Abstract: This document aims to introduce how to use SPMC65P2404A MCU to design and develop the induction cooker.
Keyword: Induction Cooker

1 System Introduction
An induction cooker is a modern electric cooker that takes electromagnetic induction principle to heat. It is easy to operate and can implement boiling, stir-frying, deep frying in oil, sauting and other cooking operations. The following features are provided: high efficiency, small bulk, light weight, low noise, power saving, pollution-free, safety and sanitation; besides, equal heating can preserve the color, aroma, taste and nutrition of food. The induction cooker has been a new and indispensable electronic cooker in modern life. The operating power ranges around 700-1800W.
Induction cookers can be classified into low frequency one and high frequency one according to the current frequency in the induction coil, here the latter has the higher heating frequency and the lower power consumption.
They can also be classified into three categories in terms of shapes:
Desktop Induction Cooker: There are one-plane type and two-plane type. Both of them are easy to place and move. The desktop induction cookers are popular for their low price.
Imbedded Induction Cooker: First, put the whole induction cooker onto the cupboard plane and then dig a hole in the plate so that the surface of the cooker and the cupboard will be in a plane. Authorities in this field consider such installation beautiful but not reasonable. Quite a number of customers treat this kind of induction cooker as hot pot. It's not convenient to stir-fry with the imbedded induction cooker.
Embedded Induction Cooker: Adaptable to various pot wares without special requirement.
This system adopts SPMC65P2404A as the main MCU. SPMC65P2404A, an industrial 8-bit microcontroller of Sunplus, boasting its competitive price, high performance, and, powerful anti-jamming capabilities, is a designated design for the target fields of industrial control and home appliance. Following are the features and functions of the induction cooker based on SPMC65P2404A:
· Six heating modes: hot-pot, stir-fry, saute, barbecue, steam, and braise.
· One automatic operation mode: water boiling.
· It can be reserved to turn on in 1~720 minutes.
· Automatically turn off power after heating for 2 hours for machine protection.
· The system will not work when detecting the following cases: No pan is on the panel, the pan is not big enough or the pan is not suitable for the induction cooker.
· Built-in temperature detection for electronic parts protection.
· Built-in over voltage & current protection circuit.
· Switching power supply makes system work well in the voltage range of 180~250V.
· System supplies fault detection and warning function, which makes it easy to discover and repair faults in time.
· Built-in self test program makes the online testing easy.

2 Device Feature
SPMC65P2404A, an industrial 8-bit micro-controller of Sunplus, provides powerful features for driving the induction cooker:
· Sunplus 8-bit micro-controller SPMC65P2404A
-182 instructions
-11 addressing modes
-Max. operating speed: 8MHz @ 5V
-Support bit operation (Set, Clear, Inverse and Test)
· 2 8-bit programmable Timer / Counter (auto initialization)
-General purpose Timer / Counter
-8-bit capture mode
-8-bit compare mode
· 2 16-bit programmable Timer / Counter (auto initialization)
-General purpose Timer / Counter
-8 /16-bit capture mode
-16-bit compare mode
-12-bit PWM waveform output
· Interrupt Management
-IRQ and NMI for the external interrupts
-4 external interrupts
-11 internal interrupts
· Operation voltage: 3.0V~ 5V
· 8 10-bit ADC (100KHz)
· Built-in 4K Bytes OTP ROM
· Built-in 192K Bytes RAM
· 23 programmable general purpose IOs
· Built-in pull-up & pull-down resistor
· 5 types of reset functions: Power-On Reset (POR), External Reset (RESET), Low Voltage Reset (LVR), Watchdog Timer Reset (WDTR), and, Illegal Address Reset (IAR)
· Clock selection: Crystal Resonator, RC oscillator, external clock
· Power saving modes: Halt and Stop
· LVR (Low Voltage Reset) and LVD (Low Voltage Detection)
· One buzzer output port
· SPI Baud rate: Max. 2Mbps at 8MHz CPU clock

3 Operation Principle of the Induction Cooker
The induction cooker adopts electromagnetic induction principle for heating. Look at Figure 3.1 below. The heat-proof ceramic panel is used as the cooker plane, and through the electrified coil under the plane the AC makes a magnetic field, which induces vortex in iron / stainless steel pan bottom and therefore heats the pan bottom quickly, and then conducts the heating to food.

