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ꢆ
SLUS591A − NOVEMBER 2003 − REVISED MARCH 2004
FUNCTIONAL DESCRIPTION
V
OVP
V
OVH
V
IN
V
SS
OUT
AUX
UDG−03150
Figure 4. Line Overvoltage Sequence
Pulse Skipping
During output load current transients or light load conditions most PWM controller 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.
D = 0 Boundary
1.25 V
FB
T
SW
OUT
AUX
UDG−03151
Figure 5. Pulse Skipping Operation
To overcome this problem, the UCC2897 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.
15
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