FSQ0365(2007_05) Просмотр технического описания (PDF) - Fairchild Semiconductor

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FSQ0365
(Rev.:2007_05)

Green Mode Fairchild Power Switch (FPS™) for Valley Switching Converter - Low EMI and High Efficiency

Fairchild Semiconductor 

4.3 Over-Voltage Protection (OVP): If the secondary

side feedback circuit malfunctions or a solder defect

causes an opening in the feedback path, the current

VO

through the opto-coupler transistor becomes almost

VOset

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

VFB

triggers. Because more energy than required is provided

to the output, the output voltage may exceed the rated

0.55V

voltage before the overload protection triggers, resulting

0.35V

in the breakdown of the devices in the secondary side.

To prevent this situation, an OVP circuit is employed. In

IDS

general, the peak voltage of the sync signal is

proportional to the output voltage and the FSQ-series

uses a sync signal instead of directly monitoring the

output voltage. If the sync signal exceeds 6V, an OVP is

triggered, shutting down the SMPS. To avoid undesired

triggering of OVP during normal operation, the peak

VDS

voltage of the sync signal should be designed below 6V.

4.4 Thermal Shutdown (TSD): The SenseFET and the

control IC are built in one package. This makes it easy

for the control IC to detect the abnormal over

temperature of the SenseFET. If the temperature

exceeds ~150°C, the thermal shutdown triggers.

FSQ0365RN Rev.00

Switching

Switching

t1

disabled

t2 t3

disabled

t4

time

Figure 26. Waveforms of Burst Operation

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 15ms, 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 26, 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.

7. Switching Frequency Limit: To minimize switching

loss and EMI (Electromagnetic Interference), the

MOSFET turns on when the drain voltage reaches its

minimum value in valley switching operation. However,

this causes switching frequency to increases at light load

conditions. As the load decreases, 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.

Because of these problems, the valley switching

converter topology has limitations in a wide range of

applications.

To overcome this problem, FSQ-series employs a

frequency-limit function, as shown in Figures 27 and 28.

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 27 and 28 (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, our

devices have a minimum switching frequency of 55kHz

and a maximum switching frequency of 67kHz, as shown

in Figure 28.

© 2006 Fairchild Semiconductor Corporation

FSQ0365, FSQ0265, FSQ0165, FSQ321, FSQ311 Rev. 1.0.4

14

www.fairchildsemi.com

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