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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 21st Jun 2018, 9:09 am   #1
kellys_eye
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Default Switched Mode Power Supplies

Has anyone built their own SMPS? If so, was it from existing plans, self-design or an adaptation of existing device?

I have a number of SMPS transformer cores and quite a few magazine articles relating to SMPS designs - most involving the rewinding of existing cores etc - and I'm considering throwing a few together and looking to avoid pitfalls, both obvious and unintended.

Having a few multi-purpose SMPS units ready for general experimentation seems a practical use of otherwise 'junk' parts and I'm thinking along the lines of them all being 12V input (only because I don't like the idea of a bridge rectified mains source) and having outputs such as 90V/1.5V (battery eliminator), 250V/6.3V (general valve experimenting) and 24V-36V (@10A) for some testing/repair work I have lined up.

Pointers to reliable (i.e. known-to-work) designs and/or advice on avoiding aforementioned pitfalls would be appreciated.
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Old 21st Jun 2018, 9:35 am   #2
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Default Re: Switched Mode Power Supplies

I would go for an input of 24 volts as there are more older chips about in thru hole format.
Mains ones have a ready wound primary and are an advantage once you pick up a bit of experience.
Your best early experiment would be an auto-transformer and a UA3842 running from 24 volts. The chip needs 15 volts to start up. I use only the start up sequence to drive an electric fence so that it always pulses every second or so.
If you want to go SMD there are loads of lower voltage parts on old computer motherboards if you are prepared to trawl through the look-up tables for the part numbers.
The manufacturers data sheets are your friends here.
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Old 21st Jun 2018, 11:13 am   #3
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Default Re: Switched Mode Power Supplies

I’ve built a few boost and buck converters over the last couple of years. Some from scratch and some with COTS ICs.

Best success so far has been the LT1073 IC. Very useful if a little expensive. I built a 90v step up from a 5v source with that to build an avalanche pulse generator.

The canonical reference for these sorts of things is The Art of Electronics. 3rd edition happens to have a lot of material on switching supply design.

Conveniently the free sample chapter is the one on power supplies! https://artofelectronics.net/wp-cont...3_chapter9.pdf - see page 636 onwards.

Edit: the MC34063 is a nice little IC. It’s rubbish compared to some designs but it does the job. Also don’t forget the cheap chinese boost/buck modules; some of them are actually quite good.
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Old 21st Jun 2018, 11:22 am   #4
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Default Re: Switched Mode Power Supplies

I've designed them professionally. Everything from battery powered voltage converters to full mains input multi-output and also some wild creatures with regulated high frequency sinewave outputs.

The first thing you need is a need. Something to set some specs for what you want to achieve.

Linear Technology did some very eloquent applications notes. Still on their website.

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Old 21st Jun 2018, 11:38 am   #5
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Default Re: Switched Mode Power Supplies

Rather than using a COTS chip, why not do it the old way with a few discrete transistors? You'll probably learn a lot more about the workings of a SMPS if you can hook a 'scope to various points in the circuit and see what waveforms are there/how they change with differing loads.

I've built a few SMPS this way over the years: deceased Pye Cambridge radiotelephones will provide a nice potted transformer and a few NKT404 transistors. Sprinkle in some additional diodes and an OC81 or AF117 or three as a multivibrator.
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Old 21st Jun 2018, 12:08 pm   #6
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Default Re: Switched Mode Power Supplies

The link to Horowitz is very useful - thanks MrB.

As already inferred in posts above, my sources for parts are the usual ex-PC and UPS devices so the TL494, SG3524/3854 etc are bountiful at the moment!

I dug up an old magazine that described an SMPS intended for in-car use, driving a high power audio amplifier setup. 12V input, +/- 35V output at around 200W which, with a modification to the secondary winding, may be a suitable starting point?

