AN-6003 Datasheet, PDF (1/6 Pages) Fairchild Semiconductor – Shoot-through in Synchronous Buck Converters

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AN-6003
“Shoot-through” in Synchronous Buck
Converters
Jon Klein
Power Management Applications
Abstract
The synchronous buck circuit is in widespread use to
provide “point of use” high current, low voltage
power for CPU’s, chipsets, peripherals etc. In the
synchronous buck converter, the power stage has a
“high-side” (Q1 below) MOSFET to charge the
inductor, and a “Low-side” MOSFET which replaces
a conventional buck regulator’s “catch diode” to
provide a low-loss recirculation path for the inductor
current.
V _IN
H ig h -S id e
Q1
L1
V_OUT
+
–
L o w -S id e
Q2
Figure 1. Synchronous Buck output stage
Shoot-through is defined as the condition when both
MOSFETs are either fully or partially turned on,
providing a path for current to “shoot through” from
V_IN to GND. To minimize shoot-through,
synchronous buck regulator IC’s employ one of two
techniques to ensure “break before make” operation
of Q1 and Q2 to minimize shoot-through:
1. Fixed “dead-time”: A MOSFET is turned off,
then a fixed delay is provided before the low-
side is turned on. This circuit is simple and
usually effective, but suffers from its lack of
flexibility if a wide range of MOSFET gate
capacitances are to be used with a given
controller. Too long a dead-time means high
conduction losses. Too short a dead time can
cause shoot-through. A fixed dead-time
typically must err on the “too long” side to allow
high C_GS MOSFETs to fully discharge before
turning on the complementary MOSFET.
2. Adaptive gate drive: This circuit looks at the
V_GS of the MOSFET that’s being driven off to
determine when to turn on the complementary
MOSFET. Theoretically, adaptive gate drives
produce the shortest possible dead-time for a
given MOSFET without producing shoot-
through.
In practice, a combination of adaptive and fixed
produces the best results, and is typically what is in
today’s PWM controllers and gate drivers as shown
in Figure 2
BOOT ^RG
D1
+5
C _BOOT
VIN
D
PWM
Delay
1V +
HDRV
C_GD
R_GATE
G
C_GS
SW
S ^Q1
D
PWM
Delay
LDRV
C_GD
R_GATE
G
C_GS
PGND
S ^Q2
1V
Figure 2. Typical Adaptive Gate drive
Even though there apparently is a “break before
make” action by the controller, shoot-through can
still occur when the High-side MOSFET turns on,
due to Gate Step.
Shoot-through is very difficult to measure directly.
Shoot-through currents persist for only a few nS,
hence the added inductance in a current probe
drastically affects the shoot-through waveform.
Shoot-through manifests itself typically as increased
ringing, reduced efficiency, higher MOSFET
temperatures (especially in Q1) and higher EMI.
This paper will provide analytical techniques to
predict shoot-through, and methods to reduce it.
04/25/2003