Fig. 6 shows the principle of a half-bridge dc-dc converter for plasma cutting machine. It can be briefly described as follows: when the control circuit drives a half-bridge dc-dc converter, power flows from the centertap transformer passing to the bridge rectifiers. In order to provide the negative polarity for plasma jet, the power supply is from the HV-HF circuit using Flyback converter. This converter is connected with the coupling with output inductor in order to activate the initial plasma jet.