JPS635876B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to an induction heating cooker, and in particular to an electromagnetic field leaking to the outside from an internal high-frequency power converter.
In particular, it relates to a structure that reduces leakage magnetic fields in the AM radio frequency band.

Conventional configuration and its problems Traditionally, induction heating cookers have been equipped with a heating coil that generates a magnetic field with high frequency and high magnetic flux density, and a high frequency power converter that supplies a high frequency large current to the heating coil. A large current with steep rises and drops flows through high-frequency circuit components such as resonant capacitors of high-frequency power converters, semiconductor switching elements, and filter capacitors for high-frequency power supplies. Therefore, the leakage magnetic field generated from the high-frequency power converter is particularly harmful to AM radio reception. In order to reduce this leakage magnetic field, conventionally, as shown in FIG.
Measures have been taken to cover the entire upper part of the internal chassis 4, which is made of a conductive material, on which the shielding material is disposed, with a shield cover 5 made of a conductive material such as aluminum. However, in such a structure, the high-density, high-frequency magnetic field of the heating coil 2 interlinks with the conductive shield cover 5, causing an induced current to flow in the shield cover 5, causing heat loss and reducing the heating efficiency of the cooker. Since the demagnetizing field generated by the induced current reduces the equivalent inductance of the heating coil, it becomes necessary to compensate by increasing the number of turns of the heating coil in the design of the high-frequency power converter. Further, since the shield cover 5 is a conductor, there are problems in that it requires a considerable amount of space in order to ensure electrical insulation from other electrical parts. All of these problems pose serious limitations when reducing the height (thinner profile) of the cooker. Furthermore, if a conductive plate like the shield cover 5 that covers the entire high-frequency power converter is installed inside the cooking device, the conductive plate will become hot due to the radiant heat received from the heating coil 2 and the bottom of the load pan, raising the temperature inside the device. This may adversely affect the cooling of semiconductor components and other electrical components, and may impair their reliability.

Furthermore, in FIG. 1, when the internal chassis 4 and the external casing 6 are made of resin, the magnetic field generated from the high frequency power converter 3 cannot be sufficiently reduced by the shield plate 5 covering the top surface thereof. Can not.

In other words, since the magnetic field leaks to the outside from the resin part, at least the internal chassis 4 or the external casing 6 must be a conductor and have a magnetic shielding effect. be. Making the case of resin is very advantageous for reducing the weight of the device and the product price, but the conventional electromagnetic shielding structure of surrounding the entire high frequency power converter with a conductive material is not completely effective. It was difficult to make the casing from resin, and the advantages of making the casing from resin could not be fully exploited. Therefore, recently, high-frequency power converters are made of resin without taking measures to reduce the radiated noise emitted from the equipment, which may cause considerable damage to external radio broadcast reception due to radiated noise in the radio frequency band. Ta.

Purpose of the Invention The purpose of the present invention is to eliminate the above-mentioned conventional drawbacks, and to reduce the electromagnetic field in the radio frequency band leaking to the outside from the high frequency power converter of an induction heating cooker with a simple configuration. Furthermore, the purpose of the present invention is to promote the use of resin, thinness, and miniaturization of the housing of induction heating cookers.

Structure of the Invention In order to achieve the above object, the induction heating cooker of the present invention includes a plurality of semiconductor switching elements and a damper diode connected in antiparallel to the semiconductor switching elements, and controls the conduction period of the semiconductor switching elements. In a high-frequency power conversion device that controls output power, a closed loop formed by high-frequency current components including a steep falling waveform that occurs when the semiconductor switching element is cut off and a sudden rising waveform of the charging current of a resonant capacitor accompanying the steep falling waveform. In particular, in the present invention, a plurality of semiconductor switching elements constituting a closed loop for the high frequency current component and a distribution line connected thereto are configured to selectively minimize the opening of the high frequency current component. By stipulating the above, it is intended to reduce the electromagnetic field to prevent damage in the radio frequency band. In conventional induction heating cookers, when the current capacity is insufficient and it is necessary to use the same semiconductor switching elements in parallel, the second
As shown in the figure, each semiconductor switching element 7
are arranged on separate metal bases 8, and the metal bases are used as one electrodes, which are connected and fixed to cooling fins 9.
For the other electrodes, the terminals 10 taken out from each packaging are connected to each other with a conductive metal rod 11 or the like.
Normally, the lead wires 12 and 13 are connected to components such as the resonant capacitor 14, but in the present invention, multiple sets of the same semiconductor junction are arranged on the same metal base, and the main current of the semiconductor junction is One electrode through which the current flows is connected to the metal base, and the other electrodes are connected to each other near the semiconductor connection and connected to other parts as a single electrode. When multiple semiconductor switching devices are connected in parallel, the opening area of the high-frequency current closed loop when the semiconductor switching devices are cut off can be made extremely small, and the magnetic field generated therefrom can be reduced. has. This effect is further enhanced by twisting or bundling the lead wires connected to the semiconductor switching element.

