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 Total Downloads : 7041
 Authors : Savitha S, Vibin C Thomas
 Paper ID : IJERTV3IS070740
 Volume & Issue : Volume 03, Issue 07 (July 2014)
 Published (First Online): 22072014
 ISSN (Online) : 22780181
 Publisher Name : IJERT
 License: This work is licensed under a Creative Commons Attribution 4.0 International License
PSIM Simulation of a Buck – Boost DCDC Converter with Wide Conversion Range
Savitha S
Department of EEE
Adi Shankara Institute of Engineering and Technology Kalady, Kerala, India
Vibin C Thomas Department of EEE
Adi Shankara Institute of Engineering and Technology Kalady, Kerala, India
Abstract Nonisolated DCDC converters with wide conversion range have many applications such as battery charging, renewable energy harvesting, fuel cell etc. Incorporating coupled inductors in nonisolated DCDC converters encourages many advantages including providing a high voltage gain. Introducing coupled inductors in the conventional buck and boost DCDC converters and the integration of both produces a two switch buckboost topology which has better conversion range. This paper provides the simulation of the converter which can stepup as well as step down the input voltage. PSIM simulation software is used.
Keywords Buck; Boost; Buckboost; Coupled Inductors; Wide Conversion Range DCDC Converter.

INTRODUCTION
The nonisolating type DCDC converters are generally used for relatively small ratio stepup or stepdown of the input voltage where there is no problem of isolation. Theoretically the conventional converters can provide almost any voltage; but practically the output voltage range is limited by extreme duty ratio operation of the switches and high component stresses. Introduction of transformers attains large stepup or stepdown voltage conversion; but it causes problems associated with the magnetizing and leakage inductances.
Among the several studies taking place in nonisolated DCDC converters, widening the conversion range is of typical interest. Authors proposed various techniques to improve the conversion range of the converters such as using converters with nonlinear characteristics. Converter topologies with singletransistor were proposed in [1] which had achieved wide stepdown conversion range. Single switch nonisolated high stepup DCDC converters with simple topologies were proposed in [2]. These uses hybrid switched capacitor technique for providing high voltage gain without extreme duty cycle operation yet enabling the use of a lower voltage and RDSON MOSFET switch so as to reduce cost, switch conduction and turnon losses. Two boost converters cascade connection is one of solution for high voltage gain. It usually integrates two boost converters by using a common switch [3]. However, the output diode reverserecovery problem and the high voltage stress across
switch are severe. The switchedcapacitor and boost converters can be integrated together to obtain a stepless voltage gain. A family of singleswitch DCDC converters with high voltage gain is proposed in [4]. It consists of a boost multiplier cell, which can obtain a voltage conversion as the conventional boost converter, and a capacitordiode multiplier, which is in series to enlarge the voltage gain and to reduce the switch voltage stress. A new, twoinductor, two switch boost converter topology and its variations suitable for applications requiring very high voltage gains were described in [5]. The output voltage regulation of the proposed converters is achieved in a wide load and inputvoltage range by employing an auxiliary transformer that couples the current paths of the two boost inductors.
A different approach to obtain wide conversion range is by utilizing coupled inductors. The voltage gain can be extended by proper turns ratio design of the coupled inductors. [6] proposes a tappedinductor buck converter which can extend the duty cycle and has a low component count. The tappedinductor boost converters proposed in [7] attains comparable voltage stepup preserving relative circuit simplicity. The concept of the coupled inductor and the switched capacitor can be combined to derive highstepup converters [8]. The outputdiode reverserecovery problem is avoided by the leakage inductance of the coupled inductor. A softswitching tapped inductor buck converter was proposed in [9]. It shows the current injection method, which gives an additional design freedom which can maximize the efficiency. A modification of tapped buck converter for power factor correction was realized in [10]. The modification involves addition of only a linefrequency commutated switch and a diode. Families of highefficiency high stepup DCDC converters were proposed in [11]. [12] presents an interleaved softswitching buck converter with coupled inductors to extend duty ratio for high stepdown voltage applications. With the proposed converter, conversion efficiency can be improved significantly. Similarly a boost converter with coupled inductors and a buckboost type of activeclamp circuit was proposed in [13]. It can yield a high stepup voltage ratio and a proper duty cycle, resulting in low component stresses.
As already said the conventional buck and boost converters can be modified by using coupled inductors. Thus a WideInputWideOutput (WIWO) DCDC converter which is an integration of buck and boost converters with coupled inductors was proposed in [14]. It retains both buck and boost converter features with wider stepup and step down DCDC conversion range. Here this nonisolated topology proposed is simulated using PSIM simulation tool.
This paper presents the openloop and closedloop simulation of the converter. In section II, the overview of the converter, its control scheme and its operating principle is discussed. Section III presents the open loop and closed loop PSIM model of the converter. Section IV presents the simulation results obtained and concluding remarks are presented in section V.

SYSTEM OVERWIEW

Converter Topology
The converter is shown in Fig. 1. It has two active switches S1 and S2, coupled inductors L1 and L2 with turns ratio n = n2: n1, a diode D, capacitive filter C and a resistive load R.
Fig. 1. Converter Topology
It can operate in Buck as well as boost mode. Its operation can be explained with assumption that the circuit comprises of ideal components, unit coupling coefficient and under continuous conduction mode.
In buck charging mode, the switch S2 is turned on and S1 is turned off. The diode D conducts and Coupled inductors are charged. In buck discharging mode, the switch S2 is turned off cutting current in L1. S1 is turned on and D conducts L2 current to the load. In boost charging state, S1 and S2 are turned on charging L1. D is reverse biased by L2 and output voltage is supported by capacitor C. In boost discharging mode, S2 is on and S1 is off. L1 and L2 conduct through D to the output.

