Pulse Width Modulation for a bench supply pre regulator.

[si2030]

Hi There,

I have been researching the idea of a PWM controller - say the TL494 - to be used as the basis for a bench power supply pre-regulator.

Basic specs would be 30volt adjustable power supply with current limiting to 10Amp.

What I would like is some input as to whether I am on the right track here. I wanted to use the TL494 (or I think, SG2524) as this was also a learning curve just as much as producing a bench power supply.

The idea was to run a linear regulator after the pre-regulator such that the pre regulator would supply a voltage just above the voltage required by the linear regulator.

The second idea was to do with the type of topology for the pre-regulator. From what I can see there are three different approaches (apart from flyback, and Buck) that I can use to drive the high frequency step down transformer. I can use Half bridge, full bridge or push pull.

I thought I might use push pull as I will be winding the transformer and can add in a centre tap without too much trouble.

Adjusting the voltage level via the pulse width is the intention... feeding this into a large filter cap and then supplying the linear reg from here...

What are the reasons you would use a half or full bridge instead of a push pull topology in this regard? Is the only difference the complexity of the transformer?? or are there other reasons?


I am hoping someone who has a good idea about these concepts might tell me the answer to the above questions and also whether the use of the PWM idea as a pre-reg has merrit....


Kind Regards

Simon

 

[MrAl]

Hi,

The push pull is when you have a center tapped primary. It's only two transistors.
The half bridge is when you have dual power supplies to feed it with and no center tap.
The full bridge is when you have single supply to feed it with and no center tap. It's four transistors.

With those older switcher parts you want to check the dead time vs operating frequency. You dont want too much dead time or you loose some of the top end of the voltage range, but you want high enough frequency to be able to use a small inductor.
I think they make a MOSFET driver version now...been a long time since i looked at these chips though so i dont remember everything about them.

The simplest way to get differential feedback is with a transistor and zener, and coincidentally it's also the fastest. Perfect voltage regulation here isnt mandatory because it's only keeping track of the voltage across the linear regulator.

 

[si2030]

Hi MrAl

Thanks for replying..

Wondering if you might give me some insight as to how to apply the transistor and zener on the linear reg... I assume it goes on the middle leg and is a function of the input voltage... not sure though.

by the way for the linear regulator I was intending to use the LT1083... I was going to use two of them in parallel.

Simon

 

[MrAl]

Hi,

Here is the basic connection diagram. People have built this circuit quite a while back and we even had a Digikey web project for the parts but it's probably not there anymore.