Basis and Analysis for Selection of Main Circuit Form of Induction Heating Power Supply

Induction heating power supplies are classified into 400Hz-10kHz thyristor (SCR) type semiconductor intermediate frequency power supplies and 100Hz-400kHz electron tube type high-frequency heating power supplies according to their operating frequencies. The intermediate frequency induction heating power supply mentioned in this paper uses IGBT as the switching device and can work in the 10Hz-10kHz frequency band. It makes full use of the principles of intermediate frequency skin effect, proximity effect and circular ring effect, and effectively converts electrical energy into heat energy through the induction heater, so as to achieve the purpose of high efficiency and energy saving.

1 Main Circuit

1.1 Scheme Selection There are many circuit forms of induction heating power supplies, and the selection basis is as follows:

Using series resonant inverter There are mainly two types of inverters suitable for induction heating devices: parallel resonant (current source inverter) and series resonant (voltage inverter). During commutation, the inverter switching device of the parallel resonant inverter may withstand reverse voltage, but IGBT cannot withstand reverse voltage. If anti-parallel fast diodes are used for protection, circulating current will occur and damage the devices. Therefore, each bridge arm must be connected in series with a fast recovery rectifier diode with the same voltage level as the switching device to withstand reverse voltage. However, this will increase the on-state loss of each arm and at the same time increase the equipment cost. In addition, due to the high frequency, when using a parallel resonant inverter, the lead between the resonant capacitor and the heating coil should not be too long, otherwise it will seriously affect the power output and efficiency. For the series resonant inverter, a longer lead will only change the operating frequency, and have little impact on the output frequency and efficiency.

2. Using single-tube IGBT module as the switching device Among power semiconductor devices, the switching speed of IGBT can meet the requirements of induction heating power supplies below 50kHz. It has a series of advantages such as high input impedance, small driving power, and small on-state loss.

3. Using transformer coupled output A single-phase inverter bridge powered by a three-phase 380V power grid has an output voltage as high as about 530V. If it is directly output, the voltage on the resonant capacitor and heating coil is Q times the output voltage (the Q value varies with the load, ranging from 3 to 15), which makes the voltage on the heating coil too high, and voltage reduction measures must be taken. Moreover, high-voltage capacitors are also difficult to solve.

4. Using PWM control mode to adjust output power There are two power adjustment modes for series resonant inverters: one is to change the DC voltage; the other is to change the power factor. The former can give the corresponding frequency according to the load condition, so that the inverter always works in the state of power factor 1. The output power is adjusted by changing the DC voltage. Although this circuit has low requirements on the surge voltage and surge current borne by the inverter switch tube, and the inverter often works in a higher power factor state, the reactive current flowing through the IGBT module is small, which is very beneficial to the IGBT. The method of changing the power factor is used to adjust the output power. The specific method is to first adjust the output frequency to make the system work in a state close to resonance, and then adjust the pulse width of PWM to achieve the required output power.