Control Scheme
Modified PWM control circuitry is used for control of operation. Open loop control circuitry is shown in Fig. 2. The modulation index m, actually defines the buck and boost operation here. WIWO operates in buck mode when 0 m <

Then the upper comparator provides the required duty cycle pulses for S2 and lower comparator outputs a 1 state; thus NAND gate produces the complimentary pulses for S1. While when 1 m < 2, it operates in boost mode. Then the upper comparator produces a 1 state which keeps S2 ON and
the lower comparator together with NAND gate produces the required pulses to S1.
Fig. 2. Openloop control Scheme for WIWO Converter
Closed loop control circuitry is shown in Fig. 3. It has two control voltages Vc and Vc . Vc is obtained by compairing the reference voltage with the output voltage. Vc is obtained by downshifting the control voltage Vc by Vm ; Vc
= Vc – Vm . Swtooth ramp has an amplitude of Vm .The relationship between Vc and Vm is Vc = mVm . So WIWO operates in buck mode when 0 Vc < Vm. While when Vm Vc < 2Vm, it operates in boost mode.
Fig. 3. Closedloop control Scheme for Converter


Operating Principle
It is assumed that the circuit comprises of ideal components, unit coupling coefficient and under continuous conduction mode. The dark connections show the current path in each mode.

Buck Mode
State 1: Buck mode charging state (0 m < 1).
Fig. 4. Buck mode charging state
State 2: Buckmode discharging state (0 m < 1).
Fig. 5. Buck mode discharging state

Boost Mode
State 1: Boost mode charging state (1 m < 2).
Fig. 6. Boost mode charging state
State 2: Boostmode discharging state (1 m < 2).
Fig. 7. Boost mode discharging state Fig. 8.


SIMULINK MODEL

Open Loop Simulink Model.
PSIM simulation tool is used to simulate the DCDC converter with the logics discussed above. The turns ratio was set to n = 1. The switching frequency of 20 kHz was chosen. L1 and L2 were chosen as 56.53ÂµH. Input voltage of 12V is given. Filter capacitor value should be high.

Openloop model
Fig. 9. Openloop model of converter topology
Fig. 8 shows the openloop circuit model of the topology. Fig. 9 shows the model of the openloop control scheme. Here m controls the mode of operation. So whenever m changes the mode of operation changes.

Openloop control scheme.
Fig. 10. PSIM model of openloop control scheme


Closedloop model

Closedloop model
Fig. 11. Closedloop model of converter

Closedloop control scheme.

Fig. 12. PSIM model of closedloop control scheme
Fig. 10 shows the closedloop model. Fig. 11 shows the model of the closedloop control scheme. A PI controller and a limiter is used to obtain the required value of duty cycle during closed loop operation.


RESULT ANALYSIS

Gain vs Duty Cycle

In Buck Mode
Fig. 13. Gain Vs Duty Cycle graph in buck mode
Fig. 12 is the graph showing the voltage gain against duty cycle in buck mode. It can be seen that the voltage gain is about 0.28 for a duty cycle of 0.1. It shows a good stepdown capability of the circuit.

In Boost Mode
Fig. 14. Gain Vs Duty Cycle graph in boost mode
Fig. 13 is the graph showing the voltage gain against duty cycle in boost mode. It can be seen that a voltage gain of 8.34 is obtained for a duty cycle of 0.8. It shows a good stepup capability of the circuit. In boost mode the duty cycle cannot be extended beyond 80%.


Buck Mode
The output waveforms for a duty cycle of 0.5 are shown here. Fig. 14 shows the output voltage waveform.

Boost Mode
Fig. 17. Voltage across S2
Fig. 15. Output Voltage (Buck Mode)
Fig. 15 shows the voltage across S1.
Fig. 16. Voltage across S1
Fig. 16 shows the voltage across S2. There occurs a turn off voltage spike across the switch as can be seen from the waveform. These can be reduced by designing effective snubber circuit. About 0.75 times and 1.46 times the input voltage appears across S1 and S2 in steady state.
The output waveforms for a duty cycle of 1.5 are shown here. Fig. 17 shows the output voltage waveform.
Fig. 18. Output Voltage (Boost Mode)
Fig. 18 shows the voltage across S1. There occurs a high transient voltage across the switch as can be seen from the waveform. These can be reduced by designing effective snubber circuit. About 2 times the input voltage appears across S1 during steady state.
Fig. 19. Voltage across S1

Closedloop Simulation Result
Fig. 19 shows the output voltage for input voltage of 100 V and reference voltage of 50 V. The voltage is found to be constant at 50 V under closed loop operation.
Fig. 20. Output Voltage in closedloop operation


CONCLUSION
This paper presented the openloop and closedloop PSIM simulations of wideOutput DCDC Converter which is an integration of buck and boost converter via coupled inductors. Simulation results were also presented. The discussed converter can be used for battery charging and discharging operation as both stepping up and stepping down the input voltage can be done. There are various advantages for this particular topology such as wider stepup and wider stepdown DCDC conversion range, moderate component count, simple structure, high efficiency, smooth transition between operating modes, avoid operation in extreme duty cycle, can operate with broadly varying input source etc. A disadvantage is the coupled inductor leakage causing transient and turn off voltage spikes across the switches. A
passive lossless snubber cell can be used to improve the turn on and turnoff transients in nonisolated pulsewidth modulated (PWM) dc/dc converters [16]. Switching losses
and EMI noise are reduced by restricting of the reverse –
recovery current and of the drainsource voltage. However
here there is a need to design two separate snubber circuits
for S1 and S2. Numerous advantages of WIWO make it suitable for many industrial applications.
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