JPS6131956B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION This invention detects the currents of transistors and freewheeling diodes used in switching elements of an inverter circuit to determine the load condition of a heating coil, and when a small object load is detected, the inverter circuit The present invention relates to an induction heating device that is capable of stopping an alarm circuit or continuously driving an alarm circuit.

Conventionally, induction heating devices have a configuration in which the heating output is varied by controlling the on/off duty of thyristors and transistors, which are switching elements of inverter circuits. Whether a small object such as a knife, kitchen knife, spoon, etc. has been loaded due to negligence or the like is determined based on the ratio of the input power to the current of the heating coil. In other words, since the area of the small load located within the alternating magnetic field of the heating coil is smaller than that of the proper load, the heating output due to the current flowing through the heating coil is not sufficiently transmitted to the load, and therefore the input power is also small. Become. However, when detecting input power, it is not easy to directly convert the input power into the voltage required for detection, so the voltage must be detected indirectly from the detected value of the input current, and it is difficult to directly convert the input power to the voltage required for detection. In addition, as the operating power source for the inverter circuit is generally a DC power source obtained by rectifying and smoothing a commercial AC power source, the power factor of the AC signal changes as the heating output is varied. An error occurs in the detection of the input power value, and the detection level is not constant. In addition, as another device, by utilizing the fact that the Q of the heating coil changes according to changes in the load condition and the resonant voltage changes, it detects when the resonant voltage exceeds a predetermined value and loads small objects. There is something that determines that
This device also requires correction due to power supply fluctuations as described above, and furthermore, it is essential to vary the detection level as the heating output varies, making the circuit configuration complicated.

This invention takes into consideration the above-mentioned conventional drawbacks, does not require any correction due to power supply fluctuations and variable heating output, and uses a simple circuit configuration to detect a small load and stop the operation of the inverter circuit, thereby reducing the input power. This invention is designed to maintain the operation of an alarm circuit that warns that the load is a small object even when the load becomes zero.Next, this invention will be explained in detail with reference to the drawings showing one embodiment of the invention.

In FIG. 1 showing one embodiment, 1 is a main DC power supply that rectifies and smoothes commercial AC, L is a heating coil whose one end is connected to the positive terminal (+) of the main DC power supply 1, and 2 is a heating coil. L is a heating pot that is a load, Q is a transistor whose collector and emitter are connected to the other end of the heating coil L and the negative terminal (-) of the main DC power supply 1, respectively, and D1 is connected between the collector and emitter of transistor Q. The first diode for freewheeling, C1, is the first capacitor for resonance connected in parallel to the heating coil L, and the heating coil L, the first capacitor C1, the first diode D1, and the transistor Q constitute an inverter circuit. ing. CT1 and CT2 are first and second current detectors provided on the emitter side of the transistor Q and the anode side of the first diode D1, respectively, and one output end of the first current detector CT1 is connected to ground. There is. R1 and VR1 are a first resistor and a first variable resistor connected between both output terminals of the first and second current detectors CT1 and CT2, respectively;
1. The current detected by CT2 is connected to the first resistor R1
and the voltage across the variable resistor VR.
D2 and D3 have their respective anodes connected to the other output terminal of the first current detector CT1 and the second current detector CT.
The second and third diodes C2 and C3 are connected to one output terminal of the second and third diodes D2 and D3, respectively, and between the cathodes of the second and third diodes D2 and D3 and the ground and the other output terminal of the second current detector CT2. The second and third capacitors R2 and R3 are respectively connected in parallel to the second and third capacitors C2 and C3, and the second diode D2, the second capacitor C2 and the A first peak detection circuit PD1 that peak-detects the detected value of the first current detector CT1 is configured by two resistors R2, and a third diode D3 and a second capacitor C.
3 and the third resistor R3 to the first current detector CT.
2nd peak detection circuit that peak detects the detected value of 1
PD2 is configured, and the first and second peak detection circuits are connected from both ends of the second resistor R2 and the third resistor R3.
Both output terminals O1, O2 and O3, O4 of PD2
has been derived. E is a sub-DC power supply connected between the other output terminal of the second current detector CT2 and the ground.

