Application of single chip microcomputer in induction cooker

       The electromagnetic cooker is an advanced electronic cooker for cooking food in modern families, which uses the principle of electromagnetic induction for heating. It is very convenient to use and can be used for various cooking operations such as boiling, frying, frying, steaming, and stir-frying. The power of the electromagnetic cooker is generally around 700-1800W. The electromagnetic cooker

       is divided into two categories: low frequency and high frequency according to the current frequency in the induction coil. Compared with the high-frequency electromagnetic cooker, the heating efficiency is high and it is more energy-saving.

       According to the style classification, it can be divided into the following three types.　
　
       Desktop electromagnetic cooker: divided into single-head and double-head, with the advantages of convenient placement and strong mobility. It is more popular because of its low price.
　　
       Embedded electromagnetic cooker: the entire electromagnetic cooker is placed in the cabinet surface, and then a hole is dug on the countertop to make the cooker surface and the cabinet countertop into a plane. Industry experts believe that this installation method is only beautiful, but not scientific. A large part of the consumer group regards the electromagnetic cooker as a hot pot, and the embedded cooking is not convenient.
　　
       Embedded electromagnetic cooker: It can adapt to the needs of different pots and no longer has special requirements for pots.

       This article mainly introduces the design of induction cooker using SPMC65P2404 chip. SPMC65P2404 is an industrial control 8-bit single-chip microcomputer launched by Lingyang. It has a high cost-effectiveness and strong anti-interference ability. It is very suitable for the design of industrial control and home appliance products. The induction cooker designed with SPMC65P2404 has the following performance:

       six heating modes: hot pot, frying, cooking, grilling, steaming, and stewing;

       one automatic working mode: boiling water; a timed start function with a maximum of 720 minutes; a 2-hour automatic shutdown protection function; a small object detection function, which does not heat inappropriate objects; the system adopts multiple protection measures such as overcurrent, overvoltage, and overtemperature; the use of switching power supply enables the system to work normally within the voltage range of 180~250V; the system is set with a fault alarm function to facilitate fault finding and maintenance; the system contains a self-test program to facilitate production testing.

       2 Principle of induction cooker heating

       The induction cooker uses the principle of electromagnetic induction to heat food. The surface of the induction cooker is a heat-resistant ceramic plate. The alternating current passes through the coil under the ceramic plate to generate a magnetic field. When the magnetic lines of force in the magnetic field pass through the bottom of the iron pot, stainless steel pot, etc., eddy currents are generated, causing the bottom of the pot to heat up quickly, achieving the purpose of heating food.

       The heating principle of the induction cooker is shown in Figure 2-1. The stove top is a high-strength, impact-resistant ceramic flat plate (crystal glass). The high-frequency induction heating coil (i.e., excitation coil), high-frequency power conversion device and corresponding control system are installed under the table, and a flat-bottomed cooking pot is placed on the top of the table.

Figure 2-1 Principle of induction cooker heating

       The working process is as follows: the current and voltage are converted into direct current through the rectifier, and then the direct current is converted into high-frequency alternating current exceeding the audio frequency through the high-frequency power conversion device. The high-frequency alternating current is applied to the flat hollow spiral induction heating coil, thereby generating a high-frequency alternating magnetic field. Its magnetic field lines penetrate the ceramic table of the stove and act on the metal pot. Strong eddy currents are generated in the cooking pot due to electromagnetic induction. When the eddy currents overcome the internal resistance of the pot body and flow, they complete the conversion of electrical energy into thermal energy. The generated Joule heat is the heat source for cooking. 
 3 Requirements for induction cooker design Induction

       cookers are a common household product. In addition to having basic heating functions, their safety performance and stability are the key to their design.

       Induction cookers are equipped with a variety of protection devices, including small object detection, automatic shutdown protection for overheating, automatic shutdown protection for overvoltage or undervoltage, automatic heating stop protection for empty burning, 2-hour power off protection, 1-2 minute automatic shutdown protection, and sound and light alarm display. In summary, induction cookers can be evaluated by the following technical characteristics and parameters:

       (1) Self-protection characteristics. The output switch tube is a key component of the induction cooker. It operates at high voltage and high power. Due to cost and device parameter limitations, it is impossible to have a large margin when designing. Therefore, during operation, if the power supply voltage is too high, there is a momentary impact when switching the working state, the current increases, the temperature rise inside the machine is too high, the iron pot is moved away from the stove or is unloaded, the switch tube may be damaged. Therefore, the overvoltage, overcurrent, overtemperature, pot detection and other protection devices should be guaranteed to be normal;

       (2) Pot bottom temperature control characteristics. The heat from the bottom of the pot is directly transmitted to the stove (ceramic glass), which is a thermal conductive material.

