Fairchild FPS Series, Руководство по конструкции

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www.fairchildsemi.com/fps
FPS
™
Design Guide
Green FPS™ Feature – Over Load Protection
Shutdown
V
SD
V
t
t1
t2
Time Constant t2 =
(Cfb * (V
SD
-3V))/5uA
0
3V
OSC.
5uA
250uA
2.8R
R
Voffset
S
R
Q
Vck
FB
Vfb*
V
SD
S
Q
R
Vo
Cfb
Thermal
Shutdown
Reset
Shutdown
Secondary Side
LEB
• Consider the load current increasing significantly in this overload scenario.
• The FPS increases its duty cycle within the limits of its pulse-by-pulse current limit.
• The output voltage falls and the feedback voltage rises in response.
• If it is just a momentary surge, then the output will rise after the surge and the feedback voltage will
resume its steady state.
• However, if it is not a surge, for example; if output is shorted to ground, then feedback voltage continues to rise.
• At 3V, the feedback voltage is pulled up with a small internal 5
µ
A current, allowing for adjusting the delay
as shown above.
• When the feedback voltage crosses the over load shutdown (VSD) threshold, the FPS stops switching.
• With the FPS not switching, bias current is not supplied to the FPS supply (VCC) storage capacitor.
• When the VCC voltage reaches its "under voltage lock out" threshold, the FPS is automatically restarted.
• The cycle will repeat until the fault is removed, providing a quick recovery from the overload condition to
normal operation.
Below are waveforms showing the over load protection and auto-restart occurring repeatedly for a static fault
condition and a close-up of the feedback voltage showing when the over load protection is triggered and
switching stops.
Over Load Protection and
Auto-Restart @ 265VAC
Feedback Over Load
Protection Level @ 265VAC