Power frequency machines and high-frequency machines are distinguished based on the operating frequency of the designed circuit of UPS. The power frequency machine is designed based on traditional analog circuit principles and consists of a thyristor SCR rectifier, IGBT inverter, bypass, and power frequency step-up isolation transformer. Due to the fact that both the rectifier and transformer operate at a power frequency of 50Hz, as the name suggests, they are called power frequency UPS. A high-frequency machine usually consists of an IGBT high-frequency rectifier, battery converter, inverter, and bypass. IGBT can be controlled to turn on and off by controlling the driver added to the gate. The switching frequency of IGBT rectifier is usually from a few K to several tens of KHz, or even up to hundreds of KHz, which is much higher than that of power frequency machines, hence it is called high-frequency UPS.

In a power frequency UPS circuit, the three-phase AC input of the main circuit is connected to a rectifier composed of three SCR bridge arms through a commutation inductor and converted into a DC voltage. Adjust the output DC voltage value by controlling the conduction angle of the rectifier bridge SCR. Due to SCR being a semi controlled device, the control system can only control the turn-on point. Once SCR is turned on, even if the gate drive is cancelled, it cannot be turned off. It can only be turned off naturally after its current is zero. Therefore, its turn-on and turn-off are based on one power frequency cycle and do not exist in high-frequency turn-on and turn-off control. Due to the fact that SCR rectifiers belong to step-down rectification, the AC voltage output by the DC bus voltage inverter is lower than the input voltage. To achieve a constant 220V voltage in the output phase, a step-up isolation transformer must be added to the inverter output.