Application of SPMC65P2404A in induction cooker

1 Introduction
The induction 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. Features: high efficiency, small size, light weight, low noise, energy saving, no pollution to the environment, safe and hygienic, uniform heating during cooking, and good preservation of the color, aroma, taste and nutrients of food. It is an indispensable new type of electronic cooker for realizing kitchen modernization. The power of an induction cooker is generally around 700--1800W. Induction cookers are divided into
two categories: low frequency and high frequency according to the current frequency in the induction coil. Compared with high-frequency induction cookers, they have high heating efficiency and are more energy-saving.
According to the style classification, they can be divided into the following three types.
Desktop induction cooker: divided into single-head and double-head types, with the advantages of convenient placement and strong mobility. It is more popular because of its low price.
Embedded induction cooker: the entire induction cooker is placed in the cabinet surface, and then a hole is dug on the countertop to make the stove surface and the cabinet countertop form a plane. Industry experts believe that this installation method is only for aesthetics, but not scientific. A large part of the consumer group regards the induction cooker as a hot pot, and it is not convenient to cook with it embedded.
Embedded induction 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 SPMC65P2404A chip. SPMC65P2404A is an industrial-grade general-purpose 8-bit microcontroller launched by Lingyang Technology. 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 SPMC65P2404A has the following performance:
six heating modes: hot pot, frying, stir-frying, 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;
a switching power supply is used to enable the system to work normally within the voltage range of 180~250V;
the system is equipped with a fault alarm function to facilitate fault finding and maintenance;
the system contains a self-test program to facilitate production testing.
2 Introduction to chip features
SPMC65P2404A is an industrial-grade general-purpose 8-bit microcontroller launched by Lingyang Technology. It has a maximum operating frequency of 8MHz, an operating voltage of 3V~5.5V, 192 bytes of RAM and 4K bytes of OTP ROM, 23 programmable IO ports, built-in pull-up/down resistors, and 8-channel 10-bit A/D converter (100KHz). Other features of SPMC65P2404A include:
◆ Lingyang Technology 8-bit microcontroller
☉ 182 instructions
☉ 11 addressing modes
☉ Support bit operations (Set, Clear, Inverse, Test)
◆ 2 8-bit programmable timers/counters (the initial count value can be automatically preset)
☉ Ordinary timer/counter
☉ With 8-bit capture function
☉ With 8-bit comparison function
◆ 2 16-bit programmable timers/counters (the initial count value can be automatically preset)
☉Ordinary timer/counter
☉With 8 or 16-bit capture function
☉With 16-bit comparison function
☉12-bit PWM output
◆Interrupt mode
☉Support IRQ and NMI interrupts
☉4 external interrupts
☉11 internal interrupts
◆5 reset functions: power-on reset, low voltage reset, watchdog reset, external reset and wrong address reset
◆Support 3 clock circuits: crystal oscillator, RC oscillation, external signal
◆2 low power modes: Halt mode and Stop mode
◆Low voltage monitoring (LVD) function
◆A buzzer drive output port
◆With SPI bus
Using these resources of SPMC65P2404A, the function of induction cooker can be realized conveniently.
3 Principle of induction cooker heating 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 generates a magnetic field through the coil under the ceramic plate. When the magnetic lines of force in the magnetic field pass through the bottom of the iron pot, stainless steel pot, etc., eddy current is 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 3-1. The stove top is a high-strength, impact-resistant ceramic flat plate (crystal glass). A high-frequency induction heating coil (i.e., excitation coil), a high-frequency power conversion device and a corresponding control system are installed under the top, and a flat-bottomed cooking pot is placed on top of the top.

The working process is as follows: the AC voltage is converted into DC through a rectifier, and then the DC is converted into high-frequency AC with a frequency exceeding the audio frequency through a high-frequency power conversion device. The high-frequency AC is applied to the flat hollow spiral induction heating coil, thereby generating a high-frequency alternating magnetic field. Its magnetic lines of force penetrate the ceramic plate 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.
4 Design requirements for induction cookers
As a common household product, induction cookers must not only have basic heating functions, but also their safety performance and stability performance 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 characteristic parameters:
(1) Self-protection characteristics. The output switch tube is a key component of the induction cooker. It works at high voltage and high power. Due to the limitation of cost and device parameters, it is impossible to have a large margin when designing. Therefore, during the working process, if the power supply voltage is too high, there is a momentary impact when switching the working state, the current increases, the temperature rise in the machine is too high, the iron pot is moved away from the stove plate 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) The temperature control characteristics of the pot bottom. The heat from the pot bottom is directly transmitted to the stove plate (ceramic glass). The stove plate is a heat-conducting material, so the thermistor is generally installed at the bottom of the stove plate to detect the temperature of the pot bottom;
(3) Power stability. The induction cooker should have an automatic output power adjustment function to improve the adaptability of the power supply and load;
(4) Electromagnetic compatibility. This performance involves interference with other household appliances and harm to the human body. Induction cookers are equipped with LC filter circuits in the power supply circuit and use metal frames to absorb leakage flux. At the same time, pulse mode is used to control the average radiation power to a minimum.
On the basis of achieving the above performance specifications of induction cookers, the induction cookers we designed also have the following functional specifications:
manual control of firepower, from 300W to 1600W, divided into 14 firepower levels, each with stable power;
manual fixed temperature selection, from 70 ~ 240℃, divided into 6 fixed temperature selections, each can achieve precise temperature setting;
automatic cooking function, with 1 automatic cooking function inside: boiling water.
6 optional functions: hot pot, stir-fry, steaming, stewing, frying, grilling, among which hot pot and stir-frying can adjust different firepower levels; steaming, stewing, frying, grilling 4 functions can choose different temperatures.
It can realize the function of scheduled start-up from 1 to 720 minutes and the function of timed shutdown from 1 to 180 minutes.
The system provides a 2-hour automatic shutdown protection function.
5 System Hardware Design
The system uses SPMC65P2404A as the main control MCU. The main modes are: keyboard scanning, pot temperature detection, IGBT
temperature detection, current overcurrent detection, overvoltage and undervoltage detection, oscillation signal detection, fan control, digital tube display control, LED control, buzzer control, system startup control.


