Design and application of active power factor corrector in voltage critical working mode

　　O 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. To this end, a variety of methods to improve the power factor have been studied, among which 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 principle of a single-phase boost pre-boost converter. It is implemented by a dedicated chip and has the advantages of high efficiency, simple circuit, and low cost. The low-cost voltage-type critical working mode APFC control chip FAN7530 introduced in this article can achieve this function.

　　1 Circuit characteristics of FAN7530

　　1.1 Internal circuit

　　As shown in Figure 1, FAN7530N DIP8 package, there is also SMD package (FAN7530M), which contains self-starting timer, orthogonal multiplier, zero current detector, totem pole drive output, over-voltage, over-current and under-voltage protection circuits.

　　1.2 Performance characteristics of FAN7530 PFC control chip

　　The biggest feature of this chip is the voltage control critical working mode. Other performance characteristics are as follows:

　　160μs built-in startup timing circuit;
　　low THD and high power factor;
　　overvoltage, undervoltage, overcurrent protection;
　　zero current detector;
　　CRM control mode;
　　operating temperature low -40℃ ~ +125℃;
　　low startup current (40μA) and low operating current (1.5mA).

　　FAN7530 is a pin-simple, high-performance active power factor correction chip. 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.

　　2 Design of active power factor correction principle

　　2.1 Principle of power factor correction

　　As shown in Figure 2, the control chip uses FAN7530, and the on and off of power MOSFET S1 is controlled by the zero current detector of FAN7530. When the current in the zero current detector drops to zero, that is, the current in the boost diode D1 is zero, S1 is turned on, and the inductor L starts to store energy at this time. The current control waveform is shown in Figure 3. This zero current control mode has the following advantages:

　　Since S1 can only be turned on when the current in the energy storage inductor is zero, the switching stress and loss of MOSFET are greatly reduced. 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 the driving pulse time of the switch tube has no dead zone, the input current is continuous and sinusoidal, which greatly improves the power factor of the system.

　　2.2 Application design example

　　Technical requirements:

　　input grid voltage range AC 90~265V;
　　output DC voltage DC 400V;
　　output power 150W.

　　2.2.1 Design of PFC inductor

　　The electrical schematic diagram of the inductor is shown in Figure 4.

　　2.2.2 Selection of Boost MOSFET

　　2.2.3 Selection of boost diode

　　2.2.4 Selection of rectifier bridge

　　FIG5 shows the application circuit of FAN7530N in the APFC pre-converter.

　　3 Problems and solutions for using FAN7530

　　The faster the bootstrap diode in PFC, the better;
　　Pay attention to the connection between the source and ground of 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;
　　Adjustment of the metallized film capacitor after the rectifier bridge can change the resonance;
　　Add R/C between pin 1 and pin 3 of FAN7530, and adjust the parameters appropriately to reduce light load instability;
　　The capacitance value between pin 1 and pin 2 of FAN7530 affects the startup time;
　　This chip has many advantages and disadvantages in use.

　　4 Conclusion

　　After repeated tests, the design has successfully designed a 150W boost pre-converter and applied it to the adapter, adjusting the parameters of the PFC boost inductor, and determining other peripheral parameters through design tests, and calculating devices such as power MOSFET. Practice has proved that this solution is feasible and has certain application value.