3, the positive side and negative side input terminals are the respective positive side output terminals O of the first and second peak detection circuits PD1 and PD2.
1, a comparison detection circuit connected to O3, and 4 an alarm circuit such as a buzzer connected to the output terminal of the comparison detection circuit 3, which is driven when a low level signal is input from the comparison detection circuit 3. 5 is an OR circuit whose one input terminal is connected to the output terminal of the comparison detection circuit 3; 6 is a reset terminal connected to the other input terminal of the OR circuit 5; when a reset button (not shown) is pressed, A high level signal is input to the OR circuit 5. A collector zero voltage detection circuit 7 has an input terminal connected to the collector of the transistor Q, and has a detection function for detecting the point in time when the collector voltage becomes zero. 8 is an AND circuit whose both input terminals are connected to each output terminal of the OR circuit 5 and the collector zero voltage detection circuit 7; 9 is a monostable multivibrator whose trigger terminal T is connected to the output terminal of the AND circuit 8; VR2; C4 is a second variable resistor and a fourth capacitor that constitute a time constant circuit that determines the driving time of the monostable multivibrator 9; 10 is a base voltage control circuit whose input end is connected to the output end of the monostable multivibrator 9; The output terminal is connected to the base of the transistor Q, and the output signal of the monostable multivibrator 9 is amplified and the output terminal is connected to the base of the transistor Q.
is applied to the base of Q, turning on transistor Q.

Next, the operation of the above embodiment will be explained with reference to FIGS. 2 to 4.

In the inverter circuit, when the transistor Q repeats on and off operations at high speed, a high frequency current flows through the heating coil L, and the resulting alternating magnetic field inductively heats the heating pan 2 placed within this magnetic field. . Note that the first capacitor C1 is connected to the heating coil L.
This improves the waveform of the high-frequency output signal, reducing the generation of harmonic noise and surge voltage. The first diode D1 also prevents a reverse voltage from being applied to the collector of the transistor Q.

As shown in FIG. 1, the current flowing through the heating coil L is IL, the current flowing through the transistor Q is IT, and the current flowing through the first capacitor C1 is IC,
Let ID be the current flowing through the first diode D1, and explain the operating principle of this inverter circuit. To explain the operating principle of this inverter circuit, t ₀ ~
During the period when the transistor Q remains on, as shown at time t ₁ , the current IT flows through the equivalent impedance and DC resistance component of the heating coil L, as shown in FIG. 2b, which shows the current in the heating coil L. In addition, the current IC flows through the equivalent impedance and DC resistance component of the heating coil L from the moment _t1 when the transistor Q is turned off, and this current IC is the same due to the back electromotive current generated in the heating coil L. It decreases as shown in b, and eventually enters a negative region, and a free oscillating signal waveform is obtained by the equivalent impedance of the heating coil L, the DC resistance component, and the first capacitor C1 for resonance. Note that c indicates the voltage between the collector and emitter of the transistor Q, and the free vibration continues until time _t2 when this voltage becomes zero.

On the other hand, the current ID flowing through the first diode D1
At time _t2 when the collector-emitter voltage of the transistor Q becomes zero, the first diode D1 becomes conductive and the current flows in the opposite direction indicated by ID in FIG. this is,
It is based on the electromagnetic energy stored in the inductance of the heating coil L by the current IT when the transistor Q is turned on, and is returned to the main DC power supply 1.

In the above operation, when transistor Q is in the on state from t ₀ to _{t 1} , the current flowing through transistor Q
IT is detected by the first current detector CT1, and this current IT is transformed into a voltage across the first resistor R1, as shown by the solid line in e, and at both output terminals of the first peak detection circuit PD1. A peak detection voltage as shown by the broken line e is output from O1 and O2. On the other hand, the first diode D1 flowing between t ₂ and _{t 3}
The current ID is detected by the second current detector CT2, and this current ID is, as shown by the solid line f,
The voltage across the first variable resistor VR is transformed, and the minute voltage of the auxiliary DC power supply E is superimposed on the voltage across the first variable resistor VR.
A peak detection voltage as shown by the broken line f is output from both output terminals O3 and O4 of the peak detection circuit PD2. And the first and second peak detection circuits
Both output voltages of PD1 and PD2 are compared by a comparison detection circuit 3.

