Digital simulation of high efficiency multi resonant forward converter using simulink.

Suresh, L. Padma ; Rajesh, R.S. ; Prasad, S.V. Muruga 等

Introduction

RECENTLY, a common trend in power industry is the quest for higher power density and higher efficiency. A general solution to achieve higher efficiency is by increasing switching frequency which leads to size and weight reduction of the capacitive and magnetic components. The forward converter remains as an industry work horse in low power DC/DC conversions. The higher power step up DC/DC conversion technique finds increasing necessities and power capability demands in applications such as electric vehicles, uninterruptible power supplies, servo -drive systems, and semiconductor fabrication equipments.

Evolution of Forward Converter

Classic Forward converter

The forward multi-resonant converter, shown in Fig. 1, is one of the topologies belonging to the family of zero-voltage-switched multi-resonant converters (ZVS-MRCs). The ZVS-MRCs are generated from PWM topologies by adding three resonant components to the power circuit: a resonant inductor L, a resonant capacitor [C.sub.S] in parallel with the active switch, and a resonant capacitor [C.sub.D] in parallel with the rectifier.

[FIGURE 1 OMITTED]

Due to the position of the resonant components, a desirable zero voltage switching is achieved for the power switch Q, and the rectifier [D.sub.1] and [D.sub.2]. The circuit is suitable for high-frequency operation, because of parasitic components, transformer leakage inductance, MOSFET's output capacitance, and junction capacitances of [D.sub.1] and [D.sub.2], in the resonant circuit. In addition, transformer reset is achieved automatically by interaction between [C.sub.D] and the magnetizing inductance of the transformer. As a result, the circuit is simplified, because no resetting winding is needed.

Although design procedures are available for the basic ZVS-MRCs, such as buck, boost, and flyback, the design process for the forward ZVS-MRC has been, to a large extent, a matter of trial and error. The reason why the results of the analysis performed for the basic converters cannot be directly applied to the forward converter, is the presence of the forward diode [D.sub.1] and the transformer magnetizing inductance [L.sub.m]. In all basic ZVS-MRCs, voltage across the resonant capacitor [C.sub.D] is unidirectional, because [C.sub.D] is shunted by a single diode. In the forward ZVS-MRC, a forward diode, [D.sub.1] is added to avoid transformer's saturation. As a result, voltage across [C.sub.D] is bidirectional. This makes the analysis process for forward converter quite different from that for the basic ZVS-MRCs.

Among the dc-dc converters examined so far, the fly back circuit comes close to an ideal dc transformer. The coupled inductor in the fly back circuit is suggestive of a conventional magnetic transformer. Unlike a transformer, the coupled inductor stores energy and carries a net dc current. The buck and boost circuits lack a transfer source, so simple substitution of a coupled inductor will not give them isolation properties. In the buck and boost cases, we can seek out an ac signal within the converter, and insert a magnetic transformer at the ac location. Circuits based on this idea are called forward converters.

The diode voltage is a pulse train and it would be very convenient to insert an ac transformer at that location. However [V.sub.out] has a positive average value. The magnetic transformer's magnetizing inductance cannot sustain this dc voltage. Each pulse would impose positive voltage on the transformer so that the flux would build up during each cycle. There are physical limits on flux just as there are physical limits on current or voltage. Thus simple insertion of a transformer is not feasible. There are two ways to avoid this problem and embed a transformer in a buck-type circuit.

1. Construct the converter as an ac link circuit by cascading an inverter and rectifier. This arrangement explicitly creates an ac point in the circuit, then inserts a transformer at the ac point.

2. Add a third catch winding on the coil so that flux can be conserved and a nonzero d[PHI]/dt value can be maintained without causing other problems.

Operational Principle

The circuit diagram of the proposed TCB ZVS forward converter is the same as that of the conventional active-clamp forward converter, as shown in Fig. 1. The switch M1 is operated in a duty ratio of D, and the switch [M.sub.2] is operated with complementary to [M.sub.1] with the time delay between their gate pulses. Synchronous rectifiers are employed instead of Schottky diodes to reduce the conduction loss in the secondary side. It shows the gating pulses for synchronous switches and key operating waveforms of the proposed converter in the steady state. The gating pulse for SR1 is imposed before [S.sub.2] is turned off. Each switching period is subdivided into eight modes, with their topological stages. In order to illustrate the steady-state operation, several assumptions are made as follows.

