1 Main Circuit

1.1 Scheme Selection

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

Adoption of Series Resonant Inverter
The main types of inverters suitable for induction heating devices include 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 class 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, however, a slightly longer lead wire will only change the operating frequency, and have minimal impact on the output power and efficiency.

2. Adoption of Single-Tube IGBT Modules as Switching Devices
Among power semiconductor devices, the switching speed of IGBTs (Insulated Gate Bipolar Transistors) can meet the requirements of induction heating power supplies with a frequency below 50kHz. It boasts a series of advantages, including high input impedance, low driving power, and low on-state loss.

3. Adoption of Transformer-Coupled Output
A single-phase inverter bridge powered by a three-phase 380V power grid has an output voltage of approximately 530V. If the voltage is output directly, the voltage across the resonant capacitor and heating coil will be Q times the output voltage (the Q value varies with the load, ranging from 3 to 15), resulting in excessively high voltage on the heating coil. Therefore, voltage reduction measures must be adopted. Furthermore, high-voltage capacitors also pose significant challenges in terms of implementation.

4. Adoption of PWM Control for Output Power Regulation
There are two power regulation methods for series resonant inverters: one is changing the DC voltage, and the other is changing the power factor.

For the former (changing DC voltage), the corresponding frequency can be set according to the load condition, ensuring the inverter always operates at a power factor of 1. Output power is regulated by adjusting the DC voltage. Although this circuit has low requirements for the surge voltage and surge current borne by the inverter's switching tubes, the inverter often operates at a relatively high power factor, and the reactive current flowing through the IGBT module is small-this is highly beneficial to the IGBT.

The latter method regulates output power by changing the power factor. The specific approach is: first adjust the output frequency to make the system operate in a state close to resonance, then adjust the pulse width of PWM to achieve the required output power.