Designing a simplified chopper power supply

[HandyMan]

Hello everyone

I want to design a circuit. I'm a repairer of electronic goods but unfortunately I am not a circuit designer, not to the level whereby I can create a fairly complex circuit that works smoothly - my design tends to be a bit rough and ready.

Recently I've been repairing a good number of TFT monitors which operate on 12V DC supplies. Sometimes the monitors are supplied without the 12V chopper power supply adapters which means I have difficulty testing them as my benchtop power supply uses a standard mains transformer and a regulator connected to a pot which allows voltage control from 0 - 30V and the output current just isn't high enough despite the huge size of the transformer.

As I want the higher current but with the continued ability to alter the voltage level, I thought that a chopper power supply hooked up to a regulator would be better. I can use LM338T for voltage regulation.

I want to build the chopper power supply myself. I know a lot of people gawp at the thought because they are fairly complex circuits, but what about if a lot of the 'extras' in the circuit were removed, like the optocoupling and other perhaps unnecessary complexities.

Most TFT monitors (and laptops that come in for repair) need from 2.5A to 4A and a variable supply using a chopper circuit would be ideal, while giving me high current and variable voltage to use on other devices when needed.

I'd take mains voltage, rectifying and smoothing it I can design myself, but I'm not sure how to go about creating the high frequency which chopper circuits use. I have plently of chopper transformers about (from TV chopper supplies) and planned to use one winding with a center tap and then use a push-pull design at high-frequency. Nothing fancy, if that's workable.

I'd like to then use any of the secondary windings that produce around 35V, rectify, smooth, and supply to a lead.

Is it possible to design a very simple circuit to do this and does anyone feel up to the task of helping me please? I can make the PCB using photo-etch board and I have a good stock of components and can get hold of others easily.

I'd really appreciate any technical help and advice on this.

Thanks.

 

[Russlk]

If your time is worth anything, it is more efficient to buy than build. You can buy a 12 volt, 5 amp supply from Mouser (www.mouser.com) for $32. You can't buy the parts and build it for less. Mouser part number: 418-CFM60S120.

 

[HandyMan]

Thanks for finding that for me, I appreciate it. I think I may go with your suggestion.

Incidentally, I don't suppose you or anyone else knows where a good place is on the net where I can learn more about the workings and theories to do with switchmode (chopper) circuits?

Thanks a lot,

James

 

[Nigel Goodwin]

Thanks for finding that for me, I appreciate it. I think I may go with your suggestion.

Incidentally, I don't suppose you or anyone else knows where a good place is on the net where I can learn more about the workings and theories to do with switchmode (chopper) circuits?

I would suggest looking at IC manufacturers datasheets, however it's a very complex subject, and the really critical component is probably the transformer.

 

[HandyMan]

Do you feel like explaining it and answering a few of my hundred and one questions?

I wouldn't blame you for wanting to switch your computer off right now!

James

 

[Nigel Goodwin]

Do you feel like explaining it and answering a few of my hundred and one questions?

I wouldn't blame you for wanting to switch your computer off right now!

I'll offer what advice I can, but my experience is in repairing them, not designing them.

As I see it, the BIG problem is when things go wrong, generally it's catastrophic - it's like a chain of dominoes, ones goes down - THEY ALL GO DOWN.

Have you considered using an old PC PSU?, there's been a number of threads on this in the last few months.

 

[HandyMan]

Yes, I have! I've got 3 in the workshop from old computers, but they are the type which are turned on by a momentary switch on the computer case and apparently the circuits are so contrived that they aren't really suitable.

I did think of using one though, they have a great deal of current available and a 12V line...

 

[Nigel Goodwin]

Yes, I have! I've got 3 in the workshop from old computers, but they are the type which are turned on by a momentary switch on the computer case and apparently the circuits are so contrived that they aren't really suitable.

If you search for the previous threads, they discuss how to make them suitable - basically just adding a minimum amount of load to make the supply run correctly.

 

[HandyMan]

I'll have a search and try to get one up and running, sounds ideal although I'd like more than 12V available, it's good enough for now.