1. The switches, [M.sub.1] and [M.sub.2], are ideal except for their internal diode and output capacitor.

2. The output voltage [V.sub.o] and clamping capacitor voltage [V.sub.c] are constant.

3. The transformer magnetizing current iLm(t) and leakage inductor current iLr(t) are constant during the time interval [t.sub.1]-[t.sub.2].

4. The output capacitors of switches, [C.sub.1] and [C.sub.2], have the same value of Cs.

Simulation Results

Modified forward converter is simulated and the results are presented here.

[FIGURE 3a OMITTED]

[FIGURE 3b OMITTED]

[FIGURE 3c OMITTED]

[FIGURE 3d OMITTED]

[FIGURE 3e OMITTED]

[FIGURE 3f OMITTED]

[FIGURE 4a OMITTED]

[FIGURE 4b OMITTED]

[FIGURE 4c OMITTED]

[FIGURE 4d OMITTED]

[FIGURE 4e OMITTED]

[FIGURE 4f OMITTED]

Open loop system with disturbance applied at the input is shown in Fig. 4a. A step decrease in input voltage is applied as shown in Fig. 4b. The output of forward converter with disturbance is shown in Fig. 4c. In an open loop system, DC output decreases as shown in Fig. 4c.

Closed loop circuit model is shown in Fig. 4d. The output is sensed and it is compared with the reference voltage. The error is processed through a PI controller. The step reduction in input voltage is shown in Fig. 4e. The response of closed loop system is shown in Fig. 4f. It can be seen that the voltage increases and reaches the set value.

Conclusion

Double forward converter system is modeled and simulated using Matlab simulink version 7.1. The system is simulated in open loop and closed loop. The circuit model for closed loop system is developed and it is successfully used for simulation studies. The closed loop system is found to maintain the voltage constant when there is a disturbance at the input. The double forward converter has advantages like smaller transformer and smaller filter. Therefore double forward converter is a viable alternative to the existing DC/DC converters. The simulation results closely agree with the analytical results.

References

[1] F. D. Tan, 2002,"The forward converter: From the classic to the contemporary," in Proc. IEEE APEC, Mar. 10-14, vol. 2, pp. 857-863.

[2] F. Dong Tan, March, 1996 "Basic Circuits in Power Electronics", Department of Electrical and Computer Engineering, University of California at Irvine,

[3] Rudy Severns, 2000,"The history of the forward converters," Switching Power Magazine, Vol. 1, No. 1, My, pp. 20-22.

[4] A. K. S. Bhat and F. Dong Tan, 1989,"A unified approach to characterization of PWM and quasi-PWM switching converters: topological constraints, classification, and synthesis," IEEE Transaction on Power Electronics, Vol. 6, No. 4, Oct., pp. 719-725.

[5] F. Dong Tan, March 2001, "The forward converter: from the classic o the contemporary," IEEE APEC'01, Seminar #6.

[6] J K. Liu and F. C. Lee, 1984, "Resonant switches--a unified approach to improve performances of switching converters," in IEEE International Telecommunications Energy Conf. Proc, pp. 334-341.

[7] J K. Liu, R. Oruganti, and F. C. Lee, 1985,"Resonant switched-topologies and characteristics," in IEEE Power Electronics Specialists Conf. Rec, pp. 106-116.

[8] W.A. Tabisz and F.C. Lee, 1988, "Zero-voltage-switehing multi-resonant technique--A novel approach to improve performance of high-frequency quasi-resonant converters," IEEE Power Electronics Specialists Conference Rec., pp. 9-17.

[9] W. Tang, W. Tabisz, A. Lofti, F.C. Lee and V. Vorperian, 1990, "DC analysis and design of forward zero-voltage-switched multi-resonant converter," IEEE Power Electronics Specialists Conference Rec, pp. 333-340.

[10] R. Hiramatzu et al., 1989, "ZVS PWM converter utilizing partial resonance," IEEE INTELEC Proceeding, No. 20.9.

L. Padma Suresh, Dr. R.S. Rajesh and S.V. Muruga Prasad

Research Scholar, Reader, Asst. Prof., Dr. M.G.R University, M.S. University, Tirunelveli. Noorul Islam College of TamilNadu, Kumaracoil, TamilNadu, India [email protected] [email protected]