ON Semiconductor NCP1351B, Application Note

Page: 9 / 28

AND8344/D
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9
Output Short Protection:
Figure 10. The Output Short Protection Schematic
This feature protects against shorts on the output, shorts
on the output diode and secondary winding failure. This
event is detected on the primary side through the charge
pump created by D12, D13, C24 and R25. Voltage from this
pump is further divided by R50 and R77 and filtered on C43.
Once voltage on C43 reaches V
BE
of Q7, Q7 is turned on and
the current from V
cc
starts to flow through junction EB of Q5
and R58 and starts to charge C33. Due to the current flowing
through junction EB of Q5, Q5 is turned on and a positive
voltage is driven to the base of Q8 through R71. By turning
on Q8, the collector of Q7 is grounded even if the voltage on
the base of Q7 is already low. If the collector of Q7 is
grounded, soft start capacitor C51 is discharged through
D18 and transistor Q6 is turned on through diode D19 and
R86. Transistor Q6 pulls the BO pin over 2 V, and the
NCP1392B immediately stops output pulses. Once C33 is
charged and the flow of current through the EB junction of
Q5 stops, Q5 as well as Q8 are turned off. The collector of
Q8 is freed and is pulled up by R52. The capacitor C33 is
then discharged through D15 and R52. Because the collector
of Q7 is not grounded, diodes D18 and D19 are reverse
polarized. Thus, the soft start capacitor C51 is not grounded
and Q6 is turned off as well. This means that the voltage of
the BO pin can drop below 1.9 V and the controller starts
working with a regular soft start. Please see Figure 41.
NOTE: The output short protection is optional. The short resistance
can be achieved by using a pair of diodes D3 and D8. In this
case only the primary current is limited, the secondary current
remains high until the short circuit is removed.