Datasheet

 
 

SLUS542F − OCTOBER 2003 − REVISED JULY 2009
16
www.ti.com
FUNCTIONAL DESCRIPTION
OUT
V
IN
V
SS
V
OVP
V
OVH
UDG−03150
AUX
Figure 4. Line Overvoltage Sequence, P−Channel
Pulse Skipping
During output load current transients or light load conditions most PWM controllers needs to be able to skip
some number of PWM pulses. In an active clamp topology where the clamp switch is driven complementarily
to the main switch, this would apply the clamp voltage across the transformer continuously. Since operating
conditions might require skipping several switching cycles on the main transistor, saturating the transformer is
very likely if the AUX output stays on.
OUT
AUX
1.25 V
UDG−03151
FB
T
SW
D = 0 Boundary
Figure 5. Pulse Skipping Operation, P−Channel
To overcome this problem, the UCC2891 family incorporates pulse skipping for both outputs in the controller.
As can be seen above, when a pulse is skipped at the main output (OUT) because the feedback signal demands
zero duty ratio, the corresponding output pulse on the AUX output is omitted as well. This operation allows to
prevent reverse saturation of the power transformer and to preserve the clamp capacitor voltage level during
pulse skipping operation.