Re: Minimum current spec for power supply


jibberjabber wrote:
I'm looking at the little open frame power supply (PSA-25L-201-R) that has a
5V .2-2A rating. When my device is in idle mode it draws 750mA which is
below the minimum Amp rating. What happens when a similar PS is run below
the rated current? The spec *** isn't much help.
thx

Hi, Jib. You're saying you've got a small switching power supply which
has a rating for minimum current as well as maximum, and you're running
the power supply at below minimum current.

The PSA number looks like a Phihong switcher. If it's the PSA-25L-201

http://www.phihong.com/assets/pdf/PSA25L.PDF

it shows two outputs, 5V and 12VDC. The 5V supply is shown with a
minimum load of 0.2A, and a maximum load of 2.6A. The 12VDC supply
shows no minimum load, and can supply up to 1A. (See page 1 of the .pdf
data ***.)

Most switchers these days are built to operate properly under no-load
conditions (like most of the PSA-25L models). Your model, though, has
a minimum curent rating. If you run a switcher at below minimum
current, it will not meet specifications for output regulation or
ripple voltage, and will not respond properly to load transients.
Worst case, this instability can cause the power supply to fail at
turn-on, or when the load changes (i.e. from idle to run mode).

If your load will ever fall below the rated minimum of 0.2A, load the
power supply with an output resistor such that the current never goes
below rated minimum.

You're saying your load is 750mA, which is above rated minimum. In
that case, you should have no problem. You need to be between minimum
and maximum, which is where you are. But if you ever run into this
again, or if your supply is something other than described above, get
out the junkbox and scrounge a power resistor that will get you above
minimum current.

By the way, this is another good reason to scrounge components from any
non-working electronics you come across. Power resistors seldom fail,
and scrounged ones can be handy when you've got a problem like this.

Good luck
Chris

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