With many government regulations  encouraging energy savings around the world, most industry experts agree that new technology advances in power MOSFETs  play a critical role in providing these benefits to power conversion applications. Higher power density, system efficiency and reliability are always critical factors for server / telecom power systems. Super junction MOSFETs have been used in resonant converters in these systems, but generally, their body diode performance is not attractive for these topologies. Newly developed 650V fast recovery super-junction MOSFETs, called SuperFET® II FRFET® MOSFET have rugged body diode, higher threshold voltage (Vth=4V), ultra low on-resistance and extremely fast switching speed. It can provide improved reliability and efficiency needed in server / telecom power applications.

In high voltage MOSFET technologies, the most remarkable achievement for on-resistance reduction has been charge-balance technology. SuperFET® II MOSFETs that combine faster switching and Qrr of body diode performance with 40% RDS(ON) per given die area reduction compared to previous generation super-junction MOSFETs called SuperFET® MOSFETs. RDS(ON) reduction is just one of the benefits, while the following parameters improvements truly benefit resonant converters applications. Both zero voltage switching and LLC resonant technologies related to the body diode will be further described.

TABLE I Critical Specification Comparison of DUTs


Figure 1Figure 1. Comparisons of reverse recovery behavior under ISD=10A, di/dt=100A/μs, VDS=400V, Tj=25ºC

As shown in table 1, the gate charge, Qg of 650V/190mΩ SuperFET® II FRFET® MOSFET is dramatically reduced by 27% compared to previous generation 600V/190mΩ SuperFET® I FRFET® MOSFET. Figure 1 shows the reverse recovery behavior comparison at ISD=10A, di/dt=100A/μs, VDS=400V and Tj=25ºC. It can be clearly seen that the reverse recovery charge, Qrr of SuperFET® II FRFET® MOSFET reduced by 47% compared to previous generation. Furthermore, voltage spikes of a SuperFET® II FRFET® MOSFET during reverse recovery behavior is lower than SuperFET® I FRFET® MOSFET due to its soft reverse recovery characteristics and small Qrr As shown in figure 1. MOSFET output capacitance is another crucial parasitic parameter to understand for zero voltage switching (ZVS) topologies. It determines how much inductance is required to provide ZVS conditions because MOSFET output capacitance can be used as a resonant component in ZVS topologies. Understanding the L-C and switching timing relationship makes this part of the switching cycle (almost) lossless for resonant topology. Therefore, if the stored energy in output capacitance of the MOSFET is small, less resonant energy is required to achieve soft switching without increasing the circulating energy. As shown in figure 2, a SuperFET® II FRFET® MOSFET has approximately 23.3% less stored energy in output capacitance than SuperFET® I FRFET® MOSFET at 400V across the MOSFET generated from a typical switching power supply bulk capacitor voltage. Figure 3 shows the switching loss comparison. A SuperFET® II FRFET® MOSFET has much better switching performance, that is 22~42% less switching losses according to load current, compared to previous generation SuperFET® I FRFET® MOSFET in clamped inductive switching tests under the following condition : Vdd=400V, Rg=4.7ohm and Id=2~20A

Figure 2Figure 2. Comparisons of stored energy in output capacitance, EOSS

Figure 3Figure 3. Comparisons of switching losses (Eon + Eoff) under Vdd=400V, Rg=4.7ohm and Id=2~20A

The LLC resonant converter requires a device with body diode ruggedness characteristic because there are high current stresses in over-load, output short-circuit condition and inrush current during start-up. Even though voltage and current of power MOSFETs are within safe operating area, some unexpected cited device failures associated with shoot through current, reverse recovery dv/dt, and breakdown dv/dt still happen in various conditions, such as overload and output short circuit. The worst case is a short-circuit condition. During short circuit, the MOSFET conducts extremely high (theoretically unlimited) current. When short circuit occurs, operation mode during short circuit is almost the same as the overload condition, but the short-circuit condition is worse because reverse-recovery current, which flows through the body diode of the switch, is much higher. Figure 4 shows the failed waveforms of the Normal MOSFETs in LLC resonant converter at short-circuit condition. The current level during short-circuit condition is much higher and can lead to increased junction temperature of the MOSFET and make it easier to fail. Fast recovery MOSFETs can prevent this failure thanks to their robust body diode performance.

Figure 4Figure 4. Waveforms of Power MOSFETs in LLC Resonant Converter at Short-Circuit Condition

New fast recovery SuperFET® II MOSFET combines a faster and more rugged intrinsic body diode performance with fast switching, aimed at achieving better reliability and efficiency in applications including resonant converters. With reduced gate charge and stored energy in output capacitance, switching efficiency is increased and driving and output capacitive losses are decreased. Performance of fast recovery SuperFET® II MOSFET allows designers to significantly increase system efficiency and reliability, particularly for in phase shifted full-bridge converters or half-bridge LLC resonant converters under abnormal conditions.

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