Research on Hard Switching and Soft Switching Power Factor Correction Circuits

1 Introduction

The front stage provides direct current from the 220V AC grid rectification, which is a basic current conversion scheme widely used in power electronics technology and electronic instruments. However, the rectifier- capacitor filter circuit is a combination of nonlinear devices and energy storage elements. Therefore, although the input AC voltage is a sine wave, the input current waveform is severely distorted and pulse-shaped, containing a large number of harmonics, making the power factor of the input circuit less than 0.7.

The low input power factor of electrical equipment will mainly cause the following hazards: harmonic current seriously pollutes the power grid and interferes with other electrical equipment; it is easy to cause line faults such as overheating of lines and distribution devices, and grid resonance; it increases the capacity of lines, transformers and protection devices; the superimposed three-phase third harmonic current flows through the neutral line, causing the neutral line to overcurrent and easily damaged.

Therefore, we must take appropriate measures to reduce the distortion of the input current waveform and improve the input power factor to reduce grid pollution. For example, the Ministry of Information Industry requires that the power factor of power supply equipment above 1500W must be higher than 0.92 in the network access inspection of communication power supply ; the power factor of power supply equipment below 1500W must be higher than 0.85.

At present, the main methods used to improve the power factor are: passive inductor filtering, which is effective in suppressing high-order harmonics, but is large in size and weight, and its application in product design will become less and less; inverter active filtering, which responds quickly to each harmonic, but the equipment is expensive; three-phase high power factor rectifier, which has high efficiency and good performance, and its control strategy and topology are in continuous development in recent years. Single-phase active power factor correction (APFC), usually using Boost circuit and CCM working mode, is currently being used more and more widely in product design due to its good correction effect.

This paper mainly introduces the working principle, functional characteristics and experimental waveform analysis of two commonly used APFC chips UC3854 and UC3855, and makes a comparative study.

Hard-switching active power factor correction based on 2UC3854

Circuit

2.1 Working Principle

UC3854 is a high power factor correction integrated control circuit chip . Its main features are: PWM boost circuit, power factor up to 0.99, THD <5%, suitable for any switching device, average current control mode, constant frequency control, accurate reference voltage. Its structure is shown in Figure 1:

UC3854 includes: voltage error amplifier, analog multiplier/divider, current amplifier, fixed frequency pulse width modulator, gate driver for power MOS tube, comparator for overcurrent protection, 7.5V reference voltage, as well as soft start, input voltage feedforward, input voltage clamp, etc.

The analog multiplier/divider is the core of the power factor correction chip. Its output IMO reflects the line current and is therefore used as the reference current. The relationship between IMO and the multiplier input current IAC (IAC is proportional to the instantaneous value of the input voltage) is:

IMO=IAC(UAO－1.5)/KU2ms

(corresponding to IM=AB/C in Figure 1)

Where IMO and UAO are the output signals of the voltage error amplifier, which are

Figure 1 Circuit diagram of UC3854

Subtracting 1.5V is a requirement of chip design; K is a constant in the multiplier, equal to 1; Ums is the feedforward voltage, approximately 1.5~4.77V, provided by the input voltage of APFC after voltage division.

The analog multiplier/divider divides by U2ms to play a feedforward role. On the one hand, the chip internal clamps Ums, eliminating the influence of the input voltage on the voltage loop gain, making the voltage loop gain independent of the input voltage; on the other hand, the output of the voltage error amplifier can also stabilize the input power and does not change with the change of the line voltage. For example, when the input voltage is doubled, IAC and Ums, which reflect the change of the input voltage, are both doubled. From the above formula, it can be seen that IMO will be halved, and the input current is halved by modulation, thereby keeping the input power unchanged. In addition, the voltage error amplifier has an output clamp that can limit the maximum power of the circuit . The input of the feedforward voltage uses a second-order low-pass filter, which can not only improve the anti-interference ability, but also does not affect the rapid response of the feedforward voltage input to the grid fluctuation.

The output voltage range of the voltage error amplifier is 1 to 5.8V. When the output voltage is lower than 1V, the output of the multiplier will be suppressed. The maximum output of the voltage error amplifier is internally limited to 5.8V to prevent output overshoot; in order to reduce the crossover dead zone generated when the input voltage is too low, the nominal voltage of the AC input terminal is 6V, and a resistor is also used to connect this port to the internal reference, so that the crossover distortion of the line current will be minimized.

The switch tube and diode of UC3854 both work in the hard switching state, which mainly brings the following problems:

(1) When the switch is turned on, the current rises and the voltage drops at the same time, and when the switch is turned off, the current drops and the voltage rises at the same time, resulting in large turn-on and turn-off losses of the switch;

(2) When the switch device is turned off, the inductive element induces a large peak voltage, which may cause the switch tube voltage to break down;

(3) When the switch device is turned on, the energy stored in the junction capacitance of the switch device may cause the switch device to overheat and be damaged;

(4) When the diode changes from on to off, there is a reverse recovery problem, which can easily cause the DC power supply to short-circuit instantly.

