JP3073099B2 - Induction heating device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an induction heating apparatus for heating a work or the like by, for example, high-frequency heating.

[0002]

2. Description of the Related Art Conventionally, an induction heating apparatus used for high-frequency heating, for example, when brazing a carbide tip to a circular saw base, is disclosed in Japanese Patent Publication No. 63-50830. This induction heating device comprises a field effect transistor (F
ET) is connected to the push-pull, and the capacitance of the parallel tank capacitor of the tank circuit connected to the transistor is set to be larger than the capacitance of the series resonance capacitor so as to obtain high frequency high power. ing.

[0003]

However, in this induction heating device, the cost of the heating device is increased,
There is a problem that the device itself becomes large. That is, to obtain high-frequency high power, it is necessary to increase the capacitance of a parallel tank capacitor or the like, and a plurality of commercially available capacitors must be connected in parallel. Therefore, a large number of expensive capacitors with high withstand voltage are used, and the cost of the heating device itself is increased due to an increase in parts cost. In addition, a space for accommodating a plurality of capacitors is required, and the size of the device is increased. Furthermore, in the above-mentioned heating device, there were problems that the rising and falling characteristics of the signal waveform for switching operation of the field-effect transistor were inferior, stable output characteristics were hardly obtained, and the FET was easily damaged. . SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems, and to provide an induction heating device capable of significantly reducing the cost and size of the heating device itself and obtaining stable output characteristics.

[0004]

In order to achieve the above object, according to the first aspect of the present invention, at least a pair of switching elements, a main coil, and a main coil and a substantially intermediate portion of the main coil are connected in parallel. And a coil having both ends connected to the output side of the switching element, a primary coil connected to both ends of the main coil, and a heating coil connected to the secondary coil. An output transformer, a first capacitor forming a parallel resonance circuit with the primary coil of the output transformer, and a second capacitor grounding an intermediate portion of the primary coil of the output transformer. And the invention of claim 2 is
2. The induction heating device according to claim 1, wherein a main coil of the coil is formed by a single-turn coil.

[0005]

In the induction heating apparatus according to the present invention, for example, a current accumulated in a choke coil is supplied to the coil by an on / off operation of a switching element such as an FET. As a result, a high voltage is generated at both ends of the coil, and the parallel resonance circuit including the primary coil of the output transformer and the first capacitor resonates, generating a predetermined sine waveform. On the other hand, when the parallel resonance circuit is in a resonance state,
Two second resonance circuits are formed with one side of the primary coil of the output transformer, and the resonance circuits generate harmonics, which are superimposed on the sine waveform. At this time, since the second capacitor grounds the intermediate portion of the primary coil in an AC manner, the reference point of the resonance circuit becomes clear, and a symmetrical harmonic is generated, which is superimposed on the sine waveform. You. As a result, a sharp rising and falling signal waveform is output as a drive signal for the switching element, the switching characteristics are improved, and a predetermined stable output can be obtained on the secondary side of the output transformer.

[0006]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 shows a circuit diagram of a main part of an induction heating apparatus according to the present invention. In the figure, FETs 1 and 2 (including SIT) as a pair of switching elements have their sources grounded via resistors 3 and 4 (for example, 1Ω) (connected to the negative side of a DC power supply. The same applies hereinafter). Resistances 5 and 6 (for example, 33Ω) are connected to each gate. Further, each drain is connected to an intermediate portion of the coil 7, whereby the FETs 1 and 2 are in a push-pull connection. The choke coil 9 is connected to the intermediate tap 8 of the coil 7, and the primary side of the output transformer 10 is connected to both ends thereof. Output transformer 10
A heating coil 11 is connected to the secondary side of the capacitor, and a capacitor 12 is connected in parallel to the primary side. The capacitor 12 and the primary side coil of the output transformer 10 form a parallel resonance circuit. The primary side of the output transformer 10 is grounded at both ends via capacitors 13 and 15 and capacitors 14 and 16 connected in series, and an intermediate tap is grounded via a capacitor 17. Thereby, the capacitors 13, 15, 17 and half of the coil of the output transformer 10 (upper half in the figure)
Also, the capacitors 14, 16, 17 and the output transformer 10
The resonance circuit described later is formed by a half of the coil (lower half in the figure). The above capacitor 1
The connection point (G2) of the FETs 3 and 15 is connected to the input side of the resistor 6 connected to the gate of the FET2,
The 16 connection points (G1) are respectively connected to the input sides of the resistors 5 connected to the gate of the FET1. The resistors 5 and 6 are connected to a bias circuit 18. The bias circuit 18 includes a smoothing capacitor 19 connected between DC power supplies and a pair of resistors 20 and 21 and resistors 22 and 23 connected in series. The connection point between the resistors 22 and 23 is connected to the resistor 5, and the connection point between the resistors 20 and 21 is connected to the resistor 6.