Figure 4‑1 System Block Diagram[page]

       4.1 Power Board Circuit Analysis

Figure 4-2 Power board circuit diagram

       4.1.1 Heating coil working circuit 

       

After the front-end filtering, the 220V AC power is transformed into about 310V DC power through the rectifier bridge. The MCU controls the on and off of the IGBT to control the working state of the heating coil.

       4.1.2 Switching power supply circuit

 

       The switching power supply uses the latest integrated circuit VIPer12A launched by TI to achieve outputs of different voltages. After the AC is connected, it undergoes half-wave rectification and is connected to the voltage input pin of VIPer12A. The output end obtains 18V and 5V DC power through voltage stabilization and voltage transformation to provide power for IC and other peripheral components.

       4.1.3 Voltage value measurement circuit

       After AC is connected, it undergoes half-wave rectification and is divided by R10 and R17 to measure the circuit voltage proportionally to determine whether the circuit voltage exceeds or exceeds the limit.

    4.1.4 Temperature Measurement

       The temperature of the IGBT and the bottom of the tile are tested by two thermistors respectively to protect the IGBT and provide a reference for temperature control of the system.

       4.1.5 IGBT control circuit

 

The circuit includes a current detection part. After the total loop current is reduced by comparison through the current transformer, it is rectified and converted into DC. The resistor is connected to the ground. The system determines the current size of the loop by checking the voltage at the resistor end. At the same time, if the loop current exceeds a certain value, the protection signal at the other end is fed back to the control end of the IGBT, the control signal is pulled low, the IGBT stops working, and is sent to the MCU to stop the system and generate an alarm signal. [page]

       4.2 Control board circuit analysis

 

Figure 4-3 Main control board circuit diagram
      
       The main control board is mainly composed of MCU, digital tube, light emitting diode, button, reset circuit. The digital tube adopts common anode type, the light emitting diode driving method is dynamic scanning, the button and SEG line are multiplexed, the COM port is controlled, and the I/O of the SEG data is read back to scan the button. The reset circuit is a low-voltage reset circuit. When the voltage is lower than 2.6V, the system resets.

       5 System software design

       5.1 Program flow analysis

       The main process adopts a time-sharing structure, and different work is performed in each different time slice. The time slice can refresh and scan the dynamically scanned LED regularly, which is convenient for program control. The time round-robin method is adopted when working, which can effectively utilize time resources. In the process, information is mainly transmitted to other modules by means of signs.

Figure 5-1 Main program flow chart 
 5.2 Interrupt subroutine flow chart

       The overcurrent interrupt is the only interrupt in the entire system. When an interrupt occurs, the system immediately stops the control signal and then sets the overcurrent flag to allow the system to detect whether the overcurrent state lasts for 3 seconds in other places. If so, an overcurrent alarm signal is generated and the system stops working.

       5.3 &

The power regulation module

       system needs to calculate whether the set power value has been reached according to the size of the external voltage and current, and adjust the PWM value by comparing the power size relationship to output a relatively constant power.

       Assuming that the external voltage is V1, the voltage value detected by the MCU is V2. According to the circuit calculation, V2=5.1*V1/(330+5.1), the A/D value DATA obtained is: DATA="V2"*256/5, and the relationship between the external current and the test value of the voltage converted by the MCU is: external current value/converted voltage=2.4.

       According to the above relationship, the power value is converted to: P=V*I=0.06*AD(V)*AD(I), and it is deduced that: AD(I)=100*P/(6*AD(V)). After determining AD(I), adjust the PWM value to make AD(I) reach the calculated value.

       5.4 System Resource Allocation

 

      

References

　　[1] Xiao Jianhua, Jing Shunlin. Application and Prospect of Fuzzy Control in Home Appliances. Journal of Wuyi University (Natural Science Edition), 2001

　　[2] Zhang Chao, Sun Zhifeng, Jin Gaoxian. Research on main resonant circuit and power control of induction cooker. Power Technology Application, 2004.

　　[3] VIPer12A datasheet http://www.dzsc.com/datasheet/VIPer12A_693114.html.

　　[4] R10 datasheet http://www.dzsc.com/datasheet/R10+_1193166.html.

　　[5] COM datasheet http://www.dzsc.com/datasheet/COM+_1118194.html .