Hello,
>From Keith Billings "Switchmode Power Supply Handbook":
One of the major advantages of switchmod supplies is their ability to
maintain the output voltages constant for a short period after line
failure. This "holdup time" is typically 20ms minimum, but depends on the
part of the input cycle where the power supply failure occurs and the
loading and the supply voltage before the line failure. ... most
specifications define holdup time from nominal input voltage and loading.
Holdup times may be considerably less if the supply voltage is close to its
minimum value immediately prior to failure.
>From Power-One Technical reference Guide-
"It is important to specify the conditions under which the holdup time is
measured. Since the AC input may fail at any point during a cycle, it is
convenient to use the last AC peak as the reference point for measuring
holdup. ... Longer holdup time is difficult to achieve without tradeoffs,
the primary one being cost. ... "
I suggest you determine what is needed for your specific design and come to
agreement amoung your team. Are you designing or specifying? either way,
the application is key since some chip families require a certain holdup
time to prevent latching. If this is what you are trying to avoid, consult
the chip manufacturer for their definition of holdup.
John Loiselle
Product safety engineer
3Com Corporation
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