some comments on linear power supplies: was : RE: [sdiy] Power Supply Design Questions

[harrybissell]

Another good solution for inrush current is to use Polyswitch fuses by
Raychem (I think they were bought recently)... they have some fraction
of an ohm resistance cold...  This is often enough to take the edge off the

inrush, without too much voltage drop.  Side effect, if you short the
supply
they will go high impedance.

An N channel VMOS transistor in the negative lead of the cap... driven
by a small RC makes a good soft-start as well.  Have not done the negative
side but a P channel would work....

H^) harry

ASSI wrote:

> Some more comments...
>
> To the question of how the load capacitor should be dimensioned: simply
> design for ripple voltage (Upeak-Ulow) at maximum load current and cut
> a few corners on the way. The ripple frequency is known (mains
> frequency for half and double that for full wave rectification) and for
> simplicity assume that the cap supplies the current for the full
> period. Then chose C=Imax*T/Uripple and select the next standard cap
> size. The conduction angle is approx. the arc cos of the normalized
> droop (Ulow/Upeak) for half wave rectification and twice that for full
> wave rectification. You normalize to 2pi or pi and take the inverse and
> that tells you the factor with which to multiply the load current to
> get the inrush current for the cap. Now check the specs of your
> capacitor, rectifier and anything else upstream carefully to see if
> they're designed for that.
>
> So, for 1A load current and a full wave rectifier, 10V peak and 10%
> ripple we get a cap of  1000µF, which sounds about right. The inrush
> current to the cap is around 7A. For ripples of 1% and 0.1% we arrive
> at 10mF/22A and 100mF/70A respectively. Now you know why these caps get
> hot and why severely oversizing them is a bad idea, as is designing for
> exceedingly low ripple. Adding chokes to the load path limits the peak
> current by extending the conduction angle and makes for a more quickly
> decaying spectrum of the ripple hum, but their use is limited by
> various factors in practice. If the hum is the problem, then shunting
> the known frequencies after the cap is a more effective solution.
>
> Having said all that, I really think a primary switched power supply
> with PFC and slow start as the raw DC source plus point of use
> regulation is a better solution for large currents. If you have an
> assortment of wall-warts and individually powered gear, then the best
> solution to slow start is power sequencing, i.e. never switch more than
> one at the same time. I've got some distribution sockets from Conrad
> with a switch on each outlet and that works fine for me. There are more
> fancy things that sequence slaves after a master has been switched on
> or even *gasp* switch power outlets on demand from MIDI.
>
> Achim.
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>
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