FSL126MRT Просмотр технического описания (PDF) - Fairchild Semiconductor

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FSL126MRT

Green-Mode Fairchild Power Switch (FPS™)

Fairchild Semiconductor 

4.4 Over-Voltage Protection (OVP): If the

secondary-side feedback circuit malfunctions or a

solder defect causes an opening in the feedback path,

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 triggered. Because more energy

than required is provided to the output, the output

voltage may exceed the rated voltage before the

overload protection is triggered, resulting in the

breakdown of the devices in the secondary side. To

prevent this situation, an OVP circuit is employed. In

general, the VCC is proportional to the output voltage

and the FS136MRT uses VCC instead of directly

monitoring the output voltage. If VCC exceeds 24.5V,

an OVP circuit is triggered, resulting in the termination

of the switching operation. To avoid undesired

activation of OVP during normal operation, VCC should

be designed to be below 24.5V.

4.5 Thermal Shutdown (TSD): The SenseFET and

the control IC on a die in one package makes it easier

for the control IC to detect the over temperature of the

SenseFET. If the temperature exceeds 140°C, the

thermal shutdown is triggered and stops operation.

The FSL126MRT operates in Auto-Restart Mode until

the temperature decreases to around 80°C, when

normal operation resumes.

5. Soft Burst-Mode Operation: To minimize power

dissipation in Standby Mode, the FSL126MRT enters

Burst-Mode operation. As the load decreases, the

feedback voltage decreases. The device automatically

enters Burst Mode when the feedback voltage drops

below VBURL (400mV), as shown in Figure 22. At this

point, switching stops and the output voltages start to

drop at a rate dependent on standby current load. This

causes the feedback voltage to rise. Once it passes

VBURH (550mV), switching resumes. The feedback

voltage then falls and the process repeats. Burst Mode

alternately enables and disables switching of the

SenseFET, reducing switching loss in Standby Mode.

6. Random Frequency Fluctuation (RFF): Fluctuating

switching frequency of an SMPS can reduce EMI by

spreading the energy over a wide frequency range. The

amount of EMI reduction is directly related to the

switching frequency variation, which is limited internally.

The switching frequency is determined randomly by

external feedback voltage and an internal free-running

oscillator at every switching instant. This random

frequency fluctuation scatters the EMI noise around

typical switching frequency (67kHz) effectively and can

reduce the cost of the input filter included to meet the

EMI requirements (e.g. EN55022).

Figure 23. Random Frequency Fluctuation

Figure 22. Burst-Mode Operation

© 2012 Fairchild Semiconductor Corporation

FSL126MRT • Rev. 1.0.0

11

www.fairchildsemi.com

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