Increase Current Capability and Simplify Thermal Design of Flyback Converters with Secondary-Side Synchronous Rectifier Driver in a 5-Pin SOT-23

Increase Current Capability and Simplify Thermal Design
of Flyback Converters with Secondary-Side Synchronous
Rectifier Driver in a 5-Pin SOT-23
Wei Gu
In a flyback converter, current levels in high current applications are constrained
by heat produced in the output rectifier diode. The clear way to lift this restriction
is to replace the diode with a much lower voltage drop MOSFET, significantly
reducing heat produced in the rectifier—reduced heat dissipation increases output
current capability and efficiency, and simplifies thermal design. The LT8309 is a
secondary-side synchronous MOSFET driver that replicates the behavior of the
output diode by sensing the drain-to-source voltage of the MOSFET to determine
if should be on or off, allowing it to replace a less efficient rectifier diode.
The LT8309 can be combined with any
of Linear’s line of no-opto boundarymode flyback ICs (such as the LT3748
primary side controller) to produce high
performance isolated power supplies
with a minimal number of components.
5V, 8A ISOLATED SUPPLY
Figure 1 shows a low voltage, high
current, low parts count flyback
supply. The traditional output diode is
replaced by an ideal diode consisting
of the LT8309, a MOSFET and a few
small external components.
For a MOSFET to act as a diode it must
turn on as soon as the body diode starts
Figure 1. A low voltage high current flyback converter
VIN
36V TO
72V
LT8309’s internal LDO generates a
7V output at the INTVCC pin for the
MOSFET gate drive. The strong gate
drive with 1Ω pull-down resistance
speeds up turn-on and turn-off of the
MOSFET, resulting in better efficiency.
Efficiency is shown in Figure 2 with
a comparison to a diode-only design.
Thanks to higher efficiency, the operating temperature of a board built
around an LT8309-based design remains
significantly lower than a diode-only
design, as shown in Figures 3 and 4.
PA1735NL
10µF
402k
EN/UVLO
VIN
D2
LT3748
RFB
RREF
COUT : LMK325ABJ107MM
D1: CMMRIU-02
D2: SMBJ85A
D3: 1N4148
D4: BAV21W-7-F
60.4k
D5: CMZ5920B
M1: BSC900N20NS3
M2: BSC028N06LS3
1µF
•
D1
15.8k
TC
GATE
SS
VC
SENSE
GND INTVCC
12.1k
15nF
•
D4
10Ω
147k
2k
M1
M2
VOUT +
5V, 8A
DRAIN
INTVCC
GATE
D5
GND
4.7µF
13mΩ
•
LT8309
VCC
COUT
100µF
×6
1µF
6.04k
D3 100Ω
4.7µF
32 | January 2014 : LT Journal of Analog Innovation
a 150V voltage rating, making it suitable
for wide input voltage design as well.
conducting current, and turn off as soon as
its current decreases to zero. The LT8309’s
fast comparator produces the required
near instantaneous action. The current
sensing comparator monitors the drain
voltage of the MOSFET. When the body
diode begins to conduct, the drain voltage
goes well below ground, the comparator trips and turns on the MOSFET. After a
minimum on-time, the LT8309 waits for the
MOSFET turn-off trip point to be reached
to turn off the MOSFET. The turn-off trip
point can be adjusted through an external
resistor connecting the part’s DRAIN pin to
the drain of the MOSFET. The DRAIN pin has
2.2nF
VOUT–
design ideas
The LT8309 can be combined with any of Linear’s line of
no-opto boundary-mode flyback ICs, such as the LT3748
primary side controller, to produce high performance
isolated power supplies with minimal components.
The 40V VCC pin rating allows the LT8309
to be driven from the output voltage or
the rectified drain voltage of the MOSFET.
If The VCC pin is connected to the output of the flyback converter, during
an output short-circuit condition, the
LT8309 is off and the body diode of the
MOSFET must handle the short-circuit
condition. This puts additional thermal
requirements on the MOSFET. Instead, if
VCC is connected to the drain voltage of
the MOSFET as shown in Figure 1, VCC is
equal to VIN /N in short-circuit, allowing the LT8309 to operate during a short.
The short-circuit current flows through
the MOSFET instead of the body diode.
CONCLUSION
LT8309 is an easy-to-use, fast, secondaryside synchronous flyback rectifier driver in
SOT-23 package. High efficiency, high current isolated power supplies require a minimal number of components, and thermal
design is simplified when the LT8309 is
combined with one of Linear’s line of
no-opto boundary-mode flyback ICs. n
PDS760 SCHOTTKY
Figure 3. Thermal image at 5V/5A output with diode PDS760
92
90
88
EFFICIENCY (%)
Figure 2. Efficiency of
LT8309-based flyback
converter compared same
converter with a traditional
secondary-side diode
rectifier
LT8309 & MOSFET
86
84
82
DIODE
80
VIN = 36V
VIN = 48V
VIN = 72V
78
76
0
1
2
3
4
5
ILOAD (A)
6
7
8
9
LT8309 AND FET
Figure 4. Thermal image shows 5V/5A output runs much cooler with the LT8309
January 2014 : LT Journal of Analog Innovation | 33