High Step-Up DC-DC Converter for Distributed Generation System

This study proposes a method which consists of High step up DC-DC converter with a coupled inductor for distributed generation system. Theoretically the conventional boost converter provides high step up voltage gain but in practical it is limited by reverse recovery problem of diode, effective series impedance of inductors and capacitors and switching losses. High charged current and conduction losses occur in the switch when voltage lift and switched capacitor techniques are used. In the proposed strategy a coupled-inductor and two capacitors is utilized to achieve high step-up voltage gain. High power loss and voltage spike on the switch is avoided using passive clamp circuit that recycles the leakage inductor energy. The operating principle and steady-state analysis are discussed. The Proposed topology was simulated using PSPICE SOFTWARE and the following results were obtained. For an input voltage of 24V, an output of 333V was obtained.


INTRODUCTION
Distributed Energy Resources (DERs) are becoming integral components of electric power distribution systems nowadays.Distributed generation, also called on-site generation, dispersed generation, embedded generation, generates electricity from many small energy sources.The DG system is mainly based on renewable energy resources.But, usually, the voltage output of these renewable sources is low and hence cannot be directly connected to the grid.Thus stepping up of voltage is necessary.Thus, in most of the cases, an isolated dc-dc converter forms part of the interface required to connect the DER output to the distribution system.The conventional boost converter is not good candidates for extremely high step-up applications, because the voltage gain is only determined by the duty cycle.This results in extreme duty cycles, which means very narrow turnoff pulses in high step-up conversions.Hence the conventional boost converter cannot provide such a high dc voltage gain even for an extreme duty cycle and also may result in serious reverse-recovery problems and increase the rating of all devices.As a result, the conversion efficiency is degraded and the Electromagnetic Interference (EMI) problem is severe under this situation (Mohan et al., 1995).
In order to get high voltage gain and to increase the conversion efficiency, many modified boost converter topologies have been investigated in recent years.Abutbul et al. (2003) proposed a step-up switchingmode converter with high voltage gain using a switched-capacitor circuit.This converter can achieve any voltage ratio and can operate at relatively low duty cycle.Thus the reverse recovery problem of diode can be reduced.But, the diode will have high voltage stress and switch will suffer high charged current.The conduction loss is also high.Da Silva et al. (2001) proposed a boost soft-single-switch converter.The proposed topology has only one single active switch.The converter is operated with soft switching in a Pulse-Width-Modulation (PWM) manner, hence the voltage and current stresses is low.But in order to achieve the soft switching the voltage gain is limited below four.Jovanovic and Jang (1999) and Duarte and Barbi (2002) proposed voltage clamped techniques.Although voltage-clamped techniques are manipulated in the converter design to overcome the severe reverserecovery problem of the output diode in high-level voltage applications, there still exists overlarge switch voltage stresses and the voltage gain is limited by the turn-on time of the auxiliary switch.Papanikolaou and Tatakis (2004) proposed using active voltage clamp circuits in flyback converters operating in continuous conduction mode under wide load variation.This topology provides isolation and also limits over voltages.A transformer is used and by adjusting the turn's ratio of the transformer, high step up voltage gain can be achieved.But the transformer is utilized during one half cycles only.Since voltage on secondary will be reflected onto the primary, high rating is required for the main switch.In Hirachi et al. (2002) and Roh et al. (1999), coupled inductors were employed to provide a high step-up ratio and to reduce the switch voltage stress substantially and the reverse-recovery problem of the output diode was also alleviated efficiently.In this case, the leakage energy of the coupled inductor is another problem as the switch was turned off.It will result in the high-voltage ripple across the switch due to the resonant phenomenon induced by the leakage current.In order to protect the switch devices, either a highvoltage-rated device with higher or a snubber circuit is usually adopted to deplete the leakage energy.But the power conversion efficiency will be degraded.
Zhao and Lee ( 2003) proposed a high-efficiency, high step-up dc-dc converters by only adding one additional diode and a small capacitor.It can recycle the leakage energy and alleviate the reverse-recovery problem.In this scheme, the magnetic core can be regarded as a flyback transformer and most of the energy was stored in the magnetic inductor.However, the leakage inductor of the coupled inductor and the parasitic capacitor of the output diode resonated after the switch was turned on, a proper snubber is necessary to reduce the output rectifier peak voltage.
In the proposed strategy a coupled-inductor and two capacitors is utilized to achieve high step-up voltage gain.High power loss and voltage spike on the switch is avoided using passive clamp circuit that recycles the leakage inductor energy.The operating principle and steady-state analysis are discussed.This section presents the operating principle of the proposed converter.In CCM operation, there are five operating modes as shown in Fig. 2 to 6 in one switching period.Fig. 7 shows typical waveforms

STEADY-STATE ANALYSIS OF THE PROPOSED CONVERTER
Modes 1 and 3 are very short and hence for steady state analysis only modes 2, 4 and 5 are considered.
The time for which capacitor C1 is charged by the leakage inductor energy:  In mode 5, let the voltages across primary, secondary and leakage inductor be  1 5 ,  2 5 and   5 .Average voltage across inductor is zero.Therefore, for winding 1, 2 and leakage inductor: On solving: Considering the capacitor voltages by KVL: Figure 8 shows the plot of gain versus duty ratio, we can infer that as duty ratio increases, the gain also increases exponentially for increase in turns ratio.

SIMULATION OF THE PROPOSED CONVERTER
The Proposed topology was simulated using PSPICE SOFTWARE with the following specifications: The proposed converter is simulated using PSPICE Software as shown in Fig. 9.
The output wave form is shown in Fig. 10.The input voltage of 24V is applied to the converter and the Figure 12 shows the continuous current mode of operation of the converter with inductor current varying from 7-12 Amp.The Fig. 13 shows gate pulses and the current through diode D1 and it is clear that, soft switching is achieved and diode reverse recovery problem also alleviated.

CONCLUSION
A soft-switching dc-dc converter with high voltage gain for DG system has been proposed in this study.By using coupled inductor high gain was obtained.Using passive clamp circuit the leakage inductor energy was recycled.Hence reverse-recovery problem is alleviated.It provides a continuous input current and the ripple in the output voltage is also very low.The different modes of operation and steady state analysis were discussed.The setup was validated by simulation using Orcad software.

Mode 3 :
Switch remains turns off.Diodes D1, D2 and D3 remains turned off.Do turns on.The parasitic capacitor of the switch is charged by the energies of leakage and magnetizing inductor.Co provides energy to load.Mode ends at the instant when D1 conducts.Mode 4: Switch remains off.D1 and Do are on and D2 and D3 are off.Leakage inductor energy is recycled.

Fig. 8 :
Fig. 8: Plot of gain and duty ratio for CCM Fig. 9: Orcad schematic By substituting the above values in the equation for Vo, Voltage gain: