12-08-2013, 03:58 PM
Novel High Step-Up DC-DC Converter for Distributed Generation System
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Abstract
In this paper, a novel high step-up DC-DC
converter for distributed generation (DG) system is proposed.
The concept is to utilize two capacitors and one coupled-inductor.
The two capacitors are charged in parallel during the switch-off
period and are discharged in series during the switch-on period
by the energy stored in the coupled inductor to achieve high step-
up voltage gain. In addition, the leakage-inductor energy of the
coupled inductor is recycled with a passive clamp circuit. Thus,
the voltage stress on the main switch is reduced. The switch with
low resistance RDS(ON) can be adopted to reduce the conduction
loss. In addition, the reverse-recovery problem of the diodes is
alleviated, and thus, the efficiency can be further improved. The
operating principle and steady-state analyses are discussed in
detail. Finally, a prototype circuit with 24-V input voltage, 400-V
output voltage, and 200-W output power is implemented in the
laboratory to verify the performance of the proposed converter.
INTRODUCTION
In recent years, distributed generation (DG) systems
based on renewable energy sources (RES) have rapidly
developed. The DG systems are composed of microsource like
fuel cells, photovoltaic cells, and wind power [1-7]. However,
fuel cells and photovoltaic (PV) source are low-voltage source
to provide enough DC voltage for generating AC utility
voltage. Although PV cells can connect in series to obtain
sufficient DC voltage, it is difficult to avoid shadow effect [8-
10]. Thus, high step up DC-DC converters are usually used as
the front-end converters to step from low voltage up to high
voltage which are required to have a large conversion ratio,
high efficiency, and small volume [11].
Theoretically, the boost converter can provide high step-
up voltage gain with extremely high duty cycle [12]. In
practice, the step-up voltage gain is limited by the effect of
power switch, rectifier diode, and the equivalent series
resistance (ESR) of the inductors and capacitors. Also, the
extreme duty cycle operation may result in serious reverse-
recovery problem and electromagnetic interference (EMI)
problem [13].
CCM Operation
This section presents the operation principle of the
proposed converter. The following analysis contains the
explanation of the power flow direction of each mode. In CCM
operation, there are five operating modes in one switching
period. Fig. 2 shows typical waveforms and Fig. 3 shows the
current-flow path of each modes of the circuit. The operating
modes are described as follows:
1) Mode I [t0, t1]: During this time interval, S is turned on.
Diodes D1 and Do is turned off, and D2 and D3 are turned
on. The current-flow path is shown in Fig. 3(a). The
voltage equation on the leakage inductor and magnetic
inductor of coupled inductor on the primary-side is
expressed as Vin=VLk+VLm. The leakage inductor Lk starts to
charge by Vin. Due to the leakage inductor Lk, the
secondary-side current is of the coupled inductor is
decreased linearly. Output capacitor Co provides its energy
to load R. When current iD2 becomes zero at t = t1, this
operating mode is end.
CONCLUSION
This paper proposed a novel high step-up DC-DC
converter for DG system. By using the capacitor charged in
parallel and discharged in a series by the coupled-inductor,
high step-up voltage gain and high efficiency are achieved.
The steady-state analyses are discussed in detail. Finally, a 24-
400 V / 200 W prototype circuit of the proposed converter is
put into operation in the laboratory. Experimental results
confirm that high efficiency and high step-up voltage gain can
be achieved. The peak efficiency is 95.88%. Additionally, the
voltage on the main switch is clamped at 84 V, thus low
voltage ratings and low on-state resistance RDS(ON) switch can
be selected.