1.Background
In AC-DC SMPS applications, bridge rectifiers are used to convert AC inputs to DC bus voltages and power a second-stage isolated DC-DC converter. Among them, the mismatch between the current and the input voltage will bring a lot of harmonic feedback to the power grid. Therefore, electronic instruments need to follow the power factor specifications and harmonic limits specified in the relevant standards when they are connected to the power grid. To address these issues, power factor correction techniques are commonly used in most AC-DC applications.
2.Single-stage AC-DC topology
In this paper, we propose a single-inductor structure LLC resonant topology that integrates PFC functions, as shown in Figure 1. This topology consists of a boost circuit and a half-bridge LLC circuit that uses the same pair of switches, MOS Q1 and Q2. L1 is the main inductor of the boost circuit. When the MOSFETs Q1 and Q2 of the boost circuit start to switch alternately, L1 can smooth the input current, reduce phase mismatch, increase the PF value, and realize LLC resonant conversion. Both Q1 and Q2 on the primary side can operate in ZVS mode, and the SR MOS on the secondary side can operate in ZCS (Zero Current Switch) mode. This can effectively improve the efficiency of the entire system.
Figure 1 Single-stage AC-DC topology with high power factor.
3.How it works and state analysis
In a full switching cycle, we can divide this unipolar AC-DC converter into 8 operating states (including dead time). To deepen our understanding, we will analyze each of these work statuses.
Figure 2: Working state 1 (t0-t1).
Status 1 (T0-T1): As shown in Figure 2, the part circled in the blue box does not participate in the working state, and the colored arrows indicate the flow direction of the current, where the red is PFC and the green is LLC. In state 1, Q1 and Q2 are turned off, L1 is in discharge mode, and continuous inductor current flows through the body diode of QD1, the energy storage capacitor C3, and then flows through D6 and C2 back to L1. At the same time, in the LLC resonant loop, the current flows from the upper end of the resonant loop through QD1 and C3 back to the other end of the resonant loop. On the secondary side, D7 turns on, charging the output capacitor C4 and supplying power to the load. Since the body diode QD1 operates in on-mode, the VDS of Q1 is limited to the body diode forward voltage, and at the end of this cycle, Q1 is ready to turn on, and ZVS is realized.
a)
b) Figure 3: Working status 1 (T1-T2).
State 2 (T1-T2): As shown in Figure 3, in this operating state, Q1 switches to the ON state, L1 continues to discharge, and the inductor current flows through Q1, C3, D6, and C2 before returning to L1. Capacitor C3 is still in charging mode. In an LLC circuit, the resonant loop continues to discharge until it is exhausted, at which point current is still flowing out of the LR and CR to charge C3 (as shown in Figure 3a). When the charge current drops to zero, the depleted resonant network is charged with a boost inductor for a short time, and the current becomes reversed (as shown in Figure 3b). The polarity of the transformer magnetic inductance LM remains at positive ground throughout the operating state 2. On the secondary side, D7 remains on, and supplies power to the output load.
Figure 4: Working state 3 (t2-t3).
Working state 3 (T2-T3): As shown in Figure 4, L1 is fully discharged, and C3 becomes discharge mode to supply power to the whole system. Capacitor C1 discharge current flows through Q1, charges L1, and loops back through D5. The discharge current of C3 also passes through the resonant network and transmits power through the transformer, the polarity of the primary side winding remains as the upper positive pole, while the secondary side winding current continues to flow through D7 to supply power to the output load.
a)
b) Figure 5: Working state 4 (T3-T4).
Working state 4 (T3-T4): As shown in Figure 5, during T3, the resonant current is equal to the excitation current in the excitation inductor LM, no longer has the current flowing to the primary side winding of the transformer, the power transmission ends, the diode D7 on the secondary side is naturally closed in ZCS mode, and the positive half-cycle power transmission is completed. The output capacitor C4 starts discharging and maintains a constant output power. L1 remains charged by the input voltage until Q1 is turned off, and the charge current cycles between C1, D5, Q1, and L1 (as shown in Figure 5a). Once Q1 is turned off, the COSS of Q2 begins to discharge and participates in resonance. During T4, the COSS of Q2 is fully discharged, the VDS drops to 0, and the ZVS conduction is achieved.
Figure 6: Working state 5 (T4-T5).
Operating state 5 (T4-T5): As shown in Figure 6, after the COSS of Q2 is fully discharged, the ZVS is turned on during T4. L1 starts discharging and supplying power to the system, and the inductor current flows through C1, D5, C3, Q2, and then loops back. CR continuously charges LR, LM works in demagnetization mode, the polarity of the primary side winding of T1 becomes positive and negative, rectifier D8 becomes positive, and electric energy is transmitted to the load through D8.
