Power supply: Single-stage PFC converter without input rectifier bridge

1 Introduction

Traditional AC/DC converters have large input harmonics, low power factors, and pollution to the power grid because they are directly connected to large energy storage capacitors behind the input rectifier bridge. In order to reduce harmonic pollution, AC/DC converters must be corrected for power factors. A more common method is to add a first-stage active power factor correction link to the converter, that is, a two-stage converter. However, the two-stage converter increases the cost and complexity of the converter, especially in low-power situations. For this reason, the concept of single-stage PFC was proposed, that is, the PFC stage and the DC/DC stage are integrated together and share the switch tube. Subsequently, a new type of single-stage PFC family was proposed, which received widespread attention, and various topologies and control methods of single-stage PFC emerged.

2 Single-stage PFC converter without input rectifier bridge

The common method for PFC stage is to add full-bridge rectification after the grid input. However, the power frequency rectifier bridge is not only large in size but also causes losses. Reference [1] integrates the single-phase PWM rectifier into the PFC stage, eliminating the input rectifier bridge and thus improving efficiency. Figure 1 shows two topological structures of PWM rectifier.

The working principles of Figures 1(a) and (b) are similar, both of which are equivalent to two Boost circuits, which can switch between the positive and negative half-waves of the input AC voltage. The advantage of Figure (b) is that the two MOSFETs have a common source, so there is no need to use an isolated drive, simplifying the design. There is a half-bridge switch arm hidden in Figure (a). Reference [2] successfully applied the PWM rectifier in Figure 1(a) to the electronic ballast, designed a half-bridge structure single-stage PFC electronic ballast without an input rectifier bridge, and obtained experimental results.

Based on this, this paper integrates the PWM rectifier and the symmetrical half-bridge in Figure 1(a) to design a single-stage PFC without an input rectifier bridge.

3 Working modal analysis

The single-stage PFC circuit shown in Figure 2 integrates a PWM rectifier and a half-bridge circuit, thereby eliminating the input rectifier bridge. When the AC input Vin is in the positive half-wave, the upper tube serves as an integrated switch tube for the PFC stage and the DC/DC stage. When the AC input Vin is in the negative half-wave, the lower tube serves as an integrated switch tube. The two sets of boost circuits implement PFC in the power frequency cycle, and the current on the inductor Lpfc always maintains a discontinuous mode so that its peak current automatically follows the input voltage. Since the subsequent stage is a half-bridge DC/DC converter, the duty cycle of the two switch tubes is D, and the output voltage is

Here Vo is the output voltage, n=ns/np is the ratio of the secondary winding to the primary winding. If the secondary winding is a balanced winding, the ratio of the two secondary windings to the primary winding is n1=n2=n. VC is the voltage on the energy storage capacitor.

Figure 3 shows the control signals for Q1 and Q2.

The following analyzes the working status of each switch mode. Before that, make some assumptions:

1. Assuming that the output filter inductance and transformer excitation inductance are large enough, the current on them can be considered as constant current.

2. All components are ideal devices.

a. When the input voltage is a positive half-wave, the duty cycle of Q1 is D, and the duty cycle of Q2 is also D:

Mode 1, Figure (4-a): In this mode, Q1 is turned on, the current on Lpfc increases to store energy, and C1 provides energy to the converter secondary through Q1.

Mode 2, Figure (4-b): In this mode, Q1 is turned off, and the inductor Lpfc current charges C1 and C2 through D1 and the body diode of Q2. The transformer leakage inductance Lr current and the excitation current charge C2 through the body diode of Q2.

Mode 3, Figure (4-c): At this time, Q2 is turned on and C2 provides energy to the secondary through the transformer.

Mode 4, Figure (4-d): In this mode, Q2 is turned off, and the transformer leakage inductance Lr current and the excitation current charge C1 through the body diode of Q1.

b. When the input voltage is a negative half-wave, the duty cycle of Q1 and Q2 is also D:

In this half power frequency cycle, Q2 is used as a common switch for the PFC stage and the DC/DC stage, but its working principle is completely similar to that of the positive half wave.

4 Simulation circuit and its waveform

This paper uses SIMetrix simulation software to simulate and analyze the single-stage PFC converter without input rectifier bridge. The circuit parameters are set as: output power = 50W, input voltage = 200, output voltage = 12V, inductance = 100µH, switching frequency = 100kHZ, output filter capacitor = 400uF, = 400uF. The chip used is uc1825, two-way control signal output, Figure 5 is the simulation circuit. The simulation output waveform is completely consistent with the theoretical analysis. The output voltage 12V and the capacitor (C1C2) voltage, the current waveform on the inductor Lpfc are shown in Figure 6. It can be seen from the figure that the input current follows the input voltage. The simulation results show that the above analysis of the operation of the single-stage PFC converter based on the non-input rectifier bridge is correct, and the circuit realizes the power factor correction function of the system.