Modelling of a Resonant Converter with a Synchronous Current- Doubler Rectifier for DC Magnet Power Supplies S. Pholboon, J. Clare, P. Wheeler Department of Electrical and Electronic Engineering Faculty of Engineering, University of Nottingham, UK E-mail: eexsp5@nottingham.ac.uk Keywords: Current doubler rectifier, DC magnet power supplies, DQ model, Resonant converter, ZCZVS converter. Abstract This paper presents the modelling of a resonant converter with a synchronous current doubler rectifier (SCDR) for DC magnet power supplies. The concepts and principles behind the modelling approach are described. A dynamic model of the converter suitable for control design is presented. The modelling is based on a DQ approach and is verified through simulation and through experiment. 1 Introduction Magnet power supplies (MPSs) for high current magnet loads are widely used for high energy physics and in industrial/ medical applications such as particle-beam excitation and control for particle accelerator systems [1]. The requirements for MPSs include very low output waveform ripple and demanding transient performance requirements. In addition, low volume and high efficiency are also a benefit in many applications. Conventional magnet power supplies employ either 6-pulse, 12-pulse, or 24-pulse phase-controlled rectifiers with bulky passive power filters [2] and a large line frequency transformer. The advantages of the conventional approach are the simple system configuration and the potential for high power capacity. However, thyristor rectifiers generate significant harmonics resulting in output waveform ripple. In order to eliminate all the low frequency characteristic and non-characteristic harmonics, large and expensive passive power filters are required. A solution to this problem is to use switch mode power supplies (SMPSs). Due to high switching frequency operation, the SMPSs can achieve small size and light weight [3]. Nevertheless, the main disadvantages of SMPSs are high switching stresses and high switching losses, resulting in low efficiency and high power rating required for the semiconductor devices. Soft switching (zero voltage/zero current switching) techniques have been suggested to overcome these problems in [4]. Using these techniques, the converter is able to operate at higher switching frequencies (20 50 kHz) than a conventional, hard switched circuit. To provide soft switching, resonant magnet power supplies are preferred [5]. The output from the resonant tank is normally connected to a high frequency transformer and an output rectifier stage where current doubler rectifiers (CDRs) are a better choice in terms of the efficiency since the current in the secondary windings is half the output current and hence the copper losses decrease. Furthermore, MOSFETs can be used to replace diodes in CDRs in order to improve the efficiency using synchronous rectification (SCDR). Since a power MOSFET has low on-resistance, its turn-on voltage drop can be smaller than that of a diode resulting in a low conduction loss [6]. The proposed converter topology shown in Figure 1 consists of a three phase diode rectifier on the AC side along with a DC link capacitor, a series resonant parallel loaded inverter with a high frequency transformer and a SCDR connected to an L-C output filter and an R-C damping network. The design parameters of this converter were presented in [7]. Lr Cr C S1 S2 S3 S4 S5 S6 D1 D2 D3 D4 Cs D5 D6 V DC I_ref V_ref R d C d C f R L L L L 1 L 2 Frequency &Phase Shift Modulation V C AB V I T B A c1 V Gate driver circuit PI + - Gate driver circuit + - I OL V OL PI Current doubler synchronous rectifier Resonant tank H-bridge inverter DC-link Diode recitifer 3-phase supply Figure 1: Magnet Power Supply Topology