Replacing failed powersupplies on qbus PDP-11s

Brent Hilpert hilpert at
Sat Jan 31 02:09:21 CST 2009

Chuck Guzis wrote:
> 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.

Hmmm.. thinking about it a bit more, maybe I better backtrack a bit:

> > I don't believe he is discussing the situation of adding an EMI filter to the
> > front of a typical 120/240VAC input SMPS. In such supplies the 'lytic filter
> > caps of the AC-DC converter at the beginning of the supply reduce the
> > current-variation frequency to 50/100/60/120Hz (unless there is inadequate
> > filtering),

When the rectifiers of the AC-DC converter turn on (conduct) at the peaks of
the 50/60Hz mains sine wave, there will be a switch-mode-frequency component in
the current drawn from the mains, so there may still be an issue for an AC
mains EMI filter. How long those bursts are depends on the conduction angle of
the rectifiers, as determined by the final load demand, but it will be many
cycles at the switch-mode f.

Perhaps this is (one reason) why these supplies have built-in EMI filters on
the AC input, so that that filter can be tailored to the rest of the SM design.

Nonetheless, I suspect that by the time you get to the front of that internal
EMI filter (to the IEC connector or outside the power supply box, so to speak),
there is no longer ((in modern supplies), much of) a problem, as this (in part)
is what the internal EMI filter is there to deal with.

I wonder if this is one reason that early SMPS's, when perhaps such issues as
this were not dealt with as well, were prone to seemingly arbitrary self-destruction.

(.. All of which serves to reinforce your original point: that the functioning
of these things involves a lot of issues beyond the basic principles as
commonly presented.)

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