FSL136HR

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10

Output−Short Protection (OSP)

If the output is shorted, steep current with extremely high

di/dt can flow through the SENSEFET during the LEB time.

Such a steep current brings high−voltage stress on the drain

of SENSEFET when turned off. To protect the device from

1.6 V and the SENSEFET turn−on time is lower than 1.0

ms,

the FPS recognizes this condition as an abnormal error and

again. An abnormal condition output is shown in Figure 20.

Figure 20. Output Short Waveforms (OSP)

D

MOSFET

Drain

Current

Rectifier

Diode

Current

output short occurs

1.6us

Turn-off delay

Minimum

turn-on time

Soft−Start

The power switch has an internal soft−start circuit that

slowly increases the feedback voltage, together with the

SENSEFET current, after it starts. The typical soft−start

time is 20 ms, as shown in Figure 21, where progressive

increments of the SENSEFET current are allowed during the

startup phase. The pulse width to the power switching device

is progressively increased to establish the correct working

conditions for transformers, inductors, and capacitors. The

voltage on the output capacitors is progressively increased

with the intention of smoothly establishing the required

output voltage. Soft−start helps to prevent transformer

saturation and reduce the stress on the secondary diode.

1.25ms

16Steps

Current Limit

t

Drain

Current

Figure 21. Internal Soft−Start

Burst Operation

To minimize power dissipation in standby mode, the FPS

enters burst mode. As the load decreases, the feedback

voltage decreases. As shown in Figure 22, the device

automatically enters burst mode when the feedback voltage

limit is fixed internally to minimize flux density in the

transformer. The fixed current limit is larger than that

further. Switching continues until the feedback voltage

output voltages start to drop at a rate dependent on the

standby current load. This causes the feedback voltage to

feedback voltage then falls and the process repeats. Burst

mode alternately enables and disables switching of the

SENSEFET and reduces switching loss in standby mode.

Figure 22. Burst−Mode Operation

Adjusting Peak Current Limit

As shown in Figure 23, a combined 6 k

W internal

resistance is connected to the non−inverting lead on the

PWM comparator. An external resistance of Rx on the

current limit pin forms a parallel resistance with the 6 k

W

when the internal diodes are biased by the main current

source of 400

mA. For example, FSL136HR has a typical

ground. The value of the Rx can be estimated by the

following equations:

2.15 A : 1.5 A + 6kW :X kW

(eq. 1)

X + Rx 6kW

(eq. 2)

Where X is the resistance of the parallel network.