Figure 3.1 Operation Principle

The working process is as follows: First the AC is converted into the DC through a rectifier, and then the DC is converted into the ultra sonic high frequency AC through a high-frequency electric power conversion device. By connecting the hi-frequency AC to the flat hollow helical heating coil, the hi-frequency alternating magnetic field will be generated. Under the ceramic panel, the electrified coil creates a magnetic field breaking through the panel and induces vortex in the iron pan bottom, which can convert electric energy into heat energy while overcoming the internal impedance stream. The generated joule heat is the heat source for cooking.

4 Requirement for Induction Cooker Design
As a widely used household product, the induction cooker must be safe and stable besides the basic heating function.
The induction cooker provides many protectors, including small pot detector, automatic power-off protector when over heated, over / under voltage protector, pan detection(if no pan over the panel, the system stops running), 2 hours turn-off protector, automatically power-off protector in 2 minutes, and warning display with voice and light.
To sum up, induction cooker can be assessed according to the following function parameters:
(1) Automatic self protection: IGBT, the key component of the induction cooker, works in high voltage and high power condition. Considering the high cost and rigid parameters, IGBT is designed conditionally, and it will be destroyed if one of the following cases occur: The over-voltage, the instantaneous impingement generated when the power switching, the proliferated current, over-temperature, the iron pan is removed from the ceramic panel or no pan is on the panel. Therefore, it's necessary to keep the protectors of over voltage, over current, over temperature and pot detection in working mode.
(2) Temperature control in the iron pan bottom: The heat in the iron bottom is directly transferred to the ceramic panel. The ceramic panel is the heat conducting material so thermo sensors are often fixed in the panel bottom to detect the temperature of the iron pan bottom.
(3) Stable power: The output power of the induction cooker can be automatically regulated to improve the adjustment of the power supply and load.
(4) Electromagnetic compatibility: This feature involves the interference to other appliances and the harm to human body. LC filtering circuit is set in the power supply loop of all induction cookers and leakage flux is absorbed with mental frame. Meanwhile, pulse mode is adopted to control the average radiation power to the minimum.

Besides the above features, the induction cooker based on SPMC65P2404A also has the following functions:
· Ranging from 300W to 1600W power, there are 14 switches for you to select a different firepower degree. Each works with a specific power;
· There are 6 switches to set temperature from 70℃ to 240℃ accurately;
· One automatic operation mode for boiling water;
· Six optional functions: hot pot, stir-frying, steaming, braising, frying, and barbecue. Among these, hot pot and stir-frying can be switched to different firepower degrees while other functions can be set to different cooking temperatures.
· It can be reserved to turn on in 1~720 minutes and power off in 1~180 minutes.
· System offers the protection that turns off machine automatically in 2 hours.

5 Hardware Design
As mentioned above, this system adopts SPMC65P2404A as the primary MCU. There are different working modes: Momentary key scan, pan bottom temperature detection, IGBT temperature detection, over current detection, over/under voltage detection, oscillation signal detection, fan control, digital display control, LED control, buzzer control, and, power-on control. See Figure 5.1.

Figure 5.1 Hardware Diagram

5.1 Power Board Circuit Analysis
The power board circuit is shown in Figure 5.2.

Figure 5.2 Power Board Circuit

5.1.1 Switching Power Supply Circuit
In this part, we adopt the VIPer12A,a new design of TI Corp., to output the different voltages. After taking the half-wave rectification, AC is outputted to the voltage input pin in VIPer12A. Then the VIPer12A outputs DC 18V and DC 5V after regulating to supply for IC and other peripherals. See Figure 5.3.

Figure 5.3 Switching Power Supply Circuit

5.1.2 Voltage Measuring Circuit
In the measuring circuit (see Figure 5.4), after AC takes the half-wave rectification, the R10 and the R17 will divide AC voltage, and voltage is measured in the circuit proportionally for judging the over / under voltage state.

