NCP1239

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24

Figure 43. An Easy Means to Add Slope Compensation is by Using an Extra RC Network Building a Ramp

from the Drive Signal

R1

C1

D1

1N4148

Rsense

R3

DRV

CS

R4

A severe fault like a winding short-circuit can cause the

switch current to increase very rapidly during the on-time.

because the current sense signal is blanked by the LEB

circuit during the switch turn on, the power switch current

can become huge causing system damage.

The NCP1239 protects against this fault by adding an

additional comparator for abnormal over-current fault

detection. The current sense signal is blanked with a shorter

the abnormal over-current fault comparator. The voltage

operation. Four consecutive abnormal over-current faults

cause the controller to enter latch mode. The count to 4

provides noise immunity during surge testing. The counter

is reset each time a DRV pulse occurs without activating the

Fault Over-Current Comparator.

Please note that like timer-based short-circuit protection,

A, F, G, H and I versions are latching off compared to B, C,

D and E versions that are auto-recovery.

pin. When the voltage on this pin exceeds 25.5 V typically

(32.0 V for F versions) for more than 20

ms, a signal is sent

to the internal latch and the controller immediately stops the

driving pulses while remaining in a lockout state. Depending

auto-recovery or latched. For latching-off versions, the part

off the power plug but also if a brown-out recovery is sensed

by the controller. This technique offers a simple and cheap

means to protect the converter against optocoupler.

PROTECTING FROM A FAILURE OF THE CURRENT SENSING

A 1-

CS pin to disable the controller if the pin is left open.

In addition the maximum duty ratio limit (80% typically)

avoids that the MOSFET stays permanently on if the switch

current cannot reach the setpoint when for instance, the input

voltage is low or if the CS pin is grounded. In this case, the

OCP timer is activated. If the timer elapses, the controller

enters in auto-recovery or endless hiccup mode depending

on the controller option. This unexpected operation can lead

to deep CCM with destructive consequences.