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Any ideas for this power supply?

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Frosty90

Wed Jan 05 2011, 01:11AM

Registered Member #1617 Joined: Fri Aug 01 2008, 07:31AM
Location: Adelaide, South Australia
Posts: 139

Hi all,

I want to build a small switch mode supply for 12 volts, to run from 240AC (340DC), with a about 5w or so ouput at most. The problem is running a low voltage controller IC, directly from the 340. I was thinking of building something like a simple blocking oscillator, like is in a 'joule theif', have a non-gapped transformer, and just have a regulator on the output. Id like to build it from discrete components. Anyone built something like this? Want to share your ideas? I pulled appart a small 'plug pack' once I was going to use this inside of this, but I broke it. The board consisted of a couple of capacitors, some SMD diodes and a tiny SMD IC which appears to do all the switching, regulation etc all in one, and run off the tertiary winding of the tranformer...how does that work? how does it start up?!

hboy007

Wed Jan 05 2011, 01:34AM

Registered Member #1667 Joined: Sat Aug 30 2008, 09:57PM
Location:
Posts: 374

These regulators can have a trickle resistor as a power-up supply which is not what I'd recommend in the first place. You could come up with some capacitive voltage divider that fills a capacitor and a TO-92 thyristor that ignites once a certain voltage is reached, connecting whatever circuit you'd come up with to the buffer capacitor. That would buy you some time to get the oscillation started and then have the circuit power itself.

In terms of efficiency it might be worth the try to read through manuals and implement a given example circuit. Newer "all-in-one" controller ICs are not as bad as one might think and they save a lot of design time, too.

Arcstarter

Wed Jan 05 2011, 02:57AM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

I think trickle charge is the way to go, with an auxiliary supply winding to keep the reservoir cap charged. Here is an excellent example:

Almost every power supply i have seen in TVs and Monitors, even in those other than CRT type, use the IC supply reservoir cap charged from mains (after rectification obviously) via a resistor, using the aux winding to keep the voltage up, and a zener to limit the voltage. It is simple, reliable, and cheap.

Crunchy Frog

Wed Jan 05 2011, 02:58AM

Registered Member #2422 Joined: Tue Oct 06 2009, 02:41AM
Location:
Posts: 85

There are also circuits like this

which need no ICs and can be adapted for line voltages.

IamSmooth

Wed Jan 05 2011, 03:03AM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

Maybe I am missing something, but why don't you use a 240:15v step-down with a regulator and run your ICs off of this?

Arcstarter

Wed Jan 05 2011, 03:12AM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

Oh yea, some TVs and ATX PSUs have stand-by power supplies of a few watts, and 7, 12, or 5 volts output, and they use only transistors with an opto isolator. I have 2 of them which run on 320v, and one has like 4-5 2n2222's on the secondary side! Then, there is one on the primary side, and a tl431 for the op amp.

Frosty90

Wed Jan 05 2011, 03:16AM

Registered Member #1617 Joined: Fri Aug 01 2008, 07:31AM
Location: Adelaide, South Australia
Posts: 139

[blockquote]Maybe I am missing something, but why don't you use a 240:15v step-down with a regulator and run your ICs off of this?[/blockquote

yeah i thought about it, but it would be dissipating about as much power as the supply would be producing! but I suppose you cant really build an efficient small device usually. Any way thanks guys, Arcstarter thats the sort of circuit i was thinking of, thanks!

hboy007

Wed Jan 05 2011, 01:04PM

Registered Member #1667 Joined: Sat Aug 30 2008, 09:57PM
Location:
Posts: 374

I just found this, it includes an overview over several controller ICs and design considerations:

Steve Conner

Wed Jan 05 2011, 01:11PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

The ST VIPER

and Topswitch "SMPS-On-A-Chip"s are popular for small power supplies. They include everything, even the power MOSFET. The manufacturer can give you datasheets and example circuits.

Usually you use a "kickstart" circuit with a trickle resistor to charge the IC's power supply capacitor straight off the 340V bus. This doesn't supply enough current to run it continuously, but once it starts, an auxiliary winding on the transformer kicks in.

Really cheap plug packs can use a self-oscillating circuit with just a single tiny IGBT and a few resistors and caps. I have no idea how these manage to work.

For 5W, it's hardly worth using an off-line SMPS. If you haven't made one before, you're talking man-weeks of troubleshooting to get it working reliably. A plugpack with a transformer inside would be much easier, and take up maybe 1 cubic inch more space.

GeordieBoy

Wed Jan 05 2011, 03:11PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

The most compact, lightweight and cheapest design in that power range is likely to be a discontinuous flyback converter. As Steve C said you can get flyback controller chips like the ST part or the Topswitch parts to make these designs very compact. You usually still have to tackle the flyback transformer design and control loop compensation yourself though, and to be honest these are where most of the challanges lie anyway with a flyback converter.

Steve C is right that you could easily waste a lot of your time going round and round the iterative design process in an attempt to arrive at a design that is stable for all imaginable line-voltage variations and load variations, open-circuit protected, short circuit protected, minimal ripple, low EMI etc.

Unless you've got loads of time spare and really want to learn about this stuff, (or are getting paid well to design it!) then it is simpler to just pick a ready made product. You can always crack it open out of curiosity if you can't help but gain the knowledge of how it works. It's much easier to start with something that works, than spend weeks pondering over a new design that doesn't!

-Richie,

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