Figure 7: Working state 6 (t5-t6).
Working state 6 (T5-T6): As shown in Figure 7, during this period, the L1 discharge loop is the same as state 5, except that the resonant loop current is in the opposite direction, the LR starts to charge the CR, and the LM is reversed to magnetize. The polarity of the primary winding of T1 is still positive and negative, D8 remains on, and the secondary current flows through D8 to supply power to C4 and the load.
Figure 8: Working state 7 (T6-T7).
Working state 7 (T6-T7): As shown in Figure 8, Q1 is in the off state and Q2 is in the ON state. The stored energy in L1 is completely depleted and the inductor starts to be charged by the input voltage source via C2. The charging current is cyclic between C2, L1, Q2, and D6. d5 is cut off naturally. In an LLC resonant loop, the polarity of the primary winding is positive and negative, and the electrical energy is delivered to the secondary side while the current flows to the load through D8.
Figure 9: Working state 8 (T7-T8).
Working state 8 (T7-T8): As shown in Figure 9, the L1 charging circuit remains unchanged. During T7, the resonant current is equal to the lm magnetic induced current, and no electrical energy is transmitted through T1. In ZCS mode, D8 on the secondary side is turned off. The output capacitor C4 starts discharging and supplies power to the load.
In the description of the above operational status, we do not analyze the dead time separately. In fact, when both switches are turned off, the current from the inductor L1 will continue to flow through the MOS body diode and discharge the MOSFET capacitor, thus achieving ZVS. The resonant loop operates in the same way as LLC and is not described in detail here.
Figure 10 shows that the entire topology work sequence starts from t0 to t8, and is divided into eight working states. The working strategy of dead time is the same as that of a traditional LLC and is easy to understand. Before T0, the VDS of Q1 has dropped to 0, so when Q1 is turned on at T0, Zvs is achieved, and then the resonant current on the primary side rises and accompanies the entire resonant period.
Fig.10 Working sequence diagram.
4.* With verification.
In order to verify the operation and control principle of the single-stage AC-DC converter, we performed a professional ** using Simetrix software. The schematic diagram is shown in Figure 11.
Figure 11 **Schematic diagram.
The schematic diagram includes bridge rectifiers D1-D4, filter capacitors C1 and C2, freewheeling diodes D5 and D6, switching MOS Q1 and Q2, bulk capacitors C3, resonant capacitors CR, resonant inductors LR, and secondary side rectifier diodes D7 and D8. **The parameters are shown in Table 1 below, in which the parameters of the main components are: C1, C2 330Nf, L1 50UH, LR 120UH, CR 22NF, LM 380UH, and the transformer turns ratio is 85:1。**Results and waveforms are shown below.
Table 1: Parameters.
Figure 12: PFC input current vs. input voltage.
Figure 12 provides a waveform comparing the AC input voltage to the AC input current. Figure 13 shows the amplified inductor current and input voltage. This topology ideally implements the PFC function. The DCM operation strategy makes this topology more suitable for small and medium-power AC-DC SMPS applications with PFC function requirements.
Figure 13: Waveforms for IL and AC inputs (amplified).
Figure 14: Q2 ZVS turn-on waveform.
Figure 15: Q1 ZVS on-waveform.
The Zvs conduction characteristics of Q1 and Q2 are shown in Figures 14 and 15, when the VDS resonance of the MOS reaches 0, the gate is turned on, the ZVS is implemented, and the ZVS behaves similarly to the LLC topology.
4.2 Demonstrate functional verification.
In order to verify the effectiveness of this working principle in a practical case, we built a high-power factor single-stage AC-DC converter based on a 300W LLC demo board. Its specifications are as follows: input voltage 180VAC, output power 12V 25A, resonant capacitance CR 66nf, resonant inductance LR 54UH, transformer magnetic inductance 690UH, turns ratio 165:1。
In the demonstration, we measured the AC input voltage and current, and the measurements were in line with the ** results, achieving the expected PFC functionality. The resonant loop can be zVS turn-on on the primary side and SR diode ZSC shutdown on the secondary side. The power is transmitted to the secondary side without any conflict with the LLC function. In addition, the harmonic currents are well matched.
5.Summary
In this paper, a single-stage AC-DC converter with a PFC functional topology is studied. Compared to the traditional two-stage topology, which is the classic PFC+LLC, this new topology combines two circuits and shares a pair of MOS in a half-bridge structure, which is beneficial for reducing bill-of-materials (BOM) costs and increasing power density. Since this topology has only one power inductor to operate in DCM mode, it is more suitable for small to medium-sized power SMPS applications that require a high power factor, such as LED lighting, fast chargers, etc.
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