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Preregulation of a linear bench PSU
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Atom:
So, first of all HI to everyone,

second...after building the blackdog psu i'm having trouble with the preregulation, let me explain, the preregulator works but is has some problems.
by the way the linear regulator is a beast and even with all the noise that is injected by the preregulator none is seen on the output.

problems of the preregulator.

1.the transformer is buzzing like crazy because of the high current that is being drawn.

probe across 10cm of 1.75mm^2 cable in the gnd return path  for measuring current drawn by the capacitors.

now making some calculations wire resistance 0.96mohm(taken from a calculator on the internet). the spikes that i see are 60mV at 3.5A load.(yellow trace)

the current that the caps are drawing is 60mV/0.96mohm = 62Amps.

the blue trace is the rectified voltage ..as you can see it drops quite a bit before returning up.

2.Loading the transformer even more 9/10 amps the spikes increase to 100mV... so 100A spikes.the yellow trace also seems to flatten towards the end (the transformers cant provide the current i think?, explain this to me ).

i tried to add an inductor as people suggested in other disscussion but you need a huge inductor (photo of mine below) i don't know the inductance either.

possible solution: my transformer has a center tapped winding 0-14-28Vac so i need a circuit that has to switch betweens the 2, if someone has an already made circuit that i could take ideas from please share it.

everything is in a drive folder i have a lot of images and the max size of the formu woudn't suffice.

note: switching preregulator are excluded because high freq noise is difficut to flatten out, maybe in a smaller bench psu i ll'try it.

https://drive.google.com/drive/folders/1aw1aTxQGjsbswE9cbhaWeTquFv80WQW1?usp=sharing
 

Atom:
So going for the tap switching solution, this is the circuit i've come up with ...i wanted to know if there is something that can be improved.

considering the ripple of the capacitor at aload of 8A is 4pk-pk ,i'v added a volt so the pass transistors can regulate the voltage properly.

the ranges are :

vout 0-15V first tap (rectified 14vac= 20VDc)

vout 0-30(or 35) second tap (rectified 28vac ) 40VDc)

the circuit consist of a nonc relay and a comparator lm393.

i wanted to know if there is something that i can improve...

switching times won't be the best and i'm worried relay contacts could weld themselves if i have a load on the output (up to 8 amps). the ncno relay that i have now is made for switching 10A at 250v, i have also 17A relays but they are only no

i thought of using mosfet but how can you turn on a n mos passing an ac signal? you need a floaring dc voltage on top the ac?

i haven't tested the circuit and multisim wont simulate it  :-// (stupid multisim uses only one core...pfff)

file of the circuit is on the drive(if you want to try and simulate): https://drive.google.com/drive/folders/1aw1aTxQGjsbswE9cbhaWeTquFv80WQW1?usp=sharing
not1xor1:

--- Quote from: Atom on December 24, 2018, 07:09:42 pm ---So going for the tap switching solution, this is the circuit i've come up with ...i wanted to know if there is something that can be improved.

--- End quote ---

I'm in a hurry at the moment so have not yet checked carefully your circuit (I find some symbols quite odd).
But here is a proof of concept I simulated with LTspice a while ago.
That would work even with just a TL431 as a comparator to switch the rails, but it is for negative rail regulation. You have to reverse it and use P-MOS (with usually higher Rds-on) for positive rail...
Kleinstein:
The type of pre-regulator shown, with switching at the line frequency can be tricky. If something is not working 100% right and symmetric, it could cause some DC current to the transformer, and thus can make the transformer to make quite some noise. The current spikes can also be quite high if the turn on is under power. The active rectifier is also a possible source of trouble. So I am not convinced it is a good idea to use active rectifier in combination with the switching for pre-regulation.
If everything is working fine it can be a good type of pre-regulator.  However filtering is difficult at the low frequency.

I can't understand the circuit shown for tap switching. It looks rather odd - somewhat missing the important part.
I think the planed turn over at 15 V  is a little optimistic, as one will need some reserve for ripple and the relay switching time. In addition Tap switching with a relay normally needs quite some hysteresis to prevent to frequent switching that wears out the contacts. So relativistic turn over would be more at 10 and 13 V.  Switching DC with a relay is tricky, switching AC causes quite some peak current from charging the capacitors. So it might be better to use more than 2 step switching - this is what many cheap linear supplies use.

With just 2 transformer taps there is also the alternative to use 2 transformer taps with linear turn over. It is a little more effective than switching 2 taps and no spike on turn over. As a downside there is a little more drop out (e.g. some 1 V) and one needs twice (compares to ideal switching) the number of power transistors, though only half of them will get hot at the same time. Ina addition it takes more filter caps - though with less peak current to them.
Atom:
really nice circuit , i'm going to try it right now, i'm going to look on how the tl431 can be used as a comparator so i can reduce the part count.
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