s30p5| Application Note
MOSFET Failure Modes in the Zero-Voltage-Switched Full-Bridge Switching Mode Power Supply Applications
Alexander Fiel and Thomas Wu
International Rectifier Applications Department El Segundo, CA 90245, USA
Abstract-As the demand for the telecom/server power is growing exponentially, the need for higher power density increases each year. Increasing power density relies on less component counts, smaller reactive component size, and/or better system efficiency. Higher switching frequency leads to the smaller reactive and filter component size. Better efficiency, which reduces the heat sink or paralleled devices, requires the reduction either in conduction losses or switching losses, or both. Therefore, phase-shifted zero-voltageswitching (ZVS) full bridge topologies are gaining popularity due to their extremely low switching losses in the power devices even at higher switching frequency. However the intrinsic body diode is required to conduct in order to create the ZVS turn-on condition for the power MOSFET. Due to the extremely low reverse voltage, the reverse recovery charges might not be swept out before turning off the MOSFET. Therefore, the body diode might be subjected to the dv/dt stress when it is not yet
capable of blocking reverse voltage. Also not able to maintain the ZVS operation at low load will force the on-state MOSFET to turn off at hard-switching condition. Like in the hard-switched full bridge topology, the cdv/dt shootthrough current might produce a voltage spike at the gate of the off-state MOSFET on the same leg and cause devices failure. Several silicon technologies will be presented to resolve the abovementioned failure modes in the ZVS topology. Fast reverse recovery time and better dv/dt ruggedness make this new MOSFET technology suitable for higher frequency ZVS full-bridge applications. Inherent with extremely high silicon density and low gate charge, these new MOSFETs can reduce the component count with the same or better performance and will enable much higher power density for the next generation telecom/server SMPS designs.
I. INTRODUCTION Demand for higher power density, especially in the telecom/ server power applications, has driven the power electronics industry to the innovation of better topologies and power devices. The design trends are higher power density and more reliable power devices. Increasing power density from 5-7 W/in3 now to 10 W/in3 in the same form factor is an incredible challenge for power supply designers. Phase-shifted zero-voltage-switching (ZVS) topology [1] seems to be the remedy for achieving higher power density and increased device reliability. The advantages of ZVS operation are minimized switching losses, higher operation frequency, less dv/dt and di/dt dynamic stresses.
Reduction of switching losses enables higher switching frequency and requires less heat sink. Higher switching frequency will also lead to smaller reactive and filter components, and result in higher power density. Less dynamic stresses increase the reliability on power devices. However, power MOSFET failures have been reported recently in the ZVS full bridge topology [2, 3]. Slow reverse recovery of the intrinsic body diode under low reverse voltage is believed to be the root cause for the MOSFET failures. Screening the reverse recovered time of power MOSFETs is one of the solutions, but customers need to pay the price penalty. Another solution is a complicated network of paralleled fast-recovery diodes to divert current from the body diode of the MOSFET [3, 4]. The best solution is to expedite the reverse recovery process on the body diode [2]. The other field failure is also reported at no or low load conditions in the ZVS circuitry. Unable to achieve ZVS condition at no or low load has forced the power M
OSFET to experience hard-switched turn-on and turn-off. The possible failure mode for this operation is the Cdv/dt shootthrough [5]. Detailed failure analysis on the Cdv/dt shoot through problem, together with the silicon and system solutions, will be described in this article. The in-depth understanding of the failure modes in the ZVS operation has led to the development of a new generation fast-body-diode MOSFET technology that reduces reverse-recovery time, increases dv/dt stress capability, minimizes the Cdv/dt shoot-through, and therefore is suitable for the high frequency, high power phase-shifted ZVS applications. II. MOSFET FAILURE MODES IN ZVS OPERATION A full-bridge ZVS topology is displayed in Figure 1 where diodes D1-D4 are the intrinsic body diodes of power MOSFETs Q1-Q4. Capacitors C1- C4 represent either the output capacitances of power... 1
