Current - Fed Multi - Resonant DC - DC Converter *

A novel topology, Current-Fed MultiResonant DC-DC Converter (CF-MRC) was studied theoretically and experimentally. The main features of the proposed converter are an inherent protection against a short circuit at the output, a high voltage gain and zero voltage switching over a iarge range of output voitage. These characteristics make it a viable choice for the implementation of a high voltage capacitor charger. I. INTRODUC110N Rapid charging of a capacitor to a high voltage [1,2] is a challenging task from several aspects. The charger must not only be immune to a shorted output but also to maintain an acceptable operating conditions while the output voltage swings over a wide range. This requirement and the general trend toward higher switching frequencies warrants a search for resonant topologies that are inherently protected against short circuit at the output while maintaining Zero Voltage Switching (ZVS) or Zero Current Switching (ZCS) over the expected output voltage swing. The objective of the present study was to explore the characteristics of a novel current fed converter which maintains ZVS under shorted output and no load conditions. Fig. I. Basic topology of the proposed Current-Fed MultiResonant Converter (CF-MRC). The basic operation of the CF-MRC is described by considering a simplified equivalent circuit in which the secondary is reflected to dIe primary (Fig.2). The replacement of dIe input inductor ~Lin) by a current source is justified by the fact that in the proposed CF-MRC 4Lin»Lr. Consequently, during one switching cycle, the AC component through Lin is low and dIe input feed can be considered to be a DC current source. Assuming an initial stage in which dIe toggle switch S (which represent the action of dIe two transistors) has just entered dIe Q2 position (Fig. 2), the input current source will charge the primary resonant circuit (Lr, Cl) and dIe voltage across dIe parallel tank (VDS 1 = V C I) will rise in a semi-sinusoidal manner (Fig. 3). This mode is recognized as dIe quasi-resonant period [3] which lasts for A degrees (referred to dIe switching frequency fs). At dIe end of dlis period, dIe voltage across dIe Ql becomes negative and dIe anti-parallel diode Dl clamps it to zero. In this boost mode, bodl dIe arms of dIe push-pull stage conduct and hence dIe voltage across Cl is zero (Fig 3). At half way of dIe switching frequency period, Q2 is turned off and Ql is turn on. This revens the situations and the invener stage enters dIe quasi resonant mode of dIe second half cycle. II. TOPOLOGY AND BASIC W A VEFORMS The proposed Current-Fed Multi-Resonant Converter (CFMRC) (Fig. I) comprises a high-frequency current fed pushpull inverter stage, a coupling current transformer (TV and an output rectifier section which is followed by a capacitive filter (Co). The coupling transformer (TV includes a split primary to ensure a balanced loading of the push-pull stage. The inverter is driven by a symmetrical square wave of a frequency fs which is lower dIan dIe basic resonant frequency formed by CI and Lr (Fig. I). A second capacitor C2 is placed at the secondary to facilitate a conduction path when the rectifier section is blocked. As will be shown below, the secondary resonance, formed by Lr and dIe reflected C2 is of prime importance in the operation of the CF-MRC. The controlled switches of the inverter are transistors (QI) and (QV to which external high speed anti-parallel diodes are connected (Dl, DV to ~rmit bidirectional conduction. * Corresponding author. Incumbent of the LuckRille Chair il) Instrumentation Design