DESCRIPTION OF EMBODIMENTS Hereinafter, embodiments of the present invention will be described based on the drawings.

FIG. 3 is an example of a high frequency power conversion device for an induction heating cooker, and is a basic circuit diagram of a transistor inverter. In this Figure 3, the commercial low frequency power supply 15
is full-wave rectified by a full-wave rectifier 17 and passed through a filter coil 18 to a high-frequency power supply capacitor 19.
It is connected to the. A heating coil 21 is connected to the positive terminal of the high-frequency power capacitor 19, and a diode is connected in antiparallel between the collector and emitter terminals between the other terminal of the heating coil 21 and the negative terminal of the high-frequency power capacitor 19. connected
Two parallel circuits of NPN reverse conduction transistors 22,
A parallel circuit of resonant capacitors 23 is connected. Further, a drive signal is supplied to the base of the reverse conduction switching transistor 22 from the control circuit block 20, and the control circuit block 20 is supplied with control power from the power transformer 16. The operation of this inverter circuit will be explained in detail with reference to FIG. FIG. 4a shows the drive current I _B of the reverse conduction switching transistor 22. At time t ₁ , the forward current I _B1 starts flowing, and as shown in FIG. 4B, the diode current I _D flows through the reverse conduction switching transistor 22. flows, the switching transistor becomes conductive, and the heating coil 2
Collector current I _C increases with a slope determined by the combined impedance of 1 and the load pot. After a predetermined time T _ON (time t ₂ ), the supply of the drive current I _B1 to the reverse conduction transistor 22 is stopped, the reverse conduction transistor 22 begins to shut off, the collector current decreases rapidly, and after a falling time T _f (time t ₃ ), the circuit is completely shut off and the collector current I _C becomes zero. On the other hand, the reverse conduction transistor 22 begins to shut off, and at the same time the collector current I _C (substantially equal current flows through the heating coil 21 during the T _ON period) begins to decrease, a back electromotive force is generated in the heating coil 21, and the resonant capacitor 23 A charging current is applied to the
The rise time of the reverse conducting transistor 22
It corresponds to the falling time T _f of , which is very steep, and then the resonant current of the heating coil 21 and the resonant capacitor 23 flows through the resonant capacitor 23 as shown in FIG. 4d. As shown in FIG. 4c, the resonant voltage of the heating coil 21 and the resonant capacitor 23 is applied between the collector and emitter of the reverse conducting transistor 22 at the same time as the collector current begins to decrease. At the point where the trailing edge of the collector voltage crosses zero (time t ₄ ), the drive current I _B flows again to the base of the reverse conduction transistor 22.
Apply ₁ . After this, the operations described above are repeated. Note that the diode of the reverse conducting transistor 22 is a damper diode. Furthermore, as shown in Figure 4e, the current flowing through the heating coil 21 is the sum of the current flowing through the reverse conduction transistor 22 (b in the figure) and the current in the resonant capacitor 23 (d in the figure), and has a steep rise and rise. There is no falling waveform.

