Improving the operation of 3-phase diode rectifiers using an asymmetrical half-bridge DC-link active filter

The PWM asymmetrical half-bridge is used to control the output current of a three-phase diode rectifier with a capacitor smoothed output voltage. The controlled-current function provided by the bridge improves the performance of the rectifier by lowering the low frequency ripple in the diode rectifier output current and by maintaining a balanced AC supply current when the AC supply voltage is unbalanced. The two-switch asymmetrical bridge can be designed with low per-unit voltage ratings, typically 15%, and can guarantee equal current sharing between parallel rectifiers. The higher conduction losses introduced by the bridge is offset by the improved diode rectifier performance, the low voltage devices used in the bridge, and the use of smaller L-C filter components. Balanced UPF sinusoidal supply currents can be obtained by using a three-phase bi-directional Y-switch and two asymmetrical half-bridges to separately control the output currents at both the diode rectifier output terminals. The asymmetrical bridge switches are exposed to only half the rectifier output voltage and the three-phase Y-switch has very low rms per-unit current ratings