FAN6208 Fairchild Semiconductor, FAN6208 Datasheet - Page 8

• Manufacturer Part Number: FAN6208
• Description: FAN6208 is a synchronous rectification (SR) controller for isolated LLC or LC resonant converters that can drive two individual SR MOSFETs emulating the behavior of rectifier diodes
• Manufacturer: Fairchild Semiconductor

© 2010 Fairchild Semiconductor Corporation
FAN6208 • Rev. 1.0.1
Function Description
Operation Principle
FAN6208 is a secondary-side synchronous rectifier
controller for LLC or LC resonant converters that drive
two synchronous rectifier MOSFETs. Figure 7 is the
simplified circuit diagram of an LLC converter. The
FAN6208 determines SR MOSFET turn-on/off timing by
detecting the drain-to-source voltage of each SR
MOSFET. The key waveforms for LLC resonant
converter for below resonance and above resonance
are shown in Figure 8 and Figure 9, respectively.
Figure 8.
LLC Resonant Converter
Figure 7.
V
IN
Cr
Lr
Key Waveforms of LLC Resonant Converter for
I
P
L
m
Simplified Schematic of LLC Converter
Below Resonance Operation
Secondary-Side Synchronous Rectifiers
DETL1 GATE1
DETL2
RP
FD
FAN6208
GATE2
GND
VDD
SR1
SR2
V
O
8
Timing Estimator
Figure 10 shows the timing diagram for FAN6208. Once
the body diode of SR begins conducting, the drain-to-
source voltage drops to zero, which causes DETL pin
voltage to drop to zero. FAN6208 turns on the MOSFET
after t
drops below 2V. As depicted in Figure 11, the turn-on
delay (after t
(150ns) and propagation delay (200ns).
FAN6208 measures the SR conduction duration (t
during which DETL stays lower than 2V, and uses this
information to determine the turn-off instant of SR gates
of the next switching cycle. The turn-off timing is
obtained by subtracting a dead time (t
measured SR conduction duration of the previous
switching cycle. The dead time can be programmed
using a resistor on the RP pin and the relationship
between the dead time and SR conduction duration
(t
Figure 5.
Figure 9.
DETL
) for different resistor values on RP pin is given in
ON-ON-DETL
Key Waveforms of LLC Resonant Converter for
SR-ON-DETL
(about 350ns), when voltage on DETL
Above Resonance Operation
) is the sum of debounce time
DEAD
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) from the
DETL
),