Two highly cost-effective induction cooker system design solutions

　　1. The basic working principle of induction cooker

　　Ordinary kitchen utensils generate heat by themselves, and the heat is then transferred to the pots. The induction cooker does not generate heat by itself to heat the food. The induction cooker converts AC power into DC voltage, and then adds it to the IGBT through the excitation coil. The IGBT is driven to control conduction and cutoff. A current with a frequency of 20-50KHz flows through the excitation coil, which generates a high-frequency magnetic field. If an iron pot is placed on the stove, eddy currents are generated at the bottom of the pot, which overcome the internal resistance of the pot and convert into heat energy.

　　Since the induction cooker adopts the heating principle of magnetic field induced current, its key component is the high-speed alternating switch of high-power IGBT. The protection of IGBT is the focus and difficulty of the induction cooker. In view of the characteristics of the induction cooker that rapid protection is required in any situation during operation, Saiyuan has launched the SC91F738 microcontroller dedicated to induction heating. The built-in 16MHz RC can provide the MCU with 16MHz operating frequency, 8K Flash ROM, 256RAM, 4-way comparator, 1-way operational amplifier, 3+1 channel ADC, buzzer, IGBT control PPG, and integrates overcurrent protection, overcurrent protection, reverse voltage regulation, synchronization and other functional modules into the MCU.

　　2. Schematic diagram of the induction cooker system solution

　　Schematic diagram of the induction cooker system based on the Saiyuan SC91F738 induction heating dedicated MCU and Saiyuan SC91F832 touch button MCU

　　3. Main functions of induction cooker solution

　　1. No pot detection

　　Automatically detect whether there is a pot on the stove, and only turn on the power device IGBT when there is a pot.

　　2. Power control

　　The PPG of Saiyuan SC91F738 outputs different PWM signals, and the load current is different. According to the power of the set gear, the mains voltage and current values ​​tested by SC91F738, PPG outputs the appropriate PPG duty cycle to achieve automatic power control, so that the power is not affected by the mains.

　　3. Temperature control

　　A temperature sensor is set under the bottom of the microcrystalline plate to realize automatic temperature control, and the user can select the appropriate temperature level.

　　4. Timing

　　Scheduled startup and shutdown

　　5. Alarm

　　Alarms will be given for mains overvoltage, undervoltage, IGBT overtemperature, pot bottom temperature too high, current too large, sensor failure, etc.

　　4. Induction Cooker Function Module Description

　　1. Main circuit, power control PPG

　　As shown in the block diagram, the IGBT is driven by pulses. When the IGBT is turned on, the current in the excitation coil increases rapidly. When the IGBT is turned off, the L and C circuits resonate and a pulsed high voltage is generated at the IGBT collector. When the high voltage drops to nearly 0, the driver turns the IGBT on again. The drive pulse width determines the size of the induction cooker load. Usually, the IGBT is completed by peripheral circuits such as an oscillation circuit and a pulse width control circuit.

　　Saiyuan SC91F738 integrates these peripheral circuits into the internal MCU, called PPG (Programmable Pulse Generator). SC91F738 has a 16MHz high-speed RC clock source, 11Bits adjustable pulse width, ranging from 0.0625μs to 128μs, with a step unit of 0.0625μs and 2K levels adjustable, making the pulse width adjustment more delicate and smooth.

　　The PPG of SC91F738, in addition to the self-reduction function of reverse pressure protection, it integrates the function of automatically tracking the target value.

　　The PPG of SC91F738 adopts open-drain output with reverse duty cycle to reduce the IGBT drive circuit again and again.

　　2. Synchronous detection

　　The synchronization circuit monitors the working condition of the main circuit, and when the IGBT voltage drops close to 0V, it outputs a trigger pulse to turn on the IGBT. This can prevent the current in the excitation coil from changing too much instantly, thus protecting the power device IGBT.

　　Saiyuan SC91F738 integrates synchronous detection function and automatically triggers PPG output, which greatly simplifies the external circuit. At the same time, the delay function of SC91F738 PPG can directly set different delay output times through software to achieve the phase shifting effect. Compared with external hardware phase shifting, the software setting is more flexible.

　　3. Automatic back pressure adjustment

　　When the IGBT is turned off, the excitation coil generates reverse voltage. In order to protect the IGBT, the IGBT collector voltage must be closely monitored. Once the voltage is too high, the drive pulse is generally shut down immediately through the peripheral circuit and then restarted through software. In the common practice, due to the slow restart, part of the sinusoidal envelope will be cut off, reducing the power utilization rate.

