The duty cycle
of the switching stage, varies according to load. Notice the
word "feedback" in the schematic here - that is how the
supply checks the output voltage, and adjusts the output
voltage to maintain the correct value. The adjustment is
made by varying the duty cycle of the switching stage.
http://www.pavouk.org/hw/en_atxps.html
The ATX supply, can maintain correct output voltages, while
the input is 90-120VAC. That means, even under input brownout, the
supply continues to function. It could operate over a wider range
than that, but for the need to have an undervoltage lockout
to protect the switching transistors.
Say the DC output of the supply, is currently drawing 120 watts.
On the AC input, we'd guess the supply was using 120V 1 amp.
If the AC voltage suddenly dropped to 60V and there was no UVLO,
the supply would draw 2 amps (60V * 2A primary side, to make 120W
for the secondary side). The equivalent of those two amps (a
doubling of current), would be needed in the primary switching
transistors. And if continued unchecked, the switcher would
be handling too large currents for comfort. So at some point,
switching converters should switch themselves off, and limit
operation to an acceptable range.
*******
In terms of holdup time (the time the supply can provide DC
output), that is a function of output loading.
The spec sheet for the supply, will say it can provide full
load current, for 16 milliseconds or more. The primary side
capacitor, is made big enough, to protect it against line
transients (transients where the voltage goes to zero for
periods of milliseconds). The holdup time is generally
enough, to make it compatible with the cheapest UPS types
(standby UPS). A standby UPS needs some time to switch
to battery, during which the AC output will be zero.
Utility power Running on battery
Standby UPS ----------------+ +-----------
| |
+-----------+
notices power
is out
ATX primary cap ---------------___ +---------
\___ |
\___ |
\_|
No glitch or variation
ATX DC outputs --------------------------------------
Why? Because it's regulated
At the other extreme, if your PC is sleeping, and the
motherboard is only using +5VSB rail, the power supply
can maintain a correct 5V level on +5VSB, for 30 seconds
or longer. So if the AC power goes off for a few seconds
while the computer is sleeping, you won't lose your session
stored in RAM. It's good for times in the vicinity of
thirty seconds or so. That's how long mine can go, before
the +5VSB LED goes off on the motherboard.
Paul
