Replacing failed powersupplies on qbus PDP-11s

Chuck Guzis cclist at
Fri Jan 30 18:31:45 CST 2009

On 30 Jan 2009 at 16:13, Brent Hilpert wrote:

> I believe in that discussion he's referring to objects in their pure form: when
> he says SMPS he means just the DC-to-DC converter; not what we colloquially
> refer to as a SMPS, which is really a conventional AC-DC converter (rectifier &
> filter) followed by a switch-mode DC-DC converter.
> As I would understand it, the problem with adding the EMI filter to the front
> of the SM DC-DC converter is that the converter actually draws current from the
> DC source at a frequency corresponding to the switch-mode frequency, so the
> current through an input EMI filter is nothing like DC, and you can get AC
> interaction between the filter and the DC-DC converter. The EMI filter becomes
> the resonant circuit, and negative resistance characteristics appearing at the
> the input of the DC-DC converter turn the combination into an oscillator.

Thanks for the elaboration!  I hadn't read the whole thing yet, but 
was mildly surprised at the lede.  Your explanation makes sense.


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