JPS6127114Y2 - - Google Patents

Description

[Detailed description of the invention] Industrial application field The present invention relates to induction heating cookers, and in particular to a temperature correction circuit that prevents an inverter current from increasing or decreasing due to changes in ambient temperature and prevents excessive current from flowing through the inverter. This invention relates to an induction heating cooker with added features.

Prior Art In the case of a cooking device that has a function of detecting the oscillating current of an inverter using a current transformer, controlling the oscillation of the inverter based on this detection signal, and adjusting the input to the load, the secondary side winding of the current transformer is The disadvantage is that the wire is affected by the ambient temperature and the oscillating current is not accurately reflected in the sensing current. For example, as the ambient temperature increases, the output voltage of the current transformer decreases. As a result, the input to the load may increase. As a result, even though the setting input is added, the input increases and exceeds the allowable power of the cooker (for example, 1200W), and there is a danger that excessive current will flow to the inverter and cause heat generation and damage to the switching elements. be. Conventionally, in order to deal with this problem, a configuration has already been proposed in which a thermistor is used as an overcurrent protection means, and the detection signal of the current transformer is temperature-corrected using the thermistor and then applied to the input adjustment circuit. However, the resistance value-temperature characteristic of the thermistor depicts a quadratic curve, and a new drawback arises in that the resistance value increases significantly with respect to temperature, especially in a low temperature region, and excessive temperature correction is performed. In Figure 3, the straight line a shown by a dashed line is a characteristic diagram showing the relationship between the input current to the load and the temperature when no temperature correction measures are taken, and the broken line b is a characteristic diagram when a thermistor is provided as the temperature correction means. It can be seen that when the temperature drops from around 5° C., the current value begins to increase and exceeds the set current value (12 A in the illustrated case).

Purpose of the invention The present invention uses a thermistor as a temperature correction means, and furthermore, by adding a new correction means in the low temperature range that cannot be corrected by the thermistor, the present invention completely implements temperature correction measures over a wide temperature range. The purpose is to make something.

Input adjustment circuit that adds the detection current from the current transformer to a capacitor and stores it there, divides this charge using a resistor, changes it to an arbitrary voltage level, and obtains an input corresponding to this level. In this configuration, when dividing the voltage accumulated in the capacitor, a resistance means consisting of a parallel circuit of a resistor, a thermistor, and a Zener diode is used,
The main configuration is such that a thermistor is used to correct the temperature above room temperature, and a Zener diode is used to correct the temperature at low temperatures.

Embodiment In FIG. 1, 1 is a DC power supply, 2 is a filter capacitor, 3 is an induction heating coil, 4 is a resonant capacitor, 5 is a switching element such as a transistor, and 6 is a damper diode, which constitutes an inverter 7. . 8 is a cooking pot, 14
is a current transformer that detects the current flowing through the induction heating coil. 9 is a starting signal generation circuit that outputs a starting signal to start oscillation of the inverter 7; 10 is an input adjustment circuit that changes the oscillation frequency of the inverter 7 in the range of about 20 to 40 KHz;
Approximately 1200W at 20KHz, approximately 200W at approximately 40KHz
and change its output. This input adjustment circuit 10
is changed in its output by an external operating means (not shown), while the temperature correction circuit 11 described later
It also functions to automatically control the oscillation frequency and adjust the output by receiving input from the oscillator. Reference numeral 12 denotes a drive circuit that actually turns on and off the switching element 5, and starts its operation when an activation signal is applied via the input adjustment circuit 10. Reference numeral 13 denotes a self-excited oscillation circuit that generates a self-excited oscillation signal from the detection signal of the current transformer 14. After the inverter 7 starts oscillating in response to a start signal, it outputs a signal at a predetermined timing to continue the oscillation. 11 is
When an excessive current flows into the inverter 7 upon receiving the current transformer 14 detection signal, the function detects this and prevents the excessive current from flowing into the inverter 7, and the function adjusts the input according to changes in ambient temperature. A temperature correction circuit having a temperature correction circuit whose signal is applied to the input adjustment circuit 10 to control it.

FIG. 2 shows a specific circuit example of the temperature correction circuit 11, and 15 is a capacitor 18, 19, 20 in which a detection signal from the current transformer 14 is inputted and stored via a diode 16 and a resistor 17.
is a divided resistor connected in parallel to this capacitor 15, 21 and 22 are a thermistor and a Zener diode connected in parallel to the low resistance resistor 18, respectively. is installed in a position where it can be detected. 2
3 is a variable resistor connected in parallel to the resistor 20;
24 and 25 are resistors and a Zener diode connected in parallel to the variable resistor 23; 26 is a transistor whose base is supplied with the terminal voltage of the variable resistor 23 via a resistor 27 and a Zener diode 28; The potential is applied to the input adjustment circuit 10 via a resistor 29. This transistor 26 is used in its amplification region. Variable resistor 23 and Zener diode 2
8 sets the maximum allowable current value. In this example, the allowable current value is set to 8.5V by the 12A variable resistor 23, the Zener voltage of the Zener diode 28 is set to 8.2V, and the Zener voltage of the Zener diode 21 is set to 4.7V. There is. Furthermore, the voltage between the terminals of the variable resistor 23 is adjusted by the thermistor 22, Zener diode 21, resistors 18, 19, 20, and variable resistor 23 so that it becomes approximately 8.3V when the temperature drops to about 0°C. Therefore, when the ambient temperature is at room temperature (20°C), the current sensed by the current transformer 14 is accumulated in the capacitor 15, and its divided potential appears across the terminals of the variable resistor 23; The level corresponding to
Since it is within the range of 8.3V to 8.5V, it only slightly exceeds the Zener voltage of the Zener diode 28, and a small current flows between the collector and emitter of the transistor 26, and the input voltage is determined by the collector potential obtained accordingly. A regulating circuit 10 is controlled.

