Three-phase PFC converter

The three-phase uncontrolled bridge rectifier circuit consists of a three-phase (diode) rectifier bridge and an output filter, as shown in Figure 1 (a). The positive and negative ends of the rectifier output are represented by the p-terminal and the n-terminal, respectively. The output voltage Upn of the rectifier bridge in one power frequency cycle has 6 pulsating waves, as shown in Figure 1 (b). The peak value of the output pulsating voltage Upn is 1.73Um, the minimum value of Upn is 1.5Um, and Um is the amplitude of the AC phase voltage.

By connecting two sets of three-phase rectifier bridges in parallel and shifting the input voltage phase by 30°, a three-phase 12-pulse rectifier circuit can be obtained. Compared with the three-phase 6-pulse rectifier circuit, the three-phase 12-pulse rectifier circuit has a higher power factor at the input end, and smaller voltage ripple and current THD.

Figure 1 Three-phase 6-pulse bridge rectifier

The main circuit of the three-phase PFC converter is a controllable switching rectifier circuit. The control method is used to achieve an input power factor of 1 or close to 1, which is called a three-phase UPF (Unity Power Factor) (PF≈1) rectifier or a high power factor (HPF) rectifier.

Compared with the single-phase PFC converter, the three-phase PFC converter has the following advantages:

(1) The maximum power of a single-phase PFC converter is limited. Loads with power greater than a few kilowatts are more suitable for power supply by a three-phase PFC converter;

(2) The instantaneous power of a single-phase system pulsates at twice the grid frequency, while the instantaneous total power of an ideal three-phase system is constant;

(3) Under the same power, if the effective value of the single-phase system current is 1, the effective value of the three-phase system phase current is only 1/3;

(4) In a balanced three-phase system, only two phase variables (voltage and current) are independent, which gives the three-phase PFC converter greater freedom in design.

(5) The low-frequency ripple of the output voltage of the three-phase PFC converter is 6 times the frequency of the input voltage. The output filter capacitor can be smaller and the output voltage can be quickly controlled. The input filter is easy to design.

In terms of circuit structure, three-phase PFC rectifiers have the following basic forms: three single-phase PFC converters form a three-phase PFC rectifier (hereinafter PFC rectifier is also referred to as rectifier), three-phase single-switch DCM rectifier, three-phase CCM Boost rectifier, three-phase Buck rectifier, three-phase three-level Boost rectifier, etc.