By the way, the first and second peak detection circuits PD
1. To explain the relationship between both output voltages of PD2 using FIGS. 3 and 4, if we assume that the minute voltage of the auxiliary DC power supply E is not superimposed on the second peak detection circuit PD2, the broken line in FIG. indicates the case of proper load condition, and the dashed line indicates the case of small object load condition, and the solid line indicates the discrimination level of comparison detection circuit 3, and the first and second peak detection circuits
Both output voltages of PD1 and PD2 are in an equal relationship.
That is, in the case of a proper load state, the output voltage of the first peak detection circuit PD1 is equal to that of the second peak detection circuit PD.
If the output voltage is higher than No. 2 and the load is on a small object, the above-described situation is reversed. Next, when a minute voltage is superimposed on the second peak detection circuit PD2 by the auxiliary DC power supply E, the output voltages of both peak detection circuits PD1 and PD2 will have the characteristic straight lines indicated by the broken line and the dashed-dotted line, as shown in FIG. , the relationship is shifted in parallel from the state shown in FIG. 3 by a minute voltage E' of the auxiliary DC power source E in the Y-axis direction. Here, when the output voltage of the first peak detection circuit PD1 becomes a value less than or equal to the intersection point A between the solid line and the broken line, the comparison detection circuit 3 determines that the small object is loaded even if the load condition is a proper load condition. In addition, the first variable resistor VR1
By varying the second peak detection circuit PD
The output voltage of 2 changes, and the detection level of small loads can be adjusted.

Therefore, at the aforementioned time t ₂ , Fig. 2 e
As is clear from and f, the output voltage of the first peak detection circuit PD1 is higher than the output voltage of the second peak detection circuit PD2, so the comparison detection circuit 3 determines that the load is appropriate and outputs a high level signal. The signal is then input to the AND circuit 8 via the OR circuit 5.

Here, the transistor Q, the heating coil L, the first
Capacitor C1, monostable multivibrator 9,
Collector zero voltage detection circuit 7 and base voltage control circuit 10 form a self-oscillation loop. That is, during the period from time t ₀ to time t ₁ in FIG.
After holding in the on state, the collector zero voltage detection circuit 7 detects the time _t2 at which the voltage between the collector and emitter of the switching transistor Q shown in FIG. As shown in d, a high level signal is output and applied to one input terminal of the AND circuit 8. At this time, since the high level signal is applied to the other input terminal of the AND circuit 8 from the OR circuit 5 as described above, the monostable multivibrator 9 is triggered by the AND circuit 8, and t ₂ of the same g
As shown at ~t ₄ o'clock, the transistor Q is driven for a certain period of time determined by the time constant of the second variable resistor VR2 and the fourth capacitor C4, and as shown in FIG. By repeating a series of operations in which the device is held in the on state for a certain amount of time, a self-oscillating operation is obtained and the device continuously operates as a transistor inverter. In this case, it is preferable to select the point at which the transistor Q is turned on at time _t3 , but since no collector current flows between time _t2 and time _t3 , the transistor Q is turned on at time _t2. Even if it is turned on, there is no problem at all in the operation of the inverter circuit. Further, by varying the second variable resistor VR2, the driving time of the monostable multivibrator 9 changes and the on-time of the transistor Q changes, so that the heating output can be adjusted as desired.