Thanks for your help so far. The main thing I don't understand with chopper circuits is how such small transformers can deliver so much current when an equivalent mains transformer has to be the size of a brick to do the same thing. I know the frequency difference is involved (~60KHz compared to mains transformer operating at 50/60Hz) but not sure how this fits into it.

 

[Nigel Goodwin]

Thanks for your help so far. The main thing I don't understand with chopper circuits is how such small transformers can deliver so much current when an equivalent mains transformer has to be the size of a brick to do the same thing. I know the frequency difference is involved (~60KHz compared to mains transformer operating at 50/60Hz) but not sure how this fits into it.

As you say, it's down to the higher frequencies involved - basically the higher the frequency the smaller the transformer can be.

I presume it's down to core saturation, a lower frequency transformer needs to be larger to prevent the core saturating?. But I'm sure there are plenty of people here who will know!.

 

[k7elp60]

I have built a designed a built a number of switching power supplies. All have been the buck(reduce) or boost(increase) the DC voltage. Even in these one has to be aware of the minimum load current to keep them stable. One thing I noticed was that the higher the current the lower the
value of the inductance.
Some were built from published data sheets and manufacture furnished software. These generally used a specific IC that had a lot of the circuit in the package.
Others where built around a specific IC like the LM3524 switching regulator. In this case the math for the design was sometimes extensive depending upon the application.

I have also built numerous linear power supplies, they are simple and seem to work for ever. Like Nigel says you can usually buy them cheaper than you can build them.

 

[HandyMan]

I have found a good tutorial for learning about switch mode power supply circuits:

**broken link removed**

Hope this helps others who wish to learn more about them.

 

[M.HAROON]

Choper Power Supply Desine Help

Hi
I Want To Desine A 5 &12v ,5a Choper Power Supply.
So Pls Help Me. Pls If You Have Any Circuit Diagram Or Technial
Data About It So Pls Send Me.
Thank U.

 

[blueroomelectronics]

Just salvage an old ATX power supply, they usually have high current 5V & 12V supplies.

 

[M.HAROON]

220v To 110 1500 Watt Convetor

I Need Help Or Circuit Diagram Of 220 V To 110v Transformer-less Convetor . And Wattage Should Be 1500.

 

[MrAl]

Hi there,


If any of you are new to switching power supplies i would have to suggest
starting with a buck (step down) configuration. These are somewhat easy to
understand.
There are a number of things to consider when you start to design one, such
as input voltage, output voltage, output current, and max allowable ripple voltage.
You then move on to what kind of circuit and what parts might get you
there.

A good way to start out is to start with one of the National Semiconductor
buck converters from the line called "Simple Switchers". These are ic's
that perform most of the functions needed to do a buck converter except
the catch diode and capacitors. The data sheet suggests real component
values and manufacturers too.
If you can get one of these to work (and it's not that difficult really) and
understand the basics of why it is working the way it does then you
can move on to designing more complex circuits like this.
If you cant get this to work then you should keep trying and asking questions
here until you do.
To give you a rough comparison of these chips to some linear types,
some of the Simple Switcher chips work almost like the LM317 chips,
except you need a catch diode and more care in choosing the input
and output capacitors, but that's about it. The linear chip of course
uses lots of power, while the Switcher chip attains much higher
efficiency due to the switching nature of the circuit.
There are chips like these that can do zero to 12v with a 24v input
for example, but the output is also regulated.

After you get past that, you can start using the controller chips made
by various companies that control larger transistors that have to
handle the higher currents.

BTW, if you have never used a chip like the LM317 i would suggest
starting with that first. It's a linear chip but some of the basic
principles involved are also found in switching circuits, so it's good
to start there. It shouldnt take more than a day to get one
working and understand how the feedback works and such.
It's much easier to understand too because for one thing you
can measure various quantities with a simple volt meter. With
switching circuits sometimes you need a scope, or you have
to understand the circuit good enough to assume a certain
waveshape, so it's more complex. Knowing how the LM317
chip works will help with understanding the switchers.


Feel free to ask any questions you might think of, the more specific the
better, and im sure someone here will be glad to help you with your
first design.