Datasheet
SLUS486B − AUGUST 2001 − REVISED JULY 2003
14
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APPLICATION INFORMATION
comparison between predictive and adaptive gate drive techniques
The first synchronous rectifier controllers had a fixed turn-on delay between the two gate drivers. The advantage
of this well-known technique is its simplicity. The drawbacks include the need to make the delay times long
enough to cover the entire application of the device and the temperature and lot-to-lot variation of the time delay.
Since the body-diode of the synchronous rectifier conducts during this deadtime, the efficiency of this technique
varies with different MOSFETs, ambient temperature, and with the lot-to-lot variation of the deadtime delay.
To combat the variability of the internal time delays, second generation controllers used state information from
the power stage to control the turn-on of the two gate drivers. This technique is usually referred to as adaptive
gate drive technique and is pictured in FIgure 8.
UDG−01031
V
IN
V
OUT
ON
OFF
ON
+
+
Figure 8. Adaptive Gate Drive Technique
The main advantage of the adaptive technique is the on-the-fly delay adjustment for different MOSFETs and
temperature-variable time delays. The disadvantages include the body-diode conduction time intervals caused
by delays in the cross-coupling loops and the inability to compensate for the delay to charge the MOSFET gates
to the threshold levels. Additionally, it is difficult to determine whether the synchronous MOSFET channel is off
by solely monitoring the SR MOSFET gate voltage. Some devices actually add a programmable delay between
the turn-off of the synchronous rectifier and the turn-on of the main MOSFET via an external capacitor. This
added delay directly affects the power stage efficiency through additional body-diode conduction losses. Since
these losses are centralized in the synchronous MOSFET, the stress and temperature rise in this component
becomes a major design headache.
The third-generation predictive control technique is different from the adaptive technique in that it uses
information from the previous switching cycle to set the deadtime for the current cycle. The adaptive technique
on the other hand uses the current state information to set the delay times. The inherent feedback loop
propagation delays cause body-diode conduction.