AN-6206 Datasheet, PDF (1/9 Pages) Fairchild Semiconductor – Primary-Side Synchronous Rectifier (SR) Trigger Solution for Dual-Forward Converter

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AN-6206
Primary-Side Synchronous Rectifier (SR) Trigger Solution
for Dual-Forward Converter
Introduction
In any switching converter, rectifier diodes are used to
obtain DC output voltage. The conduction loss of diode
rectifier contributes significantly to the overall power losses
in a power supply; especially in low output voltage
applications, such as personal computer (PC) power
supplies. The conduction loss of a rectifier is proportional to
the product of its forward-voltage drop and the forward
conduction current. Using synchronous rectification (SR)
where the rectifier diode is replaced by MOSFET with
proper on resistance (Rds_ON), the forward-voltage drop of a
synchronous rectifier can be lower than that of a diode
rectifier and, consequently, the rectifier conduction loss can
be reduced.
The highly integrated FAN6210 is a primary-side SR
controller for dual-forward converter that provides control
signals for the secondary-side SR driver FAN6206.
FAN6210 also provides drive signal for the primary-side
power switches by using an output signal from the PWM
controller. FAN6210 can be combined with any PWM
controller that can drive a dual-forward converter. To obtain
optimal timing for the SR drive signals, transformer winding
voltage is also monitored. To improve light-load efficiency,
green-mode operation is employed, which disables the SR
turn-on trigger signal, minimizing gate drive power
consumption at light-load condition.
This application note describes the design procedure of SR
circuit using FAN6210 and FAN6206. The guidelines for
printed circuit board (PCB) layout and a design example
with experiment results are also presented.
V_in
V_ac PFC stage
C_bulk
Drv
n:1
D₁
FAN6210
PWM control signal
(From PWM controller)
R₁
1 XP GND 8
2 XN SOUT 7
3 SIN VDD 6
4 RDLY DET 5
C₁
D₅
D₃ R₃
D₆
D₄
R₄
R₂
Drv
D₂
From power supply
of PWM controller
R₅
PT
L_o
V_o
R₈
Q₁
R₉
R₆
R₇
FAN6206
1 LPC1GATE1 8
2 LPC2 GND 7
3 SN GATE2 6
4 SP VDD 5
Q₂
C₂
Figure 1. Typical Application
© 2010 Fairchild Semiconductor Corporation
Rev. 1.0.0 • 4/27/10
www.fairchildsemi.com