Analysis on the principle of over-temperature protection circuit of induction cooker

 Principle of induction cooker over-temperature protection circuit

When the induction cooker is operating normally, a large amount of heat is generated because the IGBT is in a high current, high voltage, and high frequency on and off state. In order to ensure that the IGBT is not damaged due to excessive temperature, in addition to taking appropriate cooling measures, at the same time There must also be an over-temperature protection circuit; in addition, in order to prevent the induction cooker from being damaged by dry burning for a long time while the pot is placed, or even a fire accident, an over-temperature protection circuit must also be installed in the induction cooker. The over-temperature protection circuit signal is also a feedback signal for the induction cooker to realize intelligent functions such as heat preservation.

The over-temperature protection sampling and testing points of the induction cooker are as follows: place a temperature-sensitive thermistor on the IGBT, install a thermal relay contact or temperature-sensitive thermistor on the heat sink, place a temperature-sensitive thermistor under the stove, and place a temperature-sensitive thermistor on the heat sink. A temperature-sensing thermistor is placed on the coil plate.

The structure of the over-temperature protection circuit is relatively simple. Generally, the structure of the temperature detection circuit on the heating coil plate is: connect the +5V DC power supply through the temperature detection thermistor in series with another ordinary fixed-value resistor and then divide it to the ground. (the thermistor can be connected to the positive terminal of the power supply or the negative terminal of the power supply), and the voltage value that changes with temperature is taken out from the middle voltage dividing point and sent to the microcontroller. The microcontroller compares this changing voltage signal with the numerical change range set by the program to determine whether to output a protection control signal.

The structure of the IGBT temperature detection circuit is generally: one end of the IGBT temperature detection thermistor is connected to the negative pole (ground) of the power supply, and the other end is connected to the +5V voltage through a resistor with a fixed resistance value. It is taken out from the middle voltage dividing point and changes with the temperature. The changing voltage signal is fed into the microcontroller. The microcontroller compares the voltage changes with the reference value set by the internal program to determine whether to output a protection control signal.

According to the different connection methods of the temperature detection thermistor in the circuit, over-temperature protection is divided into two categories: low-potential action and high-potential action: Some induction cooker over-temperature protection is low-potential action, that is, the temperature sensing voltage drops to a certain level. When the temperature sensing voltage rises to a certain level, the microcontroller will issue a shutdown protection command; some induction cooker over-temperature protection is a high-potential action, that is, when the temperature sensing voltage rises to a certain level, the microcontroller will issue a shutdown protection command. Some induction cookers use high and low potential action wiring forms for the thermistors on the heating coil plate and the thermistors on the IGBT respectively.

Fujibao IH-P205C induction cooker over-temperature protection circuit

Fujibao IH-P205C induction cooker over-temperature protection circuit schematic diagram

One end of the IGBT thermistor SENSOR of the induction cooker is connected to +5V voltage, and the other end is connected to ground through resistor R32, which is a high-potential action. When the temperature on the IGBT is higher, the resistance value of its temperature detection thermistor is smaller, and the divided voltage on resistor R32 is higher. This voltage is sent to pin ⑦ of the microcontroller through resistor R31. The microcontroller adjusts the voltage according to this voltage. Compare the high and low with its internal program setting value to decide whether to perform shutdown protection. One end of the heating coil plate thermistor TM is connected to ground, and the other end is connected to +5V voltage through resistor R40. The protection of this end is low-level action. The higher the temperature on the heating coil plate, the smaller the resistance value of the thermistor, the lower the voltage division value at the midpoint, and the lower the voltage sent to pin ⑥ of the microcontroller through resistor R41. The microcontroller compares the level of this voltage with the action value set by the program and determines whether to issue a shutdown command.

Grid voltage detection, high voltage detection and protection circuit

Principles of power grid voltage detection and high voltage detection and protection circuits

The grid voltage detection circuit is mainly composed of D101, D102, R101, R102, R103, C101, etc. After the grid voltage is full-wave rectified by D101 and D102, it is divided by R101 and R102. After the voltage is divided, a voltage drop is formed on C101 through R103, and then sent to the ⑩ pin of the microcontroller IC01. This input voltage is proportional to the grid voltage. Based on this voltage, the microcontroller compares it with the internal set value to determine whether the operating voltage is within its allowable value range. If it exceeds the allowable value range, the IGBT drive output signal will be turned off, that is, it will be shut down, thus serving as overvoltage or undervoltage protection.

The high voltage detection and protection circuit is mainly composed of R001, R305, R306, etc. After R001, R305 and R306 are connected in series, they are sampled at the c-pole of the IGBT tube, and their partial voltage values ​​are sent to the pin of the microcontroller IC01. Based on this voltage, the microcontroller compares it with the internal set value to determine whether the c-pole voltage of the IGBT tube is too high. If it is too high, the conduction time of the IGBT tube will be limited, the high level of the IGBT tube drive signal will become narrower, the conduction time of the IGBT tube will become shorter, and the c-pole voltage of the IGBT tube will become lower, thereby providing overvoltage protection. role.

Voltage and current surge protection circuit

Principle of voltage and current surge protection circuit

The voltage surge protection circuit is mainly composed of C601, R603, R606, R609, D601, D604, R604, R018, C602, etc. When there is surge pulse interference in the power grid, it is directly coupled through C601, divided by R606 and R609, and rectified by D601, and then sent to the ① pin of the microcontroller IC01. The microcontroller turns off the IGBT tube drive signal and the IGBT tube is cut off. Among them, C602 is a high-frequency bypass capacitor, and D604 is a clamping diode. Its main function is to prevent the surge sampling voltage sent to the microcontroller from being too high and burning the microcontroller.

The current surge protection circuit mainly consists of: constantan wire JP3, operational amplifier IC601 (LM358), R010, R0ll, D603, R014, etc. (see Figure 2). When there is a current surge pulse in the power grid, the constantan wire generates a higher surge pulse (negative voltage relative to GND). This pulse is divided by R010 and R011 and sent to the ② pin of the op amp IC601. ③Pin is the base reference voltage.

After the surge pulse signal is amplified by the inverting amplifier, it is also sent to the IC01① pin of the microcontroller through D603 and R014. After processing by the microcontroller, the driving signal of the IGBT tube is turned off, and the IGBT tube is cut off.

Power-on delay protection circuit

Principle of power-on delay protection circuit

The power-on protection circuit is mainly composed of R601, R602, D602, R810, Q803, etc. When the power supply is plugged in, the +310 V voltage is stepped down by R601 and R602, and after rectification by D602, Q803 is turned on, making the base voltage of Q801 low level, and the IGBT tube is turned off, thereby protecting the IGBT tube.

Current detection protection and pot detection circuit

The current detection protection circuit mainly consists of: constantan wire JP3, operational amplifier IC601 (LM358), R012, C012, R016, C013, R017, VR1, R013, C010, C015, etc. When the current flows from the e pole (GND) of the IGBT tube back to the "-" terminal of the rectifier bridge through the constantan wire, a negative voltage relative to GND is generated on the constantan wire. The voltage passes through the reverse circuit composed of R012, C012, R016, and IC601. It is amplified by the phase amplifier, and then divided by R017, VR1, R013 and then input into the roller pin of the microcontroller IC01 for processing. The voltage input to the microcontroller is proportional to the current flowing through the constantan wire. R016 is a negative feedback resistor, C012 is an anti-self-excitation capacitor, C013 and C015 are high-frequency bypass capacitors, and C010 is a filter capacitor.