A single-stage PFC circuit design based on three levels

O Introduction

At present, switching converters with power factor correction function are usually divided into two types: two-stage structure and single-stage structure. The two-stage structure circuit has good performance, but the number of components is large, and the cost will increase compared with the circuit without PFC function. In the single-stage PFC converter, the PFC stage and the DC/DC stage share the switch tube, and there is only one set of control circuits, which can realize the shaping of the input current and the regulation of the output voltage at the same time. However, there is actually a very serious problem in the single-stage PFC circuit: when the load becomes lighter and reaches the critical continuous state, the excess input energy will charge the intermediate energy storage capacitor. This process will cause the voltage across the intermediate energy storage capacitor to reach a very high value. In this way, in the circuit, for the 90-265 V AC power grid, the voltage will reach or even exceed 1000 V. In terms of current capacitor technology and power device technology, such a high voltage is not practical. Therefore, reducing the bus capacitor voltage and adapting to wide voltage input occasions and load changes have become the hot spots of single-stage power factor correction technology.

This paper studies the circuit topology and control method suitable for high-power single-phase single-stage converters, and proposes a design scheme for a single-stage power factor correction AC/DC converter. The PFC converter is based on a three-level LCC resonant converter topology. The entire converter consists of a boost power factor regulator and a three-level resonant converter. The multi-level resonant converter can limit the voltage drop when the switch tube is turned off to half the DC bus voltage. At the same time, the converter can stably adjust the output voltage and obtain a stable DC bus voltage within a wide load variation range. The control method of the converter is realized by two control loops, in which the output voltage is adjusted by controlling the switching frequency of the DC converter; the DC bus voltage is adjusted by controlling the duty cycle of the boost regulator.

1 Circuit topology and working principle

The circuit topology of the three-level single-stage PFC given in this article is shown in Figure 1. In the figure, the converter input boost inductor is directly connected to a pair of switches below, and the DC-DC part is composed of a three-level LCC resonant circuit. The boost inductor can work in CCM or DCM mode. The intermediate energy storage capacitors Cb1 and Cb2 have equal capacity, so they can evenly divide the input DC voltage under the stable working state of the circuit, and work with the clamping diodes Dc1 and Dc2 to reduce the voltage stress of the switch. The working timing of the switch in the circuit is shown in Figure 2.

Before analyzing the working mode of the converter, the following assumptions can be made:

(1) All switches, diodes, inductors, and capacitors are ideal devices.
(2) Capacitors Cb1 and Cb2 are large enough and equal, and their voltages are both Vbus/2.
(3) The output filter capacitor Co is large enough, and its voltage is Vo.

Based on the above assumptions, in phase 1 [t0, t1]: switches S3 and S4 are turned on. The boost inductor (Lin) stores energy and the inductor current increases linearly. The current flowing through the switch is the sum of the resonant circuit and boost inductor currents. The voltage VAB across the resonant circuit is -Vbus/2;

Phase 2 [t1, t2]: Switch S4 is turned off, and clamp diode Dc2 clamps its voltage at Vbus/2. The boost inductor current will flow through the upper pair of switches and discharge its body capacitance. At this time, VAB is zero;

Phase 3 [t2, t3]: Switch tube S3 is turned off (because its body capacitance is discharged, S3 will be turned off with zero voltage), the inductor current continues to charge the intermediate energy storage capacitor, and the body capacitance of S1 and S2 is discharged. After they are fully discharged, their body diodes are turned on. At this time, the VAB voltage is Vbus/2.

Phase 4 [t3, t4]: Switches S1 and S2 are turned on at zero voltage at the same time. The boost inductor current and the resonant circuit current flow through S1 and S2 at the same time. At this time, the VAB voltage remains unchanged and is still Vbus/2;

Phase 5 [t4, t5]: The switch tube S1 is turned off, and the voltage is clamped at Vbus/2 by the clamping diode Dc1. The resonant current flows through S2 and Dc1, and the voltage drop of VAB is zero at this time;

Phase 6 [t5, t6]: Switch S2 is turned off, the direction of the resonant current is reversed, and the body capacitance of S3 and S4 is discharged; after complete discharge, their body diodes are turned on. Until the next cycle begins, S3 and S4 will be turned on with zero voltage.

2 Control strategy and steady-state analysis

2.1 Control strategy

The converter in this article is composed of multiple switching tubes. It has more than one control variable. Therefore, when designing, the switching frequency of the resonant circuit and the duty cycle of the boost circuit can be used to control the output voltage and DC bus voltage respectively. In this article, the duty cycle of the boost circuit is selected to obtain the required DC bus voltage. The advantage of this control method is that the required DC bus voltage can be obtained regardless of how the load changes.

2.2 Boost Mode

The boost circuit in this paper is set to work in DCM state. In this way, when the boost inductor is charged, the inductor current will increase linearly from zero, and its current peak value is:

Therefore, in one cycle, its average current is:

Since the DC bus voltage can vary according to different AC input voltage peaks, it can be expressed as:

Therefore, when the input AC voltage range is 90Vms~265Vms, its DC bus voltage is 350~650V.

3 Simulation results

During the simulation, it is assumed that the specific circuit parameters of a single-stage PFC circuit designed through the above analysis are: output voltage 48 V, power 2.3 kW, Vin=90~265Vms, Lr=7μH, Cs=10nF, Cp=15 nF, N1/N2=4, Lin=0.95 μH, energy storage capacitor Cbl=Cb2=4700μF.

If Figure 3 shows the waveforms of its input voltage and input current, and the AC input voltage Vin is 265Vms at this time, then Figure 4 shows the simulation diagram of the power factor and DC bus voltage under load changes. As can be seen from Figure 4, under different load conditions, the DC bus voltage is basically maintained at around 650 V; at the same time, the converter also has a high input power factor.

4 Conclusion

This paper studies and analyzes a relatively novel single-stage power factor correction circuit topology. The converter consists of a three-level resonant converter combined with a boost inductor. The converter has input and output isolation, high power factor, low switch tube voltage stress, and can achieve zero voltage switching. The simulation results show that the output voltage of the circuit is stable, and at the same time, even under light load conditions, its DC bus voltage can still be maintained within a stable range. This proves the high efficiency of the circuit and control method in this article.