AN-6069 Datasheet, PDF (1/12 Pages) Fairchild Semiconductor – Application Review and Comparative Evaluation of Low-Side Gate Drivers

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AN-6069
Application Review and Comparative Evaluation
of Low-Side Gate Drivers
Summary
Power MOSFETs require a gate drive circuit to translate the
on/off signals from an analog or digital controller into the
power signals necessary to control the MOSFET. This paper
provides details of MOSFET switching action in
applications with clamped inductive load, when used as a
secondary synchronous rectifier, and driving pulse/gate
drive transformers. Potential driver solutions, including
discrete and integrated driver designs, are discussed.
MOSFET driver datasheet current ratings are examined and
circuits are presented to assist with evaluating the
performance of drivers on the lab bench.
Introduction
In many low-to-medium power applications, a low-side
(ground referenced) MOSFET is driven by the output pin of
a PWM control IC to switch an inductive load. This solution
is acceptable if the PWM output circuitry can drive the
MOSFET with acceptable switching times without
dissipating excessive power. As the system power
requirements grow, the number of switches and associated
drive circuitry increases. As control circuit complexity
increases, it is becoming more common for IC
manufacturers to omit onboard drivers because of grounding
and noise problems.
Synchronous rectifiers (SRs) are increasingly used to replace
standard rectifiers when high efficiency and increased power
density are important. It is common for isolated power
stages delivering tens of amps to parallel two or more low-
resistance MOSFETs in each rectifying leg, and these
devices require current pulses reaching several amps to
switch the devices in the sub-100ns timeframe desired.
External drivers can provide these high-current pulses and a
means to implement timing to eliminate shoot-through and
optimize efficiency to control the SR operation. In addition,
drivers can translate logic control voltages to the most
effective MOSFET drive level.
Low-side drivers are also used to drive transformers, which
provide isolated MOSFET gate drive circuits or
communication across the power supply isolation boundary.
In these applications, a driver is required to handle concerns
specific to transformer drive, discussed later.
Low-side drivers may seem a mundane topic; several papers
have been written on the subject. Though often presented as
an ideal voltage source that can source or sink current
determined by the circuit’s series impedance, the current
available from a driver is, in fact, limited by the discrete or
integrated circuit design. This note reviews the basic
requirements of drivers from an application viewpoint, then
investigates methods for testing and evaluating the current
capability of drivers on the lab bench.
Clamped Inductive Switching
The simplified boost converter in Figure 1 provides the
schematic for a typical power circuit with a clamped
inductive load. When the MOSFET Q is turned on, the input
voltage V_IN is applied across inductor L and the current
ramps up in a linear fashion to store energy in the inductor.
When the MOSFET turns off, the inductor current flows
through diode D1 and delivers energy to C_OUT and R_LOAD at
voltage V_DC. The inductor is assumed large enough to
maintain current constant during the switching interval.
VIN
VDC
L
D1
V_DD
CBYP
RG
VOUT
IG
COUT
Q
RLOAD
Figure 1. Simplified Boost Converter
© 2007 Fairchild Semiconductor Corporation
Rev. 1.0.3 • 1/6/10
www.fairchildsemi.com