[Boatanchors] [BoatAnchors] 12 volt supply question

[don davis]

The reason you don't need min load applied externally to the PS is that each
output has its own bleeder/min load applied.  All modern switch mode power
supplies have outputs that will drift up due to high speed rectifying diodes
(reverse recovery energy) causing switching spikes when they transition.
Without some small load the spike peaks get rectified and the output caps
charge up and they stay charged since only leakage will bleed the charge off
(slowly).  Many years of designing these tells me I usually needed ~1% of
nominal load to stay within eol +/- 5% voltage reg at no load.   In the
example you provided each 5 volt output has 100 Ohm resistors dissipating
0.25 W which should be enough to keep no-load voltages under control.  Note
the coupled inductor in the outputs, it helps keep the light load condition
voltages under fairly good control.

Ben's right that the noise level of these "depends".  If these are very old
they might follow MIL-STD-462 / 462 RQMTS.  If so they might have good
Common-mode rejection (as shown in your example) and sealed EMI filters with
cavities, isolation between PRI/SEC, good ground bonding, etc.  Isolation
from mains or PRI side depends on the end-user RQMTs.  Most space
applications require ~ 1.0 MOhm or more, and in fact most have total
isolation.  Most switchers nowadays use pulse xfmrs to send the control
signal from SEC to PRI.  Opto-couplers aren't used in space anymore due to
catastrophic failures of the HP-xx pattern part on orbit a long time ago,
dunno if comm'l stuff uses them anymore.

73 de don ad6pb

-----Original Message-----
From: BoatAnchors [mailto:boatanchors-bounces at theporch.com] On Behalf Of Ben
Hall via BoatAnchors
Sent: Thursday, December 01, 2016 4:34 PM
To: boatanchors at theporch.com
Subject: Re: [BoatAnchors] [Boatanchors] 12 volt supply question