AN-6099 Datasheet, PDF (1/11 Pages) Fairchild Semiconductor – New PowerTrench MOSFET with Shielded Gate Technology Increases System Efficiency and Power Density in Synchronous Rectification Applications

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AN-6099
New PowerTrench^® MOSFET with Shielded Gate
Technology Increases System Efficiency and Power
Density in Synchronous Rectification Applications
Abstract
Synchronous rectification in a high-performance converter
design is essential for low-voltage and high-current
applications because significant efficiency and power
density improvements can be achieved by replacing
Schottky rectification with synchronous rectification
MOSFETs. Many critical parameters for synchronous
rectification MOSFETs and even parasitic components in
devices and printed circuit board directly affect the system
efficiency of synchronous rectification. Optimization of the
MOSFETs plays an important role in improving efficiency.
The PowerTrench^® MOSFET with shielded-gate technology
can dramatically reduce both on-resistance and gate charge,
which are usually in conflict. With soft-body diode
characteristics, the new power MOSFETs reduce voltage
spikes that cause additional losses in the snubber circuits.
For better system efficiency and power density, the
characteristics of new PowerTrench^® MOSFETs are
introduced and compared to other power MOSFETs
available in the market. The benefits of these MOSFETs are
shown in synchronous rectifier of target application.
Introduction
As our economy moves from paper-based to digital
information management; data centers for data processing,
storage, and networking play an important role in many
industries. However, data centers are becoming increasingly
expensive to power and cool. Higher system efficiency and
power density in modern data and telecommunication power
systems are the core focus since making a small high-
efficiency power system means saving space and energy
bills. From a topology point of view, synchronous
rectification that converts the AC voltage from the
transformer back to DC becomes an essential building block
for the secondary side of the switched-mode power supply
(SMPS) in many applications. This solution offers improved
efficiency for these conversion stages with both lower
conduction loss and switching losses.^[1]-[3] For this reason,
synchronous rectification is very popular in low-voltage and
high-current applications, such as server power supplies or
telecom rectifiers. As shown in Figure 1, it replaces
Schottky rectifiers, allowing lower voltage drop. From a
device point of view, the power MOSFET transistor has
© 2013 Fairchild Semiconductor Corporation
Rev. 1.0.1 • 3/12/13
enjoyed significant evolution in the last decade, which
enabled new topologies and high power density in power
supplies. The key requirements for synchronous rectification
MOSFETs are:
 Low R_SP
 Low dynamic parasitic capacitances. This also reduces the
gate drive power since synchronous rectification circuit is
generally operated at high frequency.
 Low Q_RR and C_OSS reduces reverse current. This becomes
a problem when this topology is operated at high
switching frequency. At high switching frequency, this
current acts as high leakage current.
 Low t_RR, Q_RR, and less snappy body diode is needed to
avoid momentary shoot-through and reduce loss. Snappy
diode may require a snubber across each MOSFET.
 Low Q_gd/Q_gs ratio prevents dynamic turn-on.
Np:Ns
L1
D1
C_o
R _LOAD
D2
L2
(a) Diode Rectification
Np:Ns
L1
Q1
C_o
R _LOAD
Q2
L2
(b) Synchronous Rectification
Figure 1. Diode Rectification and Synchronous
Rectification
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