JPS592154B2 - induction heating device - Google Patents

Description

[Detailed description of the invention] Containers (pots, etc.) used for cooking etc. using high frequency signals.
A so-called induction heating device, which is a device designed to heat a furnace (such as a furnace), is already known.

As shown in Fig. 1, this device has an insulating plate 1 made of relatively mechanically strong ceramic or the like, and a work coil, which is generally called a work coil, is placed at a desired position on one surface, that is, the lower surface 1a. An induction coil 2 is provided.

The induction coil 2 is wound spirally around the center of the insulating plate 1, for example, and is attached to the lower surface 1a. This induction coil 2 is driven by a signal obtained by a drive system shown in FIG. In Figure 2,
4 represents a commercial AC source, 5 represents a power switch, and 6 represents a rectifier circuit.

The rectified output obtained by the rectifier circuit 6 is supplied as a drive signal to the above-mentioned induction coil 2 via the choke coil T and the resonance capacitor 8, respectively. A switching element, in this example a transistor Q, is connected to which a desired switching signal (its frequency is approximately 20 to 100 kHz) is supplied. 12 indicates an oscillator for obtaining this switching signal.

Note that the capacitor 13 is for smoothing, and the diode 14 is for damper. Therefore, with the container 3 to be heated such as a pot or kettle placed on the other surface of the insulating plate 1, that is, the upper surface Ib, the induction coil 2 is driven intermittently by a high frequency signal (20 to 100 kHz). When a signal is supplied, eddy current loss occurs in the container 3 due to the high frequency magnetic field generated by the coil 2, and this eddy current loss causes the container 3 to
Since it generates heat, only the container 3 is heated and can be used for cooking.

For the container 3, a material that is easily induction heated by high frequency heat is selected, and as is well known, this can include a magnetic material such as iron (in other words, a material with high electrical resistance).

By the way, in such an induction heating device, since it is necessary to drive the device with a relatively large amount of power, a large current of 20 to 30 A flows through the switching transistor Q during resonance.

Since such a large current cannot be handled by a single transistor Q, a plurality of transistors are generally connected in parallel. For example, as shown in the figure, two transistors Qa and Q
b is used to reduce the amount of current handled by the individual transistors Qa and Qb, thereby preventing the transistors from being destroyed. However, even if a plurality of transistors are connected in parallel for such a purpose, almost no resonance current flows through each transistor.

This is because there are variations in the characteristics of these transistors, and as a result of this characteristic mismatch, resonant current concentrates on the transistor where current flows easily, resulting in the disadvantage that this transistor is easily destroyed. In order to eliminate this drawback, transistors with large withstand currents may be used for each transistor, but this method inevitably becomes expensive and is not a good idea.

Another method that can solve the above drawback is to select and use the characteristics of the switching elements so that the characteristics of the switching elements match. Although this method can solve the above-mentioned drawbacks, since it is necessary to select the characteristics of the elements, it not only lacks mass productivity but also increases costs. In order to solve this drawback, the induction coil 2 is divided into two as shown in FIG.
It is conceivable to attach the coils 2A and 2B to a predetermined position on the insulating plate 1 as shown in the figure, and drive these coils 2A and 2B at the same time using separate drive systems.

In this way, if you divide the coil into two and drive each at the same time,
Since each resonance current is halved, for example, in the above example, a transistor that can withstand a resonance current of 15 A may be used. As a result, in addition to the feature that it is not affected much by variations in the transistors Qa and Qb, element selection becomes easy and it can be provided at a low cost. By the way, when the induction coil 2 is divided into two parts and each is driven, the divided induction coils 2A and 2B are arranged so as to include the bottom surface of the container 3 as shown in FIG. , the container 3 has induction coils 2A, 2B as shown in FIG.
It has the same effect as an iron core on.

Therefore, if a drive signal to turn on transistors Qa and Qb is now supplied, an interlinkage magnetic flux ω is generated, so that the induction coils 2A and 2B are magnetically coupled by the iron core and influence each other. Therefore, for example, the inductance in one of the induction coils 2A is greater than or equal to 1 (L is the total inductance of the coil 2), and a resonant current greater than a set value flows through the transistor Qa connected to this induction coil 2A. As a result, it becomes difficult to set the current value in transistor Qa, and both transistors Qa and Qb must have a large withstand current to ensure safety, making it impossible to solve the above-mentioned drawbacks. Therefore, in the present invention, the above-mentioned drawbacks are reliably avoided even when the induction coils are divided, by taking these points into consideration and driving the divided induction coils 2A and 2B alternately at different times without driving them simultaneously. It was designed so that it could be wiped away.

Therefore, the induction heating device according to the present invention is configured to be driven in synchronization with the power supply frequency.

