Datasheet
SLUS486B − AUGUST 2001 − REVISED JULY 2003
13
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APPLICATION INFORMATION
synchronous rectification and predictive delay
In a normal buck converter, when the main switch turns off, current is flowing to the load in the inductor. This
current cannot be stopped immediately without using infinite voltage. For the current path to flow and maintain
voltage levels at a safe level, a rectifier or catch device is used. This device can be either a conventional diode,
or it can be a controlled active device if a control signal is available to drive it. The UCC27222 provides a signal
to drive an N-channel MOSFET as a rectifier. This control signal is carefully coordinated with the drive signal
for the main switch so that there is minimum delay from the time that the rectifier MOSFET turns off and the main
switch turns on, and minimum delay from when the main switch turns off and the rectifier MOSFET turns on.
This scheme, Predictive Gate Drive delay, uses information from the current switching cycle to adjust the
delays that are to be used in the next cycle. Figure 7 shows the switch-node voltage waveform for a
synchronously rectified buck converter. Illustrated are the relative effects of a fixed-delay drive scheme
(constant, pre-set delays for the turnoff to turn on intervals), an adaptive delay drive scheme (variable delays
based upon voltages sensed on the current switching cycle) and the predictive delay drive scheme.
Note that the longer the time spent in body-diode conduction during the rectifier conduction period, the lower
the efficiency. Also, not described in Figure 7 is the fact that the predictive delay circuit can prevent the body
diode from becoming forward biased at all while at the same time avoiding cross conduction or shoot through.
This results in a significant power savings when the main MOSFET turns on, and minimizes reverse recovery
loss in the body diode of the rectifier MOSFET.
The power dissipation on the main (forward) MOSFET is reduced as well, although that savings is not as
significant as the savings in the rectifier MOSFET.
During reverse recovery the body diode is still forward biased, thus the reverse recovery current goes through
the forward MOSFET while the drain−source voltage is still high, causing additional switching losses. Without
PGD during this switching transition, Vds = Vin and Ids = Iload + Irr in the main MOSFET. With PGD however,
Vds = Vin and Ids = Iload. The reduction in current accounts for additional power savings in the main MOSFET.
UDG−02175
GND
Fixed Delay
Adaptive Delay
Predictive Delay
Channel Conduction
Body Diode Conduction
V
IN
V
D
0 V
Figure 7. Switch Node Waveforms for Synchronous Buck Converter