Embodiments of the present invention will be described using the inverter circuit described above as an example. 5 and 6 show an embodiment of the present invention, and in the inverter circuit diagram of FIG. 3, a block A surrounded by a dashed line consisting of a reverse conduction transistor 22, a resonant capacitor 23, and their wiring is shown. A block diagram is shown in FIG. 5, and a cross-sectional block diagram of two reverse conducting transistors 22 connected in parallel is shown in FIG. In Figure 6, reverse conduction NPN
Two identical silicon chips 22 constituting a transistor are arranged on one metal base 25,
The metal base 25 is connected to the collector so as to have the same potential. On the other hand, the two emitter electrodes are connected to each other near the silicon chip by conductive leads and connected to the emitter terminal 27, and similarly, the two base electrodes are also connected to each other near the silicon chip and connected to the base terminal 28. These are integrally molded with the resin case 26 and the injected resin 29 to form a single component. On the other hand, in FIG. 5, a reverse conduction NPN transistor having the structure shown in FIG. 6 is mounted on the cooling fin 24 with screws, and the lead wires 31 and 30 are connected to the emitter terminal 27 and the metal base 25 (collector terminal). The other terminal is connected to a connecting terminal 33 of a resonant capacitor 32. The resonant capacitor 32 and its lead wire near the connection terminal 33 are surrounded by a conductor 34 such as an aluminum plate.

The effects of the above configuration will be explained below. In the above inverter, as described above, a substantially continuous current as shown in FIG. 4e flows through the heating coil 21, and there is no sudden change in current. On the other hand, the current shown in FIG. 4b flows through the reverse conduction transistor 22, and the current _shown in _FIG . Change occurs. In order to analyze this waveform, a reverse conducting transistor 22 and a resonant capacitor 2 are used.
A closed circuit consisting of 3 is taken out and shown in Fig. 7. FIG. 8 shows the waveform of the current flowing through each element.
The direction of the current is defined as the direction shown by the arrow in FIG. 7 as positive. The steep current change from time t ₂ to t ₃ described above can be equivalently considered as the high-frequency pulse current shown in FIG. 8c flowing in the closed circuit in FIG. 7 in the direction shown by the arrow. The time interval between times t ₂ and t ₃ corresponds to the falling time of the collector current of the switching transistor, and when the output power of the inverter is about 1.2KW,
The peak current of the collector current is about 50A,
The interval between times t ₂ and t ₃ is approximately 1 μsec. Therefore, the frequency spectrum of the high-frequency current component when this transistor is cut off is predominantly from 500 KHz to 2 MHz. On the other hand, the strength of the magnetic field due to the high frequency current component when the transistor is cut off increases or decreases in proportion to the area (shaded area) surrounded by the closed loop of the high frequency current component when the transistor is cut off as shown in FIG. Therefore, in this embodiment, as shown in FIG. 5, the terminal 33 of the resonant capacitor 32 is connected from the collector and emitter of the reverse conducting transistor 22 in order to minimize the area created by the closed circuit through which the high frequency current component flows when the transistor is cut off. The wirings 30 and 31 are twisted together, and the element of the resonant capacitor 32 and the connection terminals 30, 3 are connected.
A shield plate 34 made of a conductive material surrounds the opening of the high-frequency current closed loop generated in the wiring near 1, and further includes a conductive metal rod 11 connecting the emitters of the reverse conduction transistors, a reverse conduction transistor, and a heat dissipation fin. As shown in FIGS. 5 and 6, the closed loop consisting of 24 is made extremely small by arranging semiconductor chips on the same metal base. It shows the difference in the opening area of the closed circuit formed by the high-frequency current component when the transistor is cut off. FIG. 9a shows a conventional example, and FIG. 9b shows an example of the configuration of this embodiment. The high frequency reduction effect according to this embodiment can reduce radiation noise by about 10 to 15 dB at a frequency of 500 KHz to 2 MHz, for example, when measured at a location about 3 m away from the induction heating cooker.

As described above, this embodiment has the effect that the radiation noise in the radio frequency band generated from the induction heating cooker can be reduced with a simple configuration and with almost no effect on the arrangement of other parts or the cooling configuration. . In other words, in this embodiment, the radiation noise in the radio frequency band generated from around a transistor having a plurality of switching elements can be reduced without using a shielding plate or the like to surround and shield the switching elements. It does not adversely affect the cooling of transistors where current flows and high voltage is applied and high losses occur, and electrical insulation between the heat dissipation fins to which high voltage is applied and the shield plate, and between the shield plate and other high voltage live parts. There is no need to consider insulation, and this greatly contributes to making the induction heating cooker thinner and smaller.

