Design and Application of Active Power Factor Correction Pre-Boost Converter

1 Introduction

Improving the power factor of the switching power supply can not only save energy, but also reduce the harmonic pollution of the power grid and improve the power supply quality of the power grid. For this reason, a variety of methods for improving the power factor have been studied, among which the active power factor correction technology (APFC for short) is one of the effective methods. It is achieved by adding a power factor correction device in series between the power grid and the power supply. The most commonly used method is the single-phase BOOST pre-boost converter , which is implemented by a dedicated chip and has the advantages of high efficiency, simple circuit, and low cost. The low-cost zero-point flow APFC control chip FAN7528N introduced in this article can achieve this function.

2 Circuit Features of FAN7528

2.1 As shown in Figure 1, FAN7528N has a DIP8 package and an SMD package (FAN7528M). It contains a self-starting timer, an orthogonal multiplier, a zero current detector, a totem pole drive output, overvoltage, overcurrent, and undervoltage protection circuits.

2.2 Performance characteristics of FAN7528 PFC control chip

The biggest feature of this chip is the zero current conduction variable frequency control mode. Other performance features are as follows:

« Built-in start timing circuit;

« Built-in R/C filter, eliminating the need for external R/C;

« Overvoltage and undervoltage comparator;

« Zero current detector;

« Single quadrant multiplier;

« 1.5% internal adjustable bandwidth;

« Low starting current and low operating current

FAN7528 is a high-performance active power factor correction chip with simple pins. It is an optimized, stable, low-power, high-density power chip with few peripheral components, saving PCB wiring space. Built-in R/C filter, strong anti-interference ability, and special circuits are added to suppress light load drift. There is no requirement for the auxiliary power range, and the output totem drive circuit limits the risk of power MOSFET short circuit, greatly improving the reliability of the system.

3 Active Power Factor Correction Principle Design

3.1 As shown in Figure 2, the control chip uses FAN7528. The on and off of the power MOSFET Q1 is controlled by the zero current detector of FAN7528N. When the current in the zero current detector drops to zero, that is, when the current in the boost diode D1 is zero, Q1 is turned on. At this time, the inductor L starts to store energy. The current control waveform is shown in Figure 3. This zero current control mode has the following advantages:

« Q1 can only be turned on when the current in the energy storage inductor is zero, which greatly reduces the switching stress and loss of the MOSFET. At the same time, there is no strict requirement for the recovery time of the boost diode. On the other hand, it eliminates the switching loss caused by the long recovery time of the diode and increases the reliability of the switch tube.

« Since there is no dead zone in the driving pulse time of the switch tube, the input current is continuous and sinusoidal, which greatly improves the power factor of the system.

3.2 Application Design Example

Technical requirements:

« Input grid voltage range: AC90V-265V

« Output DC voltage: DC400V

« Output power: 150W

Design of PFC inductor

Determine the core model

Core selection: EI40 Material: PC40 (AL ₌ 4860 ± 25%) nH / N ²

Output power: P ₀ =V ₀ I ₀ where V ₀ is the output voltage and I ₀ is the output current

_{Calculate the peak current I pk} of the inductor (η ₁ =0.98)

I _pk =2V ₀ I ₀ /(η ₁ ×V _in(peak) ), substitute the input voltage V _in =85V, 264V respectively to obtain,

Ipk1 = _2.71A , Ipk2 ₌ 0.87A

Calculate the inductance L of the inductor (set the minimum switching frequency f _sw(min) =33kHz)

L=η ₁ /(4 f _sw(min) V ₀ I ₀ (1/V ² _in(peak) +1/ (V _in(peak) ( V ₀ - V _in(peak) ))), substitute V _in =85V, IV _in =264V into the above formula to obtain L ₁ =560μH, L ₂ =530μH, and the actual inductance of L=535︿550μH is taken. Schematic diagram : as shown in Figure 4

Boost MOSFET selection:

_{Calculate the maximum effective current I Qrms} flowing through the MOSFET

I _Qrms =2√2 V ₀ I _0(max) /(η ₁ × _Vin(LL) )×(1/6-4√2 V _in(LL) /(9π×V ₀ )) ^1/2

Substituting the relevant values, I _Qrms = 0.955A

Therefore, the peak current flowing through the MOSFET is Ipk ₌ 1.2 × IQrms ₌ 1.15A

_{Calculate the maximum reverse voltage V DS(max)} that the MOSFET can withstand

V _DS(max) =1.2×264×√2=450V

Determine the specifications of the MOSFET

According to I _pk , V _{DS (max)} and the principle of reducing power consumption , Fairchild's MOSFET is selected. Its model and technical indicators are as follows:

FQP13N50 , V _DSS =500V , I _D =12.5A , R _DS(on) =0.43 Ω , P _D =170W TO-3P

Boost diode selection:

_{Calculate the average current I Davg} flowing through the diode

I _Davg =I _0(max) =0.4075A

Therefore, the peak current flowing through the diode is Ipk ₌ 1.2 × I0 _(max) = 0.489A

Calculate the maximum reverse voltage of the diode, VR _(max)

VR _(max) =1.2×V0 ₌ 480V

Determine the specifications of the diode

According to I _pk and VR _(max) , IXYS's HiPerFREDT ^M diode is selected. Its model and technical indicators are as follows:

DSEP 6-06AS , V _RRM =600V , I _FAV =6A , P _tot =55W , TO-252 A

Selection of rectifier bridge

_{Calculate the maximum reverse voltage VR(max)} that the rectifier bridge can withstand

V _R(max) = √ 2 × V _in(max) =375V

_{Calculate the effective current I rms} flowing through the rectifier bridge

I _rms =P _in /V _(in-max)rms =1.36A

Therefore, the maximum current flowing through the rectifier bridge is: 1.4×I _rms =1.90A

Determine the specifications and models of the rectifier bridge

According to the above conditions, RECTRON's rectifier bridge is selected, and its specifications and technical indicators are as follows:

RS406L , VRRM ₌ 600V , 6A

Other parameters are based on conventional APFC and refer to the FAN7527 user manual, which will be omitted here.

As shown in Figure 5, the application circuit of FAN7528N in APFC pre-converter

4 Problems and solutions when using FAN7528

« The faster the bootstrap diode in PFC, the better;

« Pay attention to the connection between the source and ground of the MOSFET to reduce the occurrence of resonance;

« The capacity of the high-voltage capacitor after PFC boost should be sufficient, and standard values ​​should be used as much as possible;

« The adjustment of the metallized film capacitor after the rectifier bridge can change the resonance;

« Add R/C between pin 1 and pin 3 of FAN7528 and adjust the parameters appropriately to reduce light load instability;

« The capacitance value between pin 1 and pin 2 of FAN7528 affects the startup time;

« The chip has many advantages and disadvantages in use.

5 Conclusion

The design has been repeatedly tested many times, the PFC boost inductor parameters have been adjusted, and other peripheral parameters have been designed and tested, and the power MOSFET and other devices have been calculated. A 150W boost pre-converter has been successfully designed, and the post-stage DC-DC has been successfully used in a 24VDC/5A output, 120W power factor correction switching power supply with a power factor of up to 0.998 and an overall efficiency of up to 88%.

According to this scheme, a 200W-300W power supply can also be designed. Practice has proved that this scheme is feasible and has certain application value.

References:

[1] FAN7528N User Manual and Application Design, February 2007

[2] Zhao Ke, Su Dayi. Application research of MC34262 series PFC control chip [J]. Power Technology Application, 2001, No. 1 • 2: P36-38.