A review of improved single-stage power factor correction AC/DC converter topologies

1 Overview

In order to reduce harmonic pollution to the AC power grid, relevant standards for limiting current harmonics have been formulated both at home and abroad (such as IEC1000-3-2). Therefore, it is required that the AC input power supply must take measures to reduce the current harmonic content and improve the power factor. There are two widely used active power factor correction methods, namely two-stage PFC and single-stage PFC. The two-stage PFC scheme [1] is shown in Figure 1. The output end of the PFC stage is connected in series with the DC/DC converter, and the two-stage control circuits are independent of each other.

The PFC stage makes the input current follow the input voltage, makes the input current sinusoidal, improves the power factor, and reduces the harmonic content. The subsequent DC/DC stage realizes the rapid regulation of the output voltage. Due to the two-stage structure, the circuit is complex, the device cost is high, and the efficiency is low. In low-power applications, the two-stage PFC is not applicable. Therefore, the study of single-stage PFC and conversion technology has become an important topic in the field of power electronics.

Single-stage PFC[2][3] combines the PFC stage and the DC/DC stage to share a switch tube and a set of control circuits, and simultaneously realizes input current shaping and output voltage regulation. It is different from the two-stage solution in that the control circuit only regulates the output voltage to ensure the stability of the output voltage. In steady state, the duty cycle is constant. Therefore, the current of the PFC stage is required to automatically follow the input voltage. Although the input current of the single-stage PFC converter is not a sine wave and the PF value is not as high as the two-stage solution, since IEC1000-3-2 only has requirements for the current harmonic content and no strict requirements for the PF value, the input current harmonics of the single-stage PFC converter are sufficient to meet IEC1000-3-2. In addition, due to the single-stage structure, the circuit is simple, the cost is low, and the power density is high.

Therefore, single-stage PFC converters are widely used in low-power applications. This paper mainly analyzes the topology of single-stage PFC, points out the existing problems, and introduces several improved topologies to solve these problems.

2 Analysis and Problems of Single-Stage Isolated Boost PFC Circuit

A typical single-stage isolated Boost PFC circuit is shown in Figure 2. This topology is composed of a boost PFC stage and a forward DC/DC converter. The active switch S is a shared switch, and CB is a buffer capacitor. By controlling the on and off of S, the circuit simultaneously shapes the input current and regulates the output voltage.

As we all know, the current of the Boost converter in discontinuous current mode (DCM) automatically follows the input voltage at a fixed duty cycle. Therefore, the PFC stage can obtain a higher power factor when working in DCM. However, the peak values ​​of the input and output inductor currents are high, which increases the current stress and switching loss of the active switch; the efficiency of the converter is low; and the circuit requires a larger EMI filter. If it is required to reduce the voltage and current stress of the switching device, the PFC stage needs to work in continuous current mode (CCM), which can improve the efficiency of the entire converter and reduce EMI. For example, adding an inductor L1 between a and b in Figure 2 can make the PFC stage work in CCM. For DC/DC converters, in order to improve the efficiency of the converter, they generally work in CCM, so the duty cycle does not change with the load. When the load becomes lighter, the output power decreases, and the PFC stage input power is the same as when it is heavily loaded, then the capacity charged into the energy storage capacitor is greater than the energy drawn from the energy storage capacitor, resulting in an increase in the voltage of the energy storage capacitor. In order to keep the output voltage consistent, the voltage feedback loop adjusts the output voltage to reduce the duty cycle and the input energy accordingly. This dynamic process stops when the input and output power are balanced. The consequence of the load reduction is a significant increase in the DC bus voltage, which means a significant increase in the capacitor voltage, even up to thousands of volts.

There are usually two ways to reduce the capacitor voltage: one is to use variable frequency control [4], which can make the capacitor voltage lower than 450V, but the frequency variation range may be as high as ten times, which is not conducive to the optimization design of magnetic components; the other is to use transformer windings to achieve negative feedback. If both the PFC stage and the DC/DC converter work in CCM, when the output power decreases, although the duty cycle remains unchanged, the input power will also decrease accordingly, suppressing the increase of the energy storage capacitor voltage. Its efficiency is the highest and the PF value is reduced. However, it is difficult to find a topology that works completely in CCM, and the design is relatively complex. The power flow diagram of the series single-stage PFC converter is shown in Figure 3. It can be seen from the figure that the power is transferred from the input to the output after two conversions, which is inefficient.

