ON Semiconductor NCP1351B, Application Note

Page: 10 / 28

AND8344/D
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10
Over-Current Protection:
Figure 11. The Over-Current Protection Schematic
The output over power is detected by the same charge
pump used for detection of output shorts, the divider made
by R29 and R75 and filtered on C42. Once the voltage on
TLV431 (IC2) reaches its reference level of 1.25 V, TLV431
starts to conduct from the cathode to ground and increases
current from the RT pin of the NCP1392B through D17 and
R72. The increased current means higher frequency and thus
a lower output power. The response speed of this current
loop depends on the value of capacitor C32. In case the LLC
starts with a shorted output, the current loop controls
regulation and a very large current flows through the
secondary winding and diodes, which can be damaged. To
avoid this situation, there is a circuit around Q10 and Q12.
If the current loop starts to regulate, the voltage on the
cathode of TLV431 is lower than V
cc
. In this situation,
current starts to flow through the EB junction of Q12 and
through resistor R89, turning on the transistor. Capacitor
C53 is charged through Q12 and R94. Once the voltage on
C53 after division reaches the level necessary to turn on
transistor Q10, the collector of Q10 is grounded, and C53 is
discharged through Q10. Thereafter, the same happens as
was described above (i.e. the situation of the short [see
Figure 40]). If the applied over-current on the output
disappears before charging C53 to the level necessary to turn
on Q10, the LLC continues in normal operation and C53 is
discharged through R91 and R95. This situation occurs, for
example, during transients on the output (see Figure 38). If
the over-current (even during transients) is present for a long
time, C53 does not have enough time to discharge and its
voltage reaches critical level, thus causing the output voltage
to be stopped (see Figure 39).
NOTE: The use of the OCP is optional. If the circuit is not used, the
overload of the secondary side is not monitored.