LTC3900_1 Datasheet, PDF (7/20 Pages) Linear Technology – Synchronous Rectifier Driver for Forward Converters

LTC3900
Applications Information
Overview
In a typical forward converter topology, a power trans-
former is used to provide the functions of input/output
isolation and voltage step-down to achieve the required
low output voltage. Schottky diodes are often used on
the secondary-side to provide rectification. Schottky
diodes, though easy to use, result in a loss of efficiency
due to relatively high voltage drops. To improve efficiency,
synchronous output rectifiers utilizing N-channel MOSFETs
can be used instead of Schottky diodes. The LTC3900
provides all of the necessary functions required to drive
the synchronous rectifier MOSFETs.
Figure 1 shows a simplified forward converter application.
T1 is the power transformer; Q1 is the primary-side power
transistor driven by the primary controller, LT1952 output
(OUT). The pulse transformer T2 provides synchronization
and is driven by LT1952 synchronization signal, S_OUT or SG
from the primary controller. Q3 and Q4 are secondary-side
synchronous switches driven by the LTC3900’s FG and CG
output. Inductor L_O and capacitor C_OUT form the output
filter to provide a steady DC output voltage for the load.
Also shown in Figure 1 is the feedback path from V_OUT
through the optocoupler driver LT4430 and an optocoupler,
back to the primary controller to regulate V_OUT.
Each full cycle of the forward converter operation con-
sists of two periods. In the first period, Q1 turns on and
the primary-side delivers power to the load through T1.
SG goes high and T2 generates a negative pulse at the
LTC3900 SYNC input. The LTC3900 forces FG to turn on
and CG to turn off, Q3 conducts. Current flows to the
load through Q3, T1 and L_O. In the next period, Q1 turns
off, SG goes low and T2 generates a positive pulse at the
LTC3900 SYNC input. The LTC3900 forces FG to turn off
and CG to turn on, Q4 conducts. Current continues to
flow to the load through Q4 and L_O. Figure 2 shows the
LTC3900 synchronization waveforms.
External MOSFET Protection
A programmable timer and a differential input current sense
comparator are included in the LTC3900 for protection
of the external MOSFET during power down and Burst
Mode^® operation. The chip also shuts off the MOSFETs
if V_CC < 4.1V.
When the primary controller is powering down, the primary
controller shuts down first and the LTC3900 continues to
operate for a while by drawing power from the V_CC bypass
cap, C_VCC. The SG signal stops switching and there is no
SYNC pulse to the LTC3900. The LTC3900 keeps one of
the drivers turned on depending on the polarity of the
last SYNC pulse. If the last SYNC pulse is positive, CG
will remain high and the catch MOSFET, Q4 will stay on.
The inductor current will start falling down to zero and
continue going in the negative direction due to the voltage
that is still present across the output capacitor (the current
now flows from C_OUT back to L_O). If Q4 is turned off while
the inductor current is negative, the inductor current will
produce high voltage across Q4, resulting in a MOSFET
avalanche. Depending on the amount of energy stored in
the inductor, this avalanche energy may damage Q4.
GATE
(OUT)
SG
(S_OUT)
SYNC
FG
CG
3900 F02
Figure 2. Synchronization Waveforms
3900fb
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