Therefore, the main problem of the single-stage PFC converter is to reduce the capacitor voltage and improve efficiency while making the input current harmonics meet IEC1000-3-2 and quickly adjust the output voltage; in addition, when the single-stage PFC converter works in the hard switching state, the voltage and current stress on the switching device is high, so the switching loss is very large. Therefore, people have proposed using transformer windings to achieve negative feedback, using soft switching technology and parallel PFC to reduce capacitor voltage, switching loss and improve efficiency. The following introduces several improved topologies to solve these problems.

3 Introduction to Several Improved Topologies

3.1 Single-stage parallel PFC converter[1][6][7]

As mentioned above, the efficiency of the series topology is low, whether it is a single-stage or two-stage structure. In order to improve the efficiency of the converter, people have proposed a parallel PFC method. The basic idea is as follows: Assuming PF = 1, the relationship between PFC input power and output power is shown in Figure 4.

It shows that 68% (P1) of the average input power Pin reaches the load after one power conversion, and the remaining power (P2) of 32%, which is the difference between the input and output power in half a grid cycle, reaches the load after two power conversions [1]. Figure 5 is the power flow diagram of this method. P2 reaches the output after two power conversions, and the remaining part P1 reaches the output after one power conversion, thereby improving the circuit efficiency and is higher than that of two-stage and series single-stage converters.

A typical single-stage Boost PPFC converter [1] is shown in Figure 6. The circuit adds D1, S5 and Cb to the original Boost topology with isolation transformer. When the circuit is working, when it is detected that the input power (Pin) is less than the output power (P0), S5 is turned on and the energy in Cb is released to the output. This part of energy is P2. When the input power (Pin) is greater than the output power (P0), S5 is turned off, and the excess energy is stored in Cb by controlling S1~S4. Therefore, the control of the circuit must achieve three functions, namely input current control, output voltage control and capacitor voltage control. The main advantage of this PPFC converter is high efficiency. Due to the coupling relationship between the three controlled quantities, the control circuit is complex and the controller design is difficult; in addition, the number of switch tubes is large and the cost is high, which are the main disadvantages of the converter. Therefore, it is suitable for larger power applications but not for small power applications. Therefore, the literature [6] proposed a single-stage flyback PPFC converter, as shown in Figure 7. T1, S, D3, Cf, RL form the main branch of the circuit, and T2 and D2 form the auxiliary branch of the circuit. The energy storage capacitor CB is charged to the peak voltage of the input voltage through D1 as the input voltage of the auxiliary branch. Since the two parallel flyback branches work at the same time, diodes D2 and D3 are used to prevent the generation of circulating current between the two branches. The converter provides energy to the load from both the input voltage Vin and the energy storage capacitor CB. Although the input voltage Vin provides most of the energy to the load. However, when the input voltage is very small, the energy of the load is mainly provided by the energy storage capacitor CB. The two transformers can work in DCM or CCM. For low-power applications, in order to improve efficiency, both transformers work in DCM. The power distribution between the main branch and the auxiliary branch determines the harmonic content of the input current, and the inductance value of the transformers T1 and T2 determines the power distribution. Therefore, by properly designing the inductance value of the transformers T1 and T2, the harmonic content of the input current can meet the requirements of IEC1000-3-2. The converter can quickly regulate the output voltage using only one active switch and one control loop.

它的主要优点是结构简单、效率高、储能电容电压被箝位，电压值的大小等于输入电压的峰值，对功率开关管没有产生附加的电压应力。另外，在S开通时，由T1直接传递大部分能量到负载，降低了开关管的电流应力，提高了变换器的效率。它的主要缺点是元件数目多，成本较高。

3.2 Single-stage PFC converter using transformer winding to realize negative feedback

A single-stage PFC converter using transformer winding to realize negative feedback [8] is shown in Figure 8. N1 is the transformer-coupled winding.

Transformer winding N1 is used to achieve negative feedback to suppress capacitor voltage Vc. When S is turned on, Vc is applied to the primary winding Np of the transformer, so the voltage on winding N1 is proportional to Vc. Only when the input rectified voltage is greater than the voltage on N1, there is current on inductor LB; when S is turned off, the energy on LB is released to CB through D1. Load changes cause Vc to change, and the voltage applied to LB changes immediately, thereby changing the input current and input power, effectively suppressing the growth of Vc. However, the addition of N1 reduces the power factor and increases the current harmonic content.

Add another winding N2[3][7] between A and B in Figure 8, as shown in Figure 9. After adding winding N2, when S is turned off, the reverse voltage applied to the inductor LB is the sum of the voltage on Vc and N2 minus the input voltage, which reduces the input power, thereby further reducing Vc and improving the power factor. The selection of N2 should satisfy N1+N2

If the voltage and current stress of the switching device is required to be reduced to a minimum, an inductor Lr is added between the diode D2 and the winding N1 in Figures 8 and 9 to make the input current work in CCM. Lr can use the transformer leakage inductance or add another inductor.

