Induction cooker working principle:
Generally, the kitchenware is heated by itself, and the heat is transferred to the pot. The induction cooker does not heat the food by itself. The induction cooker converts the alternating current into a direct current voltage, and then applies the excitation coil to the IGBT. The IGBT is controlled to be turned on and off by the drive. The excitation coil has a current with a frequency of 20-50 KHz, and the excitation coil generates a high-frequency magnetic field. If the iron pot is placed on the surface of the furnace, a vortex is generated at the bottom of the pot, and the eddy current is converted into heat energy by overcoming the internal resistance of the pot.
Since the induction cooker adopts the heating principle of the magnetic field induced current, its key component is the high-power IGBT high-speed alternating switch. The protection of the IGBT is the key point and difficulty of the induction cooker. For the operation of the induction cooker, it must be quickly protected in any situation. Features, Saiyuan introduced the electromagnetic heating dedicated controller SC91F738, built-in 16MHz RC can provide MCU 16MHz operating frequency, 8K FlashROM, 256RAM, 4 comparators, 1 operational amplifier, 3+1 channel ADC, buzzer Buzzer The IGBT controls the PPG and integrates overcurrent protection, overcurrent protection, back pressure regulation, synchronization and other functional modules into the MCU.
Induction cooker function
1, no pot detection
Automatically detect whether there is a pot on the stove, and only when there is a pot, the power device IGBT can be turned on.
2, power control
The PPG of the SE91F738 outputs different PWM signals, and the magnitude of the load current formed is different. According to the power level of the gear set, the mains voltage value and current value tested by SC91F738, PPG output the appropriate PPG duty cycle to achieve automatic power control. Make the power unaffected by the city.
3, temperature control
A temperature sensor is arranged under the bottom of the microcrystalline plate to automatically control the temperature, and the user can select a suitable temperature file.
4, timing
Scheduled boot, timed shutdown
5, the alarm
Mains overvoltage, undervoltage, IGBT temperature is too high, the bottom temperature of the pot is too high, the current is too large, the sensor is faulty, etc.
Block diagram of induction cooker system based on Sayuan SC91F738 electromagnetic heating dedicated MCU and Saiyuan SC91F832 touch button MCU
The following is a brief description of some induction cooker function modules:
1, the main circuit, power control PPG
As the block diagram, the IGBT is driven by the pulse. When the IGBT is turned on, the excitation coil current increases rapidly. When the IGBT is turned off, the L and C circuits resonate, and the IGBT collector generates a pulse high voltage. When the high voltage drops to near 0, the drive makes the IGBT again. Conduction, the driving pulse width determines the size of the induction cooker load, usually the IGBT is completed by peripheral circuits such as an oscillating circuit and a pulse width control circuit.
The SE91F738 integrates these peripheral circuits into the internal MCU section, called PPG (Programmable Pulse Generator). The SC91F738 has a 16MHz high-speed RC clock source, 11Bits adjustable pulse width, in the range of 0.0625μs~128μs, the step unit is 0.0625μs, 2K level adjustable, making the pulse width adjustment more delicate and smooth.
The PPG of the SC91F738, in addition to the self-reduction function of the back pressure protection, integrates the function of automatically tracking the target value.
The PPG of the SC91F738 uses an open-drain output with a reverse duty cycle to reduce and reduce the IGBT drive circuit.
2, synchronous detection
The synchronous circuit monitors the working condition of the main circuit. When the IGBT voltage drops close to 0V, a trigger pulse is output to turn on the IGBT. This avoids a sudden change in the current in the field coil and protects the power device IGBT.
The SE91F738 integrates the synchronous detection function and automatically triggers the PPG output, which makes the external circuit greatly simplified. At the same time, the delay function of the SC91F738 PPG directly sets different delay output times through the software to achieve the effect of phase shifting. Software settings are more flexible compared to external hardware phase shifts.
3, back pressure automatic adjustment
When the IGBT is turned off, the excitation coil generates back pressure. In order to protect the IGBT, the IGBT collector voltage must be closely monitored. Once the voltage is too high, the drive pulse is usually turned off immediately by the peripheral circuit, and then re-opened by software. Slower, sinusoidal packets will be cut off, reducing power utilization.
