FSL136HR

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9

Figure 17. Pulse−Width−Modulation Circuit

Overload Protection (OLP)

Overload is defined as the load current exceeding a preset

level due to an unexpected event. In this situation, the

protection circuit should be activated to protect the SMPS.

However, even when the SMPS is operating normally, the

overload protection (OLP) circuit can be activated during

the load transition or startup. To avoid this undesired

operation, the OLP circuit is designed to be activated after

a specified time to determine whether it is a transient

situation or a true overload situation.

current−mode feedback path limits the current in the

SENSEFET when the maximum PWM duty cycle is

attained. If the output consumes more than this maximum

voltage. This reduces the current through the opto−coupler

LED, which also reduces the opto−coupler transistor

exceeds 2.4 V, the feedback input diode is blocked and the

5

when the switching operation is terminated, as shown in

Figure 18. The shutdown delay is the time required to charge

Figure 18. Overload Protection (OLP)

Abnormal Over−Current Protection (AOCP)

When the secondary rectifier diodes or the transformer pin

are shorted, a steep current with extremely high di/dt can

flow through the SENSEFET during the LEB time. Even

though the power switch has OLP (Overload Protection), it

is not enough to protect the FPS in that abnormal case, since

severe current stress is imposed on the SENSEFET until

OLP triggers. The power switch includes the internal AOCP

(Abnormal Over−Current Protection) circuit shown in

Figure 19. When the gate turn−on signal is applied to the

power SENSEFET, the AOCP block is enabled and monitors

the current through the sensing resistor. The voltage across

the resistor is compared with a preset AOCP level. If the

sensing resistor voltage is greater than the AOCP level, the

set signal is applied to the latch, resulting in the shutdown of

the SMPS.

Figure 19. Abnormal Over−Current Protection

Thermal Shutdown (TSD)

The SENSEFET and the control IC are integrated, making

it easier to detect the temperature of the SENSEFET. When

the temperature exceeds approximately 137

°C, thermal

shutdown is activated.

Over−Voltage Protection (OVP)

In the event of a malfunction in the secondary−side

feedback circuit or an open feedback loop caused by

a soldering defect, the current through the opto−coupler

a similar manner to the overload situation, forcing the preset

maximum current to be supplied to the SMPS until the

overload protection is activated. Because excess energy is

provided to the output, the output voltage may exceed the

rated voltage before the overload protection is activated,

resulting in the breakdown of the devices in the secondary

side. To prevent this situation, an over−voltage protection

24 V, OVP circuit is activated, resulting in termination of

the switching operation. To avoid undesired activation of

below 24 V.