Is there a rule of thumb to determine maximum power in the transformer core? Some so-called 350W PC power supplies have transformers that are less than half the size of transformers found in 200W supplies.... I dread to think I'll have to test the core for saturation/frequency etc.
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Old 21st Jun 2018, 12:14 pm   #7
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Default Re: Switched Mode Power Supplies

I've searched and downloaded a number of application notes from Linear Technology (thanks for the heads up David) to peruse when the football is on (can't stand it).
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Old 21st Jun 2018, 12:17 pm   #8
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Default Re: Switched Mode Power Supplies

Quote:
Originally Posted by G6Tanuki View Post
Rather than using a COTS chip, why not do it the old way with a few discrete transistors?
Great idea - certainly until I grasp all the principles. I could do this alongside the I-want-it-now idea of published designs!
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Old 21st Jun 2018, 1:11 pm   #9
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Default Re: Switched Mode Power Supplies

Yes, I've designed and built, in the 'day job'

Pitfalls? Make a prototype that is easy to repair. With 24V input, things die relatively quietly, but switching devices can still take out their driver stage. If you have a transformer to drive the switching devices, this is less likely. Expect to have to replace switching devices many times.

You'll learn that an inch of wire or PCB track in the wrong place can have enough inductance to completely upset operation.

If you use bipolar transistors to do the switching, you'll learn about storage time and how to turn off with reverse base current. If you use MOSFETs then you'll miss out on this fun.

SG3524 and 3854 are good IC's. They are popular for a reason. Read the application notes, they'll get you going. Trying to design with logic IC's, comparators, and op-amps is only for the masochistic! And doing entirely with discretes, again it can be done, but you will need hours and hours of debugging.

The test gear you will need is a couple of bench power supplies, some load resistors, a twin-beam 'scope, and a transient load switch. One supply is for the 'housekeeping' control circuitry, which you leave on so drive to the switching devices is present and robust; one is for the power-stage input, which you wind up from zero while watching waveforms. With care you can see problems before the voltages/currents are high enough to blow the devices up.

When you get the circuit running and stable, at some point you'll want to optimise the
regulation circuitry. This is where the transient load switch comes in. Mine is a 555 oscillator running at 10Hz - 1kHz (switchable) and driving a power MOSFET. You use it to switch in an additional load in parallel with a steady load, on and off continually, while monitoring with your 'scope the output voltage, to see how the circuit copes with load changes. Expect to see all sorts of behaviour, from continuous oscillation, through damped ringing, before you achieve the ideal of an brief excursion of output voltage followed by a rapid return to the 'correct' value without overshoot or undershoot, each time the load is switched in our out by the transient load switch.

You'll have lots of fun, and learn loads!
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Old 21st Jun 2018, 1:51 pm   #10
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Default Re: Switched Mode Power Supplies

Just a point. Wear safety glasses. MOSFETs go off in style. I had to pick bits of IRF640 out of my face a while back. If I hadn’t been wearing glasses I’d probably have been blinded.
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Old 21st Jun 2018, 6:39 pm   #11
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Default Re: Switched Mode Power Supplies

Some really sound advice coming - thanks kalee and again MrB.

I'm now rather looking forward to this - indoor fireworks! Excellent!
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Old 21st Jun 2018, 7:42 pm   #12
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Default Re: Switched Mode Power Supplies

Indoor fireworks are really for the off-line supplies. You won't see much with 24V.

I worked with a colleague who didn't bother with fuses for his prototypes. Fireworks were the order of the day when he worked on developing a 600W off-line switcher. A quick-blow low-rated fuse in series with the supply line to the switching stage won't save the devices, but it will prevent the rest of the circuit from turning into a charred mass with the flames leaping from exploding MOSFETS (as MrBungle says). You get an idea how much grunt is behind the mains, even with a standard 13A fuse in line!
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Old 21st Jun 2018, 8:15 pm   #13
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Default Re: Switched Mode Power Supplies

I was running mine off a 12V SLA which can kick a few amps out

Mains I’d kill myself in two minutes thus I will only use linear supplies I built myself there.

Most of my experiments were deriving typical linear and CRT circuit bias from 12V supplies so I could run home brew designs off grid (ie ground referenced -15v, +15v, 5v, 200v, +2kv, -2kv). Managed to destroy about about 25 quids worth of coil formers learning about oscillator stalling
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Old 22nd Jun 2018, 9:49 am   #14
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Default Re: Switched Mode Power Supplies

Quote:
Originally Posted by kalee20 View Post
Indoor fireworks are really for the off-line supplies. You won't see much with 24V.

