Abstract
With a view to regulate the output voltage with fixed frequency, without using any additional component or complex modulation technique, a topology of a variable duty controlled three-level LLC converter is proposed and an equivalent small signal model for Electric Vehicle (EV) charger applications is deduced in this paper. The steady state equations of each operating region are derived in time domain. Based on an Extended Describing Function (EDF) approach, a small signal equivalent circuit is modeled which includes both frequency and duty controlled terms. The equivalent circuit is further simplified to derive a transfer function of duty control to output voltage. The transfer function is verified through simulation software. Analyzing the transfer function, a voltage controller is designed and implemented with a PI compensator. The simulation results of the proposed control schemes are illustrated and discussed. The topology is compared to a conventional frequency control topology and the merits of the proposed topology are presented.
Highlights
Electricity based transportation methods are being researched all over the world for the past two decades [1] in order to find alternatives to fuel consumption [2,3]
Both the duty control and frequency controlled outputs are dependent on load condition, the desired voltage for duty controlled converter can be achieved at the fixed switching frequency, only by adjusting the duty cycle
The proposed circuit topology of three-level duty controlled LLC converter is presented in this paper for a potential Electric Vehicle (EV) charger application
Summary
Electricity based transportation methods are being researched all over the world for the past two decades [1] in order to find alternatives to fuel consumption [2,3]. Even though the converter in [36] uses fixed frequency scheme, the voltage stress in two lagging leg switches is equal to half of the input voltage. The waveforms are highly nonlinear and the real time calculation is far more complex than frequency controlled LLC converters. The state plane average method [41] is widely applied for PWM converters, it provides a solution up to half of the switching frequency only. The EDF method applied for LLC converter in [43,44] leads to an equivalent circuit model of fifth order. With the aim of linearizing the circuit and developing a duty control to output voltage transfer function, an equivalent small signal model approach has been modeled applying the reduced third order EDF concept. A comparison between steady state wave forms of frequency control and duty control method is presented and a conclusion is drawn
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