3.3 Single-stage PFC converter with low-frequency auxiliary switch[9]

Using an additional transformer winding to achieve negative feedback reduces the capacitor voltage and improves efficiency. However, it also reduces the power factor and increases the current harmonic content. In response to this problem, reference [9] proposed a single-stage PFC converter with a low-frequency auxiliary switch, which not only effectively suppresses the capacitor voltage and improves efficiency, but also improves the power factor and reduces the current harmonic content.

The CCM single-stage PFC converter with low-frequency auxiliary switch is shown in Figure 10, where S is the main switch and Sr is the auxiliary switch.

The driving waveform of the auxiliary switch Sr is shown in FIG11 . When the input voltage is near zero, the auxiliary switch Sr is turned on to short-circuit the additional winding N1 , thereby improving the waveform of the input current, reducing the harmonic content of the input current, and improving the power factor.

When the input voltage is greater than a certain value, the auxiliary switch tube Sr is turned off; the rest of the working conditions are similar to Figures 8 and 9. The auxiliary switch Sr is turned on only when the input voltage is very small, and does not work at other times. Therefore, the current flowing through Sr is very small, and the power loss of Sr is very small. As shown in Figure 11, the operating frequency of the auxiliary switch is twice the frequency of the AC power supply. Therefore, during the entire working period, the switching loss of Sr is very small. In addition, the control circuit of the auxiliary switch Sr is also very simple. From the above analysis, it is known that the single-stage PFC converter with a low-frequency auxiliary switch reduces the harmonic content of the input current; improves the power factor and efficiency; and reduces the capacitor voltage.

The auxiliary switch Sr can also be placed in other positions to obtain different topological structures, as shown in Figure 12. The circuit shown in Figure 12(a) bypasses L1, that is, when the input voltage is near zero, the switch Sr is turned on to short-circuit L1, and the circuit works in DCM, thereby increasing

The input current is added, and this method cannot eliminate the dead corner of the input current. Therefore, compared with the circuit of Figure 10, the input current distortion of the circuit of Figure 12(a) is larger. Another implementation method of Sr is shown in Figure 12(b), which bypasses both L1 and N1. That is, when the input voltage is near zero, turn on switch Sr to short-circuit both L1 and N1. This method can completely eliminate the dead corner of the input current and improve the power factor. However, compared with the circuit of Figure 10, the voltage of the energy storage capacitor in the circuit of Figure 12(b) is higher. Because the circuit of Figure 12(b) works in DCM for a small part of the time. In addition, this method can also be applied to other DCM/CCM single-stage PFC converters, such as the DCM single-stage PFC converter with a low-frequency auxiliary switch shown in Figure 13.

3.4 带有源箝位和软开关的单级PFC变换器

单级隔离式PFC变换器与普通的DC/DC变换器相比有电压、电流应力高，损耗大的缺点。因此，采用有源箝位和软开关等先进技术来减小单级隔离式PFC变换器的开关损耗和电压应力。

带有源箝位和软开关的单级隔离式PFC变换器[10]如图14所示。S为主开关，Sa为辅助开关。Cc为箝位电容，CB为储能电容，Cr为开关S和Sa的寄生电容以及电路中其他的寄生电容之和。Boost单元工作在DCM下，保证有高的功率因数；为避免DCM有较高的电流应力，Flyback设计为CCM。采用有源箝位和软开关技术限制了开关的电压应力，再生了储存在变压器漏感中的能量，为主开关和辅助开关提供了软开关条件，减少了开关损耗，提高了变换器的效率。主开关与辅助开关用同一个控制/驱动电路，进一步提高了电路的实用性。

4 Conclusion

Single-stage PFC converters have been widely used in small and medium power applications due to their simple circuits, low cost, and high power density. By analyzing the topological structure of single-stage PFC, some problems are pointed out, such as the voltage of the energy storage capacitor changes with the input voltage and load. When the input is high voltage or light load, the capacitor voltage may reach thousands of volts; the converter efficiency is low; the switching loss is large; the voltage and current stress of the active switch are high. The methods of using transformer windings to achieve negative feedback, soft switching technology, low-frequency auxiliary switches, and parallel PFC to reduce capacitor voltage, switching losses, reduce current harmonic content, and improve efficiency are reviewed, and the working principles of several improved topologies are analyzed, and their advantages and disadvantages are compared.