5.1 Power board circuit analysis
The power board circuit is shown in the figure

Figure 5-3 Power board circuit diagram

5.1.1 Switching power supply circuit
The switching power supply circuit is shown in Figure 5-3. The switching power supply part uses TI's latest integrated circuit VIPer12A to achieve outputs of different voltages. After 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 through voltage stabilization and voltage transformation to provide power for IC and other peripheral components.

Figure 5-3 Switching power supply circuit
5.1.2 Voltage value measurement circuit
The voltage value measurement circuit is shown in Figure 5-4. After AC is connected, it undergoes half-wave rectification and is divided by R10 and R17 to measure the voltage of the circuit in a proportional manner to determine whether the circuit voltage exceeds or is insufficient.

5.1.3 Temperature value measurement
The temperature measurement circuit is shown in Figure 5-5. Two thermistors are used to test the temperature of the IGBT and the bottom of the tile respectively, so as to protect the IGBT and provide a reference for temperature control of the system.

Figure 5-5 Temperature measurement circuit
5.1.4 IGBT control circuit
The IGBT control circuit is shown in Figure 5-6. The circuit contains a current detection part. After the current of the total loop 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 it is sent to the MCU at the same time, so that the system stops working and generates an alarm signal.

Figure 5-6 IGBT control circuit
5.2 Main control board circuit analysis
The main control board circuit is shown in Figure 5-7. The main control board is mainly composed of MCU, digital tube, light-emitting diode, button, and reset circuit. The digital tube adopts common anode type, and the light-emitting diode driving method is dynamic scanning. The button and SEG line are multiplexed to control the COM port and read back the I/O of SEG data to scan the button. The reset circuit is a low-voltage reset circuit. When the voltage is lower than 2.6V, the system resets.

Figure 5-7 Main control board circuit diagram
6 System software design
6.1 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 main program flow is shown in Figure 6-1.
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 6-1 Main program flow chart
6.2 Interrupt subroutine flow chart
The current 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 current 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.
6.3 Power regulation module
The system needs to calculate whether the set power value has been reached based on the external voltage and current, and adjust the PWM value through the compared power size relationship to output a relatively constant power.
Assuming the external voltage is V1,
the voltage value detected by MCU is V2. According to the circuit calculation,
V2=5.1*V1/(330+5.1).
The A/D value DATA is:
DATA=V2*256/5.
The relationship between the external current and the test value of the voltage converted by MCU is:
external current value/converted voltage=2.4.
According to the above relationship, the power value is converted:
P=V*I=0.06*AD(V)*AD(I).
It is concluded that: AD(I)=100*P/(6*AD(V)).
After determining AD(I), adjust the PWM value to make AD(I) reach the calculated value.
6.4 System Resource Allocation

7 Conclusion
The advantage of induction cooker is firstly reflected in its extremely high thermal efficiency. As a high-tech product advocating "green kitchen culture", the application principle of induction cooker is that the current passes through the coil to generate a magnetic field. When the magnetic lines of force in the magnetic field pass through the bottom of the iron-containing material (iron pot, stainless steel pot, enamel pot, etc.), the iron molecules are prompted to move at high speed, generating countless small eddy currents, so the thermal efficiency is high.
Hygiene, cleanliness and environmental protection are another advantage of induction cooker. Iron material is heated by magnetic field induction, does not release any substance, has no fire, no smoke, no taste, and does not raise the room temperature, which truly realizes the cleaning of the room and protection of the environment.
Based on the above advantages, induction cooker is more and more widely used in family life. Now many families are using induction cooker as one of their essential cooking utensils, and the market capacity is still growing.
SPMC65P2404A chip is very suitable for designing induction cooker products. It has rich resources to design an induction cooker product with rich functions and easy to use. And SPMC65P2404A has strong anti-interference ability, which makes the designed product have high stability and safety.