Other configuration examples of a plurality of semiconductor switching elements are shown in FIGS. 10a and 10b. Figure 10b shows two semiconductor junctions having the same junction arranged close together on the same metal base, while Figure 10a
constitutes a transistor junction having a plurality of identical junctions on the same semiconductor chip, and a damper diode 3
7 are made of separate chips and placed close to each other on the same metal base, and FIG. 10b shows an example in which a plurality of switching transistors 36 and diode junctions are formed on the same semiconductor chip 35. In the above embodiment, the damper diode is placed near the switching diode junction because the damper diode is also loaded when a large and steep current change occurs at time _t1 in FIG. This is because it comes out due to This is because the diode current increases especially when near no load.

Effects of the Invention As described above, according to the present invention, in a high frequency power converter having a plurality of semiconductor switching junctions and a steep breaking current waveform, the semiconductor switching element is Alternatively, radiation noise in the radio frequency band can be reduced by devising the wiring without covering the entire high frequency power converter, resulting in the following effects.

(1) The cooling conditions inside the induction heating cooker, including the cooling conditions of the switching element, are hardly deteriorated.

(2) Since there is no need for a conductive shielding plate around at least the switching element or damper diode to which high voltage is applied, there is no need to consider the insulation distance between the shielding plate and the components, and other component placement is easy. The parts arrangement of the entire induction heating cooker can be made extremely compact, and the device can be made smaller and thinner.

(3) In the present invention, by selectively electromagnetically shielding components and wiring other than the switching transistor, such as a resonant capacitor, among the components and wiring that constitute the closed circuit through which the high-frequency current component flows when the switching transistor is cut off. The radiation noise reduction effect is even higher, and almost the same noise reduction effect can be obtained whether the housing is made of resin or metal. Therefore, the casing can be easily made of resin, which has the effect of reducing the weight of the induction heating cooker and improving its ease of assembly.

[Brief explanation of the drawing]

Figure 1 is a cross-sectional view of a conventional electromagnetic induction cooking device.
Fig. 2 is a perspective view of a main part showing the relationship between a resonant capacitor, a reverse conduction transistor, and its distribution line in a conventional high-frequency power converter, and Fig. 3 is an illustration of an electromagnetic induction cooking device showing an embodiment of the present invention. A circuit diagram of the high frequency power converter, Figure 4 is a signal waveform diagram of the same main part, Figure 5
The figure is a perspective view of the main parts showing the relationship between the resonant capacitor, the semiconductor switching element, and its distribution line in the same high-frequency power converter, FIG. 6 is a cross-sectional view of the semiconductor switching element of the same high-frequency power converter, and FIG. An electric circuit diagram of a closed loop consisting of the semiconductor switching element, a resonant capacitor, and its distribution line, FIG. 8 is a signal waveform diagram of the main part of the electric circuit, and FIG. 9a is a front view of the main part showing the closed loop in the conventional example.
FIG. 9b is a front view of essential parts showing a closed loop in this embodiment, and FIGS. 10a and 10b are sectional views of semiconductor switching elements in other embodiments of the present invention, respectively. 19... High frequency power supply capacitor, 21... Heating coil, 22... Reverse conducting transistor, 23... Resonant capacitor, 25... Metal base, 30, 31... Lead wire, 32... Resonant capacitor, 34... Conductor.

Claims (1)

[Claims] 1. A semiconductor switching element in which a heating coil, a resonance capacitor, a high frequency power supply filter capacitor, and a damper diode are connected in antiparallel, and output power by controlling the conduction time of the semiconductor switching element. The semiconductor switching element has a plurality of sets of the same semiconductor junction, which are arranged on the same metal base, and one of the semiconductor junctions through which the main current flows. An induction heating cooker in which the electrodes are connected to the metal base, and the other electrodes are connected to each other near the semiconductor junction to form a single electrode connected to other parts. 2. The semiconductor junction of the damper diode is arranged on a metal base and connected in its vicinity to the semiconductor junction of the semiconductor switching element.
Induction heating cooker as described in section. 3. The induction heating cooker according to claim 1, wherein the distribution lines connected to the semiconductor switching elements and through which the main current flows are twisted or bundled and placed close to each other. 4. The induction heating cooker according to claim 1, wherein the resonance capacitor or the power supply capacitor is surrounded by a conductive metal plate together with a part of the power distribution line to which it is connected.