　　Saiyuan SC91F738 has an internal integrated reverse pressure automatic adjustment module. When the reverse pressure is too large, it will not only automatically reduce the pulse width according to your program preset, but also automatically and flexibly adjust according to the number of reverse voltage overvoltages. What is more special about this module is that it will automatically track the target value function. The MCU hardware helps you achieve dynamic balance at the highest efficiency, making the package closer to a sine wave. The same external components can achieve higher induction cooker power.

　　4. Overvoltage protection

　　When the power supply itself or the voltage is too high due to interference, such as when the motor, electric fan, fluorescent lamp, etc. are turned on and off, the power supply will be disturbed and a voltage surge will occur. In order to protect the power tube IGBT from being burned out, the microcontroller must immediately enter the protection state. Saiyuan SC91F738 has an overvoltage protection comparator integrated inside.

　　5. Overcurrent protection

　　Generally, induction cookers only have voltage surge protection function, but Saiyuan SC91F738 has specially added a current surge protection comparator, which has dual protection of voltage surge and current surge, making the product safer and more reliable.

　　6. Voltage, current and temperature test

　　Saiyuan SC91F738 has a 3+1 channel ADC, which provides furnace surface temperature test, IGBT temperature test, and voltage test. The other channel ADC is connected to the internal amplifier to provide current amplification test.

　　7. Communication interface with panel

　　Saiyuan SC91F738 provides an IIC-like high-speed communication interface, which reduces the number of cables connected to the panel to a minimum. There are only two communication lines. It can communicate with slow IO and high-speed MCU with IIC interface, such as Saiyuan SC91F832 touch button MCU.

　　5. Saiyuan SC91F832 induction cooker panel solution

　　Induction cooker panel solution touch key and LED share IO

　　In this solution, the 28-pin SC91F832 is used as the main control chip of the key display panel, with 16 touch keys, 24 LEDs + 4-digit digital tube display, and communication with the mainboard IIC. At the same time, the IIC-like interface is also the debugging interface for engineers. Compared with the touch key IC plus display IC solution, the advantage of Saiyuan SC91F832 is that the touch key IO and LED SEG can be shared, making the entire display panel as simple as only resistors, and completing the control of multiple keys, multiple LEDs and application functions with a single IC.

　　SC91F832, operating voltage 2.5~5.5V, internal high-speed RC oscillator circuit can provide MCU 16MHz operating frequency, provide peripheral clock, a total of 25 I/O available, 16 touch keys realized by charge transfer. 512B RAM and 8KB FLASH, as well as 256B EEPROM data storage with 100,000 erase and write times, can be read and written in single Byte.

　　6. Working principle diagram of touch button

　　Touch button principle

　　1. The regulated power supply (or VDD) charges the external distributed capacitance Cxn and the reference capacitance Cadj at the frequency set by the frequency of CTIME.

　　2. The Cadj voltage will gradually rise to the setting value of the charging circuit and give a signal after passing through the filtering circuit.

　　3. TKCNT will record the time from the start of charging to the time the signal is seen.

　　4. If a hand touches the button, the value of Cxn will increase, and the time for the Cadj voltage to rise will decrease, that is, the value of TKCN will decrease.

　　5. The user can confirm whether a touch key is pressed by scanning the value of TKCNT.

　　7. Touch Button Interference Solution - Saiyuan Touch Button Library

　　Compared with mechanical buttons and resistive touch buttons, capacitive touch buttons are not only durable, low-cost, simple and easy to install, waterproof and anti-fouling, but also provide functions such as scroll wheels and sliders. However, capacitive touch buttons also have many problems. Because there is no mechanical structure, all detections are small changes in electricity, so they are much more sensitive to various interferences. Saiyuan provides a complete solution library. There is no need for user software to process touch buttons. You can easily read the button status by simply calling the Saiyuan library file. The complex key judgment process is completely handled by the Saiyuan library file:

　　1. Electromagnetic interference

　　When the induction cooker is working, it generates an electromagnetic field, which interferes with the changing value of the touch button. Benefiting from the familiarity with the power control of the induction cooker, Saiyuan has made corresponding software algorithm processing on the touch button library to make the touch button more reliable.

　　2. Impact of water and oil splashing

　　When using an induction cooker, water or oil often splashes onto the touch panel, which may cause false triggering of the buttons. The Saiyuan touch button library file uses a special software algorithm to reliably distinguish between water or oil splashes and finger pressing.

　　3. Environmental Adaptability

　　When the induction cooker is working, it will generate a lot of heat and moisture. The panel temperature, humidity, circuit board temperature and humidity will fluctuate in a wide range. As time goes by, the microcrystalline panel and PCB board will age to different degrees, thus affecting the accuracy of key detection. Saiyuan touch key library file realizes the automatic calibration function, provides real-time environmental detection, and realizes the mechanism of environmental adaptation.