JPH0125437Y2 - - Google Patents

Description

[Detailed description of the invention] The present invention relates to an inverter device used in induction heating cookers, etc., and provides a device that allows the user to arbitrarily set the optimum temperature for cooking, regardless of the volume or shape of the object to be heated. The purpose is to

That is, the present invention includes a means for detecting the temperature of the object to be heated and a temperature setting means operated by the user, and an error signal obtained by comparing a signal from the detection means with the temperature setting means is used to control the inverter circuit. By controlling the oscillation output, it is possible to easily set the temperature of the heated object to the user's desired temperature.

An essential feature of induction heating cookers is that the temperature rise of the heating coil itself is extremely low compared to heating equipment that uses low heat generation (heaters). Therefore, the highest temperature part is the pot itself, so if a temperature sensing element is placed under the pot plate, the influence of heat from the nearby heating coil is extremely small, and the temperature of the bottom of the pot above can be accurately measured. It is possible to detect In other words, highly accurate temperature detection is only possible by adopting the induction heating method in a cooking appliance. In addition, in induction heating cookers, there are no red-hot parts like heaters near the temperature detection element, so the heat resistance of the element only needs to be considered from the temperature rise from the bottom of the pot, and semiconductor elements with low heat resistance can be used. It is.

An embodiment of the present invention will be described below with reference to the accompanying drawings.

In FIG. 1, a commercial power supply 1 is connected through a main switch 3 and a current fuse 2 to the AC input side of a full-wave rectifier 4. The positive side of the output is
The negative electrode side is directly connected to one end of the smoothing capacitor 6 through the other end of the smoothing capacitor 6.

Further, the positive electrode side of the smoothing capacitor 6 is connected to an inverter 8 (high frequency static power conversion circuit) via an overcurrent protection switch 7, while the negative electrode side is directly connected to the inverter 8.

In this way, the alternating current from the commercial power source is converted to direct current by the full-wave rectifier 4, the choke 5, and the smoothing capacitor 6, applied as a power source to the inverter 8, and further converted to a high frequency, and the high frequency output is sent to the inductor 1.
0 and heats the object to be heated, which is the load.

On the other hand, to explain the gate circuit of a thyristor,
The DC input voltage of the inverter 8 is supplied to the low voltage circuit 12 and used as a voltage source for the gate circuit.

Further, the low voltage DC output from the low voltage circuit 12 serves as a power source for an output control circuit 13, an oscillation circuit 14, and a flip-flop circuit 15. On the other hand, a semiconductor temperature detection element, that is, a temperature sensing element 11 (a thermistor in this embodiment) for detecting the temperature of the heated object.
The detection signal 18 from the inverter 8 and the voltage detection signal 17 for protecting the thyristor which detects the terminal voltage of the thyristor 9 located in a part of the inverter 8 are fed back to the output control circuit 13, and the oscillation circuit 14 is activated according to the signals. The oscillation frequency of is determined. The flip-flop circuit 15 is driven at this frequency and supplies a gate signal to the gate of the thyristor of the inverter 8. Therefore, the inverter oscillates at that frequency, supplies power to the inductor 10, and heats the object to be heated. Temperature sensing elements such as thermistors and positors described above have low heat resistance but high measurement accuracy. On the other hand, thermocouples, bimetals, and the like have high heat resistance, but are difficult to use due to complicated detection signal processing and shapes.

Further, when heating a certain metal, the relationship between the heating frequency and high frequency output power is as shown in FIG. 2 in high frequency induction heating. As can be seen, when the oscillation frequency is changed from 2 to 4.5KHz, the high frequency output P changes from 0.3 to 3KW, and the current I flowing through the overcurrent protection switch 7 changes from 2 to 20A.
Change.

Therefore, when the temperature of the temperature sensing element 11 reaches a predetermined temperature, the oscillation frequency is lowered and the high frequency output P is lowered. As a result, the temperature of the object to be heated decreases, but if the temperature decreases too much, the oscillation frequency is increased to raise the temperature, and the temperature becomes constant near a predetermined temperature.

