From: Harold Hallikainen <harold@mai.hallikainen.org>
To: Microcontroller discussion list - Public. <piclist@mit.edu>
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Date: Mon, 12 Jul 2021 11:16:37 -0700
Subject: Re: [EE] Redundant PSU for critical NAS server
Thread-Topic: [EE] Redundant PSU for critical NAS server
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It would be interesting to look at the schematic of the power supplies. If
it's a typical flyback switcher, the output is just the transformer
secondary, a diode, and some capacitors. Typically there is a voltage
sample that is optically coupled back to the controller to adjust the
pulse width of the drive to the transformer, though some measure the
primary flyback voltage when the switching transistor is opened and adjust
pulse width based on this. In any case, I wonder if the outputs of such
supplies could just be connected in parallel. There's already a diode in
each supply. If the combined voltage is higher than the setpoint of one of
the supplies, that supply would adjust down by decreasing the pulse width.
There's a possibility that one supply could take the total load and the
other one ending up below its minimum load requirement, if any. When a
switcher is operated below its minimum load, it often "chirps" between on
and off. I'd probably adjust the voltage setpoints to result in current
sharing between the two supplies. This may not track over temperature, but
perhaps the difference between the two would be small enough to avoid
minimum load issues (or, better yet, the supply would not have a minimum
load requirement).

Since switching power supply failures are typically in the high voltage
primary side, this could increase overall reliability with the addition of
zero parts.

Harold






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