As shown in FIG. 2, the coil 7 includes a main coil 24 and an auxiliary coil 25 fitted in an intermediate portion of the main coil 24. The main coil 24 is formed by winding, for example, a small diameter copper pipe 26 a plurality of times, and couplers 27 and 28 for supplying and discharging the cooling water are connected to both ends thereof. Further, taps 29 and 30 are fixed to both ends, and three taps 31, 32 and 33 are fixed to an intermediate portion. Thereby, the main coil 24 is divided into four parts, and as shown in FIG.
4b, 24c = 24d. As shown in FIG. 3, an insulating tube 34 made of, for example, glass fiber is fitted in the copper pipe 26. On the other hand, similarly to the main coil 24, the auxiliary coil 25 has couplers 36, 37 at both ends of the wound copper pipe 35.
And the taps 38 and 39 are fixed. A tap 40 is fixed to an intermediate portion of the auxiliary coil 25, and the number of turns is set so that 25a = 25b. The inner diameter of the auxiliary coil 25 is
Set larger than the outer diameter of. And this auxiliary coil 2
5 is moved to the outside of the main coil 24 as shown by the arrow A in FIG.
2, 33 are electrically connected to each other by fixing them with bolts, nuts and the like (not shown). Thereby, the coil 7 of the circuit diagram shown in FIG. 4 is formed. In addition, cooling water is supplied to the couplers 27 and 36 from a cooling water supply device (not shown) via a hose or the like, and the cooling water passes through pipes 26 and 35, and from the couplers 28 and 37 to the cooling water supply device. Will be returned again. Thereby, heat generation of the coil 7 is suppressed. The FET 1 is, for example, as shown in FIG.
Eight FETs 1a to 1h, eight resistors 3a to 3h respectively connected to respective sources of the FETs 1a to 1h,
It consists of eight resistors 5a to 5h connected to each gate, respectively, and is connected in parallel. FET2 is also FET1
The configuration is exactly the same. The number of FETs 1 and 2 connected in parallel is appropriately set according to the power required by the induction heating device.

Next, the operation of the above circuit will be described.
First, it is assumed that a predetermined bias is applied from the bias circuit 18 to the gates of the FETs 1 and 2 and, for example, the FET 1 is turned on. When the FET 1 is turned on, the choke coil 9
Is supplied to the coil 7 through the intermediate tap 8. By supplying this current, a high voltage is generated at both ends of the coil 7, whereby the parallel resonance circuit including the output transformer 10 and the capacitor 12 resonates. The resonance frequency at this time is determined by the inductance of the primary coil of the output transformer 10 and the capacitance of the capacitor 12.
In this parallel resonance state, capacitors 13, 15, 1
7 or a resonance circuit (second resonance circuit) including the capacitors 14, 16, 17 and (half of) the primary coil of the output transformer 10 resonates. A harmonic, for example, a fifth harmonic, is generated from this resonance circuit, and is superimposed on a sine waveform generated in the parallel resonance circuit. That is, as shown in FIG. 6, the fifth harmonic B generated in the second resonance circuit is superimposed on the sine waveform A generated in the parallel resonance circuit, and the waveform becomes like C. This waveform is supplied to the gate as a drive signal for FET2, turning on FET2. And FET1,2
Are alternately repeated, so that a sine waveform having a predetermined frequency and power is output to the secondary side of the output transformer 10. Thus, the work 41 (see FIG. 1) disposed at the tip of the heating coil 11 is heated, and for example, a brazing operation is performed. Since the coil 7 has a low impedance with the auxiliary coil 25 fitted in the middle part of the main coil 24, when a current is supplied from the choke coil 9 to the middle tap 8, the middle of the main coil 24 is stopped. Parts 24c, 24d and auxiliary coil 25
A circulating current flows through the circuit composed of the coil and the current increases the current flowing through the coil 7. Thereby, the magnetic flux of the coil 7 increases, and the voltage induced at both ends becomes high. Next, an example of an experimental result in the above circuit is shown. Capacitor constant Capacitance of the capacitor 12: 1000 pF １３ 13, 15 〃: 10 OpF 、 14, 16 ：: 600 pF １７ 17 ：: 200 to 1000 pF Output characteristics Frequency: 2 MHz Power: about 3 KVA

