Research on a new control strategy for single-phase active PFC

Single-phase active power factor correction (APFC) technology is widely used in switching power supplies, variable frequency home appliances and other fields, and plays a very important role in eliminating harmonic current pollution. So far, there have been a variety of PFC control algorithms, such as conventional multiplier algorithms, voltage follower algorithms, single-cycle controller algorithms, etc., all of which have certain advantages, disadvantages and different scopes of application. Based on the working principle and physical meaning of power factor correction, this paper derives a new direct control algorithm, and conducts simulation analysis and experimental research for verification. If the input voltage and output power are known, the direct control algorithm can be further simplified. In addition, under the premise of ensuring good power factor correction, in order to well characterize the output power support capability of various PFC algorithms, the concept of power adjustment range is proposed. In addition, with the development and application of smart grid technology and distributed generation technology, single-phase standard sinusoidal voltage sources, quasi-sinusoidal voltage sources, AC square wave voltage sources and DC voltage sources have emerged. In order to improve the utilization rate of these power sources and improve the power supply status of microgrids, the above voltage sources must adopt power factor correction technology, that is, the so-called square wave AC PFC and DC PFC are proposed.

1 Principle of single-phase APFC direct control algorithm

The essence of the working principle of the traditional single-phase active PFC is: in each switching cycle, with the help of the regular on-off process of the power switch S1, the power supply uac is short-circuited through the rectifier bridge and the inductor L, so that the inductor L stores energy, and then all or part of the stored energy is released to the DC electrolytic capacitor on the load side, and a synchronous sinusoidal input current waveform and a stable DC output voltage are obtained at the same time. The control strategy of the traditional single-phase active PFC is current closed loop (inner loop) and voltage closed loop control (outer loop), which can achieve good control effects. However, for APFC using analog control, it is difficult to take into account the correction effects of light load and heavy load with the same set of parameters.

The single-phase AC input voltage equation is:

The input current equation for unity input power factor is:

For the convenience of analysis, it is approximately assumed that the output DC voltage u0 = U0, the ripple voltage is zero, the switching period is Ts, the switching frequency is fs, and the duty cycle is d. According to the relationship between the output and input voltage of the Bootst DC/DC converter, we get:

When the input current is at unity input power factor, equation (1) can be rewritten as:

When high-frequency components are ignored, equation (2) can be approximately rewritten as:

Formula (3) can be approximately rewritten as:

In the formula, .

It can be seen that k is a function of the equivalent resistance of the rear stage of the rectifier bridge, which is proportional to the effective value of the fundamental wave of the inductor current (i.e. the effective value of the input current), and the proportionality coefficient is. Theoretically, the value range of k is k∈(0, +∞). In this way, by detecting the changes in the effective value of the inductor current and the effective value of the grid voltage, the changes in Ri and k can be deduced, thereby obtaining the calculation formula for the duty cycle d.

According to formula (4), the MCU can be used to store the relationship curve between the duty cycle and the effective value of the inductor current of the APFC system at different input voltage effective values; then, the duty cycle can be calculated in real time or by looking up the table based on the measured effective value of the inductor current.

For applications such as distributed power generation or digital power generation, since the AC voltage is a high-quality AC sine wave voltage, AC square wave voltage or DC voltage, that is, the input voltage of APFC is stable, the duty cycle can be directly calculated by only detecting the effective value of the inductor current. At this time, there is no need to detect the output voltage. This is the working principle of APFC without detecting the input AC voltage and the output DC voltage. Referring to the concept of power factor of single-phase sinusoidal AC power supply, in order to improve the utilization of the power supply, the input current waveform should be similar to the input voltage waveform, thus proposing concepts such as square wave AC PFC and DC PFC. Square wave AC PFC is PFC with an input voltage of AC square wave voltage, and DC PFC is PFC with an input voltage of DC voltage.

Since the algorithm can directly calculate the duty cycle of PFC, it has the advantages of good correction effect and wide supported power range. On the surface, the control algorithm has hysteresis due to the detection of the effective value of the inductor current. In order to improve the rapidity and maintain stability, a sliding average filter algorithm can be used. In order to characterize the ability of the APFC system to support output power, the concept of power adjustment range is proposed, that is, the ratio of the maximum output power to the minimum output power that the APFC system can support when ensuring good power factor correction.

