[Uram-rejections] Low Noise SMPSU

Tue Sep 19 17:34:33 BST 2017

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> From webstump at chiark.greenend.org.uk Tue Sep 19 13:41:03 2017
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> From: Jerry Stuckle <jstucklex at attglobal.net>
> Newsgroups: uk.radio.amateur.moderated
> Subject: Re: Low Noise SMPSU
> Date: Tue, 19 Sep 2017 08:41:03 -0400
> Organization: A noiseless patient Spider
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> On 9/19/2017 5:40 AM, Custos Custodum wrote:
> > On Mon, 18 Sep 2017 20:58:35 -0400, Jerry Stuckle
> > <jstucklex at attglobal.net> wrote:
> > 
> >> On 9/18/2017 6:02 PM, Custos Custodum wrote:
> >>> On Mon, 18 Sep 2017 16:17:07 -0400, Jerry Stuckle
> >>> <jstucklex at attglobal.net> wrote:
> >>>
> >>>> On 9/18/2017 12:08 PM, Custos Custodum wrote:
> >>>>> On Mon, 18 Sep 2017 10:13:05 -0400, Jerry Stuckle
> >>>>> <jstucklex at attglobal.net> wrote:
> >>>>>
> >>>>>> On 9/18/2017 5:31 AM, David Woolley wrote:
> >>>>>>> On 18/09/17 03:28, Jerry Stuckle wrote:
> >>>>>>>>
> >>>>>>>> It depends on the amount of power required.  They typically are less
> >>>>>>>> efficient than the electronic versions, which means more heat
> >>>>>>>> dissipation for the same power level (and they are definitely much
> >>>>>>>> heavier).  But the are clean.
> >>>>>>>
> >>>>>>> They are not clean.  They may be relatively clean at HF, but they put a
> >>>>>>> lot of ELF harmonics onto to the mains supply, which is why they are
> >>>>>>> mainly illegal in the EU now, for new supply.  The problem is that they
> >>>>>>> only conduct for a small part of the main cycle, and therefore draw a
> >>>>>>> very non-sine wave current waveform.
> >>>>>>>
> >>>>>>> SMPS can be designed so that the input voltage to the inverter varies
> >>>>>>> greatly throughout the main cycle and therefore the input stage
> >>>>>>> rectifiers conduct on much more of the mains cycle.
> >>>>>>
> >>>>>> Excuse me?  A transformer and full wave rectifier "only conduct for a
> >>>>>> small part of the main cycle"?  The only time they do NOT conduct is the
> >>>>>> small amount of time in the cycle between 0 and +/- 0.7V (when the
> >>>>>> diodes start conducting).
> >>>>>
> >>>>> Er, no. Consider the ripple voltage on the reservoir capacitor - it is
> >>>>> usually sawtooth-shaped. The only time that the capacitor can be
> >>>>> charged is when the output voltage of the rectifier is greater than
> >>>>> the standing voltage on the capacitor. This will be on the relatively
> >>>>> short rising edge of the sawtooth. The circuit therefore only draws
> >>>>> current over a comparatively small part of a cycle. It's why
> >>>>> valve/tube rectifiers were limited as to how much capacitance you
> >>>>> could hang on their output.
> >>>>>
> >>>>
> >>>> No, the input is a full-wave rectified sine-wave.  The output from the
> >>>> filter capacitor is a full-wave rectified sine wave on top of a (much
> >>>> larger) DC voltage.  There is no converting to a sawtooth (which would
> >>>> take some switching).  But the circuit draws power over the entire
> >>>> cycle, except within the range of +/- 0.7 volts.
> >>>>
> >>>
> >>> <sigh>
> >>>
> >>> http://www.zen22142.zen.co.uk/Design/dcpsu.htm
> >>>
> >>> Figures 7, 8 and 9 in particular.
> >>>
> >>
> >> <sigh>
> >>
> >> "This article covers the design of basic unregulated DC power supplies.
> >> It is not designed to replace the training given by any college or
> >> university, but you may find it a useful study supplement."
> > 
> > Looks like a standard, and sensible, disclaimer to me. Shooting the
> > messenger might be seen as a sign of desperation, Jerry.
> >
> 
> Nope.  It means exactly what it says.  It's not designed to replace 
> formal training.
> 
> >>
> >> Maybe you should look at the training given by a college or university.
> >>
> > 
> > I don't need to - it's exactly what I was taught at university almost
> > 50 years ago. I used that link because it was the first one I came to
> > that made the point graphically.
> > 
> 
> Which is much different than what I was taught at university about the 
> same time.  And different than what is in use today.
> 
> >>>> You are limited as to how big a capacitor you can use by the inrush
> >>>> current of the initial charge.  To large of a capacitor will overload
> >>>> the diodes and potentially the transformer (although the diodes usually
> >>>> go first).
> >>>
> >>> Remember there is no inrush with a valve rectifier. The voltage comes
> >>> up gradually as the valve heats up.
> >>>
> >>
> >> Who's talking about valves?
> > 
> > I was. I threw it in as an example of how this behaviour could be a
> > limiting factor when selecting components. I've been out of the
> > business for 15 years now, but ISTR that semiconductor rectifiers are
> > a lot more forgiving wrt surge currents than valves were.
> >   
> 
> You're the only one.  But changing the subject is a sign of desperation.
> 
> >> Which, BTW, have poorer regulation than
> >> solid state because of valves.  Valves haven't been used in new gear for
> >> over 40 years.
> > 
> > Apart from guitar amplifiers and possibly 'boutique' hi-fi for the
> > 'golden ears' fraternity.
> > 
> 
> Even many of those use solid state rectifiers and valves only for the 
> amplification.
> 
> -- 
> ==================
> Remove the "x" from my email address
> Jerry Stuckle
> jstucklex at attglobal.net
> ==================
> 
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