AN-9065 Datasheet, PDF (1/5 Pages) Fairchild Semiconductor – FRFET® in Synchronous Rectification

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AN-9065
FRFET^® in Synchronous Rectification
Introduction
A synchronous rectifier becomes essential building block for
greater efficiency and higher power density in switching
power supplies. It is popular in applications from high-end
servers to laptop adapters. This application note shows
potential failure modes of power MOSFETs and solutions for
synchronous rectification in flyback topology. An enhanced
FRFET^®, power MOSFET with fast recovery body diode,
provides higher reliability at the same performance.
Synchronous Rectifier Operation
and Potential Failure Modes
Most of today’s power MOSFETs have a vertical DMOS
structure. In this structure, many parasitic elements exist,
such as capacitances, bipolar junction transistor (BJT), and a
diode. The inherent anti-parallel diode formed by drain-
source P-N junction is sometimes called body diode. The
body diode is fully functional as general rectifiers, but it
does not perform as optimally as general rectifiers. It has a
very long reverse recovery time and large reverse recovery
charge. In spite of its limited performance, the body diode
has been utilized as freewheeling diode because it can make
simple circuit without adding system cost. Among many
applications, a synchronous rectifier is a good example of
actively utilizing body diode. It replaces the rectifier for
better efficiency. Power losses can be lowered when the
product of MOSFET’s on-resistance and drain current is less
than the diode forward voltage drop. Increasing power
density and achieving higher energy efficiency are most
challenging issues in power conversion today. The popular
flyback topology is aligning with this trend. A flyback
converter with a synchronous rectifier is shown in Figure 1.
Conduction periods of primary MOSFET and secondary
MOSFET should not overlap to guarantee proper operation
of the converter. A delay between primary MOSFET turn-on
and secondary MOSFET turn-off, and vice versa, must be
introduced to prevent simultaneous conduction of the
MOSFETs. This delay results in current flow through body
diode of the secondary MOSFET. Then the body diode is
switched off when the primary MOSFET turns on. The
switching process of the diode from on state to reverse
blocking state is called reverse recovery. A device failure
may happen during body diode reverse recovery. Another
failure mode is breakdown dv/dt. It is a combination of
breakdown and static dv/dt. A device undergoes avalanche
current and displacement current at same time.
Secondary
Gate Drive
V_IN
Primary
Gate Drive
V_OUT
Figure 1. Flyback Converter with Synchronous Rectifier
© 2009 Fairchild Semiconductor Corporation
Rev. 1.0.1 • 1/13/09
www.fairchildsemi.com