Design of single-stage power factor correction switching power supply

1. Introduction

In order to reduce the pollution of internal switching power supplies of office automation equipment, computers and household appliances to the power grid, the International Electrotechnical Commission and some countries and regions have introduced standards such as IEC1000-3-2 and EN61000-3-2, which set limits on current harmonics. To meet the requirements for input current harmonic limits, the most effective technical means is active power factor correction (active PFC).

The currently widely used active PFC technology is a two-stage solution, namely active PFC boost converter + DC-DC converter, as shown in Figure 1.

The two-stage PFC converter uses two switches (usually MOSFETs) and two controllers, namely a power factor controller and a PWM controller. Only when a PFC/PWM combination controller IC is used can one controller be used, but two switches are still required. Two-stage PFC is very mature in technology and has long been widely used, but this solution has disadvantages such as complex circuit topology and high cost.

The PFC stage and DC-DC stage in a single-stage PFC AC-DC converter share a switch tube and a set of control circuits using PWM, simultaneously achieving power factor correction and output voltage regulation.

2. Basic circuit topology of single-stage PFC converter

2.1 Basic circuit of single-stage PFC converter

A single-stage PFC converter is usually composed of a boost PFC stage and a DC-DC converter. The DC-DC converter is divided into two types: forward and flyback. Figure 2 shows a basic single-stage isolated forward boost PFC circuit. The two circuits share a switch (Q1). The current through diode D1 charges the energy storage capacitor C1, and D2 prevents current backflow when Q1 is turned off. By controlling the on and off of Q1, the circuit simultaneously completes the shaping of the AC input current and the regulation of the output voltage.

Since the input of the full-wave bridge rectifier circuit is connected to the AC power supply line, the instantaneous input power changes at any time. To obtain a stable power output, it is necessary to rely on energy storage capacitors to achieve power balance. For DC-DC converters, they usually work in continuous mode (CCM), and the duty factor does not change with the load. The output voltage of the full-bridge rectifier has nothing to do with the load size. When the load is reduced, the output power decreases, but the PFC level input power is the same as when it is overloaded, so that the energy charged into C1 is equal to the energy extracted from C1, causing the DC bus voltage to rise significantly. The voltage stress on C1 often reaches more than 1000V, and the withstand voltage requirements for the switching device are very high. Since the voltage of the switching device is high, the current stress is large, the switching loss is large, and the power must be converted twice from input to output, the efficiency is low.

2.2 Improved single-stage PFC converter circuit

In order to reduce the high voltage on the energy storage capacitor and the converter efficiency, the basic circuit topology of the single-stage PFC shown in Figure 2 must be improved.

A single-stage PFC converter circuit using a transformer double-wire group to achieve negative feedback is shown in Figure 3. N1 and N2 windings are coupled windings of transformer T1.

When switch Q1 is turned on, voltage VC1 is applied to the primary winding of T1. Current will flow through the boost inductor L1 only when the rectified voltage is greater than the voltage on N1. When Q1 is turned off, the reverse voltage applied to L1 is the sum of VC1 and the voltage VN2 on N2 minus the input voltage. The addition of two coupled coils N1 and N2 provides a negative feedback voltage, reduces the voltage stress on C1, and improves efficiency. However, the addition of N1 and N2 will reduce the power factor and increase the current harmonic content. If an inductor is added between D2 and N1 to make the input current work in CCM, the voltage on C1 can be further reduced. In Figure 3. N1+N2 is required.

Figure 4 shows a CCM single-stage PFC converter circuit with a low-frequency auxiliary switch. Q1 is the main switch and Q2 is the auxiliary switch. Near the zero crossing of the input current, Q2 is turned on, short-circuiting the additional winding N1. When the input voltage is greater than a certain value, Q2 is turned off. Since Q2 is turned on only when the input voltage is very small and is blocked at other times, the current flowing through Q2 is very small, and the power loss of Q2 is also very small. Compared with the circuit in Figure 3, this circuit topology reduces the harmonic content of the input current, improves the power factor and efficiency, and reduces the voltage on the capacitor (C1).

FIG5 shows a single-stage isolated PFC converter circuit with active clamping and soft switching. In the figure, Q1 is the main switch, Q2 is the switch, C1 is the energy storage switch, C2 is the clamping capacitor, and Cr is the sum of Q1, Q2 and the parasitic capacitance in the circuit. The boost stage of the circuit works in DCM to ensure a high power factor. The flyback converter stage is designed to work in CCM to avoid high current stress. The circuit uses active clamping and soft switching technology to limit the voltage stress of the switching MOSFET. The regenerative energy stored in the transformer leakage inductance provides soft switching conditions for the main switch Q1 and the auxiliary switch Q2, thereby reducing switching losses and improving converter efficiency. Q1 and Q2 use the same control circuit and drive circuit, which simplifies the topology.

3. Single-stage PFC AC-DC converter based on Flyboost module

The single-stage PFC AC-DC converter circuit based on the Flyboost module is shown in Figure 6. The converter is based on the flyback boost topology, and its working state is divided into two working states: flyback transformer state and boost state. If Vin(t) is the instantaneous value of the Ac input voltage, Vc1 is the voltage on the energy storage capacitor C1, and n is the voltage ratio of the transformer T1, in a switching cycle of the flyback transformer state, when the switch Q1 is turned on, T1 is charged and stores energy; when Q1 is turned off, since (Vin(t))<(Vc1-nVo), D6 cannot be turned on, and all the energy stored in T1 is transferred to the output end. In this working state, the converter input current lin waveform at the output end of the full-bridge rectifier is a right triangle, and the average input current lin(avg) is:

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