Further, if the metal to be heated is removed from the inductor 10 while the inverter 8 is energized, the inductor 10 acts on the inverter almost as an inductive load. As a result, the terminal voltage of the thyristor 9 becomes abnormally high, which may eventually lead to destruction of the thyristor. Therefore, the terminal voltage of the thyristor 9 is detected, and when it exceeds a predetermined voltage, the oscillation frequency is lowered, the resonance frequency is shifted, and the current I is lowered, thereby protecting the thyristor terminal voltage from exceeding a predetermined voltage.

In the event that the thyristor makes a commutation error while the inverter is operating, an overcurrent will flow and the overcurrent protection switch 7 will open instantly to prevent the thyristor and other parts from being destroyed, ensuring protection and safety. Further, the current fuse 2 operates in the event of an abnormality such as a short-circuit failure of the entire circuit including the full-wave rectifier 4.

Next, FIG. 3 will be explained.

FIG. 3 shows a specific circuit example of the output control circuit 13, where A is an output power setting circuit, B is a starting timer circuit, C is a temperature control circuit, and D is a thyristor 9.
A semiconductor protection circuit such as E is a circuit that combines the outputs of the above-mentioned A to D using an OR circuit and aggregates the output signals of each control circuit, and its output becomes the input of the swing circuit 14. To explain the basic operation of the output control circuit 13, each circuit output signal A to D is collected by an OR circuit, and the signal amount that gives the most negative bias to the transistor 40 among the signals is used to 42 is driven and shows a conduction state corresponding to the signal amount.

Therefore, the combined resistance value of the resistor 43 and the resistor 44, which determines the oscillation frequency of the oscillation circuit 14, changes depending on the conduction state of the transistor 42. If the combined resistance value changes, the time constant for charging the capacitor 45 changes, so the resistance 48 changes depending on the oscillation frequency of the unidirectional transistor (UJT) 47.
The terminal pulse period changes. now transistor 40
When the base 40' potential of the transistor 40 is at a potential sufficient to completely conduct between the collector and emitter of the transistor 40, the base-emitter potential of the transistor 42 becomes the knee voltage of the transistor 40, so that the transistor 42 is disabled. As it becomes conductive, the sum of the low resistor 43 and the resistor 44 becomes the oscillation constant, so the frequency at this time becomes 2KHz.

Furthermore, when there is no signal from A to D at the base potential 40' of the transistor 40, the transistor 40
0 is non-conductive, and the transistor 42 is supplied with the base potential by the resistor 41 and becomes completely conductive, so the resistor 43 is short-circuited and the oscillation constant is only the resistor 44. The frequency at this time is 4.5KHz. Therefore, the potential of the transistor base 40' is determined by the output signals A to D, and the oscillation frequency changes within the range of 2 KHz to 4.5 KHz depending on the potential. Therefore, the high frequency output power also changes according to the relationship shown in FIG.

Next, the circuit contents of A to D will be explained.

Output power setting circuit A is a zener diode 19
The voltage between the output terminals a and b from the low voltage circuit is applied to the series circuit of the and resistor 20 as shown in the figure, and the variable resistor 21 for adjusting the output power is connected to the zener diode 1.
9 in parallel.

Therefore, a prescribed constant voltage appears on the sliding piece 22, and that potential becomes the potential of the base 40' via the diode 39 of the OR circuit. Furthermore base 4
A potential of 0' acts on transistors 40 and 42,
It contributes to the resistor 43, which is an oscillation constant, and determines the oscillation frequency as described above.

Therefore, the output power can be adjusted within the range of 0.3 to 3KW by the sliding piece 22 of the output power adjustment variable resistor 21.

