Switching power supply help

You don't want to use a 60HZ transformer. You need a transformer that's designed to do the job. You should be able to find anything you need online.

Both Digi-Key and to a lesser extent Mouser offer transformers made specifically for National simple switcher use. That would be the quickest, least risky route to follow unless you have a sleleciton of bobbins, cores, and a at least a lathe equipped with a veeder root counter, operated in back gear (and a selection of magnet wire). They will both ship overnight.

Bernie Katz

Transformer E core or toroids of iron ferrite construction are what you need. As I do not know your location I don't know what supplies are in your area. go on the net and look up mail order company's.

its no harm using a 60 HZ E core / ferric oxide transformer for your job, secondly you can even use directly 220v bridge rectifier or SCR's to convert into DC but than to get 24v dc you need to use a voltage divider circuit and for 500mA your resistance combination would be of 48ohms and 392ohms but keep in mind one thing that always there would be a 196v drop against your 392oms resistance resulting in flow a current of 500mA that means the wattage of that resistance should be 192 X 0.5 = 98 watt as P= V X I, other wise that resistance will gonna burn out. and you cannot find this much wattage resistance easily, so its best to get transformer isolation and its no harm using 60 hz frequency transformer if your line frequency in not more than 60 Hz

You need an isolating transformer for safety's sake to seperate primary high-voltages from secondary (user accessible) voltages. I'd suggest that you full-wave rectify the 220Vac coming in and decouple it with a 450Vdc rated electrolytic cap. This gives you about 311V +/- some ripple to work off of. You can use a single stage isolated topology like a discontinuous-mode flyback to give you the 1.23-20Vdc output you desire or create an intermediate DC bus voltage and buck that down to the 1.23-20V. You will have some problems with the buck regulator because of the wide output voltage variation you're looking for. This will result in an extremely narrow duty cycle when programmed for 1.23V (probably something like 5%). Your efficiency will go to hell. If this design is for a commercial product then you'd better familiarize yourself with the IEC safety standards for switching power supplies. Isolate and float your test equipment so that you can monitor primary and secondary without destroying something. Be careful what you touch or you'll suffer a nasty shock!

For switching power supplies you'll need a reasonably good ferrite material. Look in the catalogs from Magnetics Inc, Epcos, FDK. The choice in material is dependent on many factors (switching frequency, peak flux density, core loss, cost...)

Try http://www.powerint.com/

I may be missing something from your description, but it sounds like you are converting 220 VAC mains voltage to 24 VDC, and then converting the 24 VDC to a variable output of 1.23 VDC to 20 VDC. Is this correct?

"Vin(regulator) = 24V

Vin(power supply) = 220V"

If this is correct then the transformer used to convert the 220 VAC to 24 VDC is just a simple 220 V to 24 V step-down power transformer rated for your mains frequency (50 HZ or 60 HZ). The AC output of the transformer is converted to DC using a full wave bridge (or 4 diodes) and a large filter capacitor. The capacitor output then connects to the input of your voltage regulator, assuming I understand what you are trying to do.

Jim

You got my point jim.

Basically, im experimenting on using a 60Hz transformer to convert the 220 VAC to 24VDC right now. Also im using a full wave bridge rectifier and capacitor before connecting it to the regulotor. I just hope i could finish it soon and it works well.

But my problem with this design it that its quite heavy. One important factor im considering with my design it its weight and size. I want my power supply as small and as light as possible compared to linear power supplies. Using 50 or 60hz transformers adds up to much weight to the power supply.

With that problem, im trying to find an alternative which would make my power supply lighter.

Jamchris:

There is quite a bit to be learned by designing your own power supply, which in your case would best be a ferrite transformer (isolated) coupled flyback topology operating directly from the AC mains. If that is your objective, see if you can find some design software to help you. A good package is "Power 4-5-6" by Ray Ridley. Alternatively you can use an all-in-one power supply controller in a TO-220 case, like items made by Power Integrations. The offer free design software (called "PI Designer" if I'm not mistaken) which also covers transformer design. Beware of one consideration: A larger ferrite core is required if the supply will operate at higher temperatures, perhaps higher than 70C. PI designer does not make provision for this, but Ridley's software does.

But if you are more interested in just getting 24V, there are plenty of off-the-shelf sources for ready-made items. One is at the link below:

http://www.cui.com/pdffiles/DTS240210UT-P5P-SZ.pdf

Output is 24V at 2.1A, more than you need, but it is a stock item. Weight is 491g.

Your personal profile does not state where you are located, but Cui and other manufacturers products are sold worldwide. Here in the states, a well-known broad line distributor is Digi-Key. Their prices are not low, but they carry quite a varied inventory. Their price for the power supply in the link is about $34 US in single quantities.

Hope this helps.

Bernie Katz