Electromagnetic heating equipment converts power frequency alternating current or pure direct current into high frequency alternating current (1KHz-1MHz) through half-bridge/full-bridge inverter technology. High-frequency alternating current will generate high-frequency alternating current magnetic field after passing through various inductive loads. When a metal object is in a high-frequency AC magnetic field, the metal molecules will generate countless small eddy currents. The eddy current makes the metal molecules move randomly at high speed, and the metal molecules collide with each other to generate heat energy, and finally achieve the purpose of converting electrical energy into heat energy. Electromagnetic heating equipment is often used in our work and life. For example, household induction cooker/electromagnetic tea stove, commercial induction cooker, high frequency quenching machine, sealing machine, industrial furnace, etc. This paper takes the three-phase high-power commercial induction cooker as an example to analyze its application. Film capacitors in electromagnetic heating equipment.

Commercial induction cooker three-phase full-bridge circuit topology

C1-C6 Function Description

C1 / C2: Three-phase AC input filter, which can absorb ripples and improve the ability of equipment to resist grid interference

C1,C2 and three-phase common mode inductors form a Pi-type filter, which plays the role of electromagnetic interference suppression and absorption in the device. On the one hand, the circuit suppresses the electromagnetic interference generated by the IGBT due to high-speed switching, and transmits it to the three-phase power frequency grid through the power line. This will affect the normal use of other grid-connected equipment. On the other hand, it prevents the electromagnetic interference signal generated by other equipment in the same power grid from being transmitted to the three-phase power frequency power grid through the power line, thereby affecting the normal use of the electromagnetic heating equipment itself. (Internal suppression) self-generated interference, external resistance to interference generated by other equipment, double-sided) EMC = EMI EMS

In actual use, C1 can choose the MKP-X2 type (fixed capacitor used to suppress electromagnetic interference), the capacity range is between 3µF-10µF, and the rated voltage is 275V.AC-300V.AC. Y-type connection is adopted, and the common terminal is suspended without grounding. C2 can choose MKP type metalized film capacitor, the capacity range is between 3µF-10µF, the rated voltage is 450V.AC-500V.AC, and the delta connection is adopted.

In principle, the larger the capacitance selected for C1 and C2, the better the suppression and absorption of electromagnetic interference. However, the larger the capacitance, the greater the reactive current when the device is in standby. The withstand voltage shall be based on the area of equipment used. When the power consumption at night is very small, it is reasonable to keep a certain margin to prevent the voltage breakdown of the capacitor or the life of the capacitor.

C3: Rectifier, DC link, absorb ripple and finish smoothing and filtering after AC component circuit.

C3 and choke coil L form an LC circuit, which converts the pulsating direct current rectified by the three-phase bridge into a smooth direct current for use by the subsequent inverter bridge and load. In the actual circuit of the magnetic core of a commercial induction cooker, C3 usually consists of several tens of microfarads of film capacitors. In fact, the film capacitor located in this position plays the role of DC support (DC-LINK), which is responsible for absorbing the ripple and completing the AC component loop, instead of (filtering) tens of microfarads as many people think. For loads of tens of kilowatts, the filtering effect is very small, and the voltage waveform of the DC bus cannot be smoothed at all. Due to the high-speed switching of IGBT, it will generate a large amount of high-order harmonic current and peak harmonic voltage. If there is no capacitor to absorb harmonic current and peak voltage, the DC bus circuit will produce a large amount of self-excited oscillation, which will affect the safe use of IGBT, etc. and shorten the service life. So. Film capacitors as DC bus ripple voltage and ripple current absorption is one of the most commonly used methods at home and abroad.

In principle, the larger the capacitance chosen for C3, the better the absorption. However, it should be noted that the capacitance is too large, because the instantaneous charging current of the capacitor is too large, when the device is just turned off and powered on, it is easy to cause a rectifier bridge. Fuses and other overcurrent breakdown. In commercial induction cooker cores, the general selection principle is: half-bridge solution (1.5 μF/ KW) full-bridge solution (1.2 μF/ KW). This configuration is based on the regular film capacitor energy. It is derived from the 2A / µF design process.

For example, for a commercial induction cooker with a 20KW half-bridge model, the required C3 capacity is 20*1.5=30 µF. For the full-bridge 20KW model, the total ripple current of C3 is 30*2 = 60A, and the required C3 capacity is 20*1.2=24 µF (actually 25-30µF is acceptable). The total ripple current of C3 is 25*2 = 50A. It is recommended that the allowable ripple current value of the actual selected capacitor and capacitor should not be lower than the above recommended value.

The position of C3 must consider whether the ripple current value actually required by the circuit is less than the total ripple current value that the selected film capacitor can withstand (still has a certain current margin), otherwise, if the circuit requires a ripple current of 60A, and the selected The capacitor can only withstand the total ripple current of 40A, which will cause the film capacitor to heat up severely, long-term overheating operation, and greatly reduce the service life of the film capacitor, and seriously cause the film capacitor to expand and swell. Bulging, even fire. In terms of withstand voltage, the rated voltage is generally 800-1000V.DC.

