AN-6208 Datasheet, PDF (1/10 Pages) Fairchild Semiconductor – Secondary-Side Synchronous Rectifier (SR) for LLC Resonant Converter

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AN-6208
Secondary-Side Synchronous Rectifier (SR) for
LLC Resonant Converter Using FAN6208
Introduction
The LLC resonant converter has drawn a lot of attention
recently due to its advantages over a conventional series
resonant converter and parallel resonant converter: narrow
frequency variation over wide load, input variation, and
Zero Voltage Switching (ZVS) for the entire load range.
In an LLC resonant converter, rectifier diodes are typically
used to obtain DC output voltage from the transformer
secondary winding. The conduction loss of diode rectifier
contributes significantly to the overall power losses in an
LLC resonant converter; especially in low output voltage
applications. 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 a small on resistance (R_DSON), 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.
FAN6208 is a synchronous rectification controller for
isolated LLC or LC resonant converters that can drive two
individual SR MOSFETs emulating the behavior of rectifier
diodes. FAN6208 measures the SR conduction time of each
switching cycle by monitoring the drain-to-source voltage of
each SR and determines the optimal timing of SR gate
drive. FAN6208 also uses the change of opto-coupler diode
current to adaptively shrink the duration of SR gate drive
signals during load transients to prevent shoot-through. To
improve light-load efficiency, Green Mode disables the SR
drive signals, minimizing gate drive power consumption at
light-load conditions.
This application note describes the design procedure for a
SR circuit using FAN6208. The guidelines for printed
circuit board (PCB) layout and a design example with
experiment results are also presented. Figure 1 shows the
typical application circuit of FAN6208.
© 2010 Fairchild Semiconductor Corporation
Rev. 1.0.1 • 3/10/11
Figure 1. Typical Application
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