The SE91F738 integrates a back pressure automatic adjustment module. When the back pressure is too large, it will automatically reduce the pulse width according to your program preset. It will also automatically adjust according to the number of back pressure and overpressure. The module is more special. The target value function will be tracked automatically, and the MCU hardware will help you to achieve dynamic balance at the highest efficiency, so that the package is closer to the sine wave, and the same external components can achieve higher induction cooker power.
4, overvoltage protection
When the power supply itself is disturbed and the voltage is too high. If the motor, electric fan, fluorescent lamp, etc. switch, 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. An overvoltage protection comparator is integrated inside the SE91F738.
5, overcurrent protection
Generally, the induction cooker only performs the voltage surge protection function, but the SE91F738 specially adds a current surge protection comparator, which has the dual protection of voltage surge and current surge, which makes the product more safe and reliable.
6, voltage and current temperature test
The SE91F738 has a 3+1 channel ADC that 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 the panel
Saiyuan SC91F738 provides high-speed communication interface of Class IIC, which minimizes the connection with the panel. There are only two communication lines, which can communicate with slow IO and high-speed communication with MCU with IIC interface. Yuan SC91F832 touch button MCU.
V. Saiyuan SC91F832 Induction Cooker Panel Solution
Induction Cooker Panel Solution Touch Key and LED Share IO
In this solution, the 28pin SC91F832 is used as the main control chip of the button display panel, 16 touch buttons, 24 LED + 4 digit digital tube display, and the motherboard IIC communication, and the class IIC interface is also the engineer debugging interface. Relative to the touch button IC plus display IC scheme, the advantage of the game element SC91F832 is that the touch button IO and the LEDSEG can be shared, so that the entire display panel is simple to only the resistor, and the single IC completes the multi-button multi-LED and the application function control.
SC91F832, working 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 charge transfer touch buttons. 512B of RAM and 8KB of FLASH, there are 100,000 erase and write times of 256B EEPROM data memory, single-byte read and write.
Sixth, the working principle of the touch button
Touch button principle
1. The regulated power supply (or VDD) charges the external distributed capacitor Cxn and the reference capacitor Cadj at the frequency set by the frequency of CTIME.
2. The Cadj voltage will gradually rise to the set value of the charging circuit, and the signal will be given after the filtering circuit.
3. TKCNT will record the time from the start of charging to the time the signal is seen.
4. If the value of Cxn increases when a button is touched by a hand, 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 button has been pressed by the scan value of TKCNT.
Seven, 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 stain resistant, but also provide functions such as rollers and sliders. However, capacitive touch buttons also have many problems. Because there is no mechanical structure, all the detections are small changes in the amount of electricity, so they are much more sensitive to various disturbances. Saiyuan provides a complete solution library, without the need for user software to process touch buttons, as long as the game library file is simply called, the button status can be easily read, and its complicated key-finding process is completely processed by the game library file:
1, electromagnetic interference
When the induction cooker works, it generates an electromagnetic field, which interferes with the change value of the touch button. The game element benefits from the familiarity with the power control of the induction cooker, and the touch button library makes corresponding software algorithm processing, making the touch button more reliable.
2, the impact of splashing water, splashing oil
In the use of induction cooker, water or oil splashing on the touch panel often occurs, which may cause the button to be triggered by mistake. The game key touch button library file uses a special software algorithm to reliably distinguish the water splashing from the state of the finger press.
3. Environmental adaptive ability
When the induction cooker is working, it will generate a lot of heat and moisture. The panel temperature, humidity, board temperature and humidity will float in a wide range, and with the passage of time, including microcrystalline panels and PCBs. The aging of different programs occurs in the board, which affects the accuracy of button detection. Saiyuan touches the key library file to realize the automatic calibration function, providing environmental detection in real time and realizing the environment adaptive mechanism.
Eight, summary
Saiyuan's overall solution for induction cooker, SC91F738 highly integrates various functional modules for electromagnetic heating applications. SC91F832 shares the touch on the panel with the LED display, which has ultra-low cost and reliable operation. At the same time, Saiyuan provides perfect and reliable touch buttons. Library, development engineers only need to focus on functional applications, which can greatly shorten the application development cycle.
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