The high-frequency link transformation mode of UPS

This paper introduces the input rectifier filter circuit and sine wave inverter circuit of the online UPS, that is, the main conversion links of AC/DC and DC/AC in the UPS. With the high-frequency transformation of the online UPS conversion link and the application of PWM control technology, not only has the UPS input circuit adopted the high-frequency PWM high input power factor AC/DC conversion method, but also in order to make the high-frequency sine wave inverter operate within the optimal input voltage range, a DC/DC high-frequency inverter has been added in the conversion link. The transformation methods of high-frequency links in online UPS and the structural sequence of the entire machine's transformation circuit are different.

The circuit structure of the UPS with a high power factor converter at the input end. The AC/DC conversion part of this UPS adopts a high-frequency PWM power factor compensation (PFC) circuit, which enables the input current waveform to track the input sinusoidal voltage waveform and maintain phase consistency, making the input phase-shift power factor and distortion power factor approach 1. The AC filter on the AC input side is used to filter out the high-frequency ripple of the mains power supply to ensure that the input current compensated by the PFC converter is a sinusoidal current with less distortion.

The DC output voltage of the PFC converter can ensure a certain degree of stability. The backup battery pack is connected to the DC output terminal of the PFC converter. When the input AC power supply is interrupted, the battery pack immediately switches to supply power to the DC/AC inverter, ensuring uninterrupted AC output power supply. When the inverter malfunctions or the output is overloaded, the UPS will automatically switch to bypass power supply. At this time, the PFC converter will lose its input current waveform compensation function.

The harmonic current generated by the rectified nonlinear load at the output end will undergo reactive power exchange with the AC input power supply, reducing the input power factor of the UPS. Therefore, an active filter or a passive filter resonating with a larger harmonic distortion frequency can be set at the automatic bypass input end. This way, the input harmonic current can be limited within a smaller range during bypass operation.

Due to the setting of active or passive filters at the input end, especially when the three-phase input is in six-pulse or twelve-pulse rectification mode, the PFC converter can be omitted. In the above-mentioned UPS, since no isolation measures are taken before the output static switch, a transformer can be used at the output end to achieve isolation between the load and the power supply.