Sinrace Power Supply Technology (Shenzhen) Co., Ltd.

As the full-wave bridge rectifier circuit is connected to AC power line input, the instantaneous input power should change at any time, to get a stable power output, depends on the storage capacitor to achieve power balance. For DC-DC converter, usually in a continuous mode (CCM) to work, the duty cycle won’t change following the load. The full-bridge rectifier output voltage is independent of the load, when the load is reduced, the output power decreases, but the PFC input power level is same as that of  heavy-duty, so the energy filled in C1 is equal to the energy extracted from the C1, causing the DC bus voltage significantly rise, the voltage stress on C1 is often up to more than 1000V, the withstand voltage requirement of the switching device is very high. As the voltage of switching device is high, current stress is large, switching losses large, and from input to output the power should go through two transformations, so it is inefficient.

Improved single-stage PFC converter circuit

To reduce pressure on the energy storage capacitor and the converter efficiency, must improve the basic circuit of single-stage PFC topology

Two-group of one kind of transformer achieves the negative feedback single-stage PFC converter circuit shown in Figure 3. N1 and N2 winding is coupled windings of transformer T1.

When the switch Q1 is on, the voltage VC1 is applied to the primary winding T1. The whole voltage is greater than the voltage on N1, the boost inductor L1 will have current flowing .. When Q1 is off, the reverse voltage in L1 is that plus the voltage VN2 on VC1 and N2 and than minus the input voltage. Joining the two coupled coils N1 and N2, provides a negative feedback voltage, reduce the voltage stress on C1 and improve efficiency. However, adding N1 and N2, will lower the power factor, increase the current harmonic content. If insert an inductor in D2 and N1, the input current work in the CCM, the voltage on C1 can be reduced. Requirement N1+ N2

 the CCM single-stage PFC converter circuit with low-frequency auxiliary switch. Q1 is the main switches, Q2 is the auxiliary switch. When the input current is near to zero, Q2 turns on, so that additional winding N1 is short-circuit, when the input voltage is greater than a certain value, Q2 turns off. As the input voltage Q2 is very small ,it will turn on, the rest of the time is blocking-up, the current flowing through Q2 is small, power loss of Q2 is also small. This circuit topology is compared with the circuit in Figure 3, reducing harmonic content of input current, improve the power factor and efficiency, reducing the voltage on the capacitance (C1).

the single-stage isolated PFC converter circuit with clamping and soft switching. The figure, Q1 is main switch, Q2 is the switch, C1 is the storage switch, C2 is the clamp capacitor, Cr is the sum of the parasitic capacitance of Q1, Q2 and the circuit. Circuit boost stage operating is in DCM, thus ensuring a higher power factor. Flyback converter-level design work is in the CCM, thus avoiding the higher current stress. The circuit uses active clamp circuit and soft-switch technology to limit the voltage stress of switching MOSFET. the renewable energy stored in the transformer leakage inductance, provide the soft-switching condition for switch Q1 and auxiliary switch Q2, thus reducing switching losses and improve the converter efficiency. Q1 and Q2 use the same control circuit and driver circuit, so that make the topology simplification.