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ꢆ
SLUS591A − NOVEMBER 2003 − REVISED MARCH 2004
FUNCTIONAL DESCRIPTION
V
OFF
V
IN
V
CLAMP
V
SS
T
SW
OUT
AUX
UDG−03149
Figure 3. Line Undervoltage Shutdown Waveforms
To eliminate this potential hazard the UCC2897 controller safely discharge the clamp capacitor during power
down. As shown by the timing diagram in Figure 4, the undervoltage lockout circuits stop the power transfer in
the converter by disabling the gate drive signal for the main switch (OUT). The AUX output keeps switching while
the soft-start capacitor C is being slowly discharged. Notice that the AUX pulse width gradually increases as
SS
the clamp voltage decreases never applying the high voltage across the transformer for extended period of time.
During the slow discharge of the timing capacitor the converter can not be restarted even if the input voltage
returns to the acceptable range.
Line Overvoltage Protection
When the line overvoltage protection is triggered in the UCC2897 controller, the gate drive signals are
immediately disabled. At the same time, the slow discharge of C is initiated. While the soft-start capacitor is
SS
discharging the gate drive signals remains disabled. Once C = 0.5 V and the overvoltage disappears from
SS
the input of the power supply, operation resumes through a regular soft-start of the converter as it is
demonstrated in Figure 5.
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