Topological Study on Suppressing the Voltage of Energy Storage Capacitor in Single-stage PFC-EEWORLD

Collect

1 Introduction

In order to reduce the harmonic pollution of AC power grid, relevant standards (such as IEC 1000-3-2 standard) have been formulated at home and abroad to limit current harmonics. 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. Two-stage PFC [1] connects the output end of the PFC stage in series with the DC/DC converter. Due to the two-stage structure, the circuit is complex, the device cost is high, and the efficiency is low. In low-power applications, two-stage PFC is not suitable. 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 the shaping of the input current and the regulation of the output voltage. Unlike the two-stage solution, the control circuit only adjusts the output voltage to ensure the stability of the output voltage. In steady state, the duty cycle is constant, so 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, the PF value is not as high as the two-stage solution, but since the IEC1000-3-2 standard 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 the IEC1000-3-2 standard. Due to the single-stage structure, the circuit is simple, the cost is low, and the power density is high. Therefore, the single-stage PFC converter has been widely used in low-power occasions.
This paper mainly analyzes the topology of the single-stage PFC. Suppressing the voltage of the energy storage capacitor is the main problem to be solved in the single-stage power factor correction. 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, and the converter efficiency is low. Several improved topological structures are introduced to reduce the capacitor voltage, and their advantages and disadvantages are discussed respectively. Through the analysis of the existing topology, a new topological structure is obtained.

2 Main problems of single-stage power factor correction

Single-stage PFC combines the PFC stage and the DC/DC stage to share a switch tube and a set of control circuits, and simultaneously realizes the shaping of the input current and the regulation of the output voltage. 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. When the output power of the PFC stage is constant, the input power is a periodically changing quantity, so the energy storage capacitor is used to solve the problem of imbalance between the instantaneous input power and the output power. As we all know, the current of the boost converter in the discontinuous current mode (DCM) automatically follows the input voltage at a fixed duty cycle, so the PFC stage can obtain a higher power factor when working in DCM. In order to improve the efficiency of the converter, the DC/DC converter generally works in the continuous current mode (CCM). In the CCM case, when the load becomes lighter, the output power decreases. Since the duty cycle does not change with the load, the input power of the PFC stage is the same as when it is heavily loaded, and 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 only when the input and output power are balanced. The consequence of the load reduction is that the capacitor voltage rises significantly, even reaching 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. Using transformer windings to achieve negative feedback can reduce the capacitor voltage, but at the same time it reduces the power factor and increases the harmonic content of the current. This paper analyzes the existing topology and obtains a new power factor correction circuit with low-frequency auxiliary switches, which not only reduces the capacitor voltage, but also improves the power factor and reduces the harmonic content of the current.

3 Analysis of several improved topologies

3.1 Single-stage PFC with transformer winding to suppress capacitor voltage

The single-stage PFC converter with transformer winding to suppress capacitor voltage [5] is shown in Figure 1. N1 is the transformer coupled winding.

Figure 1 Single-stage PFC converter with transformer winding suppression capacitor voltage
[page] Transformer winding N1 is used to implement negative feedback to suppress capacitor voltage VC. When Q 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 the inductor LB; when Q 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. In addition to reducing VC, adding N1 can also directly transfer part of the energy to the load when Q is turned on, reducing the current stress of the switch tube and improving the efficiency of the converter. However, the addition of N1 reduces the power factor and increases the current harmonic content. Add another winding N2[3][8] between AB in Figure 1, as shown in Figure 2.

Figure 2 Single-stage PFC converter using dual windings to suppress capacitor voltage
After adding winding N2, when Q is turned off, the reverse voltage applied to the inductor LB is the sum of the voltages on VC and N2 minus the input voltage, which increases the current drop rate of the inductor LB when it is turned off, reduces the input power, and thus further reduces VC, while also improving the power factor. The selection of N2 should satisfy N1+N2 If it is required to reduce the voltage and current stress of the switching device to a lower limit, then add an inductor Lr between the diode D2 and the winding N1 in Figures 1 and 2 to make the input current work in CCM. Lr can use the transformer leakage inductance, or add another inductor [3].

3.2 Single-stage flyback parallel power factor correction (PPFC) converter

Single-stage flyback PPFC converter [6], as shown in Figure 3.

Figure 3 Single-stage flyback PPFC converter
TX1, Q, D3, Cf, and RL form the main branch of the circuit, and TX2 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 C2. 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 TX1 and TX2 determines the power distribution. Therefore, by properly designing the inductance value of the transformers TX1 and TX2, the harmonic content of the input current can meet the IEC1000-3-2 standard. The converter can quickly adjust the output voltage with only one active switch and one control loop.
Its main advantages are simple structure, high efficiency, the voltage of the energy storage capacitor is clamped, the voltage value is equal to the peak value of the input voltage, and no additional voltage stress is generated on the power switch tube. In addition, when Q is turned on, most of the energy is directly transferred to the load by TX1, which reduces the current stress of the switch tube and improves the efficiency of the converter. Its main disadvantages are the large number of components and high cost.

