JPS5952519B2 - High frequency induction heating device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a high-frequency induction heating device, in which a high-frequency current is intermittently supplied to a work coil thereof, with a current smaller than the current normally supplied at the beginning of use. In this way, the presence or absence of a load, that is, a pot, placed on the work coil is detected, and when there is a load, a normal current is supplied to the work coil, and this switching is performed in a non-contact manner and automatically. It has been made possible to do so.

The induction heating device according to the present invention will be explained below with reference to the drawings.

In FIG. 1, 1a and lb are terminals connected to a commercial AC power source, and 2 is a power switch. For example, a double-wave rectifier circuit 3 is connected to the AC power terminals 1a and lb,
On the output side of this circuit 3 is a coil 4a and a capacitor 4b.
A filter 4 consisting of the following is connected. This filter 4
This is to prevent the high frequency signal from the induction heating device from leaking to the AC power terminals 1a and lb. A series circuit consisting of a work coil 5, a coil 6 constituting the filter, and a switching element 7 is connected to the output side of the filter 4. As this element 7, for example, a semiconductor controlled rectifier element called GCS is used. 8 is element 7
A drive signal source or oscillator from which a square wave signal or drive pulse having a repetition frequency of, for example, 20 kHz is obtained, which is supplied to the gate of the element 7.

9 is a so-called damper diode.

Reference numerals 10 and 11 designate capacitors that form a resonant circuit in cooperation with the work coil 5 and the coil 6 forming the filter.

Since such a high frequency induction heating device is well known, a detailed explanation thereof will be omitted. However, when the switching element 7 is on, current is supplied from the power supply 3 to the coil 5, and when the switching element 7 is off, the coil 5 is turned off. A pulse occurs at both ends of
After this pulse, a damper current is supplied to the coil 5 through the damper diode 9, and by repeating this operation, a high frequency current is supplied to the coil 5.
The signal Sa shown in FIG. 2A is an AC voltage waveform between terminals 1a and 1b, and the signal Sb shown in FIG. It is a double-wave rectified waveform, and the peak voltage V in this case is
1 is approximately 140V. In the present invention, a second power supply 1 having a voltage V2 (for example, 18V) sufficiently lower than this voltage V1 is used.
2, from which coil 5 is installed at the beginning of use of the device.
A current is supplied to the coil 5 to check whether there is a load on the coil 5 or not.

Therefore, the rectifier circuit 3 including the capacitor 4b is used as the first power source. In this case, the second power supply 12 includes terminals 1a, 1
This is a case in which a double-wave rectifier circuit 14 is connected to the output side of the step-down transformer 13 connected to the voltage converter 13, and a smoothing capacitor 16 is connected to the output side of the step-down transformer 13 through a diode 15.
The direct current obtained across this capacitor 16 is supplied to the coil 5 through a resistor 17 and a diode 18. Note that the voltage across this capacitor 16 is V2. Further, in the present invention, the voltage V1 of the first power source 3 is periodically applied to the second power source 1.
A step-down circuit 19 that steps down the voltage V2 of V2 to a voltage lower than that of V2.
has been established.

To explain an example of this down-down circuit 19, the double-wave rectifier circuit 1
Also at the output terminal Q of No. 4, a full-wave rectified signal with a waveform similar to that shown in FIG. 2B, that is, a ripple voltage Sb is obtained. This signal Sb is supplied to the discharge pulse generating circuit 20, so that the signal shown in FIG. 2C, that is, the discharge pulse Sc, is obtained at its output side. Since the circuit for obtaining the signal shown in FIG. 2C, that is, the discharge pulse Sc, from the double-wave rectified signal Sb shown in FIG. 2B is conventionally known, detailed explanation thereof will be omitted. The diode 15 is a diode for preventing backflow of current from the capacitor 16 to obtain the full-wave rectified signal Sb shown in FIG. 2B at the output terminal of the rectifier circuit 14'' (7). The charge pulse Sc is supplied to the base of the transistor 21, and the collector emitter is connected to both ends of the capacitor 4b through the resistor 22.Furthermore, in the present invention, within the period of the above-mentioned discharge pulse Sc, A pulse generation circuit 24 is provided which generates a pulse PO having a time width τ, and by supplying 7 drive pulses from the oscillator 8 to the switching element 7 only during the period of this pulse PO, the switching element 7 is supplied with 7 drive pulses within this period τ. A minute high frequency current is supplied to the coil 5.

The time point at which this pulse PO is generated is preferably close to the trailing edge of the signal Sc, and in the embodiment shown in FIG. As mentioned above, the circuit for obtaining the pulse PO shown in FIG. 2D from the signal Sb shown in FIG. 2B, ie, the pulse generation circuit 24, is well known in the art, so a detailed explanation thereof will be omitted. Therefore, this circuit ``24'' is used as a zero-cross pulse generation circuit, and the pulse PO from this circuit is used as a zero-cross pulse.In this example, the oscillator 8 is designed to oscillate only within the time width τ of this pulse PO. In this case, the output of the oscillator 8 reaches the element 7 within the time width τ of this pulse PO.
A gate circuit or the like may be provided for control so that the signal is supplied to the source. Furthermore, the magnitude of the current supplied to the coil 5 from the second power supply 12 during the period when the current is supplied to the coil 5 from the power supply 12 in FIG. A load detection circuit 23 is provided to detect the load.