I worked with a colleague who didn't bother with fuses for his prototypes. Fireworks were the order of the day when he worked on developing a 600W off-line switcher. A quick-blow low-rated fuse in series with the supply line to the switching stage won't save the devices, but it will prevent the rest of the circuit from turning into a charred mass with the flames leaping from exploding MOSFETS (as MrBungle says). You get an idea how much grunt is behind the mains, even with a standard 13A fuse in line!
In the late '80s I worked for a manufacturer of temperature control systems, most of these systems featured in-house designed universal SMPSUs. The power supply design engineer's work bench was situated next to a window for good reason; often one of his creations became a little warm or caught fire and he would toss it out of the open window
On several occasions I walked along the path outside this window to see something smouldering in the flower bed.

John
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Old 22nd Jun 2018, 9:51 am   #15
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Default Re: Switched Mode Power Supplies

I'd recommend starting 'really simple' - so that you don't get disheartened if any of your own inputs throw up a hurdle to jump (transformer or inductor, or control circuitry, or wiring, or scope probing techniques ...).

Perhaps also buy a cheap small ebay pcb dc/dc or dc/ac 50-150W module to assess and probe. I've reverse engineered a few recently - they often have something like a 494.

Unless you are good at pcb layout/production and savvy with noise and high current paths, then that can be a newb heartache to wonder where scope glitches and erratic control behaviour and low efficiency is coming from.

Oh, and I hope you're keen!
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Old 22nd Jun 2018, 9:55 am   #16
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Default Re: Switched Mode Power Supplies

Quote:
Originally Posted by kellys_eye View Post
Has anyone built their own SMPS? If so, was it from existing plans, self-design or an adaptation of existing device?

Pointers to reliable (i.e. known-to-work) designs and/or advice on avoiding aforementioned pitfalls would be appreciated.
One of the early SMPS's for computers was the one in the IBM5155 computer from the early 80's . Recently I documented it, (as IBM didn't), there are remarks and schematics & waveforms:

http://worldphaco.com/uploads/The_IB...WER_SUPPLY.pdf
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Old 22nd Jun 2018, 11:05 am   #17
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Default Re: Switched Mode Power Supplies

I seem to remember a paperback book called Electronic Fault Diagnosis,by Loveday which gave a circuit for a SMPS using discretes and a transformer,with an explanation of course.Maybe useful. Les
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Old 22nd Jun 2018, 5:39 pm   #18
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Default Re: Switched Mode Power Supplies

Quote:
Originally Posted by Argus25 View Post
One of the early SMPS's for computers was the one in the IBM5155 computer from the early 80's . Recently I documented it, (as IBM didn't), there are remarks and schematics & waveforms
That's not that early. There were SMPSUs in computers at least 10 years earlier, and switching regulators (taking in AC from a normal mains transformer) before that. And some were quite exotic. The HP2100A (The earliest version of PSU that the manual I have covers dates from 1971) had thryrstors in the mains bridge rectifier as a pre-regulator stage. Boschert did their infamous 2-stage PSUs in the late 1970s

You might look at the DEC 'PSU bricks' like the H744. This takes in 20-30VAC and gives out 5V at 25A max. It's a switching regulator (no isolation between input and output) based on the LM723 chip. The only custom component is the inductor. There are schematics on Bitsavers, but you will probably have to look through the CPU printsets to find them rather than there being specific manuals for the power supplies.
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Old 22nd Jun 2018, 6:01 pm   #19
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Default Re: Switched Mode Power Supplies

Is there any way to get information on the ferrite cores that I'd salvage from ex PC PSU's? most of which are unmarked.

I've found that generous use of a heat gun (and gloves) allows one to dismantle them neatly and save core, former and wire for re-use but there will come a point in my experimentation where the ferrite (or dust iron?) will become an issue for me - or not?

Although my initial thoughts were aimed at the larger cores, I do have a number of much smaller ones that were used in the standby supply circuitry which may be a better place to start experimenting by limiting the actual power levels achievable whilst still getting me the experience I need.
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Old 22nd Jun 2018, 8:11 pm   #20
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Default Re: Switched Mode Power Supplies

The unitrode chips are common, work well and have been on the go for yonks. Apps notes will be rather old and probably bipolar based.

National semiconductor did some ore up to date parts designed to drive Mosfet devices. All their stuff is on the Texas Instruments website since TI bought out Natsemi

The driver circuits for the switch transistor may be the most critical area. Stick to switches and controllers which go together.

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