Study on the principle of voltage source high frequency AC link AC converter

Abstract: For the first time, a family of circuit topologies of voltage source high-frequency AC link AC/AC converters is proposed. This type of circuit topology consists of an input cycloconverter, a high-frequency transformer, and an output cycloconverter. The steady-state principle and phase-shift control strategy of this type of converter are analyzed and studied, and the external characteristic curve of the converter is drawn. This type of converter has the advantages of simple circuit topology, two-stage power conversion (LFAC/HFAC/LFAC), bidirectional power flow, high-frequency electrical isolation, improved grid-side current waveform, and strong load adaptability. The PSPICE simulation waveform fully confirms the correctness and advancement of this type of converter. Keywords: AC/AC converter; high-frequency AC link; cycloconverter; voltage source; phase-shift control Research on Principles of Voltage Mode

1 Introduction

Power electronics researchers have achieved remarkable results in the study of high-frequency conversion technologies for DC/DC converters, AC/DC converters, and DC/AC inverters[1]. Research on AC/AC conversion technology is limited to thyristor phase-controlled inverters and matrix converters in which the AC load is not electrically isolated from the AC grid[2].

This paper proposes and studies in depth for the first time a voltage source high-frequency AC link AC/AC converter based on a forward converter with high-frequency electrical isolation between the AC load and the AC power grid. It is of great significance to the development of power electronics and the realization of new electronic transformers and sinusoidal AC regulators.

2 Voltage source high frequency AC link AC/AC converter circuit

Topology families and control principles

2.1 Circuit structure and topology family

The circuit structure of the voltage source high frequency AC link AC/AC converter based on the forward converter is shown in Figure 1(a). The circuit structure consists of an input cycloconverter, a high frequency transformer, and an output cycloconverter, and can convert a sinusoidal AC power into another sinusoidal AC power of the same frequency. Both the input and output cycloconverters are composed of four-quadrant power switches (capable of withstanding bidirectional voltage stress and bidirectional current stress). There are 8 types of voltage source high frequency AC link AC/AC converter circuit topology families, as shown in Figures 1(b) to (i).

Figure 1 Voltage source high frequency AC link AC/AC converter circuit structure and topology family

2.2 Control principle

Voltage source high frequency AC link AC/AC converter adopts phase shift

This work was supported by the National Natural Science Foundation of China (59977010) and the Natural Science Foundation of Jiangsu Province (BK99121).

(a) Circuit structure

(b) Single forward type (c) Parallel interleaved forward type

(d) Push-pull full-wave (e) Push-pull bridge

(f) Half-bridge full-wave type (g) Half-bridge bridge type

(h) Full-bridge full-wave type (i) Full-bridge bridge type

The control strategy is shown in Figure 2. The driving signals of the four-quadrant power switches S1 (S1′) and S2 (S2′), S3 (S3′) and S4 (S4′) are complementary high-frequency square wave signals, but the driving signals between S3 (S3′) and S1 (S1′), S4 (S4′) and S2 (S2′) have a phase difference θ (0≤θ≤180°). In Figure 2, Ts=1/fs is the switching period, fs is the switching frequency, and uAB is the front-end voltage of the filter. The common conduction time DTs/2 between S3 (S3′) and S1 (S1′), S4 (S4′) and S2 (S2′) in one switching period can be expressed as

DTs/2=Ts(180°-θ)/(2×180°)(1)

Where: D is the duty cycle.

3 Voltage source high frequency AC link AC/AC converter steady state

Principle and external characteristics

3.1 Steady-state principle

Taking the full-bridge full-wave circuit topology as an example, the steady-state principle and external characteristics of this type of converter are studied, as shown in Figure 3(a). When the converter works in steady state and CCM mode, it can be divided into four switching states within a switching cycle Ts, and its equivalent circuit is shown in Figure 3(b) to (e).

Figure 2 Voltage source high frequency AC link AC/AC converter phase shift control principle

Figure 3(b), (d) and Figure 3(c), (e) can be represented by the equivalent circuits shown in Figure 4(a), (b), respectively, where r is the equivalent circuit of the transformer winding.

Figure 3 Full-bridge full-wave voltage source high-frequency AC link AC/AC converter and its switch state circuit in CCM mode

(a) Full-bridge full-wave circuit topology

(b) S1 (S1′), S3 are on, S2 (S2′), S4 are off (c) S2 (S2′), S3 are on, S1 (S1′), S4 are off

(d) S2 (S2′), S4 are on, S1 (S1′), S3 are off (e) S1 (S1′), S4 are on, S2 (S2′), S3 are off

The equivalent resistance includes resistance, on-state resistance of power switch, parasitic resistance of filter inductor, etc.

Since the switching frequency fs is much larger than the cutoff frequency of the output LC filter and the frequencies of the input and output sinusoidal AC voltages, the input voltage ui and the output voltage uo can be regarded as constants within one switching period Ts, and the relationship between the output voltage and the input voltage can be established using the state space averaging method.

