FTEEE-1694 Design and Analysis of a High Efficiency DC–DC Converter With Soft Switching Capability for Renewable Energy Applications Requiring High Voltage Gain – IEEE EEE Project 2016-2017
Renewable sources like solar PV and fuel cell stack is preferred to be operated at low voltages. For applications such as grid tied systems, this necessitates high voltage boosting resulting in efficiency reduction. To handle this issue, this paper proposes a novel high voltage gain, high efficiency dc-dc converter based on coupled inductor, intermediate capacitor and leakage energy recovery scheme. The input energy acquired from the source is first stored in the magnetic field of coupled inductor and intermediate capacitor in a lossless manner. In subsequent stages it is passed on to the output section for load consumption. A passive clamp network around the primary inductor ensures the recovery of energy trapped in the leakage inductance, leading to drastic improvement in the voltage gain and efficiency of the system. Exorbitant duty cycle values are not required for high voltage gain, which prevents problems such as diode reverse recovery. Presence of a passive clamp network causes reduced voltage stress on the switch. This enables the use of low voltage rating switch (with low “on state” resistance), improving the overall efficiency of the system. Analytical details of the proposed converter and its hardware results are included.
In the last few decades, there has been a drastic increase in the demand for electricity. This has led to rapid use and depletion of fossil fuels. These factors have led the researchers to renewable energy sources such as wind, solar PV and fuel cell stack. Solar Photovoltaic (PV) and fuel cell energy sources play a prominent role among the existing renewable sources poses major challenges such as: Optimal utilization of the source due to their non-linear characteristics They are usually operated at low output voltage levels (typ. 25-50V) because of safety issues. This makes their application to grid connected systems and even some stand-alone loads difficult because a large voltage boosting is required. A direct implication of points that the use of a dc-dc converter is essential at the front end, right across the source.
The converters depict some of the high voltage gain topologies that are representative of the existing configurations. Direct voltage step up using high frequency transformer renders a simple and easily controllable converter providing high gain. Isolated current-fed dc-dc converters are example of this category. However these topologies result in high voltage spikes across the switch and large ripple in primary side transformer current as the turn’s ratio in the high frequency transformer increases. The isolated systems are relatively costly, bulky and generally less efficient even though they offer more safety, eliminate issues such as ground leakage current and can provide multiple outputs among other advantages.
Most of the non-isolated high voltage gain dc-dc power converters employ coupled inductor (to achieve higher voltage gain) in contrast to a high frequency transformer used by the isolated versions. The coupled inductor based dc-dc converter has advantages over isolated transformer based dc-dc converter in minimizing current stress, using lower rating components and simple winding structure. Modelling procedure of the coupled inductor is described. For high power converter applications interleaved coupled inductor based boost converters have also been proposed.
- Overview of high voltage gain dc-dc converter topologies
- Description of the proposed converter
- Report Generation
Overview of high voltage gain dc-dc converter topologies:
A demerit of coupled inductor based systems is that they have to deal with higher leakage inductance, which causes voltage spikes across the main switch during turn-off time and current spike during turn-on time, resulting in a reduction of the overall circuit efficiency. The effects of leakage inductance can be eliminated by using an active clamp network, which provides an alternate path to recover leakage energy. But active clamp network is not as efficient as a passive clamp because of conduction losses across the power switch of the active clamp network. Active clamp network consists of a switch with passive components while passive clamp network consists of passive components such as diode, capacitor and resistor. The passive clamp circuit is more popular to reduce voltage stress across the converter switch by recycling leakage energy.
A novel topology has been proposed in this paper that achieves high voltage through a coupled inductor connected in interleaved manner that charges an intermediate buffer capacitor and a passive clamp network to recover the leakage energy. Coupled inductor leads to the incorporation of ‘turn’s ratio’ into the gain expression that leads to high efficiency without increasing the duty ratio. As compared to existing high gain dc-dc converters, the number of passive components used in the proposed converter is less, which reduces the cost and improves the efficiency. Though the proposed converter is applicable to any low voltage source application (e.g. solar PV, Fuel cell stack, battery etc.), this paper focuses only on the solar PV source.
Description of the proposed converter:
Energy conversion efficiency of solar PV is quite low. Therefore, it is essential to use a highly efficient power conversion system to utilize the PV generated power to the maximum. The proposed high gain dc-dc converter configuration. It consists of one passive clamp network, a coupled inductor (L1,L2) and an intermediate capacitor apart from other components. The symbol VPV represents the PV voltage applied to the circuit. S is the main switch of the proposed converter. The coupled inductor’s primary and secondary inductors are denoted by L1 and L2. C1 and D1 represent the passive clamp network across L1. The capacitor CO is the output capacitor while D3 is the output diode. The voltage VO is the average (dc) output across the load. The intermediate energy storage capacitor, C2 and the feedback diode D2 are connected on the secondary side.
Efficiency plot with respect to load variation of the proposed converter. The efficiency plot of the proposed converter is compared with conventional push-pull converter having identical input output voltage ratings and operating frequency (50kHz). Full load (400W) efficiency of the converter, obtained from experimentation, is 96%. A comparison is also shown with conventional, hard switched push pull converter (of identical rating, operating frequency and input/output voltage specifications) with and without active clamp across the main switch. The proposed system shows a higher efficiency under all load conditions.
A circuit efficiency of 96% is achieved under full load conditions. Provides a comparison of the key circuit variables values obtained analytically with hardware measurements. The error in the results between analytical derivation and experimental result is found to be between 0 to 5%. This could be attributed to measurement errors, presence of parasitics and use of imprecise values of circuit components for experimentation. The trend shown in the plots of suggests that a good way to design and use the proposed converter would be to use turns ratio of the inductor around 4 or 5 (fixed during the design) and vary the duty ratio from 0 to 0.7 while operating. Beyond this range of duty cycle and turns ratio, the losses become significantly higher. High voltage gain is achieved without using extreme duty cycle values, which is a big advantage over conventional step-up converters.
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