JPH0642394B2 - Induction heater - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an induction heater that operates a switching element by an induced voltage of an auxiliary coil provided near a heating coil to oscillate the heating coil.

2. Description of the Related Art In recent years, the number of components has been reduced and high-density mounting has been performed in response to demands for thinning, miniaturization, and cost reduction of induction heaters.

In such a conventional induction heater, as shown in FIG. 3, a low frequency commercial power source 1 is full-wave rectified through a switch 2 and a rectifier 3 and smoothed by a capacitor 4 to obtain a DC power source. This DC power supply was supplied to an inverter circuit including a choke coil 5, resonance capacitors 6 and 7, a heating coil 8, a transistor 9 and a diode 10.

Then, the current flowing through the heating coil 8 causes an electromotive force to be generated in the auxiliary coil 12 arranged close to the heating coil 8 by the electromagnetic coupling effect, and this electromotive force is used as the base driving source of the transistor 9. The control circuit 11 controls the on / off duty ratio of the transistor 9 and controls the heating coil 8
The output is adjusted by increasing or decreasing the current of. Also the fourth
As shown in the figure, a non-magnetic plate 15 made of aluminum or the like is provided below the heating coil 8 to block the lines of magnetic force generated from the heating coil 8, and when the induction heater is placed on a metal pedestal, the pedestal I was careful not to heat.

Then the base-emitter voltage of the transistor 9 _{(V CE),} the current flowing through the heating coil 8 _(I L), indicating the base current each waveform _{(I B)} of the transistor 9 in FIG. 5. The base driving time (t _O ) of the transistor 9 in FIG. 5 is determined by the time for opening the ab between the two terminals of the control circuit 11.

As described above, since the current generated in the auxiliary coil 12 is used as it is as the driving source of the transistor 9, it is very efficient and effective in reducing the number of parts.

Problems to be Solved by the Invention However, as high-density mounting progresses, the auxiliary coil 12
When the switch 2 is turned on, the charging current of the capacitor 4 is instantaneously applied to the auxiliary coil 12 which should originally be driven by the magnetic field generated by the heating coil 8. A voltage is excited in the auxiliary coil 12 by the generated magnetic field to drive the transistor 9, leading to malfunction of the transistor 9.

That is, as shown in FIG. 6 (a), the auxiliary coil 12 is caused to move to the auxiliary coil 12 by the transient voltage when the power of the capacitor 4 is turned on.
Is excited, and this voltage causes the transistor 9
Malfunction.

In view of the above problems, it is an object of the present invention to provide an induction heater in which the generation of unnecessary induced voltage that causes a malfunction of switching operation in the auxiliary coil is prevented even when high density mounting is performed.

Means for Solving the Problems In order to achieve the above object, the induction heating cooker of the present invention has a shield plate made of a non-magnetic material that blocks magnetic lines of force arranged on the projection surface of the auxiliary coil in a range including the projection area. In addition, an electric path for passing an electric current is arranged at a position removed from the projection plane, or is arranged between the electric path and the auxiliary coil.

Operation Even if there is a rush current in the electric path and a magnetic field is generated by the above configuration, the magnetic field lines of the magnetic field are blocked by the non-magnetic plate before intersecting with the auxiliary coil, and no induced voltage is generated in the auxiliary coil. As a result, malfunction of the switching element can be prevented.

Embodiment An embodiment of the present invention is shown in FIG. 1 and will be described. Fig. 1
In (A), electric components such as the capacitor 4 are arranged on the printed board 16 and electrically connected by the pattern foil 17 of the printed board. A non-magnetic plate 15 having a magnetic permeability that blocks magnetic lines of force, such as an aluminum plate or a copper plate, is attached to the auxiliary coil 12
If it is provided on the projection plane of and parallel to the auxiliary coil surface, the magnetic flux of the magnetic force lines generated when a current flows through the pattern foil 17 cannot cross the non-magnetic plate 15, and therefore the broken line portion in the figure. Since the magnetic flux crosses the auxiliary coil 12 at a small rate, the influence of the current of the capacitor 4 and the like disappears, and only the magnetic flux of the current flowing through the heating coil 8 crosses the auxiliary coil 12.

Next, another embodiment of the present invention will be described with reference to FIG.
In FIG. 1 (B), the heating coil 8, the auxiliary coil 12,
The electric wires 18 arranged in the order of the nonmagnetic flat plate 15 and connected to the capacitor 4 and the like are arranged below the nonmagnetic flat plate 15. Therefore, even if a current flows through the electric wire 18, the magnetic flux is blocked by the non-magnetic flat plate 15, so that no induced voltage is generated in the auxiliary coil 12. Although FIG. 2 shows the case where the non-magnetic flat plate 15 is not kept in parallel with the auxiliary coil 12, the magnetic flux due to the current flowing through the pattern foil 17 is blocked even in this case. it is obvious.

EFFECTS OF THE INVENTION The present invention provides an auxiliary coil installed in the vicinity of a heating coil, utilizes the electromagnetic coupling effect between the heating coil and the auxiliary coil, and generates a current similar to the heating coil current in the auxiliary coil. In a direct switching element drive source, without being affected by the magnetic field generated by other than the heating coil such as the charging current to the capacitor when the power is turned on,
It is possible to stably drive the switching element in high-density mounting. Further, it is only necessary to dispose one aluminum plate at an appropriate position, which can be realized with a cheaper and simpler structure than the conventional one.

[Brief description of drawings]

FIG. 1 is a diagram showing a positional relationship between a heating coil, an auxiliary coil and a non-magnetic plate in one embodiment of the present invention, and FIG. 2 is a positional relationship between a heating coil, an auxiliary coil and a non-magnetic plate in another embodiment. Fig. 3 is a diagram showing the entire configuration of an inverter circuit for directly driving a transistor by electromagnetically coupling a heating coil current with an auxiliary coil, and Fig. 4 is a positional relationship between a conventional heating coil, an auxiliary coil and a non-magnetic plate. FIG. 5 is a diagram showing voltage and current waveforms of a transistor and a heating coil in a conventional induction heater, and FIG. 6 is a diagram showing voltage waveforms generated in the same capacitor and auxiliary coils. is there. 4 ... condenser, 8 ... heating coil, 12 ... auxiliary coil, 15 ... non-magnetic plate.

Claims (1)

[Claims] 1. A heating coil which is oscillated by a switching element, an auxiliary coil which is provided concentrically and concentrically with the heating coil and which supplies a drive current for the switching element, and a shield plate made of a non-magnetic material for blocking magnetic flux. And the shielding plate is located in a range including the projected area on the projection surface of the auxiliary coil when viewed from the heating coil, and the electric path of the current flowing other than the heating coil is displaced from the shielding plate. Or an induction heater having the shielding plate arranged between the auxiliary coil and the electric path.