Bidirectional Snubberless Commutated Soft-
Switching DC/DC Converter for Fuel Cell
Vehicles

Vaseem Faizal; Geetha B; Babu Paul

Bidirectional Snubberless Commutated Soft- Switching DC/DC Converter for Fuel Cell Vehicles

Vaseem Faizal1, Geetha B2, Babu Paul3

P.G. Student, Mar Athanasius College of Engineering, Kothamangalam, Kerala, India 1
Professor, Mar Athanasius College of Engineering, Kothamangalam, Kerala, India2
Assistant Professor, Mar Athanasius College of Engineering, Kothamangalam, Kerala, India3

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Abstract

A naturally clamped zero-current commutated soft switching bidirectional full-bridge isolated dc/dc converter is implemented by eliminating the necessity for passive snubbers. Switching losses are reduced significantly owing to zero-current switching of primary side devices and zero-voltage switching of secondary-side devices. Soft switching and voltage clamping are inherent and load independent. The voltage across primary-side devices is independent of duty cycle with varying input voltage and output power and clamped at rather low reflected output voltage, enabling the use of semiconductor devices of low voltage rating. These merits make the converter promising for fuel cell vehicles application, front-end dc/dc power conversion for fuel cell inverters, and energy storage in the DC/DC converter is analysed with zero current Commutation (ZCC) and the natural voltage clamping (NVC) has been analysed. Experiment simulated using MATLAB 2010.Ra and output power of 500W is obtained which is suitable for low power automotive ac motor.

Keywords

ZCS(zero current switching), ZVS(zero voltage switching), ZCC(zero current commutation), NVC(nominal voltage clamping).

INTRODUCTION

In automotive industry most research takes place in the electric vehicles. Two major concepts are electric vehicles(EV) and fuel cell vehicles(FCV). Comparing with EV’s, FCV’s has clear upper hand as FCV need short charging period and greater range of driving. In the FCV itself, earlier implementation was using voltage fed converter but it has lot disadvantages such as high input pulsating current, limited soft-switching range, high circulating current through switches and relatively low efficiency for high voltage amplifications and high current input applications. Because of the draw backs of voltage fed based converter, the later converter based on current source had been implemented using snubbers. Usually employed current fed converters were resistor-capacitor-diode(RCD) snubber. But RCD snubber leads to low efficiency owing to clamping energy dissipated in snubber resistor. As a result a novel current fed DC/DC converter is proposed.

PROPOSED TOPOLOGY

A dual half-bridge bidirectional dc/dc converter is proposed as shown in fig.1. However, this topology requires four split capacitors that occupy a considerable volume of the converter. It may need an additional control to avoid any voltage imbalance across the capacitors. In addition, the topology is not modular and is not easily scalable for higher power. Peak currents through the primary switches are greater than2.5× the input current and the top and bottom switches share unequal currents.

Mainly four methods are used here to reduce the switching losses. ZVS, ZCS, ZCC, NVC, ZCS method is used in the primary switches, when current becomes zero in the switches gate signal is switches to zero, and voltage start to build up. ZVS is used in the secondary switches, when voltage across the switch becomes zero gate signal is switched to high, so that current start to increase. Before turning off the diagonal switch pairs of primary side switches (S1-S4), the other pair (S2-S3) is turned on. It diverts current from one switch pair to the other, causing the current through the conducting switch pair to rise and the current through conducting switch pair to fall to zero naturally resulting in ZCC. Later the body diodes across switching pairs to start conducting and their gating signals are removed leading to ZCS turnoff of the devices. Commutated device capacitance starts charging with NVC.

PRINCIPLE OF OPERATION

A. Principle of operation

Principle of operation is explained with the help of 8 modes as given below.

1) Mode1

In this interval, primary-side H-bridge switches S2 and S3 and antiparallel body diodes D6 and D7 of secondary-side Hbridge switches are conducting. The current through inductor Llkis negative and constant. Power is transferred to the load through the HF transformer. Non-conducting secondary devices S5 and S8 are blocking output voltage Vo, and nonconducting primary devices S1 and S4 are blocking reflected output voltage Vo/n. The values of current through various components are

iS2= iS3 = Iin, iS1= iS4 = 0, ilk = −Iin, and iD6 = iD7 = Iin/n (1)

2) Mode 2

At t = t1, primary switches S1 and S4 are turned on. Snubber capacitors C1 and C4 discharge in a very short period of time.