When the output voltage U0 is set unchanged and the effective value of the input voltage is unchanged, as the load increases, the instantaneous value of the output voltage tends to decrease, the output current will increase, and the instantaneous value of the inductor current will increase. At this time, k can be reduced to increase the overall duty cycle. As a result, the effective value of the input current increases, the output voltage returns to the set value, and a new stable operating point is obtained. Similarly, the situation when the input voltage and output voltage change can be analyzed.

2 Simulation Verification

According to formula (4), a Simulink simulation platform of single-phase APFC without input AC voltage and output DC voltage detection is established, as shown in Figure 1 and Figure 2. The input voltage is rated AC 220 V, the output voltage is set to DC 365 V, the boost inductor is 1.0 mH, the DC electrolytic capacitor is 5600 F, the AC absorption capacitor is 2.0 F, the shunt resistor is 5 mΩ, and the load is the designed adjustable electronic load.

Fig.1 Simulation circuit of single-phase AC sinusoidal APFC.

Fig. 2 Simulation circuit of single-phase AC square wave APFC.

The simulation results show the correctness of the theoretical analysis, the power factor correction effect is very good, and it shows a wide power regulation range.

When the AC sinusoidal voltage is input and the output power is 10 kW, the input voltage and input current waveforms are shown in Figure 3. At this time, k = 0.85, the average output DC voltage is 355 V, and the peak-to-peak value of the ripple voltage is 15 V.

Figure 3 Simulated waveforms of input voltage and input current under heavy load (10 kW).

When the AC sinusoidal voltage is input and the output power is 100 W, the input voltage and input current waveforms are shown in Figure 4. At this time, k = 9.5, the average output DC voltage is 365 V, and the peak-to-peak value of the ripple voltage is 2 V.

Figure 4: Simulated waveforms of input voltage and input current under ultra-light load (100 W).

When AC square wave voltage is input and the output power is 6.6 kW, the input voltage and input current waveforms are shown in Figure 5.

Fig. 5 Simulated waveforms of input voltage and input current under heavy load (6.6 kW).

3 Experimental verification

A 6.6 kW single-phase AC sinusoidal voltage input digital active PFC power module was designed and manufactured. The rectifier bridge uses two 25 A/100 C flat rectifier bridges in parallel, the power switch uses a single 80 A/100 C SGL160N60UF, the FRD uses a single 40 A/100 C FFAF40U60DN, the boost inductor uses a 40 A 1.9 mH silicon steel inductor, the AC absorption capacitor uses a 3.3 F/275VAC non-inductive capacitor, the electrolytic capacitor uses six 680 F/400 VAC electrolytic capacitors in parallel, the core controller uses NEC 16 bit PD18F1201, and the switching frequency is fixed at 20 kHz.

After a lot of hardware and software debugging, a digital PFC power module with an input AC voltage of 150 to 265 V, an average output DC voltage of 365 V, and an input frequency of 50 Hz/60 Hz was finally realized. When the minimum input current was less than 0.5 Arms and the maximum output DC current was close to 40 Arms, an input power factor close to 1 could be obtained, and the harmonic current distribution complies with the standards IEC61000-3-2: 2000 and IEC 61000-3-12: 2005. The input voltage and input current waveforms are shown in Figure 6 when the input voltage is AC 220 V, the input current is 33.23 A, and the grid frequency is 50 Hz.

Figure 6 Measured waveforms of input voltage and input current.

4 Conclusion

A direct control algorithm for single-phase active PFC is proposed, and its working principle is analyzed. The characteristics are as follows: no input voltage detection is required, the correction effect is good, the design is simple, and it is easy to implement digitally; at the same time, it can support higher power output and has good application prospects. When the input voltage, output power and efficiency are known, there is no need to detect the output DC voltage. In order to facilitate the description of the problem, the concept of the power adjustment range of the power factor corrector is proposed, and the concepts of sinusoidal AC PFC, square wave AC PFC and DC PFC are proposed.