2.2 Experimental Results

The parameters of the APFC device made with UC3854 are as follows:

Input voltage range: AC80~270V;

Output voltage: 410V

Switching operating frequency: 72kHz;

Input inductance: 1.6mH;

Output capacitance: 2160μF

Power: 1200W

The voltage waveforms at both ends of the switch tube and the voltage waveforms at both ends of the input inductor are tested and printed out using a digital oscilloscope as shown in Figures 2 and 3.

From the above waveforms, it can be seen that there is a voltage spike on the switch tube ; and when the switch tube is turned off and the diode is turned on, and when the switch tube is turned on and the diode is turned off, a large voltage spike is induced on the input inductor. In order to overcome the shortcomings of hard-switching APFC and further improve the performance, UC introduced UC3855.

3UC3855 Soft switch Active power factor correction

Circuit

Figure 2 Waveforms at both ends of the switch tube Figure 3 Waveforms at both ends of the input inductor

3.1UC3855 Working Principle

UC3855 is a high power factor corrector integrated control chip that can achieve zero voltage conversion . It uses zero voltage conversion circuit and average current mode to generate stable, low-distortion AC input current. It does not require slope compensation and has a maximum operating frequency of 500kHz. It has ZVS detection, a main output driver and a ZVT output driver. Due to the use of soft switching technology, the loss during diode reverse recovery and MOSFET turn-on can be greatly reduced , thus having the characteristics of low electromagnetic radiation and high efficiency. Its structure is shown in Figure 4.

UC3855 is also mainly composed of multiplication, division, and square circuits, providing programmed current signals for the current loop (IMO=IAC(UAO-1.5)/KU2ms). There is a high-performance current amplifier with a bandwidth of 5MHz inside the chip, and it has overvoltage, overcurrent and hysteresis undervoltage protection functions, input line voltage clamping function, and low current starting function. The internal multiplier current limiting function can suppress power output at low line voltage. Compared with UC3854, the circuit functions added by UC3855 are mainly: overvoltage protection; zero voltage conversion (ZVT) control circuit working up to 500kHz; with current synthesizer, only the inductor current when the main switch tube is turned on needs to be detected, and the current flowing through the inductor and diode when the main switch tube is turned off can be constructed through the current synthesizer inside the chip, so one less current transformer can be used than UC3854. This not only improves the signal-to-noise ratio, but also reduces the loss of current detection.

In general, UC3855 has higher power factor (close to 1), higher efficiency, and lower electromagnetic interference (EMI).

3.2ZVT-PFC circuit Principle

Figure 5 is a schematic diagram of the ZVT-PFC circuit. S is the main switch tube , and the resonant branch composed of S1, Lr, Cr, and VD1 is connected in parallel with the main switch tube . The auxiliary switch S1 is turned on before the main switch S, making the resonant network work, and the capacitor voltage (i.e., the main switch voltage) resonates to zero, creating the condition for the main switch to be turned on at zero voltage. When the auxiliary switch tube is turned on, the diode current line

Figure 4 Circuit diagram of UC3855

Figure 5 ZVT-PFC circuit schematic

Figure 6 Current synthesizer waveform

The voltage drops to zero, and the diode VD achieves zero current cutoff (soft turn-off). The main advantages of ZVT-PFC are: the main switch is turned on at zero voltage and maintains constant frequency operation; the diode VD is cut off at zero current; the current and voltage stress are small; the operating range is wide; the disadvantages of ZVT-PFC are: the auxiliary switch S1 works under hard switching conditions, but the current flowing through it is very small compared to the main switch, so its loss can be ignored.

Figure 6 is the waveform of the current synthesizer. The upper waveform is the waveform of the inductor current synthesized by the current synthesizer, and the lower waveform is the actual waveform of the inductor current. From Figure 6, we can see that the two waveforms match well. The measurement results also show that the error between the reconstructed waveform and the actual waveform is large when the line voltage is high, and a slight deviation in the current synthesis circuit can cause errors.

Table 1 and Table 2 show the relationship between the distortion factor, power factor and AC line voltage of UC3855.

Table 1 Relationship between distortion factor, power factor and AC line voltage (first-order error amplifier clamping circuit)

AC line voltage (V) Distortion factor (%) Power factor

Table 2 Relationship between distortion factor, power factor and AC line voltage (second-order error amplifier clamping circuit)

AC line voltage (V) Distortion factor (%) Power factor

4 Comparative conclusion

Figure 7 is a graph of efficiency data obtained by measuring the ZVT-PFC circuit and the hard-switching PFC circuit (eliminating the zero-transition part). The hard-switching PFC circuit also requires a fan to maintain the normal operation of the power device.

Figure 7 Efficiency data diagram

Operating temperature. From the above data graph, it can be seen that the efficiency of the PFC circuit with ZVT (corresponding to the chip UC3855) is significantly better than that of the hard-switching PFC circuit (corresponding to the chip UC3854). It can also be seen from the figure that the PFC circuit with ZVT is significantly better than the hard-switching PFC circuit, especially at low input voltage, because the low input voltage has a high input current , which introduces high input loss in the hard-switching circuit.