Next, the starting timer circuit B is an R/C timer circuit including a resistor 23 and a capacitor 24,
When the main switch 3 is closed, an inrush current may be caused by the commutation capacitor included in the inverter 8, or an overcurrent may flow due to the proximity of the oscillation frequency and the resonant frequency of the inverter depending on the material of the heated object. Therefore, in order to prevent the overcurrent protection switch 7 from being cut off unnecessarily, the
It oscillates at 4.5KHz and gradually shifts to steady 4.5KHz oscillation. Therefore, since the current I gradually increases, the overcurrent protection switch 7 does not operate at startup.
It only operates in the event of an abnormality such as a thyristor commutation error.

For this reason, the current value at which the overcurrent protection switch 7 should operate needs to be a little more than the 20A that flows in the case of 4.5KHz oscillation, so it will operate immediately in the event of an abnormality such as a commutation error, which will make it easier for semiconductors to operate. This burden will be reduced and the rate of destruction will be reduced.

In operation, since the capacitor 24 is not charged at the moment the main switch 3 is closed, the limiting resistor 25,
The potential of the base 40' is sufficiently negatively biased through the diode 38 to oscillate at 2KHz. The capacitor 24 is then charged by the current flowing through the resistor 23.
The terminal voltage of will gradually increase, while the base 4
The potential at 0' also changes accordingly, and the oscillation frequency also increases.

Finally, the oscillation constant reaches the predetermined 4.5KHz and the startup ends. In this way, at startup, oscillation starts at a frequency lower than the inverter's resonant frequency, so overcurrent due to thyristor commutation errors, which was a drawback of conventional systems, does not flow.
Safe startup is possible. Therefore, it is not necessary to turn on the power by first supplying the gate power and then supplying the power to the inverter circuit as in the conventional case, and it is possible to perform from startup to steady operation with a single operation.

The temperature control circuit C includes a temperature setting variable resistor 27 which is a setting means for the user to set the temperature of the heated object.
A bridge circuit is formed by the resistor 28, the temperature sensing element 11, and the resistor 26. Temperature sensing element 1
Capacitor 1 is connected in parallel with temperature sensing element 11 in order to bypass the high frequency, since capacitor 1 must be installed in a place where it is likely to be induced by high frequency output.
Connect 1'. When the temperature rises and the temperature sensing element resistance value decreases, the npn transistor 29
The collector is connected to the base 40' via the diode 37.

The base potential of the transistor 29 is determined by the resistance value of the temperature setting variable resistor 27 and the voltage division ratio of the resistor 28, and when the emitter potential is higher than that potential by the resistance value of the temperature sensing element 11, the transistor 29 is activated. It conducts and determines the potential of base 40', causing transistor 40 to be conductive and transistor 42 to be non-conductive. Therefore, the oscillation frequency decreases and the high frequency output power decreases, so
Temperature decreases. When the resistance value of the temperature sensing element 11 increases, the opposite operation is performed to raise the temperature and maintain the set temperature.

The semiconductor protection circuit D protects the thyristor 9 withstand voltage. A resistor 33 and a resistor 34 are connected in parallel between the anode and cathode of the thyristor 9,
When an abnormally high voltage is applied to the thyristor 9, the base potential of the transistor 31 becomes higher than the zener voltage of the zener diode 30, and the resistor 33 and the resistor are connected so that the base current is supplied through the base current limiting resistor 32. A partial pressure ratio of 34 is provided.

Then, when the transistor 31 becomes conductive, the potential of the base 40' is negatively biased from the collector through the diode 36, and the oscillation frequency is lowered. When the oscillation frequency decreases, it deviates from the inverter's resonant frequency, so the voltage at the thyristor terminals decreases, and the critical voltage increase rate also decreases.
It will not malfunction and will not destroy the thyristor.

In inverter type induction heating, depending on the material of the heated object or the presence or absence of the heated object, the load impedance may vary widely and the terminal voltage of the thyristor may become abnormally high, exceeding the rated voltage of the thyristor, or
Alternatively, the critical voltage increase rate may increase and the thyristor may malfunction and be destroyed, but this can be prevented by providing a protection circuit as in the present invention.

