Fuzzy Control of the Push-pull Fly-back Three-phase DC-DC Converter

The use of the DC-DC isolation converters structure has now been highly sought after by the experts, since these converters have both an inlet and outlet isolation and a range of output voltage variations that are larger than classical structures [1-2], and there are several outputs available for them. One of the structures used in low-voltage isolated DC-DC converters is the push-pull converter, which has a simple structure, and with its high efficiency in the two-quarter curve of the B-H core [3]. The important form of such structures is that they carry energy for D less than 50 %, and D cannot vary from 0 to 100 % [4]. To resolve this problem, push-pull fly-back structure of the source is used, which is as follows:

f1.png

Figure 1. Push-pull fly-back converter

In this converter, the coupled inductors make the transient winding transmit energy throughout the range of energy changes. The above structure, which is based on trans-single-phase, can be generalized in the form of three phases [5], which we will introduce in this chapter the three-phase type. This structure can operate in the entire range of D variations [6]. The high impedance created by couplings prevents trans-saturation. Transistors are driven by a reference voltage and due to the use of the three-phase structure [7], the frequency of the filters is three times the switching frequency and the filter values will decrease [8].

The figure below shows circuit of three phases push-pull flyback converter:

f2.png

Figure 2. Push-pull flyback three phase converter

The remainder of this paper is organized as follows: Section 2 introduces the push pull flyback converter. Section 3 describes the calculation of voltage gain of converter. Section 4 introduces the fuzzy control. Section 5 describes the simulation results. Section 6 describes the conclusion

For this purpose, as the following can be used volt per second balance for V_LP:

$V_{L P}=\frac{1}{T_{s}} \int_{t_{0}}^{t_{0}+T_{s}} V_{L P} d t=\frac{3}{T_{s}} \int_{t_{0}}^{t_{2}} V_{L P} d t=0$   (1)

The above equation in the regions R1, R2 and R3 will be as follows:

$\left(E_{i}-2 N_{T} V_{0}\right) t_{o n}-N_{T} V_{0}\left(\frac{T_{s}}{3}-t_{o n}\right)=0$   (2)

$\left(2 E_{i}-N_{T} V_{0}\right)\left(t_{o n}-\frac{T_{s}}{3}\right)+\left(E_{i}-2 N_{T} V_{0}\right)\left(\frac{2 T_{s}}{3}-t_{o n}\right)=0$   (3)

$\left(E_{i}\right)\left(t_{o n}-\frac{2 T_{s}}{3}\right)+\left(2 E_{i}-N_{T} V_{0}\right)\left(T_{s}-t_{o n}\right)=0$   (4)

The result of solving each of the above equations will result in the same result:

$\frac{V_{O}}{E_{i}}=\frac{D}{N_{T}(1-D)}$   (5)

which indicates that the gain of this converter is similar to the conventional fB converter.

Here, the converter is under the following simulation parameters:

Vin = 100 (V)

F= 42 (KHz)

NT= 1/2

NL=1/2

Lm=12 (uH)

Cf=2000 (uF)

R=10 (ohm)

5.1 Simulation results for a reference voltage of 100 volts

In this section, simulation results are presented for two reference voltages of 100 and 200 volts. A. simulation results per 100 volts reference voltage. In this case, the reference voltage of 100 volts is applied to the control system. In this situation, the output voltage waveform of the converter will be as follows:

f18.png

Figure 18. Output voltage

As shown in this figure, the output voltage has been able to track the reference value with a slight error and reach 100 volts. In this situation, the output of the fuzzy controller, which is the same duty cycle of the switch, will be as follows:

f19.png

Figure 19. The duty cycle

As shown in the figure above, the duty cycle of the switches is about 20 %. In this situation, the waveform of the command signal to the switches will be as follows:

f20.png

Figure 20. The command signal to switch 1

f21.png

Figure 21. The command signal to switch 2

As you can see in Figure 22, switches are switched at 42 kHz and their duty cycle is about 20 %.

5.2 Simulation results for a reference voltage of 200 volts

In this case, the reference voltage of 200 volts is applied to the control system. In this situation, the output voltage waveform of the converter will be as Figure (23).

f22.png

Figure 22. The command signal to switch 3

f23.png

Figure 23. Output voltage

As shown in this figure, the output voltage has been able to track the reference value with a slight error and reaches a value of 200 volts. In this situation, the output of the fuzzy controller, which is the same duty cycle of the switch, will be as follows:

f24.png

Figure 24. Duty cycle

As shown in the figure above, the duty cycle of the switches is about 50 %. In this situation, the waveform of the command signal to the switches will be as follows:

f25.png

Figure 25. The command signal to switch 1

f26.png

Figure 26. Command signal of switch 2

f27.png

Figure 27. The command signal to switch 2

As you can see, switches are switched at 42 kHz and their duty cycle is about 50 %.