Design of high power factor boost circuit based on L6562

0 Introduction

Boost is a voltage-boosting circuit. The advantage of this circuit is that it can make the input current continuous and can be modulated throughout the entire sinusoidal cycle of the input voltage, so a very high power factor can be obtained. The inductor current of this circuit is the input current, so it is easy to adjust. At the same time, the gate drive signal ground of the switch tube shares the same ground with the output, so the drive is simple. In addition, since the input current is continuous, the current peak of the switch tube is small, so it has strong adaptability to input voltage changes.

Energy storage inductors play a key role in boost circuits. Generally speaking, they have large inductance, many turns, and large impedance, which can easily lead to inductor saturation and increased heat generation, seriously threatening product performance and life. Therefore, the design of energy storage inductors is one of the key points and difficulties of boost circuits. This paper designs a boost circuit based on ST's L6562 and analyzes the design method of magnetic components in detail.

1 Basic Principles of Boost Circuit

The boost circuit topology is shown in Figure 1. In the figure, when the switch tube T is turned on, the current IL flows through the inductor L. Before the inductor is not saturated, the current increases linearly, and the electric energy is stored in the inductor in the form of magnetic energy. At this time, the capacitor Cout discharges to provide energy for the load; and when the switch tube T is turned off, the magnetic energy in the coil will change the voltage VL at both ends of the coil L to keep its current IL from changing suddenly. In this way, the voltage VL converted by the coil L is connected in series with the power supply Vin, and supplies power to the capacitor and the load at a voltage higher than the output. As shown in Figure 2, it is a relationship diagram between its voltage and current. In the figure, Vcont is the control signal of the power switch MOSFET, VI is the voltage at both ends of the MOFET, and ID is the current flowing through the diode D. Based on the current IL, the working mode of the boost circuit can be divided into three types: continuous mode, intermittent mode and critical mode.

2 Boost-APFC circuit design under critical state

The typical topology of the Boost-APFC circuit in the critical working mode based on L6562 is shown in FIG3 , and FIG4 is a waveform diagram of its APFC working principle.

The principle of using Boost circuit to achieve high power factor is to make the input current follow the input voltage and obtain the desired output voltage. Therefore, the parameters required for the control circuit include instantaneous input voltage, input current and output voltage. The multiplier connects the input current control part and the output voltage control part and outputs a sinusoidal signal. When the output voltage deviates from the expected value, such as when the output voltage drops, the output voltage of the voltage control link increases, so that the output of the multiplier also increases accordingly, thereby increasing the effective value of the input current accordingly to provide sufficient energy. In this type of control model, the effective value of the input current is modulated by the output voltage control link, while the input current control link keeps the input current changing in a sinusoidal manner to track the input voltage. Based on this type of control model, this paper uses ST's L6562 as the control chip and gives a design method for the Boost-APFC circuit.

The pin functions of L6562 are as follows:

INV: This pin is the inverting input of the voltage error amplifier and the output voltage overvoltage protection input;

COMP: This pin is both the output of the voltage error amplifier and an input of the chip's internal multiplier. The feedback compensation network is connected between this pin and pin INV.

MULT: This pin is another input terminal of the chip's internal multiplier;

CS: This pin is the inverting input of the PWM comparator inside the chip, and can detect the MOS tube current through resistor R6;

ZCD: This pin is the inductor current zero-crossing detection terminal, which can be connected to the secondary winding of the Boost inductor through a current-limiting resistor. The selection of R7 should ensure that the current flowing into the ZCD pin does not exceed 3 mA;

GND: This pin is the chip ground. All chip signals use this pin as a reference. This pin is directly connected to the main circuit ground. GD: This is the MOS tube drive signal output pin. To prevent the MOS tube drive signal from oscillating, a resistor of more than ten ohms to several tens of ohms is generally connected between the GD pin and the gate of the MOS tube. The size of the resistor is determined by the actual circuit.

VCC: Chip power pin. This pin is connected to both the startup circuit and the power circuit.

