When the induction cooker is working, although the input signal is 220 V AC of the mains, in order to maintain constant power and internal stable operation, real-time detection and control of each parameter is required to effectively realize over-temperature protection of the pan and dry burning of the pan. Protection, cookware sensor open/short protection, forget shutdown protection, IGBT temperature limit, IGBT over temperature protection, low temperature environmental operation mode, IGBT temperature sensor open/short protection, high and low voltage protection, surge voltage protection, VCE suppression, VCE over-protection, zero-crossing detection, small object detection, pot material detection, timing and other functions.
Many of these parameters are interrelated and mutually influential. Therefore, the use of a single-chip microcomputer for detection and control is a good choice in terms of cost, reliability, and implementation.
1 Control parameters and their relationship
1.1 Working principle of induction cooker
Induction cooker is a kitchen appliance that uses the principle of electromagnetic induction to convert electrical energy into heat. There are two types: one is induction heating using a power frequency current; the other is induction heating using a high frequency current above 15 kHz. The former is called "power frequency induction cooker" and the latter is called "high frequency induction cooker". The power frequency induction cooker does not require a power frequency to high frequency conversion circuit, and the circuit complexity is small; however, a cooking pot made of a special composite material (generally stainless steel, iron, stainless steel, aluminum four-layer composite) is required to work normally. . The high frequency induction cooker needs to be equipped with a high frequency conversion and control circuit, but does not require a composite material to form the pot body. Household induction cookers generally use high frequency mode.
The high frequency induction cooker has a thermal efficiency of up to 83%. In the main resonating part inside the induction cooker, the alternating voltage of 50 Hz/60 Hz is converted into a direct current voltage by a rectifying circuit, and the direct current voltage is converted into a high frequency voltage having a frequency of 20 to 50 kHz through a control circuit. When a high-speed current flows through the coil, a high-speed alternating magnetic field is generated; when the magnetic field lines in the magnetic field pass through the metal vessel, a large eddy current is generated at the bottom, so that the vessel itself heats up at a high speed, and then the material in the heater vessel is heated.
1.2 Induction cooker detection and control parameters
There are 14 kinds of signals or parameters that need to be detected and controlled during the operation of the induction cooker: working voltage (input, analog quantity), working current (input, analog quantity), IGBT working temperature (input, analog quantity), output to the temperature of the vessel ( Input, analog quantity), AC zero crossing (input, digital quantity). Synchronization (input, digital), fan (output, digital), buzzer (output, PWM). IG-BT turn-on and turn-off (output, PWM), power surge (input/output, digital), overvoltage at the C-side of the IGBT (input/output, digital), probe signal (output, digital), button Monitoring (input/output, digital), display control (output, digital).
1.3 Correlation of control parameters
Except for the above two parameters belonging to the human-computer interaction interface, the other 12 parameters are related to each other, and the relationship is shown in Figure 1.
In Figure 1, the parameters I, V, Vp-p, VCE, T1, T2 are: operating current set value, operating voltage set value, surge voltage set value, IGBT maximum breakdown voltage setting value, IGBT temperature value and heating coil temperature value. All parameters are added to center the IGBT reliably and normally. The reliable and normal operation of the IGBT means that the output power is constant, and the heat and the bottom temperature are within the allowable range.
2 Implementation of integrated control
2.1 Single-chip microcomputer SH69P42
The SH69P42 is a single-chip microcomputer based on the SH6610D core of the 4-bit RISC instruction set. It has a 4-channel 8-bit SAR A/D converter, 2-channel 10-bit PWM output, 2 levels of 3 interrupts (A/D interrupt, port interrupt and timer interrupt), 2 8-bit timers, 16 I /O pin; has OTP type ROM (3 072 Ã— 16 bits) and RAM (192 Ã— 4 bits); there is also a low voltage monitor reset and built-in watchdog circuit, the oscillator operates at 32 768 Hz to 8 MHz . The SH69P42 has a total of 43 instructions, each of which has an equal execution time of four times the oscillator period.
The SH69P42 is suitable for industrial grades and is used in applications where the system's anti-jamming capability is extremely high. Home appliance applications are an important aspect of it. From the analysis of the single-chip resources of SH69P42, combined with the parameters given in Section 1.2, it can be seen that the microcontroller is very suitable for the control of the induction cooker.
2.2 Integrated Control Strategy
2.2.1 Control of output power
The signal that controls the IGBT is a fixed-cycle PWM pulse. It can be known from the literature  that the instantaneous output power of the induction cooker is determined by the duty ratio of the high level in one PWM period, and the magnitude of the current flowing through the IGBT that is turned on during this period determines the magnitude of the power. Therefore, the output power of the induction cooker can be realized indirectly using the duty cycle of the PWM cycle. The calculation method can simply use linear correspondence.
