AN IMPROVED ZERO-CURRENT-SWITCHING SINGLE-PHASE TRANSFORMERLESS PV H6 INVERTER WITH SWITCHING LOSS-FREE
Abstract
In this paper, a switching loss-free (SLF) concept for the first six-switches H-bridge inverter (H6-I) topology is proposed. SLF means that its switches are able to operate with soft turn-on and turn-off transitions. In order to implement the SLF goal , a new resonance-trajectory is proposed. Compared with the zero-current-transition H6-I (ZCT-H6-I) topology published in previous literature , the proposed resonance-trajectory can precisely compensate for losses of resonant tanks every switching period. With this intention, an implementing circuit is structured based on the H6-I topology, and its detailed operation principle and performance characteristics are analyzed. As a result, all active switches of the new circuit are switched under zero-current turn-on and zero-current turn-off conditions. Also, the reverse recovery problem of freewheeling diodes is alleviated owing to the zero-current turn-off property of diodes. The SLF target is realized in theory. Finally, experimental results from a 1 kW prototype at 50 kHz switching frequency are provided to verify the effectiveness of the proposed SLF concept in practice. Specifically, the conversion efficiency of the new circuit is over 95% in a wide load range, and there is roughly a 1.5% efficiency improvement compared with the hard-switching H6-I topology.
EXISTING SYSTEM:
Looking back, this developing trend will resemble the developed route of DC switching supplies in 3C (Computer, Communication and Consumer Electronic) industry, for their switching frequencies have already reached MHz level currently. However, in conventional transformerless PV grid-connected inverters, their switches are still on hard-switching state. Because of this, High frequency transformerless PV grid-connected inverters will suffer from high losses, cooling stresses, and EMI noises . Obviously, soft-switching technique is one of the most promising techniques to reduce or even remove the switching losses, and to degrade the switching stresses, such as di/dt and dv/dt. Generally speaking, existed soft switching techniques can be roughly categorized into two sorts: the snubber-type with resonant tanks and the control-type using switching modulation strategies. In previous literature, there were a plenty of snubber-type soft switching topologies resulted from Silicon Controlled Rectifier (SCR) commutating branches. Their main aim was at combining desired features from both of the conventional PWM and resonant converters while avoiding their respective disadvantage. Particularly, according to with or without active power devices, the snubber-type soft switching techniques can be further classified as the active snubber-type and passive snubber-type. In active-snubber-types, the resonant tanks can only be activated during switching transitions of high-frequency switches. Therefore, once the switching transition is finished, this kind of converters can revert back to the familiar PWM operation mode so that the circulation loss of resonant tanks can be minimized.
PROPOSED SYSTEM:
Undoubtedly, there have been two important challenges for the ZCT-H6-I so far. They include compensating for the loss of the resonant tanks every switching period, removing the turn-off loss of the auxiliary switches and the reverse recovery loss of the freewheeling diodes. In order to solve these problems, this paper restructures a resonance-trajectory with a self-compensation mode, as well as a couple of improved resonant tanks with the resonance-trajectory are proposed. Particularly, several distinctive features have been obtained in the proposed circuit: first, the loss of the resonant tanks is reliably replenished by the self-compensation mode every switching period; second, the zero-current-switching (ZCS) conditions of turn-on and turn-off processes are achieved for all power semiconductor devices including diodes. In other words, all switching losses are almost removed, and the reverse recovery problem of the freewheeling diodes is also alleviated because of diode’s ZCS turn off. Last and the most important, the SLF concept for transformerless PV grid-connected inverters is successfully realized. These advantages will significantly improve the efficiency of transformerless PV grid-connected inverters, especially in high switching frequency applications. The major contribution of this paper is to propose and demonstrate a switching loss-free (SLF) concept based on the H6-I topology, which is attractive in high power density transformerless PV grid-connected inverter.
CONCLUSION
The soft-switching technique is one of the most promising techniques to raise the switching frequency for PV grid-connected inverters. The main contributions of this paper are that the SLF concept for single-phase transformerless full bridge topologies has been proposed and the SLF concept is realized based on the H6-I topology. Several clear merits are summarized as below. 1) A resonance trajectory with the self-compensation mode is designed, and a couple of the resonant tanks with the self-compensation mode are obtained based on the H6-I. They are able to compensate for the loss of the resonant tanks precisely. 2) The ZCS conditions are achieved for all power switches in both of turn on and turn off processes under unity power factor condition. Besides, the ZCS turn off of the freewheeling diodes is achieved naturally so that the reverse recovery problem is alleviated. 3) By integrating the resonant tank and clamping diodes with saving one diode, the freewheeling clamping function is obtained synchronously so that a constant common-mode voltage is realized at switching frequency scale. These characteristics are verified by a SLF-H6-I prototype rated at 50 kHz, 1 kW. In brief, the proposed SLF concept is a good candidate for the high frequency transformerless PV inverters, especially in the unity power factor application below 4.6 kW rated power.
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