At present, boost circuit is widely used in power factor correction (PFC) technology of single-phase rectifier power supply. The traditional boost circuit works in the hard switching state. Its characteristic is that when it works in the discontinuous conduction mode, the peak value of inductance current is directly proportional to the input voltage, and the input current waveform follows the input voltage waveform, so the control is simple; The disadvantage is that the switch not only needs to pass through the large on state current, but also turn off the large peak current, which will cause great turn off loss and serious electromagnetic interference. Therefore, the soft switching technology in the boost circuit can not only improve the switching frequency, but also solve the four difficult problems of switching on and off loss, capacitive on, inductive off and diode inverse recovery. However, in terms of soft switching technology, predecessors have proposed several circuits, such as resonant converter, quasi resonant converter and zero switching PWM converter. Although these circuits can improve power factor and system efficiency in single-phase power factor correction circuit, they are not ideal on the whole. In this paper, the boost ZVT-PWM conversion circuit is used to make it work in the soft switching state, which is characterized by working in the continuous conduction mode. The advantage is that the on loss of the power switch and the reverse recovery loss of the diode are greatly reduced, which is a big step higher than the power factor correction using the traditional hard switching control technology. Through circuit simulation and actual circuit design, it is found that the purpose of power factor correction can be well achieved, and the switching loss of power transistor is significantly reduced, electromagnetic interference is suppressed, and high efficiency can be obtained.
The resonant inductors and capacitors of boost resonant converters (including quasi resonant and multi resonant converters) are always involved in energy transfer, and their voltage and current stresses are very large. In the zero switching PWM converter, although the resonant element does not always work in the resonant state, the resonant inductance is connected in series in the main power circuit, and its loss is large. At the same time, the voltage stress and current stress of the switching tube and resonant element are exactly the same as that of the quasi resonant converter. Therefore, a zero conversion PWM converter is proposed. It can be divided into zero voltage conversion PWM converter (boost ZVT-PWM) and zero current conversion PWM converter (boost ZCT-PWM). This kind of converter is another leap in soft switching technology. It is characterized by working in PWM mode, and the auxiliary resonant circuit only works for a period of time when the main switch is switched, so as to realize the soft switching of the switch; It does not work at other times, thus reducing the power consumption of the auxiliary circuit. Moreover, the auxiliary circuit is connected in parallel in the main power circuit, the operation of the auxiliary circuit will not increase the voltage and current stress of the main switch, and the voltage and current stress of the main switch is very small.
Boost ZVT-PWM converter
Main circuit topology and working principle
Working principle of circuit zero conversion
The boost ZVT-PWM conversion circuit is shown in Figure 1. The working principle and circuit operation mode of the circuit are analyzed below: the boost ZVT-PWM converter is different from the traditional boost converter. Figure 1 and Figure 2 are its circuit diagram and waveform diagram respectively. The boost ZVT-PWM converter adds a ZVT network on the basis of the traditional boost converter. The network is composed of auxiliary switch qzvt, resonant inductor LR, resonant capacitor Cr and diodes D2 and D3. When the circuit works, the auxiliary switch qzvt is opened before the main switch qmain to make the ZVT resonant network work, and the voltage on the capacitor Cr (i.e. the voltage at both ends of the main switch qmain) drops to zero, creating the zero voltage opening condition of the main switch qmain.
Figure 1 main circuit of boost ZVT-PWM converter
Fig. 2 waveform diagram of boost ZVT-PWM converter
Operation mode analysis
Assuming that the input inductance is large enough, it can be replaced by constant current source iin, and the output filter capacitance is large enough, and the output can be replaced by constant voltage source vo. Design t
1. T0 – T1: before T0, main switch qmain and auxiliary switch qzvt are turned off, diode D1 is turned on, and all load currents flow through D1. At time T0, the auxiliary switch qzvt is turned on. With the opening of qzvt, the current in the resonant inductor LR increases linearly to iin. The current in diode D1 decreases linearly to zero, and diode D1 turns off zero current, that is, the soft turn off of diode is realized. In the actual circuit, diode D1 needs to undergo reverse recovery to remove the junction charge. At this time, the voltage on ZVT resonant inductor LR is VO, and the time T01 when the inductor current rises to iin is:
2. T1-T2: at time T1, the current in resonant inductor LR rises linearly to iin, and LR and Cr begin to resonate. During the resonance period, Cr discharges until the voltage is zero. The drain voltage conversion rate Du / DT is controlled by Cr, which is actually the sum of CDs and Coss. While CR discharges, the current in the resonant inductor continues to rise. The length of time required for the drain voltage to drop to zero shall be 1 / 4 of the resonance period. At the end of the resonance period, the body diode of the main switch is turned on. At the end of this process, the body diode of qmain is turned on.
3. T2-T3: at the beginning of this period, the drain voltage of the main switch qmain drops to zero and its diode is turned on. The current flowing through the body diode is provided by the ZVT inductor. Since the voltage at both ends of the inductor is zero, the diode is in freewheeling state. At the same time, the main switch realizes zero voltage conduction.
4. T3-T4: at time T3, when the control circuit senses that the drain voltage of the main switch qmain drops to zero, turn on the main switch qmain and turn off the auxiliary switch qzvt at the same time. After the auxiliary switch qzvt is turned off, the energy in LR is transmitted to the load through diode D2.
5. T4-t5: at T4, the current in D2 drops to zero, and the working state of the circuit is the same as that of the ordinary boost converter. In practice, LR will resonate with the junction capacitor coss of the auxiliary switch qzvt, so that the anode voltage of diode D1 is negative.
6. T5-t6: the working process of the circuit at this stage is almost the same as that of the ordinary boost conversion. The main switch qmain is turned off, its drain source junction capacitance is charged to VO, and the main diode D1 starts to supply power to the load. Since the drain voltage is zero due to the initial junction capacitance, the off loss of the main switch qmain is greatly reduced.
7. T6-t0: this stage is in freewheeling state, diode D1 is on, and the circuit provides energy for the load through inductance L.
Circuit parameter design
Design index: single phase AC input 220V, fluctuation 15%, output power 2000W, efficiency 90%, output voltage 380V, converter working frequency 100kHz.
The simulation model is established in Simulink of computer simulation software MATLAB. Simulation parameters: VIN = 220V; L=200mH; fk=100kHz; Lr=4.7mH; Cr=470pF。 The simulation results are shown in Figure 3.
Fig. 3 input voltage / current simulation diagram
As can be seen from Fig. 3, the input current tracks the input voltage well and achieves the purpose of power factor correction.
Set up the main circuit and control circuit, and observe the waveform with an oscilloscope after debugging. Fig. 4 is the experimental waveform of input AC voltage / current. It can be seen from the figure that the input AC current is in the same phase as the input AC voltage, and the input current waveform is sine wave, which realizes the high power factor of the system. Due to the influence of switching and distribution parameters of power transistor, there are still some burrs in the voltage, which can be suppressed by using common mode inductance.
Fig. 4 waveform of AC input voltage and current
To sum up, soft switching PFC can be realized by using boost ZVT-PWM converter in single-phase power factor correction circuit. The experimental results show that the circuit not only achieves the purpose of power factor correction, but also reduces the loss of switch, suppresses electromagnetic interference and improves the efficiency of the system.
Responsible editor: GT