The state equation of the equivalent circuit shown in Figure 4(a) is Lf=－riLf＋ui－uo(2.a)Cf=iLf－(2.b)

The state equation of the equivalent circuit shown in Figure 4(b) is Lf=-riLf-ui-uo(3.a)Cf=iLf-(3.b)Equation (2) multiplied by D plus equation (3) multiplied by (1-D), let =0, =0, the steady-state value of the state variable in any switching cycle is ILf=(4.a)Uo=(4.b)

Study on the principle of AC/AC converter with voltage source and high frequency AC link

(a) When S1(S1′), S3 or S2(S2′), S4 are turned on

(b) When S2 (S2′), S3 or S1 (S1′), S4 are turned on

Figure 4 Voltage source high frequency AC link AC/AC

Two equivalent circuits of the converter in CCM mode

(b) DCM mode

(a) Critical CCM mode

Figure 5 Critical CCM and DCM of filter inductor current

The waveform of a switching cycle in mode

3.2 External characteristics of the converter in steady state

3.2.1 Ideal situation (r=0)

From equation (4.b), we can see that the external characteristic of the converter in the ideal case and CCM mode is Uo=(5)

The principle waveform of the filter inductor current in critical continuous and DCM modes within a switching cycle is shown in Figure 5.

From Figure 5(a), we can see that t2-t1=DTs/2. When t=t1~t2, uiN2/N1-uo=Lf=Lf=Lf(6)

The load current when the inductor current is critically continuous is IG=Iomin=iLf(t2). From equations (5) and (6), we get IG=D(1-D)(7)

From formula (7), we can see that when D = 1/2, IG reaches its maximum value, that is, IGmax = (8)

From equations (7) and (8), we can see that in the ideal case and when the filter inductor current is critically continuous, the external characteristics of the converter are:

IG=4IGmaxD(1－D)(9)

From Figure 5(b), we can see that t3

When t=t2～t3, from the equivalent circuit shown in Figure 4(b), we can know that (r=0)uiN2/N1＋uo=－Lf=－Lf=Lf(11)

From equations (10) and (11), we can get t3-t2=(12)

The output load current is io==(13)

From equations (8), (10), (12), and (13), we can get Io=IGmax(14)

Therefore, the external characteristic of the converter in the ideal case and DCM mode is = (15)

3.2.2 Actual Situation

In actual situations, the internal resistance r of the converter is not zero, so the external characteristics of the converter can be expressed by equation (4.b).

Take N1/N2=1, and the converter's per-unit external characteristic Uo/Ui=f(Io/Iomax) can be obtained by equations (5), (9), (15), and (4.b), as shown in Figure 6. Curve A is the external characteristic curve when the filter inductor current is critically continuous, which is determined by equation (9); the right side of curve A is the external characteristic curve when the filter inductor current is continuous, the solid line is the curve in the ideal situation, which is determined by equation (5), and the dotted line is the curve in the actual situation, which is determined by equation (4.b). It can be seen that as the load current increases, the output voltage decreases; the left side of curve A is the external characteristic curve when the filter inductor current is discontinuous, which is determined by equation (15). It can be seen that the ratio of output voltage to input voltage is not only related to D, but also to the load current.

4 Simulation Examples

Full-bridge full-wave circuit topology, phase-shift control strategy, input voltage Ui=220 (1±10%) V, frequency 50Hz, output voltage Uo=220V (50Hz), rated capacity S=3kVA, load power factor -0.75~0.75, switching frequency fs=100kHz, transformer turns ratio N1/N2=1:1.3, input filter inductance Li=10μH, input filter capacitor Ci=50μF, output filter inductance Lf=0.5mH, output filter

Figure 6 Voltage source high frequency AC link AC/AC converter standard external characteristics

(a) Rated input voltage, resistive rated load

(b) Rated input voltage, no-load

(c) Rated input voltage, resistive full load

(d) Input voltage 200V, inductive light load

(e) Input voltage 240V, capacitive full load

Figure 73kVA voltage source high frequency AC link AC/AC converter simulation waveform

Study on the principle of AC/AC converter with voltage source and high frequency AC link

Wave capacitance Cf=20μF.

The simulation waveform of the 3kVA220V (1 + ± 10%) (50Hz) voltage source high frequency AC link AC/AC converter under different input voltages and different loads is shown in Figure 7. From the simulation waveform, it can be seen that the transformer operating frequency is 100kHz, the output voltage waveform THD is low, the grid-side current waveform has high sinusoidality, and has strong load adaptability and excellent voltage stabilization performance.

5 Conclusion

(1) Voltage source high frequency AC/AC converter, which has the advantages of two-stage power conversion (LFAC/HFAC/LFAC), bidirectional power flow, high frequency electrical isolation, improved grid-side waveform, and strong load adaptability.

(2) The voltage source high frequency AC link AC/AC converter circuit topology family includes eight circuits: single forward, parallel interleaved forward, push-pull full-wave, push-pull bridge, half-bridge full-wave, half-bridge bridge, full-bridge full-wave, and full-bridge bridge.

(3) The external characteristic curve of the converter is obtained.

(4) The simulation results confirm that the voltage source high frequency AC link AC/

The correctness and advancement of the new concept of AC converter.