C4:IGBT peak voltage/current absorption, buffering and suppression functions to prevent IGBT breakdown

C4 serves as the on/off spike absorption of the IGBT. It is usually connected by C- type or RC-type and is connected to the CE terminal of the IGBT. The withstand voltage is usually selected according to the rated voltage of the IGBT, and a certain voltage margin is reserved. In terms of capacitance, it can typically be between 0.01 µF and 0.033 µF. The most suitable capacitor should be selected according to the matching between the circuit and the IGBT. The capacitor in the C4 position must use a larger dv / dt value. Pay attention to whether the temperature rise is within the allowable range. If you use an RC-type connection, you need to pay attention to the huge heat of R. When laying out, you need to keep a certain distance between R and C to prevent excessive heat dissipation from the capacitor.

C5: resonant capacitor, with the load (inductance coil, transformer, etc.) to form an LC resonant circuit.

C5 acts as a resonant capacitor and forms an LC resonant circuit with L to transfer power. In use, please pay attention to whether the rated voltage of the selected capacitor is sufficient (resonance voltage and device power, load material, magnetic load rate, distance from load to inductance, circuit Q value, etc.). If the selected capacitor rated voltage value is lower than the actual resonance voltage value, the capacitor voltage may break down. As far as the current selection of the resonant capacitor is concerned, it is best to calculate the theoretical value first, and then make a preliminary selection. Current value, after the equipment function meets the requirements, adjust the equipment by measuring the actual value. Peak current/rms value of the LC circuit at maximum power. If the actual high-frequency current value passed is greater than the rated value of the capacitor, if the current value is too large, it will cause the resonant capacitor to overheat and run, and it will be easy to bulge or burst or even fire after long-term work. The resonant frequency of the circuit should also be within the allowable frequency range of the resonant capacitor.

C6: DC bus absorption capacitance, local absorption, buffer and suppress the peak voltage generated when IGBT switching.

C6 and C3 are also connected in parallel to the positive and negative poles of the DC bus. However, due to factors such as structure and wiring loop, the back-end IGBT is far away from the C3 capacitor, so it is necessary to directly lock the bus to the power terminal of the back-end IGBT module. IGBT locally generated ripple voltage and ripple current. When C6 is selected, the withstand voltage is usually selected according to the rated voltage of the IGBT. Try to choose a bus absorption capacitance with larger ripple current, larger dv / dt and smaller stray inductance. For example, a DC absorption capacitor with a rated voltage of 1200V MKPH-S models such as 0.47µF 1µF 1.5µF 2µF.

Common Problems in the Selection of Film Capacitors

Improper selection of rated voltage

The rated voltage is not selected properly, the most common location is the resonant circuit part (C5). R & D personnel should be based on the rated power of the equipment, input voltage, circuit topology, inverter control method, load materials, load magnetic load rate, circuit Q value and other parameters of comprehensive consideration for preliminary calculation. After the prototype initially meets the requirements, you need to use an oscilloscope to add a high-voltage voltage probe to actually measure the peak-to-peak voltage, peak voltage, and root mean square voltage across the resonant capacitor when the device is in operation. Maximum power. Parameters such as resonant frequency are used to determine if the selected resonant capacitor model and parameters are correct.

B. Improper selection of rated current

Improper selection of rated current, the most frequent ones are C3 (DC support) and C5 (resonant). If the actual current value required is greater than the allowable current value of the capacitor, it will cause the capacitor to heat up and run at high temperature for a long time, which will greatly shorten the life of the capacitor, and will explode or even catch fire. In equipment development, you can use a dedicated current probe or other methods to measure the actual peak current and root mean square current, and then adjust the parameters of the capacitor. Finally, the temperature rise of the capacitor can be measured in the full power aging test of the device, and the selection of the capacitor can be determined according to the allowable parameters of the capacitor temperature rise. (Comprehensive evaluation of current measurement and temperature rise)

C Wiring error

Improper wiring methods mainly occur when multiple capacitors are used in parallel. Due to factors such as wiring methods and inconsistent wiring distances, each parallel capacitor in the circuit is inconsistent. It is finally reflected in multiple parallel capacitors, and the temperature of each capacitor is inconsistent. The capacitor temperature at each location was too high and burned. Therefore, it is necessary to connect the capacitors in parallel and try to achieve current sharing and improve the service life of the capacitors.

Waveform Reference in Use of Film Capacitors

C3 voltage fundamental waveform (505V / 300HZ)C3 ripple voltage waveform (38V / 23.3KHz)

C5 resonant current waveform (Ip = 84A Irms = 60A)C4 absorption capacitance waveform (Vce = 581V F = 19.6KHz)

To sum up

The application field of electromagnetic heating equipment is expanding, and the use requirements and electrical performance parameters of film capacitors are getting higher and higher. This paper takes the three-phase full-bridge commercial induction cooker as an example to analyze the role and function of film capacitors in various positions inside the equipment. Selection principles, precautions, etc., hope to bring some convenience to the majority of R & D personnel!