AN-9067 Datasheet, PDF (1/12 Pages) Fairchild Semiconductor – Analysis of MOSFET Failure Modes in LLC Resonant Converter

www.fairchildsemi.com
AN-9067
Analysis of MOSFET Failure Modes in LLC Resonant Converter
Abstract
The trend in power converters is towards increasing power
densities. To achieve this goal, it is necessary to reduce
power losses, overall system size, and weight by increasing
the switching frequency. High reliability is also very
important for today’s switched-mode power supplies
(SMPS). The zero-voltage-switching (ZVS) or zero-current-
switching (ZCS) topologies that allow for high-frequency
switching while minimizing the switching losses are of
interest. The ZVS topology operating at high frequency can
improve the efficiency as well as reduce the size of the
application. It also reduces the stress on power switches and
therefore improves the reliability. LLC resonant half-bridge
converters are becoming a popular topology because they
can provide these benefits. It has become widely accepted
for applications from high-end servers to flat-panel display
power supplies, but the ZVS bridge topologies including
LLC resonant half bridge require a MOSFET with fast
reverse-recovery body diode for better reliability. This
application note discusses potential failure mode and
mechanism in LLC resonant converters and provides a
simple and cost-effective solution to prevent failures.
Introduction
Increasing power density and achieving higher efficiency
are the most challenging issues in power conversion market,
especially in the telecom/server power supply application.
The most popular approach for increased power density is
increasing the switching frequency, which reduces the size
of passive components. The zero-voltage-switching (ZVS)
topologies that enable high-frequency switching are
growing popular thanks to extremely low switching losses,
low devices stress, and low profile^[1][2]. These resonant
converters process power in a sinusoidal manner and the
switching devices are softly commutated; therefore, the
switching losses and noise can be dramatically reduced.
Among many topologies, the phase-shifted ZVS full-bridge
is widely used for medium- or high-power application since
it allows all switches to operate at ZVS by effective output
capacitance of power MOSFET and leakage inductance of
transformer without an additional auxiliary switch.
However, the ZVS range is very narrow and the
freewheeling current consumes high circulating energy.
Recently, power MOSFET failures have been issued in the
phase-shifted ZVS full-bridge topology^[3]. The primary
cause of failure is slow reverse recovery of the MOSFET
© 2009 Fairchild Semiconductor Corporation
Rev. 1.0.0 • 11/5/09
body diode under low reverse voltage. Another cause of
failure is due to the Cdv/dt shoot-through at no- or light-
load conditions^[4]. In LLC resonant converters, a potential
failure mode can be associated with shoot-through current
due to poor reverse recovery characteristics of the body
diode^[5][6]. Even though voltage and current of power
MOSFETs are within safe operating area, some unexpected
failures associated with shoot-through current, reverse
recovery dv/dt, and breakdown dv/dt occur in conditions
such as startup, overload, and output short circuit.
LLC Resonant Half-Bridge
Converter
An LLC resonant converter has many advantages over
conventional resonant converters, as shown below.^[7]
ƒ Wide output regulation range with a narrow switching
frequency range
ƒ Guaranteed ZVS, even at no load
ƒ Utilization of all essential parasitic elements to achieve ZVS.
An LLC resonant converter can overcome the limitations of
conventional resonant converters. For these reasons, LLC
resonant converters are widely used in the power supply
market. LLC resonant half-bridge converter topology is
shown in Figure 1 with its typical waveforms shown in
Figure 2. In Figure 1, the resonant tank consists of a
capacitor, C_r, in series with two inductors L_r and L_m. One of
these inductors, Lm, represents the magnetizing inductor of
the transformer and creates one resonating point, together
with resonant inductor, Lr and resonant capacitor, Cr. Lm is
fully shorted by a reflected load, RLOAD at heavy load or will
remain in a series with the resonant inductor Lr at light load.
As a result, the operation frequency depends on the loading
conditions. Lr and Cr determine the resonant frequency, f_r1,
and Cr and two inductors, Lr and Lm, determine the second
resonant frequency, f_r2. It shifts to higher frequency as load
gets heavier. The resonant frequency moves between a
minimum and a maximum by the transformer and the
resonant capacitance C_r, as shown by Equations 1 and 2.
f_r1 =
1
(1)
2π L_r • C_r
f_r2 = _2π
1
(L_r + L_m ) • C_r
(2)
www.fairchildsemi.com