The resistor 17 mentioned above is for detecting this voltage drop. That is, when there is a load on the coil 5, its inductance increases, the current flowing to the coil 5 increases, and the voltage drop due to this resistor 17 increases, so it is necessary to detect the presence or absence of a load on the coil 5. I can do it. When it is determined that there is a load in this way, the output of this detection circuit 23 switches the above-mentioned oscillator 8 to a continuous oscillation state, and also controls the discharge control circuit 20. to stop operation, thus transistor 2
This is to stop the discharge caused by 1. Furthermore, the discharge control circuit 20, the oscillator 8, and the load detection circuit 2
3. The operating power supply for the pulse generator 24, the anode voltage detection circuit 25, which will be described later, etc., can be obtained from the second power supply 12, for example. The operation of the above configuration will be explained.

First, when the power switch 2 is turned on, voltages (currents) are obtained from the first power source 3 and the second power source 12, respectively, and the capacitor 16 is charged. On the other hand, as the signal Sc shown in FIG. 2C is obtained from the discharge control circuit 20, the transistor 21 is repeatedly turned on and off periodically, so that the terminal voltage Se of the capacitor 4b changes as shown in FIG. 2E. However, during the period τ during which the voltage V1 becomes lower than V2, current is supplied from the second power supply 12 to the coil 5 via the resistor 17 and the diode 18. On the other hand, since the zero cross pulse PO shown in FIG. 2D is obtained from the pulse generating circuit 24, the oscillator 8 performs an oscillating operation within this period τ, thereby driving the element 7.

In this case, τ is selected to be, for example, 0.5 msec. That is, for the period τ of this pulse PO, this high-frequency induction heating device operates with a voltage lower than that of the second power source 12, and at this time, no current is supplied from the first power source 3. The load detection circuit 23 detects the voltage across the resistor 17, that is, the magnitude of the current flowing through the coil 5.

FIG. 2F shows the voltage waveform across resistor 17 when coil 5 is not loaded. However, when there is a load, this voltage becomes as shown in Fig. 2G, and this integrated value causes the oscillator 8
is switched to a continuous oscillation state, and the operation of the discharge control circuit 20 is stopped, so that the voltage across the capacitor 4b rises to V1, and the device enters a normal operating state. In this case, the second power source 12 is automatically switched to the first power source 3 because of the diode 18. In the case of such an induction heating device, a very high voltage is applied to the switching element 7, for example, the anode of the GCS, and this does not cause any problem when the coil 5 is under a normal load, but under no load or under a ferrite etc. For special loads, the resonant frequency of the resonant circuit including the coil 5 may become extremely low, and the voltage at the anode of the element 7, that is, the GCS, becomes abnormally high, destroying the element 7, diode 9, etc. There is a risk.

For this reason, in this example, an anode voltage detection circuit 25 is provided to detect the anode voltage of this element 7, and the current is supplied from the second power source 12 to the coil 5, that is, within the period τ. When the anode voltage exceeds a predetermined value, the load detection circuit 23 is controlled to prevent the oscillator 8 from continuously oscillating, or when the first power supply 3 is normally operating as the power supply for the coil 5. The oscillation is stopped to prevent the switching element 7 and the like from being damaged.

According to the present invention described above, the presence or absence of a load is automatically detected at startup, and therefore, when there is no load, the oscillator 8 does not enter a continuous oscillation state, thereby avoiding power loss. Moreover, it has the feature that it can automatically switch to normal power supply operation when there is a load, and furthermore, it has the feature that this switching can be performed electronically.

Note that when the AC power source is turned on, the first power source 3 simultaneously supplies current to the coil 5, and the oscillator 8 is caused to oscillate continuously.
The start-up is delayed due to charging of the capacitor of the rectifier circuit that is the power source (for example, the power source 12 can be used),
By the time this complete oscillation occurs, the voltage of the first power supply 3 has already reached V1 (approximately 140V), and when the element 7 is turned on and off in this state,
In the transient state at the start of operation, a portion without a damper period occurs and the element 7 is turned off while a large current is flowing through it, leading to its deterioration or destruction.

However, in the present invention described above, since the oscillator 8 starts its oscillation operation within a period in which the current supplied to the coil 5 is small, that is, within a period in which the current is supplied from the second power source 12, the element This device also has the feature that no excessive or excessive voltage is applied to the switching element 7, and therefore the switching element 7 can be protected from damage.

[Brief explanation of the drawing]

FIG. 1 is a connection diagram showing one example of a high-frequency induction heating device according to the present invention, and FIG. 2 is a waveform diagram for explaining its operation. 1a and 1b are AC power supply terminals, 3 is a first power supply, 12 is a second power supply, 5 is a work coil, 7 is a switching element,
8 is an oscillator, 19 is a voltage down-down circuit, and 24 is a pulse generation circuit for obtaining pulses within this down-down period.

Claims (1)

[Claims] 1. In a high-frequency induction heating device in which a high-frequency current is supplied to a work coil based on turning on and off of a switching element by a drive pulse, a voltage suitable for normal operation is supplied to the work coil. a first power source, a second power source that is obtained by rectifying and smoothing the current of a commercial AC power source and having a voltage lower than the voltage of the first power source; A discharge pulse generation circuit obtains a discharge pulse having a ripple voltage and is synchronized with the period of the ripple voltage. While the discharge pulse from the discharge pulse generation circuit is supplied, the voltage of the first power source is means for supplying a drive pulse to the switching element within a period during which the voltage of the first power supply is lowered, and current from the second power supply is supplied to the work coil. and a load detection circuit that detects the state of the load of the work coil based on the load state of the work coil, and the driving pulse is continuously applied to the switching element using a signal obtained from the load detection circuit when the load is in an appropriate state. A high-frequency induction heating device designed to supply