In my last blog, I introduce semi-bridgeless PFC with standard PFC controller as a candidate of low cost, high efficiency PFC. This blog focus on key design considerations of semi-bridgeless PFC with an analog transition mode PFC controller.
A standard transition mode PFC controller relies on the sensing results of current-sensing and zero-current detection (ZCD) circuits as the on/off trigger of the driving signal. A current-sensing circuit is used to detect the peak value of the inductor current to turn off the switch. A ZCD circuit detects the zero-current point of the inductor current to turn on the switch. As shown in Figure 1, semi-bridgeless PFC has two switching legs, S1 and S2, instead of one leg in a standard PFC. The most important task becomes, how to feed two current-sensing signals from switching legs to one current-sensing pin as well as two ZCD signals to one ZCD pin of the standard PFC controller.
Semi-bridgeless transition mode PFC is beneficial when the power level is high. A current transformer current-sensing circuit, shown in Figure 2, is recommended here. Instead of using current sense resistors in series with S1 and S2, using a current transformer can greatly reduce the power dissipation on the sensing circuit. In addition, diodes in the current-sensing circuit with current transformers make sure peak-current from the desired switching leg is detected.
Zero current detection design
In a standard transition mode boost PFC circuit, zero-current detection is usually done by detecting the voltage signal from an auxiliary winding of the PFC inductor (Figure 3). Internal comparator detects the voltage polarity changes on the auxiliary winding as a turn on signal for S1. However, this circuit is impractical in a semi-bridgeless PFC case. One option is to apply the ZCD circuit for transition mode boost PFC to both inductors in the semi-bridgeless PFC with blocking diodes in series with ZCD current limiting resistors. However, blocking diodes extend the VZCD falling duration and make the ZCD pin sensitive to noise, which causes incorrect trigger and protection. Instead of using the inductor auxiliary winding, series connected RC provides a simple detection option. When both S1 and S2 are turned off, there is still one switch (generally MOSFET) conducting current through its body diode. Hence, a voltage difference is created between two switch legs. The capacitors in the ZCD circuit are charged and result in VZCD > VREF. The voltage difference becomes zero when the inductor current reaches zero, which makes VZCD < VREF and triggers the turn on event.
The current sensing mentioned above and ZCD circuits have been applied to PMP9640 – 310W PSU using transition mode bridgeless PFC and LLC-SRC. Semi-bridgeless TM PFC performance in PMP9640 is compared in Figure 4 with a standard TM PFC design in PMP9531. Close to 1% efficiency improvement can be found on semi-bridgeless PFC over boost PFC in light- to mid-load range.