FAN6208 Fairchild Semiconductor, FAN6208 Datasheet - Page 9

• 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
Gate-Shrink Functions
In normal operation, the turn-off instant is determined by
subtracting a dead time (t
conduction duration of the previous switching cycle, as
shown in Figure 10. This allows proper driving timing for
SR MOSFETS when the converter is in steady state and
the switching frequency does not change much.
However, this control method may cause shoot-through
of SR MOSFETs when the switching frequency
increases fast and switching transition of the primary-
Figure 11. DETL Debounce (Blanking) Time
Figure 10. SR Gate Timing Diagram
DEAD
) from the measured SR
9
side
command of SR is given. To prevent the shoot-through,
FAN6208 has gate-shrink functions. Gate shrink occurs
under three conditions:
(a) When insufficient dead time is detected in the
(b) When the feedback information changes fast.
previous switching cycle. When the DETL goes
HIGH within 125ns of detection window after SR
gate is turned off, the SR gate drive signal in the
next switching cycle is reduced by t
1.25µs) to increase the dead time, as shown in
Figure 12.
FAN6208 monitors the current through the opto-
coupler diode by measuring the voltage across the
resistor in series with the opto-coupler diode, as
depicted in Figure 13. If the feedback current
through the opto-coupler diode increases by more
than 20% of the feedback current of the previous
switching cycle, the SR gate signal is shrunk by
t
as shown in Figure 14.
Figure 12. Gate Shrink by Minimum Dead Time
SHRINK-FD
MOSFETs
Figure 13. Typical Application Circuit
(about 1.4µs) for t
takes
Detection Window
place
D-SHRINK-FD
before
SHRINK-DT
(about 90µs),
the
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turn-off
(about