Heating Principle of Induction Cooker
Induction cooker is used to heat food based on the principle of electromagnetic induction. The furnace 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 line in the magnetic field passes through the bottom of the iron pot, stainless steel pot, etc., eddy currents will be generated, which will quickly heat the bottom of the pot, so as to achieve the purpose of heating food.
Its working process is as follows: the AC voltage is converted into DC through the rectifier, and then the DC power is converted into high-frequency AC power that exceeds the audio frequency through the high-frequency power conversion device. The high-frequency AC power is added to the flat hollow spiral induction heating coil to generate high-frequency alternating magnetic field. The magnetic line of force penetrates the ceramic platen of the stove and acts on the metal pot. Strong eddy currents are generated in the cooking pot due to electromagnetic induction. The eddy current overcomes the internal resistance of the pot to complete the conversion of electric energy to heat energy when flowing, and the generated Joule heat is the heat source for cooking.
Circuit Analysis of Induction Cooker Working Principle
1. Main circuit
In the figure, the rectifier bridge BI changes the power frequency (50HZ) voltage into a pulsating DC voltage. L1 is a choke and L2 is an electromagnetic coil. The IGBT is driven by a rectangular pulse from the control circuit. When the IGBT is turned on, the current flowing through L2 increases rapidly. When IGBT is cut off, L2 and C21 will have series resonance, and the C-pole of IGBT will generate high-voltage pulse to the ground. When the pulse drops to zero, the drive pulse is added to the IGBT again to make it conductive. The above process goes round and round, and the main frequency electromagnetic wave of about 25KHZ is finally produced, which makes the iron pot bottom placed on the ceramic plate induce eddy current and make the pot hot. The frequency of series resonance takes the parameters of L2 and C21. C5 is the power filter capacitor. CNR1 is a varistor (surge absorber). When the AC power supply voltage rises suddenly for some reason, it will be short circuited instantly, which will quickly blow the fuse to protect the circuit.
2. Auxiliary power supply
The switching power supply provides two voltage stabilizing circuits:+5V and+18V. The+18V after bridge rectification is used for the drive circuit of IGBT, the IC LM339 and the fan drive circuit are compared synchronously, and the+5V after voltage stabilization by the three terminal voltage stabilizing circuit is used for the main control MCU.
3. Cooling fan
When the power is turned on, the main control IC sends out a fan drive signal (FAN) to keep the fan rotating, inhale the external cold air into the machine body, and then discharge the hot air from the rear side of the machine body to achieve the purpose of heat dissipation in the machine, so as to avoid damage and failure of parts due to high temperature working environment. When the fan stops or the heat dissipation is poor, the IGBT meter is pasted with a thermistor to transmit the overtemperature signal to the CPU, stop heating, and achieve protection. At the moment of power on, the CPU will send out a fan detection signal, and then the CPU will send out a fan drive signal to make the machine work when the machine runs normally.
4. Constant temperature control and overheat protection circuit
The main function of this circuit is to change a temperature changing voltage unit of the resistance according to the temperature sensed by the thermistor (RT1) under the ceramic plate and the thermistor (negative temperature coefficient) on the IGBT, and transmit it to the main control IC (CPU). The CPU makes a running or stopping signal by comparing the set temperature value after A/D conversion.
5. Main functions of main control IC (CPU)
The main functions of the 18 pin master IC are as follows:
(1) Power ON/OFF switching control
(2) Heating power/constant temperature control
(3) Control of various automatic functions
(4) No load detection and automatic shutdown
(5) Key function input detection
(6) High temperature rise protection inside the machine
(7) Pot inspection
(8) Furnace surface overheating notification
(9) Cooling fan control
(10) Control of various panel displays
6. Load current detection circuit
In this circuit, T2 (transformer) is connected in series to the line in front of DB (bridge rectifier), so the AC voltage at T2 secondary side can reflect the change of input current. This AC voltage is then converted into DC voltage through D13, D14, D15 and D5 full wave rectification, and the voltage is directly sent to the CPU for AD conversion after voltage division. The CPU judges the current size according to the converted AD value, calculates the power through software and controls the PWM output size to control the power and detect the load
7. Drive circuit
The circuit amplifies the pulse signal output from the pulse width adjustment circuit to a signal strength sufficient to drive the IGBT to open and close. The wider the input pulse width, the longer the IGBT opening time. The greater the output power of the coil cooker, the higher the firepower.
8. Synchronous oscillation loop
The oscillating circuit (sawtooth wave generator) composed of synchronous detection loop composed of R27, R18, R4, R11, R9, R12, R13, C10, C7, C11 and LM339, whose oscillating frequency is synchronized with the working frequency of the cooker under PWM modulation, outputs a synchronous pulse through pin 14 of 339 to drive for stable operation.
9. Surge protection circuit
Surge protection circuit composed of R1, R6, R14, R10, C29, C25 and C17. When the surge is too high, pin 339 2 outputs a low level, on the one hand, it informs MUC to stop power, on the other hand, it turns off K signal through D10 to turn off drive power output.
10. Dynamic voltage detection circuit
The voltage detection circuit composed of D1, D2, R2, R7, and DB is used to detect whether the power supply voltage is within the range of 150V~270V after the CPU directly converts the rectified pulse wave AD.
11. Instantaneous high voltage control
R12, R13, R19 and LM339 are composed. When the back voltage is normal, this circuit will not work. When the instantaneous high voltage exceeds 1100V, pin 339 1 will output low potential, pull down PWM, reduce output power, control the back voltage, protect IGBT, and prevent overvoltage breakdown.
Post time: Oct-20-2022