Next, when the small object load state occurs at t ₁₀ , the period from t ₁₀ to t ₁₂ is basically the same as in the case of the above-mentioned proper load state, but the difference is that As shown in FIG. 2, the collector-emitter voltage of transistor Q increases. Therefore, as shown in the same f, the _first
A large current flows through the diode D1, and the output voltage of the second peak detection circuit PD2 changes to the first peak detection circuit.
The output voltage of the PD1 becomes higher than the output voltage of the PD1, the output signal of the comparison detection circuit 3 becomes a low level, the alarm circuit 4 is driven and an alarm is issued, and even if a high level signal is output from the collector zero current detection circuit 7 at _t12 . , since the output signal of the OR circuit 5 is at a low level, the AND circuit 8 does not perform the AND operation, and the monostable multivibrator 9 is not triggered, and the oscillation of the inverter circuit stops, as shown in FIG. However, even if the inverter circuit stops operating, as mentioned above, the minute voltage of the sub-DC power supply E
E′ is superimposed on the second peak detection circuit PD2 and the fourth
Since the relationship is as shown by the broken line in the figure, the output of the comparison and detection circuit 3 is latched to a low level, and the alarm circuit 4 remains active until the reset button is pressed and a high level reset signal is given from the reset terminal 6.
The alarm state is maintained. Then, the AND circuit 8 performs an AND operation based on the reset signal, triggering the monostable multivibrator 9 and driving the inverter circuit. Note that the variable range of the collector current of the transistor Q by adjusting the heating output by the second variable resistor VR2 corresponds to the variable range of the output voltage of the second peak detection circuit PD2 shown by B in FIGS. 3 and 4, Misjudgment by the comparison and detection circuit 3 due to the superimposition of the minute voltage E' does not occur, and furthermore, the value of the minute voltage E' can be made as small as possible by increasing the discrimination sensitivity of the comparison and detection circuit 3.

As described above, according to the induction heating device of the present invention, an inverter circuit is constructed by connecting a diode in parallel between the collector and emitter of a transistor, and connecting the collector and emitter of the transistor to a DC power supply via a heating coil and a capacitor. and providing first and second current detectors for detecting each current of the transistor and the diode,
A comparison detection circuit is provided that determines the load by comparing the detection value of the first current detector and the value obtained by superimposing a minute voltage on the detection value of the second current detector, and the operation of the inverter circuit is controlled by the output of the comparison detection circuit. By controlling this, small loads can be detected by comparing only the operating current of the inverter circuit flowing through transistors and diodes, and there is no need for correction due to power supply fluctuations and heating output changes. The detection level can always be kept constant. In addition, by superimposing a minute voltage on the detection side of the diode current, the detection state of a small load can be maintained with a simple circuit configuration. There is no need to provide

[Brief explanation of the drawing]

Fig. 1 is a wiring diagram of one embodiment of the induction heating device of the present invention, Fig. 2 shows the operating current and voltage of each part in Fig. 1, a is a waveform diagram of the base current of the transistor, and b is a heating The waveform diagram of the current flowing through the coil, c is the waveform diagram of the voltage between the collector and emitter of the transistor, d is the waveform diagram of the output signal of the collector zero voltage detection circuit, and e is the voltage of the detected value of the first current detector. and f is a waveform diagram of the output voltage of the first peak detection circuit, f is a waveform diagram of the voltage detected by the second current detector and the output voltage of the second peak detection circuit, and g is a waveform diagram of the output signal of the monostable multivibrator. waveform diagram,
Figure 3 is a diagram of the relationship between the output voltages of the first and second peak detection circuits under proper load conditions and small load conditions when no auxiliary DC power supply is provided.
FIG. 3 is a relationship diagram of each output voltage of the first and second peak detection circuits in the proper load state and the small object load state in the figure. 1...DC power supply, L...Heating coil, C1...
Capacitor, D1...diode, Q...transistor, CT1, CT2...first and second current detectors, 3...comparison detection circuit.

Claims (1)

[Claims] 1. A DC power supply, a heating coil connected to the DC power supply, a resonant capacitor forming a resonant circuit with the heating coil, a switching transistor for generating a resonant current in the resonant circuit, and an inverse circuit connected to the switching transistor. The diode connected in parallel and the switching transistor ON,
A control means for controlling an OFF operation, first and second current detection means for detecting each current of the transistor and the diode, and the first and second current detection means. and a comparison means for comparing detected values by the means, and when a small object load condition is detected by the comparison by the comparison means, the ON/OFF operation of the switching transistor by the control means is stopped. as an induction heating device.