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FSL126HR

Green Mode Fairchild Power Switch (FPS™)

Fairchild Semiconductor 

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 FPS 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 FPS 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 17. Auto-Restart Protection Waveforms

Overload Protection (OLP)

Overload is defined as the load current exceeding a pre-

set 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.

In conjunction with the IPK current limit pin (if used), the

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 power, the output voltage (VO) decreases

below its rating voltage. This reduces the current

through the opto-coupler LED, which also reduces the

opto-coupler transistor current, thus increasing the

feedback voltage (VFB). If VFB exceeds 2.4V, the

feedback input diode is blocked and the 5µA current

source (IDELAY) starts to charge CFB slowly up to VCC. In

this condition, VFB increases until it reaches 6V, when

the switching operation is terminated, as shown in

Figure 18. The shutdown delay is the time required to

charge CFB from 2.4V to 6V with 5µA current source.

Figure 18. Overload Protection (OLP)

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

transistor becomes almost zero. Then, VFB climbs up in

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 (OVP)

circuit is employed. In general, VCC is proportional to the

output voltage and the FPS uses VCC instead of directly

monitoring the output voltage. If VCC exceeds 24V, OVP

circuit is activated, resulting in termination of the

switching operation. To avoid undesired activation of

OVP during normal operation, VCC should be designed

to be below 24V.

© 2011 Fairchild Semiconductor Corporation

FSL126HR • Rev. 1.0.0

10

www.fairchildsemi.com

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