Selection Basis and Analysis of Main Circuit Topologies for Induction Heating Power Supplies

1. Scheme Selection

There are various circuit topologies for induction heating power supplies, and the selection is based on the following factors:

Adopting Series Resonant Inverter
The main types of inverters suitable for induction heating devices include two configurations: parallel resonant inverters (current-source inverters) and series resonant inverters (voltage-source inverters). During the commutation period, the inverter switching devices of a parallel resonant inverter may be subjected to reverse voltage, but IGBTs (Insulated Gate Bipolar Transistors) cannot withstand reverse voltage. If anti-parallel fast diodes are used for protection, circulating currents may occur and damage the devices. Therefore, each bridge arm must be connected in series with a fast recovery rectifier diode of the same voltage rating as the switching device to withstand the reverse voltage. However, this will increase the on-state loss of each arm and raise the equipment cost. In addition, due to the relatively high frequency, when a parallel resonant inverter is used, the lead wires between the resonant capacitor and the heating coil should not be too long; otherwise, the power output and efficiency will be seriously affected. For a series resonant inverter, a slightly longer lead wire only changes the operating frequency, and has minimal impact on the output power and efficiency.

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

3. Using transformer coupling output.

For a single-phase inverter bridge powered by a three-phase 380V power grid, its output voltage is as high as about 530V. If output directly, the voltage on the resonant capacitor and heating coil will be Q times the output voltage (the Q value can vary in the range of 3-15 depending on the load), making the voltage applied to the heating coil too high, so voltage reduction measures must be taken. Furthermore, high-voltage capacitors are also difficult to solve.

4. Using PWM control method to adjust output power
There are two power adjustment methods for series resonant inverters: one is to change the DC voltage; the other is to change the power factor. The former can provide a corresponding frequency according to the load condition, so that the inverter always works at a power factor of 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 switching tubes, and the inverter often works at a relatively high power factor, 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 near the resonant state, and then adjust the pulse width of PWM to achieve the required output power.