Building a switch mode power supply for the first time - Any advice appreciated

[bob_60]

Hi. I'm thinking on building my own switch mode power supply and need some general advice on actually doing it.

I have played around with linear power supplies but have never really tried to do anything with a switch mode. Dave's Video #110 have really got me going, and I think it's time to design and build one.

One of the first problems getting started, is just the large number of different switch mode regulators available so using the  MC34063 Dave is using would be really nice but I don't think it's possible with the specs i have in mind.

My preferred specs for my project are the following:
   Input voltage: 24-32 V. I have some 24v and 32V secondary transformers laying around and I'm thinking of using one of those.

   Variable output voltage: Maybe about 3V – 24V. I like to use a small microcontroller to control the output voltage (in course steps).

   Max output current: Something like maybe 2A – 5A

The regulator chip has to be:
   Readily available and not to expensive (whatever that means) – also applies to the external components
   The complete circuit has to be relatively simple to build.
   Is has to be fairly stable so I don't have to really fine tune the circuit to make it work.
   Not to noisy.
   If it's at all possible, I would like to make it relatively efficient.


This is just a hobby project and besides having fun doing it, I will see what else can be learned by experimenting with it. An example is noise. I'm interested in finding out what the best cheap way is to reduce output noise is and I'm thinking about maybe following it with some kind of a linear regulator, some filters or whatever works best.


If you have any suggestions on a particular chip or a family of chips, please let me know. Also any links you think could be useful, would be appreciated


/Bo

[Bored@Work]

My recommendation would be to start with something low power. Not Poutmax =  24V * 5 A = 120 W for a start.

The MC34063 is a crap regulator, but cheap. So, if you do a few low power experiments you can easily afford to blow up a few.

[bob_60]

My recommendation would be to start with something low power. Not Poutmax =  24V * 5 A = 120 W for a start.

The MC34063 is a crap regulator, but cheap. So, if you do a few low power experiments you can easily afford to blow up a few.

Ok. I don't think I want to start out with a cheap device just because it's cheap. I would rather start out with a device that at
least to some extent lives up to my requirements - except for the high power.

Do you have any suggestions on maybe a 1A or 0.5A device I should try out.

[free_electron]

Look at the simpleswitcher from national semiconductor ( i refuse to use their new name )

[madires]

Look at the simpleswitcher from national semiconductor ( i refuse to use their new name )

I second that. I've build several LM2576 based DC-DC converters to power small network switches, routers and so on by a server which runs 24/7 (got a 80+ gold PSU and it's more efficient and less noisy (RF wise) than the tons of cheap wall warts). The LM2576 switches at about 50kHz and causes no trouble with a shortwave receiver nearby. Regardless of which switcher IC you select you should pay attention to the PCB layout for a proper ground. Some datasheets got nice recommendations for the PCB layout. BTW, I've put each of my DC-DC converters in a small aluminium box (isolated, not grounded) for heat dissipation. The switcher IC is mounted on a small aluminium plate also acting as PCB mounting and the plate is fixed to the box by two small screws. That setup runs 24/7 for several years now and never caused any problem so far.

[Paul Price]

This one gives you 5 Amps with very few parts.

[komet]

NatSemi's WEBENCH web appliation

http://webench.ti.com/webench5/power/webench5.cgi

is really very good. You enter your parameters and it creates a complete circuit, for some chips it can even generate gerbers and everything. Why not give it a try. I've used it a number of times.

[TerminalJack505]

Here's a 40-hour video lecture series on switched mode power conversion if you want to learn some theory on the subject.

[hlavac]

If you want to play with SMPS have a look at the old LM78S40 - its basically MC34063 but with internal reference output out on a pin not hardcoded onto the error comparator, an extra compararor and a diode. Its basically a set of all the building blocks you need, without imposing actual architecture on you.

And you probably can make a 5A supply with it, you just need an external switching transistor. And the inductor will have to be big beast wound with thick wire.
Its just that things will go poof and kaboom instead of getting hot if you screw up