IR1168S
www.irf.com © 2009 International Rectifier
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General Description
The IR1168 Dual Smart Rectifier controller IC is the industry first dedicated high-voltage controller IC for
synchronous rectification in resonant converter applications. The IC can emulate the operation of the two
secondary rectifier diodes by correctly driving the synchronous rectifier (SR) MOSFETs in the two secondary legs.
The core of this device are two high-voltage, high speed comparators which sense the drain to source voltage of
the MOSFETs differentially. The device current is sensed using the R
DSON
as a shunt resistance and the GATE pin
of the MOSFET is driven accordingly. Dedicated internal logic then manages to turn the power device on and off in
close proximity of the zero current transition.
IR1168 further simplifies synchronous rectifier control by offering the following power management features:
-Wide VCC operating range allows the IC to be directly powered from the converter output
-Shoot through protection logic that prevents both the GATE outputs from the IC to be high at the same time
-Device turn ON and OFF in close proximity of the zero current transition with low turn-on and turn-off propagation
delays; eliminates reactive power flow between the output capacitors and power transformer
-Internally clamped gate driver outputs that significantly reduce gate losses.
The SmartRectifier™ control technique is based on sensing the voltage across the MOSFET and comparing it with
two negative thresholds to determine the turn on and off transitions for the device. The rectifier current is sensed
by the input comparators using the power MOSFET R
DSON
as a shunt resistance and its GATE is driven depending
on the level of the sensed voltage vs. the 3 thresholds shown below.
V
Gate
V
TH1
V
TH2
V
TH3
V
DS
Figure 1: Input comparator thresholds
Turn-on phase
When the conduction phase of the SR FET is initiated, current will start flowing through its body diode, generating
a negative V
DS
voltage across it. The body diode has generally a much higher voltage drop than the one caused by
the MOSFET on resistance and therefore will trigger the turn-on threshold V
TH2
.
When V
TH2
is triggered, IR1168 will drive the gate of MOSFET on which will in turn cause the conduction voltage
VDS to drop down to I
D
*R
DSON
. This drop is usually accompanied by some amount of ringing, that could trigger the
input comparator to turn off; hence, a fixed Minimum On Time (MOT) blanking period is used that will maintain the
power MOSFET on for a minimum amount of time.
The fixed MOT limits the minimum conduction time of the secondary rectifiers and hence, the maximum switching
frequency of the converter.
Turn-off phase
Once the SR MOSFET has been turned on, it will remain on until the rectified current will decay to the level where
V
DS
will cross the turn-off threshold V
TH1
.
