Author Topic: Sometimes even bad ideas work, poor boy buck converter  (Read 2020 times)

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Offline eidolon

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Sometimes even bad ideas work, poor boy buck converter
« on: May 20, 2017, 06:55:54 pm »
I was just reading on another thread about a 50V buck converter down to a 12V battery.
At camp a couple years ago I changed to a 36V array that gave about 50V at power point.
That higher voltage was powering some other stuff and still had lower voltage panels that
generally supplied most of the battery charging. The higher volt array was to help charge
the battery only on those desperate days with extreme overcast. I have resources at home,
but at camp only stuff I can scrounge at the town recycling center.  Fortunately they get
a lot of small UPS with dead batteries.  A lot of poor planning. Still, I manage to come
up with something out the garage when the need arises.

Should add that I think all electronics should be nearly free. There is a cornucopia of
consumer electronics with parts and sub assemblies that can be re purposed. While capable
of building some really nice electronics, If patient, nice commercial stuff can be had for
next to nothing that is not working. I just got a nice current model XANTREX 30V 10A power
supply for $46 shipped. These cost over $300 used. An hour later I had it working. Finally,
solar is becoming mainstream and broken converters will start flooding the market. Till then
I'm OK with hanging some ugly stuff on the wall for a while. It is a kick creating some of
this stuff out of the most improbable parts to show that a lot of money doesn't have to be
spent to have a working system.

Pictured is your textbook basic buck converter. This one only had to put out about 10A from
a vastly overpowered array. I have seen it put out 23A. The maximum it is now allowed to
produce is reduced by limiting the maximum PWM duty cycle. The FET came from an old UPS.
Three were riveted on the heat sink so I used three.  The high side driver is just an opto
isolator driving the gate with a 870 ohm resistor to the source. At these slow speeds the
optos work just fine.  Power for this high side driver is from a 12V wall wart. These happen
to be perfectly happy operating at reduced current with only 50V DC. The capacitor bank is a
bunch of 200V electrolytics out of PC power supplies.

I wanted a high current Schottky diode for the flyback.  PC supply ones are only good for
35V.  I did have a bunch of SB5100 diodes. These are 5A 100V and were like $3 shipped for
20. They say you shouldn't parallel diodes. You can if you are careful. I found some tin
plated steel used the shield a VCR. Two four inch squares were cut out for heat sinks. That
metal was so thin I only uses a sissors to cut it.  I think I used nine diodes with the leads
soldered close to the body on the heat sink. Leads were not cut so I could use them for other
projects at a later date. So that was my cheap $1.50 diode. A small RC snubber was placed across
them to catch any spikes.

The Inductor....

Suitable inductors are not easy to find in consumer electronics and spending money for one
is not in my vocabulary.  I rewound a generator when I was 17 and have been traumatized by
that ever since. Had a good size (400W) transformer out of a UPS.  The 24V winding looked like
it would work.  I know, it is going to saturate and should have an air gap on the iron. These
were desperate times so I used it. It worked pretty well on 480Hz. The iron core did heat up
as predicted, the windings stayed cool.  I rewrote the program to change the PWM port so I
could drop the frequency to 122Hz. That greatly reduced the iron core heating. While likely
not the best efficiency, nothing was heating up much. It has been running a couple years now.
A buck converter for next to nothing.  Just don't show it to people.


Offline lighthunter

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Re: Sometimes even bad ideas work, poor boy buck converter
« Reply #1 on: May 20, 2017, 07:59:05 pm »
Very nice! Obviously has stood the reliability test. Thanks for sharing that!
Health Warning: May contain traces of nut!
LH

Offline MadScientist267

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Re: Sometimes even bad ideas work, poor boy buck converter
« Reply #2 on: May 20, 2017, 09:53:14 pm »
Haha nice indeed.

For years, my sole source of parts for everything I home brewed was from donor boards... then a brief window where there was some new part purchases going on, but the last few years, I've been right back at it again... PC power supplies, Car audio amps, UPSs, cheap MSW inverters, among a few others are absolute gold mines for experimentation parts... and as you pointed out with the UPS/battery thing, they have not only typical but also usually fairly obvious failures, so you almost know what you've got in hand before you ever even remove the first screw to take a closer look ;)

One thing... on paralleling diodes... it's actually very common in the SMPS world, particularly the TO220/247 Schottky "KAK packs" found in higher current lower voltage supplies (such as PC supplies)... The main thing is to balance each side with the traces they're connected to as closely as possible... and while that doesn't account for the minor internal differences, it's not generally a problem if they're properly sunk and not pushed all the way to the datasheet under warmer conditions. They're pretty tough little critters when they're done right.

Good post, glad to see I'm not the only one with projects like this that somehow just kinda stick to the wall/bench/table/ceiling haha... Efficiency is always a good thing, but there's a lot to be said for things that "just work" ;)

Steve
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Offline eidolon

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Re: Sometimes even bad ideas work, poor boy buck converter
« Reply #3 on: May 21, 2017, 07:07:11 am »
Trying to make a 5A diode from two 3A ones is problematic.  I have a small welder with a number of small leaded diodes in parallel that doesn't look like it would last, hasn't failed yet. It is always a race  between higher temp lowering the forward voltage and attracting more current and internal resistance increasing forward voltage with more current. Definitely you need to keep them in close thermal contact.

The world has gone to small inductors and higher frequencies for size and weight savings. If you have the space larger inductors still work and lower frequencies make FET driving easy.  The micro was actually 30 feet away from this board. This used to be mounted on the wall next to the bedroom.  I would wake up to the drone of power. Now it is in a shed and adding the audio effect with changes in power just adds to the mood.

It is wired up a little better now.  That board is solid cherry.

Offline MadScientist267

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Re: Sometimes even bad ideas work, poor boy buck converter
« Reply #4 on: May 22, 2017, 05:15:42 pm »
Yeah I probably wouldn't do it with descrete singles as a general rule either... With KAK pack it's not much of an issue because they're on the same substrate and then sunk on the same slab of aluminum. I have seen them go off under those circumstances, but usually it's traceable back to thermal contact with the sink or shoddy balancing in the board.
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