection of the heating coil 21a is formed to form a series resonance circuit.
In addition, when the contacts of the relays 23a and 23b are switched to the B side, the relay 23a and the series connection body of the resonance capacitor 22a, the relay body 23b and the series connection body of the resonance capacitor 22b are connected in parallel, and both heating A series connection of the coils 21a and 21b is formed to form a series resonance circuit.
The inductance of the resonance circuit 20 is small when the contact of the relays 23a and 23b is switched to the A side, and is large when the contact is switched to the B side.

By the way, in such a configuration, when the relays 23a and 23b are connected in parallel, the current may not be divided evenly due to variations in the contact resistance, and the current may concentrate on the relay with the smaller contact resistance. As shown in FIG. 2, the currents I ₁ and I ₂ flowing in the relays 23a and 23b connected in parallel are divided by the reciprocal ratio of the contact resistances R ₁ and R ₂ .
For this reason, in the first embodiment, as shown in FIG. 3, relays 23a and 23b and capacitors 22a and 22b are connected in series, and are connected in parallel so as not to cause current concentration due to contact resistance variation. I have to.
In the series connection of the relays 23a and 23b connected in parallel and the capacitors 22a and 22b, the sum of the voltages V _R1 and V _R2 generated at the contact resistances of the relays 23a and 23b and the voltages V _C1 and V _C2 generated at the capacitors 23a and 23b. Are equal,
V = V _R1 + V _C1 = V _R2 + V _C2
The relationship holds.
The voltages V _C1 and V _C2 generated at both ends of each capacitor increase faster as the current flowing into the capacitor increases, and decrease faster as the current flowing out of the capacitor increases. For example, when a large current flows through the relay having the smaller contact resistance and the voltage of the capacitor connected in series with the relay increases, the voltages V _R1 and V _R2 generated at the contact resistance of the relay decrease, and the current flowing through the relay Is suppressed.
For this reason, even if the contact resistances of the relays 23a and 23b are different, the currents flowing through the relays are balanced by the capacitors 22a and 22b connected in series with the relays and flow through the relays as in the above example. The effective current values are equal.

In addition, the relays 23a and 23b also vary in contact switching time. If the contacts are switched when current is flowing through the relays 23a and 23b connected in parallel, the current that has been distributed and distributed to the relays is concentrated on the relay that has delayed switching of the contacts.
The induction heating cooker generates a high-frequency large current in the resonance circuit 20 by turning on and off the switching elements 31 and 31. However, if the switching elements 31 and 31 are not turned on and off, current flows in the resonance circuit 20. Absent. Therefore, the switching operation of the contacts of the relays 23a and 23b is performed in a state where the on / off of the switching element is stopped.
That is, load determination is performed based on the detection result of the load detection circuit 50 before heating the load 100, and the resonance circuit 20 is switched according to the load. When the resonance circuit 20 is switched, the contacts of the relays 23a and 23b are switched. Is switched within several tens of milliseconds, so that switching elements 31 and 31 are turned on and off after a time of several tens of milliseconds has elapsed since the start of switching of the resonant circuit, and a current is passed through the resonant circuit 21 to the load 100. Start heating. When switching the resonance circuit 20 during heating to the load 100, the switching elements 31, 31 are once turned off and on, the relays 23a, 23b are switched in this state, and several tens of ms or more have elapsed since the resonance circuit switching start. The switching elements 31 and 31 are turned on and off after the contacts of the relays 23a and 23b are completely switched.

As described above, according to the heating cooker of the first embodiment, the resonance capacitor that forms the series resonance circuit together with the heating coil is divided into a plurality of parts, and they are connected in series with the relay, so that the contact resistance of the relay can be reduced. It acts as a capacitor to suppress current concentration due to variation to prevent local current from flowing through one relay, which makes it possible to use multiple small relays instead of conventional large relays. The resonance circuit can be reduced in size, and the heat generation of the relay can be reduced.
Further, since the current flowing through the resonance capacitor can be divided by dividing the resonance capacitor, the heat generation of the resonance capacitor is reduced similarly to the relay, and the heat generation source is dispersed, so that no local heat generation occurs.

Embodiment 2. FIG.
4 and 5 are main part circuit diagrams showing an example in which different resonant circuits are used and the capacitance of the resonant capacitor is switched according to the load as the second embodiment.
FIG. 4 shows a series resonance circuit in which resonance capacitors 22c, 22d, and 22e are connected in series to the heating coil 21c. When the contacts of the relays 23d and 23e are opened, the resonance circuit 20 resonates with the heating coil 21c. When the capacitor 22c is connected in series and the contacts of the relays 23d and 23e are closed, the resonance circuit 20 has the resonance capacitor 22c, the resonance capacitor 22d to which the relay 23d is connected in series, and the relay 23e is connected in series. The parallel connection with the resonance capacitor 22e and the heating coil 21c are connected in series.
FIG. 5 shows a series resonance circuit in which resonance capacitors 22c, 22d, and 22e are connected in parallel to the heating coil 21d. When the contacts of the relays 23d and 23e are opened, the contacts of the relays 23d and 23e are connected. When open, the resonance circuit 20 is connected in parallel with the heating coil 21d and the resonance capacitor 22c. When the contacts of the relays 23d and 23e are closed, the resonance circuit 20 includes the resonance capacitor 22d and the relay 23d in series. The parallel connection of the connected resonance capacitor 22d, the resonance capacitor 22e connected in series with the relay 23e, and the heating coil 21d.
The capacity of the resonance circuit 20 is small when the contacts of the relays 23d and 23e are open, and is large when the contacts are connected.
Even if such a resonance circuit is used, the same effects as those of the first embodiment can be obtained.