In addition, when designing the circuit, the voltage regulator D2 should be a 15 V voltage regulator, the capacitor C2 should be a 10 μF electrolytic capacitor; the diode D5 should be a fast recovery diode (such as 1N4148); and the resistor R3 should be a resistor of several hundred kilo-ohms.

Figure 5 shows the actual circuit diagram of the APFC power supply composed of L6562. In the figure, the input AC is rectified by the rectifier bridge and converted into pulsating DC as the input of the Boost circuit; capacitor C4 is used to filter out the high-frequency signal in the inductor current and reduce the harmonic content of the input current; resistors R1 and R2 form a resistor divider network to determine the waveform and phase of the input voltage, and capacitor C10 is used to eliminate the high-frequency interference signal of pin 3; a secondary winding of the Boost inductor L, on the one hand, transmits the inductor current zero-crossing signal to pin 5 of the chip through resistor R7, and on the other hand, serves as the power supply for the normal operation of the chip; the chip drive signal is transmitted through resistors R8 and R9 is connected to the gate of the MOS tube; resistor R11 is used as the inductor current detection resistor to sample the rising edge of the inductor current (MOS tube current). One end of the resistor is connected to the system ground, and the other end is connected to the source of the MOS tube, and is connected to pin 4 of the chip through resistor R10; resistors R5 and R6 form a resistor divider network, and form a negative feedback loop for the output voltage; capacitor C9 is connected between pins 1 and 2 of the chip to form a compensation network for the voltage loop; resistor R4, capacitor C6, diode D5, voltage regulator D6 and the secondary side of the Boost inductor together form the chip power supply.

3. Design of Boost Inductor

This design uses the AP rule to design the boost inductor. The principle is to first calculate the required inductance according to the design requirements:

In the formula, Virms is the effective value of the input voltage; Vo is the output voltage, fsw(min) is the minimum operating frequency of the MOS tube, usually above 20kHz; Pi is the input power. The required AP value is calculated as:

In the formula, Ku is the core window utilization, Jc is the current density, and IL(pk) is the peak inductor current.

According to the calculation result of formula (4), the AP value of the magnetic core can be selected (greater than AP_req, AP=AeAw, unit is m4).

Then calculate the primary turns and required air gap based on the selected core. The secondary turns are generally selected at 10:1.

4 Experimental waveform analysis

In order to verify the rationality of the above design, this paper sets the minimum input voltage to 187 V, the maximum input voltage to 264 V, the input frequency to 50 Hz, the output voltage to 400 V, PF=0.99, the efficiency to 87%, the output power to 26.5 W, and the minimum operating frequency to 65 kHz for physical experiments. At the same time, according to the calculation and IL(pk)=465.3 mA, the wire is selected to be mm, Jc=4/mm2, L=2.99 mH (when L=2.7 mH, the minimum frequency is verified to be 72 kHz>65 kHz, which can meet the design requirements).

Assuming Ku = 0.3, δBmax = 0.3T, we can calculate from formula (4):

AP_req(min)=6．64×10-10m4

In this way, the magnetic core EE16/6/5 can be selected, and its AP=7.5×10-10m4 can meet the design requirements; and Np=218.1 turns is calculated by formula (5), 215 turns are taken, and it is verified that δBmax=0.304T and the air gap lgap=0.41 mm.

The results of the test model built according to the above calculation parameters are shown in Figure 6.

As shown in Figure 6, the input current can follow the input voltage well, and the current and voltage phase difference is close to zero, so high power factor control can be achieved. In addition, the current of MOSFET is a high-frequency triangular wave, and its envelope is the input voltage. Since MOSFET can achieve soft switching, it can effectively reduce switching losses. According to the test results, the PF of this circuit can reach more than 0.998, and the THD is below 5%.

5 Conclusion

This paper designs a Boost high power factor circuit based on the L6562 chip, and uses the AP rule to design its key component, the Boost inductor. Tests have shown that the circuit has a small starting current, few peripheral components, and low cost, and can simultaneously meet the requirements of light weight, good stability, and high reliability of the power supply system. Experiments have shown that the AP rule is a fast and accurate design method.