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Q0465R

Green-Mode Fairchild Power Switch (FPS™) for Quasi-Resonant Operation - Low EMI and High Efficiency

Fairchild Semiconductor 

resulting in the termination of the switching operation. To

avoid undesired activation of OVP during normal

VO

operation, VCC should be designed below 19V.

Voset

4.5 Thermal Shutdown with Hysteresis (TSD): The

VFB

SenseFET and the control IC are built in one package.

This enables the control IC to detect the abnormally high

0.55V

temperature of the SenseFET. If the temperature

0.35V

exceeds approximately 140°C, the thermal shutdown

triggers IC shutdown. The IC recovers its operation when

IDS

the junction temperature decreases 60°C from TSD

temperature and VCC reaches startup voltage (Vstart).

5. Soft-Start: The FPS has an internal soft-start circuit

that increases PWM comparator inverting input voltage

with the SenseFET current slowly after it starts up. The

typical soft-start time is 17.5ms. 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. This

mode helps prevent transformer saturation and reduces

stress on the secondary diode during startup.

6. Burst Operation: To minimize power dissipation in

standby mode, the FPS enters burst-mode operation. As

the load decreases, the feedback voltage decreases. As

shown in Figure 32, the device automatically enters

burst-mode when the feedback voltage drops below

VBURL (350mV). 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 operation alternately

enables and disables switching of the power SenseFET,

thereby reducing switching loss in standby mode.

VDS

FSQ0465 Rev. 00

Switching

Switching

disabled

t1

t2 t3 disabled

t4

time

Figure 32. Waveforms of Burst Operation

7. Switching Frequency Limit: To minimize switching

loss and Electromagnetic Interference (EMI), the

MOSFET turns on when the drain voltage reaches its

minimum value in quasi-resonant operation. However,

this causes switching frequency to increases at light load

conditions. As the load decreases or input voltage

increases, the peak drain current diminishes and the

switching frequency increases. This results in severe

switching losses at light-load condition, as well as

intermittent switching and audible noise. These problems

create limitations for the quasi-resonant converter

topology in a wide range of applications.

To overcome these problems, FSQ-series employs a

frequency-limit function, as shown in Figures 33 and

Figure 34. Once the SenseFET is turned on, the next

turn-on is prohibited during the blanking time (tB). After

the blanking time, the controller finds the valley within

the detection time window (tW) and turns on the

MOSFET, as shown in Figures 33 and Figure 34 (Cases

A, B, and C). If no valley is found during tW, the internal

SenseFET is forced to turn on at the end of tW (Case D).

Therefore, the devices have a minimum switching

frequency of 48kHz and a maximum switching frequency

of 67kHz.

© 2008 Fairchild Semiconductor Corporation

FSQ0465RS/RB Rev. 1.0.1

15

www.fairchildsemi.com

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