Reducing IGBT losses in ZCS series resonant converters

The fundamental operational parameter that controls the losses In series resonant converters was round to be the (reflected) DC voltage transfer ratio. Losses which are a function of the average current (such as conduction losses of IGBTs and diodes) are independent of the s\vltchlng frequency. Losses which are associated with the rms current are a function of both the (reflected) DC voltage ratio and the switching frequency ratio. Universal and normalized graphs, derived In this study,~.can be conveniently used to assess the expectedrms and average current conduction losses under any given operational conditions. The residual switching losses in "ZCS" series resonant converters operating in Continuous Current Mode, can be reduced by simple current snubbers placed in the commutation circuits. The experimental results of this study confirm the theoretical predictions and demonstrate that the turn on snubbers can reduce switching losses by about 1.5% at a switching frequency of 65kHz. diodes. This could cause exn-a losses aside from being harmful from the EMI point of view. This problem was dealt with in relation to BIT switches [6]. Now that IGBTs are favored, reexamination of this issue with regards to the new switching devices seems to be in order. Aside from the question of the residual switching losses, there appears to be a need for a clear delineation of the expected conduction losses in resonant converters. This problem is also not new and has been dealt with in the past. However, we could not find in the literature simple and sn-aightforward answers to the crucial engineering questions: what are the expected losses of a resonant converter operating under any given conditions. It was therefore felt that a fresh look at the problem might contribute to a better understanding of practical limitations of I GB T based resonant converters. II. MAIN SmADY -ST Am EQUATIONS Consider a vol12ge-fed (DC-DC) series resonant converter which operates at a switching frequency below resonance (Fig.l). I. INTRODUCfION I Resonant converters have many favorable advantages. They can be designed for Zero Voltage Switching {ZVS), Zero Current Switching (ZCS) in either current fed or voltage fed topologies. Indeed, they were shown to be useful in a multitude of applications ranging from basic DC-DC converters [I], active power correction circuits [2] to capacitor chargers [3] and electronic welders [4]. The main drawback of resonant converter topologies is the higher stresses as compared to PWM switchers. This however has been ameliorated, to a large extent, since the introduction of economical IGBTs. Conduction losses of these bipolar devices are a function of the average rather than the rms currents and hence high peak currents are not that detrimental. Consequently, resonant topologies have still much to offer. The main switching limitation of IGBTs stems from the current tail associated with the charge stored in the junction of the device [5]. Therefore, ZCS which is characterized by forced commutation at turn off, appears to be a better strategy for resonant IGBT converters. However, conventional "ZCS" resonant topologies which operate at frequencies below resonant are still prone to switching losses at turn on. As the switching frequency is increased, the turn on losses could become significant. Furthermore, hard switching at turn on invokes the reverse recovery problem of the anti-parallel Fig. Basic topology of a voltage fed series resonant converter. Assumptions: 1. Switches, diodes and the transformer are ideal. 2. The capacity Co of the output filter is infinitely high and therefore the output voltage Vo is constant. The equivalent circuit for the transistor conduction interval is given on Fig. 2a and for the diode conduction interval is given on Fig. 2b. Vin is the input voltage, 9 is topology constant (g = 1 for full-bridge and 9 = 0.5 for half-bridge), * Corresponding Author. Incumbent of the Luck-Hille chair of Instrumentation design