Design of Buck Type AC/AC Converter-EEWORLD

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1 Introduction

AC/AC conversion is the process of converting one form of alternating current into another form of alternating current [1-2]. The main converters that can be used for step-down conversion include industrial frequency transformers, phase-controlled AC voltage regulation circuits [2-3], AC-DC-AC converters, electronic transformers [4-5], high-frequency AC link AC/AC converters [6-7], matrix converters [8] and non-isolated Buck and Buck-Boost AC/AC converters [9].

The industrial frequency transformer is large in size and weight, and has no voltage stabilization and voltage regulation functions; the input and output of the phase-controlled AC voltage regulation circuit contain serious harmonic components and are generally only suitable for power adjustment of loads with large thermal or mechanical inertia [3]; the AC-DC-AC converter has too many conversion stages, its conversion efficiency is not high, and it has serious harmonic pollution to the power grid; the electronic transformer is small in size and weight, has a large number of switching devices, and also has no voltage stabilization and voltage regulation functions; although the high-frequency AC link AC/AC converter can achieve electrical isolation, the topology and control circuit are complex, and the number of switching devices is large; the matrix converter also has the problem of many switching devices and complex control strategy, and its maximum gain is only 0.866; the Buck-Boost type AC/AC converter can achieve the buck-boost function, but its switch tube voltage stress is high, there is no direct energy transfer path between the input and output, so the conversion efficiency is not high, and the input and output phases are opposite; in the buck-type AC/AC converter where electrical isolation is not required, the Buck type AC/AC converter has the characteristics of simple structure and easy control. This paper analyzes in detail the working principle and control strategy of the Buck type AC/AC converter, conducts simulation research on it, and develops a prototype. The simulation and experimental results are consistent with the theoretical analysis.

2 Circuit structure and working principle

Figure 1 is the circuit structure of a Buck type AC/AC converter, where S1 (S1a, S1b) and S2 (S2a, S2b) are two pairs of AC switch tubes, which are turned on complementary, with the turn-on time being DTS and (1-D)TS, respectively, where D is the duty cycle and TS is the switching period. Assuming that the input voltage uin is an ideal sine wave, then:

Where Um is the input voltage amplitude; w=2pf is the input voltage angular frequency; f is the input voltage frequency. The duty cycle D is a constant, so the output voltage uo also changes sinusoidally, and its phase is consistent with the input voltage uin, but the amplitude is not greater than Um.

The reference direction of the input voltage uin and the inductor current iLf is shown in Figure 1. According to the different polarities of the input voltage uin and the inductor current iLf, there are four different stages in one input voltage cycle: uin >0, iLf >0; uin >0, iLf <0; uin <0, iLf<0; uin <0, iLf >0.

(1) uin > 0, iLf > 0

The switch tubes S1b and S2b are always on, and S1a and S2a are turned on at high frequency. The two switching modes of the circuit in this stage are shown in Figure 2. (The solid line in the figure indicates the route through which the current flows, the dotted line indicates the route through which the current does not flow, and the arrow indicates the actual direction of the voltage and current; the switch tube that is always on is omitted and replaced by a straight line.

When the switch tube S1a is turned on and S2a is turned off, the input voltage supplies power to the capacitor Cf and the load through the switch tube S1a and the inductor Lf, as shown in Figure 2(a); when the switch tube S1a is turned off and S2a is turned on, the inductor current iLf continues to flow through Lf, Cf, the load, and the reverse diode of the switch tube S2a, and Lf and Cf jointly supply power to the load, as shown in Figure 2(b).

Figure 2 Switching mode when uin > 0, iLf > 0:

(a) S1a is turned on and S2a is turned off;

(b) S1a is closed and S2a is opened

(2) uin > 0, iLf < 0

The switch tubes S1b and S2b are constantly turned on, and S1a and S2a are turned on at high frequency and complementarily. The two switching modes of the circuit in this stage are shown in FIG3 .

When the switch tube S1a is turned on and S2a is turned off, the input voltage supplies power to the capacitor Cf and the load through the reverse diode of the switch tube S1a and the inductor Lf, as shown in Figure 3(a); when the switch tube S1a is disconnected and S2a is turned on, the inductor current iLf continues to flow through Lf, the switch tube S2a, Cf and the load, and Lf and Cf jointly supply power to the load, as shown in Figure 3(b).

Figure 3 Switching mode when uin > 0, iLf< 0:

(a) S1a is turned on and S2a is turned off;

(b) S1a is closed and S2a is opened

(3) uin < 0, iLf < 0

The switch tubes S1a and S2a are constantly turned on, and S1b and S2b are turned on at high frequency and complementarily. The two switching modes of the circuit in this stage are shown in FIG4 .