On the other hand, when the temperature rises (e.g. 40°C), the output voltage of the current transformer 142 secondary winding decreases,
The accumulated charge in capacitor 15 decreases. However, since the resistance value of the thermistor 22 also decreases at this time, it acts to increase the terminal voltage of the variable resistor 23, and the two cancel each other out to keep the terminal voltage constant. Even if a large current of more than 12 A flows through the inverter 7, the corresponding voltage appearing between the terminals of the variable resistor 23 will be 8.5 V or more, exceeding the Zener voltage of the Zener diode 28 and causing a current to flow to the base of the transistor 26. flow. Therefore, its collector potential drops, controlling the input regulation circuit 10 to reduce the input. As a result, the current of the inverter 7 is kept at a maximum of 12A.

On the other hand, when the ambient temperature drops to around 5° C., the upward slope of the resistance value curve of the thermistor 22 becomes significantly large. Therefore, below this temperature, the resistance value of the thermistor 22 increases too much, and the variable resistor 23
A phenomenon occurs in which the terminal voltage decreases beyond the desired value, which is caused by the Zener diode 21
blocked by. In other words, at around 5°C, the voltage between the terminals of the thermistor 22 decreases approximately due to the increase in resistance value.
Reaches 4.7V. Therefore, the Zener diode 21 becomes conductive, and below this temperature, the thermistor 2
2 will not operate, the voltage across the terminals of variable resistor 23 will be maintained at approximately 8.3V, and transistor 26 will not operate.
The impedance between the collector and emitter of is kept constant, and as a result, the input becomes approximately constant. The solid line shown in FIG. 3 shows the input current-temperature characteristic in the above embodiment. Temperature correction is performed by the thermistor 22 below about 5°C, and below this temperature by the Zener diode 21. Temperature correction is possible within this range. Regarding the relationship between the Zener voltages of the Zener diodes 25 and 28, the Zener diode 25 is set to be larger than the Zener voltage 28. This is due to the following reasons. In other words, with the enamel pot set at 12A, the 18-
8. When a stainless steel pot is placed, the current flowing through the inverter 7 increases rapidly, and the voltage stored in the capacitor 15 also increases. Without the Zener diode 25, if this accumulated voltage is divided and applied to the base of the transistor 26, the collector potential of the transistor 26 will drop abnormally due to the excessive base current, and the input to the pot will be approximately 600W. This causes a phenomenon in which the temperature decreases to a certain extent. The Zener diode 25 operates to prevent such a phenomenon. That is, if the voltage is higher than a certain value, the resistor 24 and the Zener diode 25
When applied between the terminals, the Zener diode 25 becomes conductive and discharges the excessive voltage as a bypass. As a result, an abnormal drop in the collector potential of the transistor 26 can be avoided, and the input to the load can be maintained at an appropriate value.

Effects of the invention According to the invention, the output of the secondary winding of the current transformer that detects the inverter current decreases as the temperature rises, and the input current is no longer properly detected.
Even if the current increases, the inverter current will not exceed a predetermined permissible value, and the inverter component circuits, especially the switching elements, can be protected.

[Brief explanation of the drawing]

FIG. 1 is a circuit block diagram of an embodiment of the present invention, FIG. 2 is a circuit diagram of the main part, and FIG. 3 is an input current-temperature characteristic diagram. 1...DC power supply, 3...Induction heating coil, 4...
...Resonance capacitor, 5... Transistor, 6...
Damper diode, 7... Inverter, 13...
...Start signal generation circuit, 10...Input adjustment circuit, 1
1...Temperature correction circuit, 12...Drive circuit, 14...
...Current trance.

Claims (1)

[Scope of utility model registration request] An inverter including an induction heating coil, a resonant capacitor, and a switching element, a drive circuit that drives the switching element and causes the inverter to oscillate, a start signal generation circuit that causes the inverter to start oscillating via the drive circuit, and a start signal generation circuit that controls the oscillation of the inverter. an input adjustment circuit that changes the output by changing the output; a current transformer that detects the current flowing through the inverter; and a temperature correction circuit that controls the input adjustment circuit to reduce the current detected by the current transformer when it exceeds a maximum allowable value. The temperature correction circuit includes a capacitor for storing the current transformer detection current, and a resistance means for dividing the voltage stored in the capacitor, and the resistance means is constituted by a parallel circuit consisting of a resistor, a thermistor, and a Zener diode. An induction heating cooker characterized by: