Research on Power Factor Correction Effect of Single-Stage PFC Converter

1 Introduction

In order to make the input current harmonics of the switching power supply meet the requirements, power factor correction (PFC) must be added. Currently, the most widely used is the two-stage solution of PFC stage + DC/DC stage, which has its own switching devices and control circuits. This solution can achieve good performance, but its disadvantages are complex circuits and high costs.

In recent years, many single-stage power factor correction AC/DC converters have been proposed ^[1] , especially in low-power applications. In a single-stage PFC converter, the PFC stage and the DC/DC stage share a switch tube and a set of control circuits to simultaneously adjust the input current and output voltage. Its advantages are simple circuits and low cost.

This paper analyzes the effect of power factor correction on a single-stage PFC converter, analyzes the distortion of the input current, and derives the power factor expression of the converter. The simulation and experimental results prove the correctness of the theoretical analysis.

2 Effects of Power Factor Correction

As shown in Figure 1, a single-stage power factor correction converter usually consists of a Boost converter and a DC/DC converter ^[1,2] . The main current waveforms of the circuit are shown in Figure 2.

2.1 Working Principle of the Circuit

Because the switching frequency is much greater than the frequency of the AC input power supply, it is assumed that v _AC is a constant value within one switching cycle .

Figure 1 Single-stage PFC converter

In the figure: v _AC is the AC input power;

L1 is the Boost inductor _;

C1 is the intermediate energy storage capacitor _;

RL is _the converter load.

Figure 2 Main current waveform of the circuit

In the figure: ugs is _the control signal of the switch tube S;

Ts _is the switching period;

D is the duty cycle.

In one switching cycle, the circuit works as follows.

State 1 [ t ₀ - t ₁ ] S, D ₂ and D ₃ are turned on, D ₁ and D ₄ are turned off, the power supply v _AC charges the inductor L ₁ , and the current flowing through the inductor L ₁ increases linearly. C ₁ discharges to L _o , C _o and R _L through T _1. S is turned off at t ₁ , and the current on the inductor L _{1 is the maximum value:}

i _{L 1,P} = DT _s （1）

i _D1 =0, i _D2 = i _{L 1} (2)

State 2 [ t1 - t2 ] S, D2 and D3 are off, D1 and D4 are on, v AC and L1 charge C1 _through D1 _, and _the voltage _across the _load RL is maintained by _{the energy} storage of _L0 and _{C0 .} At _{t2 ,} the energy in L1 _is completely _released and the current is zero. During _this period

i _{L 1} = i _{L 1,P} - ( t - DT _s ) (3)

i _D1 = i _{L 1} , i _D2 =0 (4)

State 3 [ t2- ( t0 + Ts ₎ ]S, D2 _and D3 _are cut off. Due to the existence of D1, _{the current on L1} cannot _be reversed and is therefore zero, that is, D1 _{is also cut} off, D4 _is still on, and the voltage across _{the load RL} is maintained by the energy storage _of L0 and Co0 .

2.2 Input current analysis

During states 1 and 2, the energy in the boost inductor is completely released. According to the principle of flux conservation,

| v _AC | DT _s =( V _{C 1} - | v _AC |) D ₂₁ T _s (5)

It can be obtained that D ₂₁ = D （6）

Therefore, during one switching cycle, the average input current is

i _{L 1 (avg)} = ( i _{L 1, P} D + i _{L 1, P} D ₂₁ ) = D ₂ T _s (7)

Let | v _AC |=| V _IN sin(ω t )|, where V _IN is the peak value of the input voltage. So

i _{L 1(avg)} == k β （8）

Where: k =;

β=.

In a single-stage PFC converter, the input current is decomposed into a triangular pulse wave at a fixed duty cycle, and the current peak will automatically follow the input voltage. However, the current waveform obtained by the voltage following method is not an ideal sine wave. Since the discharge time of the Boost inductor is affected by V _{C 1} , the average input current presents a certain degree of distortion [2]. It can be seen from formula (8) that there is a fixed relationship between the average input current and β, as shown in Figure 3.

Figure 3 Average input current waveform

2.3 Power factor expression

The effective value of the input current is

i _{L 1(rms)} = （9）

Let z =, then we have

i _{L 1(rms)} = k β （10）

The average input power of the converter is

P _IN =| v _AC | i _{L 1(avg)} dω t = V _IN k β= y (11)

Where: y =

The power factor of the converter can be expressed as

PF == (12)

Where: V _rms =.

It can be seen from formula (12) that there is also a fixed relationship between the power factor of the converter and β.