3) Mode 3

Now, all four primary switches are conducting. Reflected output voltage Vo/n appears across leakage inductance Llkand causes its current to increase linearly. It causes currents through previously conducting devices S2 and S3 to reduce linearly. It results in conduction of switches S1 and S4 that started conducting with zero current, which helps reduce associated turn-on loss. Since the antiparallel body diodes D6 and D7 are conducting, switches S6 and S7 can be gated for ZVS turn-on. At the end of this interval t =t3, D6 and D7 commutate naturally. Primary current reaches zero and ready to change polarity. Current through all primary devices reaches Iin/2. Final values are

iS1 = iS2 = iS3 = iS4 = Iin/2, and iD6 = iD7 = 0ilk = 0 (2)

4) Mode 4

In this interval, secondary H-bridge devices S6 and S7 are turned on with ZVS. Currents through all the switching devices continue increasing or decreasing with the same slope as in interval 3. At the end of this interval, primary devices S2 and S3 commutate naturally with ZCC and their respective currents iS2 and iS3 reach zero obtaining ZCS. The full current, i.e., input current Iin, is taken over by other devices S1 and S4and the transformer current changes polarity. Final values are

ilk= Iin, iS1 = iS4 = Iin, iS2 = iS3 = 0, and Is6 = iS7 = Iin/n. (3)

5) Mode 5

In this interval, the primary current or leakage inductance current ilk further increases with the same slope. Antiparallel body diodes D2 and D3 start conducting, causing extended zero voltage to appear across the outgoing or commutated switches S2 and S3to ensure ZCS turnoff. Now, secondary devices S6 and S7 are turned off. At the end of this interval, currents through the transformer and switches S1 and S4 reach their peak value. This interval should be very short to limit the peak current through the transformer and switches, and thus their kilovolt ampere ratings.

6) Mode 6

During this interval, secondary switches S6 and S7 are turned off. Antiparallel body diodes of switches S5 and S8 take over the current immediately. Therefore, the voltage across the transformer primary reverses polarity and the current through it starts decreasing. The currents through switches S1 and S4 and body diodes D2 and D3 also start decreasing.At the end of this interval, currents through D2 and D3 reduce to zero and are commutated naturally. Currents through S1 and S4 and the transformer reach Iin.Final values are

ilk= iS1 = iS4 = Iin, iD2 = iD3 = 0, and iD5 = iD8 = Iin/n. (4)

7) Mode 7

In this interval, snubber capacitors C2 and C3 charge to Vo/n in a short period of time. Switches S2 and S3 are in forward blocking mode now.

8) Mode 8

In this interval, currents through S1 and S4 and the transformer are constant at input current Iin. The current through antiparallel body diodes of the secondary switches D5 and D8 is Iin/n. The final values are iS1 = iS4 = ilk = Iin, iS2 = iS3 = 0, and iD5 = iD8 =Iin/n. (5) In this half HF cycle, current has transferred from one diagonal switch pair to the other diagonal switch pair, and the transformer current has reversed its polarity. DC voltage obtained at the terminals of a convertr is inverted to AC with the help of single phase full bridge inverter as shown in the figure.

SIMULINK MODEL

A. Simulink Model

In the belowSimulink model, a dc /dc converter and dc/ac converter are combined. In the operation of dc/dc converter shoot through of switches are implemented which helps to enhance the energy in the inductor. Value of transfer inductance is calculated.

B.Simulation parameter

a) Input voltage : 100 V DC

b) Input inductor : 1e-9 H

c) C1, C2,................C8 : 1e-6 F

d) Transfer inductance, Llk: 1.5e-6 H

e) Load resistance : 100 ohm

f) Frequency : 50 KHz

SIMULATION RESULTS

Simulation result of the converter is shown below. Input voltage of 100v is applied, and get an output voltage of 316 v. AC volttage is obtained in the primary of the transformer as shown below, which is converted to DC and boosted up with the help of switching. This DC voltage is converted to AC by single pahse full bridge inverter. An output power of 500w is obtained which makes it suitable to use an auxiliary source of energy in the FCV’s.

CONCLUSION

A new dc/dc to converter is proposed in which use ZCStechnique in the primary and ZVStechnique in the secondary which ensure minimum switching loss. In the converter also use the possibility of ZCC and NVC. It therefore eliminatesthe need of an active-clamp or passive snubber.Usage oflow-voltage devices results in low conduction losses in primary devices, which is significant due to higher currents on the primary side. The proposed modulation method is simple and easy to implement. This dc/dc and dc/ac converter find applications in the modern electric vehicles as interface between battery and three phase motor. These merits make the converter promising for interfacing a lowvoltage dc bus with a high-voltage dc bus for higher current applications such as FCVs. Can be employed in frontend dc/dc power conversion for renewable (fuel cells/photovoltaic) inverters, uninterruptible power system, microgrid and energy storage.

Figures at a glance


Figure 1	Figure 2	Figure 3	Figure 4

Figure 5	Figure 6	Figure 7	Figure 8

Figure 9	Figure 11	Figure 12

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