Therefore, heating can be performed without being limited to the material of the object to be heated. Furthermore, even if the heated object is removed while the inverter is operating, the thyristor is protected and operates safely.
As can be seen from the figure, since the frequency is lowered, the high-frequency output is also automatically lowered, and power consumption can also be saved. Conversely, when the object to be heated is placed inside the inductor, the high frequency output is automatically increased again, allowing for continuous heating. There is no need to put it in, and assembly lines such as conveyors can be easily used.

As described above, the output signals of the output power setting circuit A, the starting timer circuit B, the temperature control circuit C, and the semiconductor protection circuit D are ORed by the diodes 36 to 39, and the potential with the lowest bias potential is ORed.
As the output of the circuit, it acts on the next stage oscillation circuit.
As mentioned above, the OR circuit aggregates the output signals of all control circuits and ultimately drives only the oscillation frequency, which has the advantage of simplifying the circuit system.

In the above embodiment, the oscillation frequency is finally driven and used as a control means for each circuit, but the full-wave rectifier 4
The purpose can be achieved even if the inverter power supply voltage is changed by thyristor phase control or full-wave/half-wave switching. Of course, in that case, the circuit configuration must be such that the power supply voltage of the gate circuit is constant. In addition, in the above embodiment, the oscillation frequency is set to 2.
I explained the case of changing ~4.5KHz, but
The present invention can be similarly implemented to obtain operation at higher audible frequencies, for example, 20 KHz or higher. Further, the power semiconductor may use not only a thyristor but also a switching element such as a gate turn-off thyristor or a transistor.
For output control when using a gate turn-off thyristor or transistor, the conduction period may be varied in addition to frequency variation.

As is clear from the description of the above embodiments, the following effects can be obtained by the induction heating inverter device of the present invention.

(1) Regardless of the volume and shape of the object to be heated, the temperature is maintained at the temperature set by the user. Since the oscillation frequency related to the heating temperature is changed using the error signal of the temperature detection element, stable heating control can be performed near the set temperature.

(2) Since it is controlled at a predetermined temperature, there is no need for constant human supervision, increasing the efficiency of heating work.

(3) In the high-frequency induction heating method, which controls the output voltage of the temperature detection element by feedback, a high-frequency magnetic field exists around the temperature detection element in order to detect the temperature of the heated object, and the effect of this is that the output voltage of the temperature detection , contains a noise signal that is not the true temperature, but the noise signal can be removed by interposing a bypass capacitor in the means for detecting the temperature of the heated object. Therefore, a correlation output of the true temperature can be obtained from the detection signal, allowing accurate temperature control.
The inverter device for induction heating of the present invention specifically employs the technology of providing a bypass capacitor to remove noise signals in order to detect accurate temperature. When used as a cooking utensil, it is possible to cook the desired food more accurately.

[Brief explanation of the drawing]

Fig. 1 is a circuit diagram showing the main parts of an inverter device according to an embodiment of the present invention as a block diagram, Fig. 2 is a diagram showing the relationship between high frequency output and frequency, and Fig. 3 is the output control in Fig. 1. 2 is a specific circuit diagram of the circuit 13. FIG. A...Output power setting circuit, B...Start-up timer circuit, C...Temperature control circuit, D...Semiconductor protection circuit, E...OR circuit.

Claims (1)

[Scope of utility model registration request] A temperature setting means comprising an inductor arranged to inductively couple the object to be heated, a high frequency static power conversion circuit for exciting the inductor, a temperature setting circuit for a user to set the temperature of the object to be heated, a semiconductor temperature detection element that detects the temperature of a heated object; a temperature detection circuit to which the output of the semiconductor temperature detection element is connected;
a high frequency bypass capacitor provided in parallel with the semiconductor temperature detection element; and an error signal generation circuit for generating an error signal from the temperature setting means and the temperature detection circuit; An inverter device for induction heating that controls an oscillation frequency for heating a heated object of the high frequency static power conversion circuit.