Embodiment 3 FIG.
FIG. 6 is a main part circuit diagram showing an example in the case of switching the capacitance of a resonance capacitor according to a load using a further different resonance circuit as the third embodiment.
In FIG. 6, when the contacts of the relays 23f and 23g are switched to the C side, the resonance capacitor 22f to which the relay 23f is connected in series, the parallel connection of the resonance capacitor 22g to which the relay 23g is connected in series, and heating A series connection with the coil 21e is a resonance circuit.
When the contact between the relay 23f and the relay 23g is switched to the D side, the resonance capacitor 22h to which the relay 23f is connected in series, the parallel connection of the resonance capacitor 22i to which the relay 23g is connected in series, and both the heating coils 21e. , 21f is a resonance circuit.
Even if such a resonance circuit is used, the same effects as those of the first embodiment can be obtained.

Embodiment 4 FIG.
7 and 8 are diagrams for explaining the fourth embodiment.
As shown in FIG. 7, when the capacitors 22a and 22b connected in series with the relays 23a and 23b have different capacities, the effective values of the currents I ₁ and I ₂ flowing through the relays 23a and 23b are connected in series with the relays. It becomes the ratio of the capacitances C ₁ and C ₂ of the capacitors 22a and 22b. Therefore, a combination of a plurality of relays can be freely selected by changing the capacity of the resonance capacitor. Accordingly, the size and heat generation of the plurality of relays and the plurality of resonance capacitors can be selected according to the configuration inside the casing and the air path configuration, thereby increasing the degree of freedom in mounting.
That is, as shown in FIG. 8, if the wind for cooling the circuit is flowing in the direction of the arrow, when the same size relay is used (FIG. 8b), if the wind hits the leeward relay 23b, Although cooling cannot be performed efficiently, the windward relay 23a is a small relay and the leeward relay 22b is large, so that wind can be applied to both relays, and cooling can be performed efficiently ( FIG. 8a).

In the first embodiment, the case where the half bridge circuit is used as the inverter circuit for exciting the resonance circuit is shown. However, the full bridge circuit shown in FIG. 9 or the voltage resonance circuit shown in FIG. 10 is used instead. Can do.
In the half bridge circuit and the full bridge circuit, the series connection of the heating coil and the resonance capacitor is the resonance circuit 20, and in the voltage resonance circuit, the parallel connection of the heating coil and the resonance capacitor is the resonance circuit 20. That is, the resonance circuit is a series connection or a parallel connection of the heating coil and the resonance capacitor.
Further, in each of the above embodiments, the contact points of the plurality of relays that switch the circuit connection of the resonance circuit are configured to have the same operation. Therefore, the contact points are configured by a plurality of contacts and one operation coil, and the plurality of contacts perform the same operation. If a relay is used, the circuit can be further reduced in size.

It is a circuit block diagram which shows the induction heating cooking appliance by Embodiment 1 of this invention. FIG. 5 is a diagram for explaining the operation of the first embodiment. FIG. 5 is a diagram for explaining the operation of the first embodiment. It is a principal part circuit diagram in Embodiment 2 of this invention. FIG. 6 is a main part circuit configuration diagram in the second embodiment. It is a principal part circuit diagram in Embodiment 3 of this invention. It is a figure explaining Embodiment 4 of this invention. FIG. 10 is a diagram illustrating Embodiment 4; It is a principal part circuit diagram which shows the other example of the inverter circuit used for this invention. It is a principal part circuit diagram which shows the example of the voltage resonance circuit used for this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Commercial AC power supply 11 Rectification circuit 12 Smoothing capacitor 20 Resonance circuit 21a, 21b, 21c, 21d, 21e, 21f Heating coil 22a, 22b, 22c, 22d, 22e, 22f, 22g, 22h, 22i Resonance capacitor 23a, 23b, 23c , 23d, 23e, 23f, 23g Relay 30 Inverter circuit 31 Switching element 40 Current transformer 50 Load detection circuit 60 Control circuit 70 Inverter drive circuit

Claims (3)

An induction heating cooker that excites a resonance circuit formed by a heating coil and a resonance capacitor by an inverter circuit composed of a switching element and switches the circuit connection of the resonance circuit according to the output of a load detection circuit that detects the type of load. In
The resonant circuit is provided with a plurality of resonant capacitors connected in parallel to each other via relays,
An induction heating cooker, wherein the switching operation of the relay is performed in accordance with the output of the load detection circuit in a state where the on / off of the switching element is stopped.   2. The induction heating cooker according to claim 1, wherein the plurality of resonant capacitors have different capacities.   The induction heating cooker according to claim 1 or 2, wherein the plurality of relays are operated by the same operation coil.

Images