Application research of MC34262 series PFC control chip

Abstract: This paper introduces the performance and characteristics of the MC34262 series PFC control chip, focusing on how to design circuit component parameters in APFC applications, and analyzing solutions to faults that are prone to occur in experiments.

1 Introduction

The traditional method of obtaining DC voltage from the 220V AC power grid through uncontrolled rectification has been widely used in power electronics technology and electronic instruments. However, this uncontrolled rectification causes the input current waveform to be seriously distorted and pulsed. In this way, on the one hand, it causes serious pollution to the power grid and interferes with the normal operation of other electronic equipment; on the other hand, it greatly reduces the power factor of the input circuit. For example, in medium and large uncontrolled rectification equipment, the power factor of the input circuit is roughly around 0.5 to 0.7, and some are even lower. Therefore, effective technical measures must be taken to reduce the distortion of the input current waveform and improve the power factor of the input circuit.

The methods of improving power factor can be summarized into two categories: one is the passive power factor correction method, which mainly expands the conduction angle of the input current through circuit design; the high-frequency compensation method can also be used to improve the conduction angle of the input current; the other is the active power factor correction method, which inserts a power factor correction device in series between the power grid and the power supply device. Among them, the single-phase BOOST circuit has been widely used due to its high efficiency, simple circuit, low cost and other advantages, and is called the active power factor correction (APFC) circuit. There are many types of active power factor correction control chips, including peak current control method, average current control method, etc. This article focuses on a new high-performance, zero-current control mode power factor correction control chip MC34262 (MC33262) produced by MOTOROLA, and analyzes its design points and experimental results in the application of power factor correction circuits.

Figure 1 Internal structure diagram of MC34262 series PFC control chip

Figure 2 Active power factor correction circuit block diagram

Figure 3 Current control waveform

Figure 4 Practical application of MC34262 series control chip in APFC

The MC34262 series PFC control chip is an 8-pin dual in-line plastic package (also available in surface mount package) device, which contains a self-starting timer, an orthogonal multiplier, a zero current detector, a totem pole drive output (0.5A), and overvoltage, undervoltage and overcurrent protection circuits. The specific internal structure block diagram is shown in Figure 1.

The biggest feature of the MC34262 series PFC control chip is the use of zero current conduction mode control. Figure 2 shows the block diagram of the active power factor correction circuit composed of the MC34262 series PFC control chip.

In the figure, the on and off of switch Q1 is controlled by the zero current detector in MC34262. When the current in the zero current detector drops to zero (that is, the current in the freewheeling diode D1 drops to zero), Q1 is turned on, and the inductor L starts to store energy. The current control waveform is shown in Figure 3. The outstanding advantages of this zero current control mode are:

(1) Since Q1 can only be turned on when the current in the energy storage inductor is zero, the stress and loss of the switch are greatly reduced. At the same time, there is no strict requirement for the recovery time of the diode, so the selection of an ordinary fast recovery diode can meet the design requirements; on the other hand, it eliminates the switch tube loss caused by the long recovery time of the diode, which greatly increases the reliability of the switch tube.

(2) Since there is no dead zone between the driving pulses of the switching tube, the input current is continuous and sinusoidal, which greatly improves the power factor of the system.

In addition, the power factor correction circuit composed of the MC34262 series active power factor correction controller has a simple structure and few peripheral circuit components, which greatly reduces the size of the circuit, reduces the system cost, and improves the system reliability.

3 Design points of MC34262 series PFC control chip applied in APFC circuit The actual application circuit of MC34262 series PFC control chip in APFC circuit is shown in Figure 4. The main technical requirements of the system are:

(1) Input grid voltage range: AC90V～265V

(2) Output DC voltage DC400V

(3) Output power 500W

According to the above requirements, the main component parameters of the APFC circuit are calculated first.

31 Peak current ILP in inductor L ILP=(1) Where PO——required output power 500W

η——The efficiency of the converter is 0.92

VAC (L) - minimum grid input voltage 90V

Then: ILP = ≈17A (2)

Considering that the voltage resistance of the switch tube should be reduced by 75%, if the output voltage of the boost converter is 400V, a switch tube with a voltage resistance of at least 500V should be selected, and the current should be greater than the peak inductor current.

32 Inductance L L=T(3)

Where: T is the switching pulse period. When the input grid voltage range is: AC90V～265V, the value of T is 40μs. At this time: L=≈200μH(4)

33 Calculation of R1 and R2 VO ≈ Vref (5)

Where: Vref is the reference voltage provided inside the chip, which is 2.5V. According to formula (5): (6)

Let R2 = 1.8MΩ, then R1 = 11kΩ

34 Calculation of overcurrent resistor R7

When the input grid voltage ranges from AC90V to 265V, the current sampling voltage VCS is set to 1V and must be less than 1.4V. At this time:

VCS=R7ILP（7） then(8)

Take R7=0.062Ω/3W

Calculation of 35R3 and R5

Let the input voltage of the multiplier VM = 3V, then: (9)

Where VAC (H) - the maximum input voltage of the power grid is 265V

From formula (9): (10)

Let R5 = 1.2MΩ, then R3 = 10kΩ

Common faults and solutions of MC34262 series PFC control chips in APFC circuits

The most common faults of MC34262 series PFC control chips in APFC circuits are difficulty in starting and susceptibility to interference in the overcurrent protection circuit.

The reason why it is difficult to start is that the error amplifier has a high output impedance characteristic and is easily disturbed by the outside world. When the control system is working normally, the voltage on pin 2 is close to the threshold voltage of the multiplier (about 2V). If it exceeds 2V, the output drive signal is automatically cut off. Therefore, to ensure the reliable operation of the system, a small compensation capacitor is connected between pins 2 and 6.

The reason why the overcurrent protection circuit is susceptible to interference is that when a strong interference current flows through the overcurrent protection sampling resistor R7, the overcurrent comparator is easily triggered and flipped, causing false triggering. To overcome the false operation of the overcurrent comparator, an RC filter circuit can be added between R7 and pin 4. Usually, the time constant of the RC filter circuit is 200ms.

5 Conclusion

This experimental prototype uses the MC34262PFC control chip to design a 500W power factor correction circuit. Since the system adopts zero current control mode, the stress and loss of the switch are greatly reduced, and there is no strict requirement for the selection of the freewheeling diode. In addition, the system has a simple circuit structure, small size, stable and reliable operation, and has a wide range of application prospects in medium-power APFC circuits.