Figure 5.4 Voltage Measuring Circuit

5.1.3 Temperature Measuring Circuit
There are two thermistors designed here. One is used for measuring the IGBT temperature for protection purpose. The other is for measuring the clay panel temperature as the reference for system temperature control. See Figure 5.5.

Figure 5.5 Temperature Measuring Circuit

5.1.4 IGBT Control Circuit
There is a current detection module in this circuit. Firstly, the current is reduced proportionally via a mutual-inductor, then is rectified to DC and joined up a resistance to GND. The current in the loop can be obtained by measuring the voltage across resistor. Secondly, if the current in this circuit exceeds a certain value, the protection circuit will generate a signal to pull the control pin signal down to stop the IGBT. At the same time, over-current signal will be transmitted to MCU to stop system working and issue a signal to alert users. See Figure 5.6.

Figure 5.6 IGBT Control Circuit

5.2 Control Board Circuit
The main control board is composed of MCU, digital tubes, LEDs, keys, and reset circuit. The digital tubes are common-anticathode connection. LEDs are driven by dynamic scanning. Momentary keys scanning, controlled by the combination logic of COM and SEG, can be achieved via backward reading the I/O ports of SEG data bit. The reset circuit is a low voltage reset circuit, and system will generate reset when the voltage is lower than 2.6V. See Figure 5.7.

Figure 5.7 The Main Control Board Circuit

6 Software Design

6.1 Main Flow Chart

Figure 6.1 Main Flow Chart

From the flow chart we can get that it is the time-sharing structure and every time slice has its own task. In a time slice, the LED can be dynamically scanned and refreshed timely, which makes the program controlling easy.
The entire system operates via polling, which can make good use of the time source. Messages are shared among the time slices by flag.

6.2 Interrupt Routine Flow
Over current interrupt is the only interrupt in the system. Once it occurs, the control signal stops at once. System sets out over current flag to test if the over current condition lasts for 3 seconds in other parts of the circuit. If the over current lasts for 3s, the system will send out a warning signal and stop working.

6.3 Power Regulator
System gets the working voltage and current to calculate the actual power. The PWM signal is regulated depending on the difference between the actual and the rated power to output stable power relatively.
Assume the working voltage is V1,
While the voltage detected by MCU is tested data V2, the academic data is
V2=5.1*V1/(330+5.1)
The A/D DATA is given by
DATA=V2*256/5
The relation between the working current and the voltage transferred by MCU is given by
External current / transferred voltage=2.4
According the above, the power can be deduced by
P=V*I=0.06*AD(V)*AD(I)
Get: AD(I)=100*P /(6*AD(V))
After confirming AD (I), it is easy to make it satisfied to the calculated value by the change of the PWM value.

6.4 System Source Assignment

7 Epilogue
Superior for its high heating efficiency, the induction cooker is a kind of hi-technology product advocating "Green Kitchen Culture". The induction cooker adopts electromagnetic induction principle for heating. The heat-proof ceramic panel is used as the cooker plane, and through the electrified coil under the plane the AC makes a magnetic field, which induces vortex in iron / stainless steel pan bottom and therefore heats the pan bottom quickly, and then conducts the heating to food.
Sanitation, cleanness and environment protection are other advantages of the induction cooker. Magnetic field induces iron substance and performs heating. The room temperature will not rise and no fire, smoke or odor will be released in the process of heating. Therefore, cleanness, and environment protection are truly obtained.
Based on the above advantages, induction cookers have been widely used in daily life. Now many families consider it as a necessity and the market prospect of induction cookers is promising.
The SPMC65P2404A MCU is properly suitable to design induction cooker. Its abundant resources enable to design one type of induction cooker that is multi-functional and easy to operate. The strong anti-jamming ability of SPMC65P2404A guarantees the high stability and safety for products.

8 Reference
[1] Xiao Jianhua, and Jing Shunlin, The Application and Prospect of Fuzzy Control on Home Appliance, Wu Yi University Transaction (Natural Science Edition), 2001
[2] Zhang Chao, Sun Zhifeng and Jin Gaoxian Main Resonance Circuit Research and Power Control of Induction Cooker, Power Supply Technologies and Applications