The SH69P42 has two 10-bit PWMs that can be programmed to achieve a specified duty cycle output.
PWM0: The corresponding period register is three units of $22, $23, $24, corresponding to the lower 4 bits, the middle 4 bits and the upper 2 bits respectively;
PWM1: The corresponding period register is three units of $28, $29, and $2A, which correspond to the lower 4 bits, the middle 4 bits, and the upper 2 bits.
Corresponding PWM period = 10-bit binary number connected to 3 units Ã— PWM clock
PWM0: The corresponding duty cycle registers are three units of $25, $26, and $27, corresponding to the lower 4 bits, the middle 4 bits, and the upper 2 bits, respectively;
PWM1: The corresponding period register is three units of $2B, $2C, and $2D, which correspond to the lower 4 bits, the middle 4 bits, and the upper 2 bits, respectively.
Corresponding PWM duty cycle = 10-bit binary number connected to 3 units Ã— PWM clock
The PWM clock can be programmed ($20 for PWM0 and $21 for PWM1) to be set to 1, 2, 4, and 8 times the oscillation period.
The power output should also be combined with the detection of the bottom temperature of the pan, and the bottom temperature of the pan is the upper limit condition. Even if the set power value is not reached, the power level is considered as long as the bottom temperature reaches the specified value.
2.2.2 IGBT safety
When the IGBT in the induction cooker is working, the on-current is 8 to 40 A depending on the set power. The withstand voltage is around 800 V. For electronic components packaged in small spaces, this operating parameter is dangerous. Once the breakdown of the IGBT occurs, the surrounding components will be compromised. Therefore, in the control strategy of the induction cooker, protecting the IGBT from normal operation is a top priority. The protection of the IGBT is carried out from the following aspects:
1 As shown in Figure 1, after the IGBT is turned off, when the heating coil and the oscillating capacitor are free to oscillate, the IGBT cannot be turned on from the positive voltage to the zero point. The IGBT control pulse is turned on when the filter capacitor is turned on by the return diode of the IGBT. Because the IGBT is in the reverse bias state at this time, the forward voltage has not been added. The control here is carried out by the synchronization circuit.
2 When a power surge occurs, the IGBT should be turned off immediately. The withstand voltage limit of TGBT is generally 1 200 V. The detection of surge after conversion to DC is limited to 1 150 V, so the metal film precision resistor must be used in this section.
3 When the power supply has overvoltage or overcurrent, turn off the IGBT for the same reason. 4 After the induction cooker is working for a period of time, the aging of the components may occur, which may cause the parameters to shift. In particular, the parameter offset of the heating coil and the oscillating capacitor causes the voltage to be too high when freely oscillating. At this time, the voltage applied across the C and E of the IGBT will exceed the design value, thereby causing IGBT overvoltage breakdown. The protection here is similar to the surge protection principle of the 1st power supply.
5 IGBT long-term high-power operation or poor heat dissipation, will cause the IG-BT temperature is too high. According to the current-temperature working curve of the IGBT. When the temperature is 100 Â°C, the current passing capacity is 50% at 25 Â°C. Therefore, the temperature of the IGBT should be monitored at any time. Once the set value is exceeded, the IGBT should be turned off immediately and the temperature should be detected in real time.
2.3 software and hardware design
Induction cooker control system with SH69P42 as the core, PWM0 is used to control the turn-on and turn-off of IGBT, PWM1 is used to control passive buzzer; A/D converter is used to receive working voltage, working current, IGBT temperature and bottom temperature 4 signals. In addition to keyboard scanning and display, the remaining I/O is used to output probe signals, AC zero-crossing detection, and fan drive.
In terms of software, the timing interrupt is designed to cycle the IGBT temperature and the bottom temperature of the pot, and the IGBT is protected at any time. Use the clock interruption of SH69P42 to carry out pot and pan detection, because in practical applications, the pot on the induction cooker will be removed at any time, and the working state of the heating coil and the oscillating capacitor of the induction cooker will change at any time. In the absence of a pot, the LC is an undamped oscillation with a peak voltage exceeding 1 200 V and damaging the IGBT. Practice has proved that the detection cycle exceeds 8 ms, which will cause overvoltage damage of the IGBT.
The control of the induction cooker is relatively complicated in the current household appliances, especially the kitchen appliances, the working environment is more complicated, and the interior is in the state of high pressure, high temperature and high frequency. With its excellent anti-jamming performance, simple command system and powerful logic operation instructions, SH69P42 MCU can control a system like a PLD. It is foreseeable that the microcontroller will have a wider application space.
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