A low power single-stage power factor correction circuit

Abstract: The principle of a single-stage power factor correction circuit is discussed and its experimental results are analyzed.

Keywords: single-stage power factor

A Low Power Single-stage Converter to Improve Power Factor

Abstract: The paper introduces the operating principle of a low power single-stage converter to improve

power factor, analyze the result of experiment.

Keywords:Single－ stage Power factor

1 Introduction

For converters with lower power, if a complex power factor correction circuit is used to improve the power factor on the source side, it will increase the cost and lose the application value. The circuit discussed in this article uses a combination of a boost inductor and a dual forward circuit to complete power factor correction and power output.

2 Circuit Principle

The circuit schematic is shown in Figure 1. In the figure, L1, VD2, VD3, switch tube S1 and energy storage capacitor C1 form a boost converter working in DCM (discontinuous current mode).

Figure 1 Circuit Schematic

This circuit uses a UC3845 as the control chip, and the feedback signal comes from the output end. The drive signal of UC3845 passes through a small transformer and becomes two drive signals with the same phase, which drive the two switch tubes S1 and S2 respectively. Since there is no current sampling, the circuit can only work in DCM mode, otherwise the current in the circuit will be out of control. The circuit must first ensure output voltage regulation, so the duty cycle does not change much, and the current waveform is shown in Figure 2.

Figure 2 Discontinuous Current Control Mode (DCM)

In DCM mode, the peak value ip of the input current in each switching cycle T is:

ip=Uin×D×T/L1(1)

Where: D—duty cycle T—switching period

Uin—input voltage L1—input inductance

The average input current iave in each switching cycle T is:

iave=ipD=UinD2T/L1（2）

Since the switching frequency is high enough, it can be considered that Uin is constant within a switching cycle. When the duty cycle and switching frequency remain unchanged, the average value of the input current is proportional to the input voltage, and it can automatically "track" the input voltage to form a sinusoidal waveform, thereby playing a role in power factor correction.

In DCM mode, the following conditions should be met:

Uin×ton≤(Uc－Uin)×toff（3）

Where: Uin—input voltage; ton—on time;

Uc—capacitor C1 voltage; toff—turn-off time.

When the above equation takes the equal sign, there is a maximum duty cycle

Dmax=ton/(ton+toff)=(Uc－Uin)/Uc（4）

The capacitor voltage Uc is limited by the capacitor's withstand voltage and cost, so it cannot be too high. Here, it is 430V. According to the domestic power grid, when the input voltage is 260V, the duty cycle Dmax = (430-260×1.414)/430 = 14.5%. It can be seen that the duty cycle is very small at this time, which will increase the loss of the main circuit switch tube, and at the same time require a large capacity of the energy storage element, low component utilization, and low overall efficiency.

In order to increase the duty cycle, a winding N2 is drawn from the transformer and connected in series in the circuit according to the polarity shown in Figure 1. At this time, the maximum duty cycle can be deduced from the formula (Uin + UN2) × ton ≤ (Uc - Uin + UN2) × toff

Dmax=(Uc－Uin+UN2)/(Uc+2UN2)(5)

Take UN2 = 70V, then when the input voltage is 260V effective value, the duty cycle

Dmax=(430-260×1.414+70)/(430+2×70)

=23.2%(6)

If UN2=100V, when the input voltage is 260V effective value, the duty cycle

Dmax=(430-260×1.414+100)/(430+2×100)

=25.8% (7)

It can be seen that the duty cycle has been greatly improved, which is very beneficial for improving circuit performance.

In addition, the addition of winding N2 can relatively increase the input current i when the input voltage Uin is small. Since the entire current average value iave remains unchanged, the peak value of the current will inevitably decrease, making the current waveform closer to a sine wave, which helps to improve the power factor.

3. Work status analysis

For the convenience of analysis, the working condition of the converter in one working cycle is divided into three stages, as shown in Figure 3.

Figure 3 Schematic diagram of working cycle

(1) Phase I: Switches S1 and S2, diode VD2 are turned on, input voltage Uin charges inductor L1, and charging current is i=(Uin+UN2)×ton/L1. At the same time, capacitor C1 transfers energy to the load through S2, N0 and S1.

(2) Phase II: Switches S1 and S2 are turned off, and VD2 is cut off due to reverse voltage. The current in the inductor charges capacitor C1 through VD3 until the current in the inductor becomes zero. At the same time, transformer N0 generates back electromotive force, which is magnetically reset through VD4, C1 and VD5, transferring part of the energy to the capacitor. In addition, part of the transformer's magnetic energy is also released to the output end through winding N3 and VD8, which helps to expand the stable range of the output voltage.

(3) In stage III, the current in the inductor is 0, the induced voltage is also 0, and VD3 is cut off due to the reverse voltage.

4 Experimental results

Using the above principle, a small power supply was made.

The technical requirements are as follows:

Input voltage AC220V Input frequency 50Hz

Output voltage DC48V Output current 4A

Operating frequency 150kHz

Key component parameters: Energy storage capacitor 220μF/450VKMH

Transformer turns: N1: N2: N3: N0 = 15: 13: 6: 44

Switching tube IR460

The input inductor is a very important component, and its selection directly affects the experimental results. The coil lead wire must be thick enough, otherwise the lead wire voltage drop will be large and the loss will be large. The air gap of the inductor cannot be too small, otherwise the inductor is easy to saturate, causing the current waveform to have a spike at the peak value, reducing the power factor; the air gap cannot be too large, otherwise there will be too many magnetic lines outside the core, the coil will heat up, and the loss will increase. In addition, the EI type core is not suitable for inductance, and the pot type core should be used.

The input inductor should not be saturated even when the current is at its maximum, that is, when the input voltage is at its highest. When the voltage of N2 is 100V and Uin is 260V, the above formula shows that D=25.8%, and the frequency f=150kHz, so ton=D/f=1.72μS. The peak current in the inductor ip=2×(N1/N0)×Io=2.73A, according to the formula

L=U/(di/dt)≈U/(△i/△t)

=(1.414Uin＋UN2)/(ip/ton)(8)

So L=290μH.

The experimental results are shown in Table 1

Table 1 Experimental results

The input current waveform is shown in Figure 4.

From the experimental data and current waveform, it can be seen that the circuit does have a certain effect on improving the power factor and achieves a higher power factor. However, due to the requirement of a small duty cycle during operation, the loss of components such as the switch tube is large, making the overall efficiency of the circuit low.

In addition, the experiment also found that when the load is light, the output voltage regulation range is small; when the load is heavy, the output voltage regulation range is large. This is because when the load is light, the capacitor discharges weakly, and the capacitor voltage reaches the voltage limit value quickly, which reduces the duty cycle of the PWM signal, reduces the output voltage, and destroys the output stability. Adding winding N3 and VD8 controls the output voltage, that is, the voltage on N3 can be controlled, and the voltage of N3 is proportional to the voltage of the energy storage capacitor, so the output voltage can indirectly control the voltage of the energy storage capacitor, so that the capacitor enters the voltage limit later and expands the output voltage regulation range.

Figure 4 Input current waveform

5 Conclusion

This circuit does have a certain effect on improving the power factor, but since it is only a principle circuit, there is still a lot to be improved as a practical circuit.