The example shown in FIG. 5 is a case where the driving voltages are supplied to the induction coils 2A and 2B at different times. Second
Parts corresponding to those in the figure are designated with subscripts "A, B" and their explanations will be omitted. Since the induction coils 2A and 2B are driven by drive signals obtained from the oscillator 12, they are connected to resonance capacitors 8A and 8B and damper diodes 14A and 14, respectively.
It has B. In this example, since the induction coils 2A and 2B are driven separately, separate power supply terminals 21a and 2B are provided.
21b, and these power supply terminals 21a
, 21b and the AC source 4, a power supply circuit 20 is connected. This power supply circuit 20 consists of diodes 22a and 22b connected in series to a pair of power supply paths Ta and tb, respectively, and diodes 23a and 23b connected in parallel. The direction of conduction is selected so that

The latter diodes 23a and 23b are connected between the anodes of diodes 22a and 22b and the ground as shown in the figure, but one diode 23a is connected to the above-mentioned diode 22a.
The anode is used with its anode grounded so that a closed circuit can be formed. The polarity of the other diode 23b is also selected so that a closed circuit is formed with the remaining diode 22b. If the power supply circuit 20 is configured in this way, one power supply terminal 21a is supplied with the power supply voltage Ea (FIG. 6A) only during the positive half cycle of the power supply frequency.
) is supplied to the other terminal 21b, and the power supply voltage Eb (FIG. 6B) is supplied to the other terminal 21b only during the negative half cycle.

Therefore, when one induction coil 2A is in a driving state, the other induction coil 2B is in a resting state, so that the magnetic fluxes generated by the current flowing through the induction coil 2A are not interlinked. Therefore, this induction coil 2A is not influenced by the other coil 2B, and only the set resonance current flows through the transistor Qa. Exactly the same can be said about the other transistor Qb. Note that the capacitors 24a and 24b are for bypassing the drive signal. As explained above, in the present invention, the induction coil 2 is divided and these divided coils 2A and 2B are independently controlled in synchronization with the power supply frequency. Only the resonant current value determined by the inductance of 2B will flow.

Therefore, in addition to the feature that the amount of current handled by division can be reduced, only the set resonance current is applied to the transistors Qa and Qb.
Therefore, setting of transistors Qa and Qb is easy. Of course, since there is no risk of the transistors Qa and Qb being destroyed by the resonant current, it is possible to use elements with low withstand current, which has the great feature that this type of device can be constructed at a correspondingly low cost. It is also conceivable that the same effect as the present invention can be obtained by simply switching the transistors Qa and Qb alternately using positive-phase and negative-phase switching pulses obtained from the oscillator 12 without synchronizing with the power supply frequency. .

However, in this configuration, the switching frequency used is 2.
Since it is a high frequency of about 0 to 100 kHz, the switching period is short, and therefore, during the slight overlap period when transistors Qa and Qb transition from on to off and from off to on, the induction coils 2A and 2B mutually induce each other. Impact occurs. Therefore, the influence of mutual induction cannot be completely avoided. Therefore, the amount of current handled by the transistors Qa and Qb cannot be reduced compared to the case of the present invention, and it becomes difficult to select the characteristics of the transistors Qa and Qb.

According to this invention, the switching cycle is synchronized with a power supply frequency cycle that is much longer than the above-mentioned switching cycle, and the other current path is completely cut off during each positive and negative half-cycle period, so the above-mentioned drawbacks are definitely avoided. will be removed. FIG. 7 shows an example of the device of the present invention. In this example, instead of supplying the drive voltages Ea and Eb at different times, and instead of supplying the drive voltages at all times as before, the transistor Qa ，
The drive period of Qb is controlled in synchronization with the power supply frequency.

For example, a positive half cycle supplies a drive signal to only one transistor Qa, and a negative half cycle supplies a drive signal only to the other transistor Qb. In this way, the same effects as described above can be achieved. Note that 25 is a switching circuit of a switch 26. Although the above-mentioned embodiments have all been described with reference to the case in which the oscillation circuit 12 is shared, the respective induction coils 2A and 2B may be driven by driving signals obtained from separate oscillation circuits 12. In this case, for example, the oscillation frequencies of the oscillation circuits are made different, and one oscillation frequency is selected as the optimal frequency for heating the container 3 made of iron material, and the other oscillation frequency is selected as the optimum frequency for heating the container 3 made of stainless steel material, for example. The optimum frequency for heating the heated container 3 is selected and used.

In this way, whether a container made of iron or a container made of stainless steel is used, the high frequency energy applied to these containers is constant, so that a substantially constant heating state can always be obtained.

Therefore, there is no need to variably adjust the drive power depending on the difference in materials, which is extremely convenient for practical use.

[Brief explanation of drawings]

Fig. 1 is a block diagram showing →1 of the induction heating device, Fig. 2 is its electrical block diagram, and Fig. 3 is a block diagram similar to Fig. 1 showing an example of the induction heating device according to the present invention. Figure 4 is the equivalent circuit diagram, Figure 5 is the electrical connection diagram of the induction heating device, and Figure 6 is the electrical connection diagram of the induction heating device.
The figure is a waveform diagram for explaining the operation, and FIG. 7 is a waveform diagram showing another example of the present invention. 2, 2A, 2B are induction coils, Q, Qa, Qb are switching elements, 12 is an oscillator, 20 is a power supply circuit, 22a~
23b is a rectifying diode.

Claims (1)

[Claims] 1 A pair of induction coils are provided, and these are driven by separate switching elements, and the driving states of these induction coils are alternately controlled by a control circuit in synchronization with the commercial power frequency, and the drive signal of the control circuit is controlled alternately. One of the switching elements is controlled intermittently at a high frequency only during the positive cycle period of the commercial power source, and the other switching element is controlled intermittently at a high frequency frequency only during the negative cycle period of the commercial power source. Induction heating device.