Figure 4 Switching mode when uin< 0, iLf < 0:

(a) S1b is turned on and S2b is turned off;

(b) S1b is closed and S2b is opened

When the switch tube S1b is turned on and S2b is turned off, the input voltage supplies power to the capacitor Cf and the load through the switch tube S1b and the inductor Lf, as shown in Figure 4(a); when the switch tube S1b is disconnected and S2b is turned on, the inductor current iLf continues to flow through Lf, the reverse diode of the switch tube S2b, Cf and the load, and Lf and Cf jointly supply power to the load, as shown in Figure 4(b).

(4) uin <0, iLf> 0

The switch tubes S1a and S2a are constantly turned on, and S1b and S2b are turned on at high frequency and complementarily. The two switching modes of the circuit in this stage are shown in FIG5 .

When the switch tube S1b is turned on and S2b is turned off, the input voltage supplies power to the capacitor Cf and the load through the reverse diode of the switch tube S1b and the inductor Lf, as shown in Figure 5(a); when the switch tube S1b is disconnected and S2b is turned on, the inductor current iLf continues to flow through Lf, Cf, the load, and the switch tube S2b, and Lf and Cf jointly supply power to the load, as shown in Figure 5(b).

Figure 5 Switching mode when uin< 0, iLf > 0:

(a) S1b is turned on and S2b is turned off;

(b) S1b is closed and S2b is opened

3 Control strategy

The control block diagram of the Buck type AC/AC converter is shown in FIG6 .

After feedback sampling, the output voltage is compared with the reference output voltage signal uo_ref, and the output voltage error amplification signal ue is obtained after PI adjustment, and then compared with the triangle wave to obtain the high-frequency PWM control signal SP2, and the control signal SN2 is obtained after SP2 is inverted; after the input voltage is sampled, a low-frequency input voltage polarity signal SP1 is generated through a zero comparator, and the signal SN1 is obtained after SP1 is inverted; SP2 and SN2 are logically or modulated with SP1 and SN1 respectively to generate control signals K1a, K1b, K2a, and K2b for switch tubes S1a, S1b, S2a, and S2b.

4 Simulation and Experiment

In order to verify the correctness of the theoretical analysis of the Buck type AC/AC converter and the feasibility of the control strategy, simulation and experimental research on the converter were carried out.

4.1 Simulation waveform

The simulation parameters are as follows: input voltage amplitude, rated output voltage amplitude, load resistance of 48.4 W (output power of 250 W), ideal switches; input voltage frequency f = 50 Hz; switching frequency is 50 kHz; inductance Lf = 500 μH, capacitance Cf = 4.4 μF.

The simulation waveforms of the control signals K1a, K1b, K2a, and K2b of the switch tubes S1a, S1b, S2a, and S2b are shown in Figure 7(a). When the input voltage is greater than zero, the switch tubes S1b and S2b are constantly turned on, and S1a and S2a are turned on at high frequency complementarily; when the input voltage is less than zero, the switch tubes S1a and S2a are constantly turned on, and S1b and S2b are turned on at high frequency complementarily. Figure 7(b) shows the input voltage uin, the output voltage uo, and the voltage waveform uS2 at both ends of the AC switch tube S2 (i.e., between D (drain) and D (drain) of the two tubes S2a and S2b).

Figure 7 Simulation waveform of Buck type AC/AC converter:

(a) Control signals K1a, K1b, K2a, K2b;

(b) Voltages uin, uo and uS2

4.2 Experimental waveform

According to the above analysis, this paper designs a prototype. The prototype parameters are set as follows: input voltage RMS value Uin = 220V, rated output voltage RMS value Uo = 110V. The switch tube uses IRFP460A; input voltage frequency is 50Hz; switching frequency is 45kHz; inductor Lf = 500μH; capacitor Cf = 4.4μF. Its experimental waveform is shown in Figure 8. Figure 8 (a) is the experimental waveform of input voltage uin and output voltage uo, uo and uin have the same phase; Figure 8 (b) is the input voltage uin and the voltage waveform uS2 at both ends of the AC switch tube S2, uS2 is a high-frequency pulse sequence with uin as the envelope.

Figure 8 Experimental waveform of Buck type AC/AC converter:

(a) Input voltage uin and output voltage uo;

(b) Input voltage uin and voltage across S2 uS2

5 Conclusion

This paper analyzes in detail the working principle and control strategy of the Buck type AC/AC converter. The two pairs of AC switch tubes of the converter are turned on at high frequency and complementarily. When the duty cycle is constant, the output voltage and input voltage have the same phase, but the amplitude is not greater than the input voltage amplitude. By judging the polarity of the input voltage and combining the comparison results of the output voltage error amplification signal and the triangular carrier, the working state of each switch tube can be determined. The simulation and experimental results verify the correctness of the theoretical analysis and the feasibility of the control strategy.

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