As described above, according to the above-described embodiment, since the coil 7 can easily obtain a high voltage at both ends thereof with a simple structure of single winding, it is advantageous in terms of cost, and as a heating device, Significant cost reduction is possible. For example, in the case of the above-described apparatus capable of obtaining an output of 3 KVA at a frequency of 2 MHz, comparing the above-described embodiment with the case where a conventional series resonance capacitor is used, the cost of the parts to be used is about 1/20. There was found. Further, the coil 7 can be configured to be relatively small and the number of capacitors can be reduced as compared with the conventional case, so that the heating device itself can be downsized. Further, an intermediate portion of the primary coil of the output transformer 10 is connected to a capacitor 17.
, The reference point of the circuit is fixed, and a stable resonance circuit can be obtained. In addition, capacitor 1
7, a pair of resonance circuits for generating the fifth harmonic is formed, so that a symmetric and stable harmonic is superimposed on the sine waveform generated by the parallel resonance circuit, and a rising portion B as shown in FIG. The falling part becomes sharp. Thereby, the switching characteristics of the FETs 1 and 2 become good,
Output characteristics are stabilized (heating efficiency is improved) and FE
T1 and T2 can be driven with low loss, FET
1, 2 can be prevented from being damaged. In the above embodiment, a plurality of capacitors 17 having a small capacity are connected in parallel,
The circuit configuration can be appropriately changed without departing from the gist of the present invention, such as a full bridge type. Further, in the above embodiment, the bias circuit 18 for the FETs 1 and 2 has a circuit configuration that does not require fine adjustment, but it goes without saying that other circuits can be employed. Further, the configuration of the coil 7 is
The present invention is not limited to the above-described embodiment.
Alternatively, a coupling transformer can be used.

[0010]

As described above in detail, according to the induction heating device of the present invention, the cost and size of the induction heating device can be significantly reduced. Further, it is possible to prevent the switching element and the like from being damaged, and to obtain stable output characteristics.

[Brief description of the drawings]

FIG. 1 is a circuit diagram of a main part showing one embodiment of the present invention.

FIG. 2 is an exploded perspective view of a coil.

FIG. 3 is a sectional view of the main part.

FIG. 4 is a circuit diagram of a coil.

FIG. 5 is a circuit diagram illustrating an example of a switching element.

FIG. 6 is a signal waveform diagram for driving a switching element.

[Explanation of symbols]

 1, 2 FET 7 coil 9 choke coil 10 output transformer 11 heating coil 12, 17 capacitor 24 main coil 25 auxiliary coil

──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-59-181483 (JP, A) JP-B-63-50830 (JP, B2) (58) Fields investigated (Int. Cl. ⁷ , DB name) H05B 6/06 361 H05B 6/04

Claims (2)

(57) [Claims] 1. An auxiliary coil comprising at least a pair of switching elements, a main coil and an auxiliary coil connected in parallel to a substantially intermediate portion of the main coil, and both ends of the auxiliary coil are connected to an output side of the switching element. A coil, an output transformer having a primary side connected to both ends of the main coil, and a heating coil connected to the secondary side; and a first capacitor forming a parallel resonance circuit with the primary coil of the output transformer. And a second capacitor for grounding an intermediate portion of the primary coil of the output transformer. 2. The induction heating apparatus according to claim 1, wherein a main coil of said coil is formed by a single-turn coil.