Replacing failed powersupplies on qbus PDP-11s

[Brent Hilpert]

Chuck Guzis wrote:
> 
> On 30 Jan 2009 at 16:41, Roy J. Tellason wrote:
> 
> > Eh?  Why would it do that?
> 
> Read about it:
> 
> http://www.smpstech.com/filter00.htm

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.

I believe his discussion is intended to be more of pertinence to DC
distribution-bus systems such as in aircraft or telecom (note he refers in a
couple of places to 28V distribution).

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), which is not a problem for the EMI filters.
(Of course, these supplies generally do include built-in EMI filters at the front).