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Preregulation of a linear bench PSU
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tombi:
I actually did both tap switching (parallel vs series) and a pre-regulator.

I have a cheap lab PSU that I bought from a local electronics shop (Jaycar) and if you set it to say 25V then when it goes into current limit (and the output voltage drops) it will switch transformer taps.

This is fine unless the load is transient in which case it makes a horrible tap switching racket as it flicks between the taps.

I didn't want that. So if you set the target voltage on my homebrew PSU to 25V it keeps the transformer tap configuration the same if it goes into current limit. Instead the pre-regulator reduces the capacitor voltage to prevent the pass element going nuclear.

Apart from this the pre-regulator does help a bit but not a lot. You need a few volts of headroom above the output depending on the size of your capacitors/current so it never goes that low even if you set the output to say 1.8V.

Not sure if this helps but I also reduce the overhead if the output current is low. That is the pre-regulator voltage is a function of both the output voltage AND current. If the current increase then the pre-regulator increase the capacitor voltage.

Tom
Atom:
Hi seems there is an intresting discussion here  :-+ , by the way i manged to find some power mos 80V 75A 15mohm rdson to220 package.

the circuit from blackdog the tap switcher works flawlessly and i would reccomend it to anyone that needs a similar solution, the to220 package didn't even get hot, it was extremly cool to the touch.

Thanks to every one that helped!!

do you have any suggestions when it comes to books? on my to buy list there are:

The art of electronics 3rd ed
jim williams books
Power Supply Cookbook by Marty Brown (i've read online that it's good and i have to get into switchmode as fast as possible (5th year thesis).

In the past i've tryied to get into smps but was limited by the lack of various other things, now i feel ready in the end the basics are easy to understand, the calcs could be more challenging but i have professors that could help me.

 

not1xor1:

--- Quote from: tombi on January 09, 2019, 01:13:00 pm ---I actually did both tap switching (parallel vs series) and a pre-regulator.

--- End quote ---

Then it is not bad.

Actually I've been really dumb.  ;D
Obviously, in any case Blackdog's style pre-regulator would be more efficient than a center tapped one from about half max Vout up to 90% where the efficiency should be about the same.

It is when the output voltage is below half of maximum that the pre-regulator offers little if any advantage (vs. center tapped one) according to my simulations.

Anyway, since you used it both with the top and the mid tap of the transformer then that is not your case.

So in case somebody needs to use a single winding transformer, e.g. a salvaged one, a Blackdog's style pre-regulator might still be useful although much less efficient than a buck pre-regulator.

I think anyway there is still much room for improvement, especially regarding efficiency at low output voltages.
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