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

The single-phase passive power factor correction converter uses a low-frequency switch to reduce the harmonics of the input current and meet the IEC1000-3-2 standard. However, the low-frequency boost PFC converter requires a large input inductor [7-8]; using an additional transformer winding to achieve negative feedback reduces the capacitor voltage and improves efficiency. But at the same time, it reduces the power factor and increases the current harmonic content. In order to improve the performance of the active single-stage PFC converter. This paper combines the above two methods to propose a single-stage PFC converter with a low-frequency, low-cost, and low-loss auxiliary switch. It 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 a low-frequency auxiliary switch is shown in Figure 4, where Q is the main switch and Qr is the auxiliary switch.
The driving waveform of the auxiliary switch Qr is shown in Figure 5. When the input voltage is near zero, the auxiliary switch Qr is turned on, short-circuiting 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.

Figure 4 CCM single-stage PFC converter with low-frequency auxiliary switch

Figure 5 Auxiliary switch Qr drive waveform
When the input voltage is greater than a certain value, the auxiliary switch tube Qr is turned off; the rest of the working conditions are similar to Figures 1 and 2. The auxiliary switch Qr is turned on and works only when the input voltage is very small, and does not work at other times. Therefore, the current flowing through Qr is very small, and the power loss of Qr is very small. As shown in Figure 5, 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 Qr is very small. In addition, the control circuit of the auxiliary switch Qr is also very simple. From the above analysis, it can be seen 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 Qr can also be placed in other positions to obtain different topological structures, as shown in Figure 6. The circuit shown in Figure 6 (a) bypasses L1. That is, when the input voltage is near zero, the switch Qr is turned on to short-circuit L1, and the circuit works in DCM, thereby increasing the input current. This method cannot eliminate the dead angle of the input current. Therefore, compared with the circuit in Figure 4, the input current of the circuit in Figure 6a) is more distorted. Another implementation of Qr is shown in FIG6b), where both L1 and N1 are bypassed, that is, when the input voltage is near zero, the switch Qr is turned on to short-circuit both L1 and N1. This method can completely eliminate the dead angle of the input current and improve the power factor. However, compared with the circuit of FIG4, the voltage of the energy storage capacitor in the circuit of FIG6(b) is higher. This is because the circuit of FIG6(b) operates 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 FIG7.

Figure 6 Implementation of different positions of Qr (a) Bypassing L1 (b) Bypassing both L1 and N1

Figure 7 DCM single-stage PFC converter with low-frequency auxiliary switch

[page]4 Conclusion

Single-stage PFC converter has simple circuit, low cost and high power density, and has been widely used in small and medium power occasions. The main problem of single-stage PFC is analyzed - suppressing the voltage of energy storage capacitor. This paper introduces several improved topologies to reduce the capacitor voltage, analyzes their working principles, and compares their advantages and disadvantages. Through the analysis of the existing topology, a new topology is obtained.

References

[1] Yimin Jiang and Fred C.Lee, "single-stage single-phase parallel power factor corrector scheme," IEEE Trans. Power Electron, Aug 1994, pp.1145-1151.
[2] CMQiao and KM Smedley, "A topology survey of single-stage power factor corrector with a boost type input-current-shaper," IEEETrans.PowerElectron, May , 2001, pp.360-36 8.
[3] L. Huber and MMJovanovic, "single-stage single-switch isolated power supply technique with input-current shaping and fast output-voltage regulation for universal line input-voltage-range application," IEEE Trans. Power Electron, 1997, pp .272-280
[4] M. Madigan, R. Erickson and E. Ismail, "Integrated high quality rectifier regulators" IEEE-PESC1992, pp. 1043-1051.
[5] Y. Jiang, GC Hua, W. Tang, and FC Lee, “a novel single-phase power factor corrector scheme,” IEEE Applied Power Electronics Conference, 1993, pp.287-292.
[6] Oscar Garcia, Pedro Alou, Roberto Prieto and Javier Uceda, “a simple single-switch single-stage AC/DC converter with fast output voltage regulation,” IEEE Transaction Power Electronics , 2002, pp.163-170.
[7] J.Pomil and G.Spiazzi, "A Double-Line-Frequency Commutated Rectifier Complying with IEC 1000-3-2 Standards," IEEE Power Electronics Conference, 1999, pp.349-355.
[8] L. Rossetto, G. Spiazzi and P. Tenti, "Boost PFC With 100 Hz Switching Fre quency Providing Output Voltage Stabilization and Compliance with EMC Standards,” IEEE Trans. 2000, pp.188-193.

Reference address：Topological Study on Suppressing the Voltage of Energy Storage Capacitor in Single-stage PFC

Previous article：Research on a new type of boost power factor pre-regulator
Next article：Electrostatic protection measures for electronic components

Popular Resources
Popular amplifiers