48V Off-Line Switcher

[PilotPTk]

Good day everyone,

I've been lurking here for quite some time, but this is my first post. So first off, Hello!

Now, my question.. I'm a hobbiest/tinkerer, and I design a lot of little gadgets (most of them even work!). I have a need, however, for something that is outside of my field of knowledge.. An Off-Line SMPS..

This may or may not be tricky - not sure, but here are the requirements..

85ish - 265ish VAC 50/60Hz Input
48V DC @ 2A Output (100W) Continous

Those are the basics.. Now, the more tricky parts..

Cost is very much an issue. I intend to release my whole design (including the SMPS) for free, but I hate giving things away that end up costing a forture to build.

Size is equally an issue. PCB Footprint is somewhat important, but height even more so. This has to fit in a pretty slim case, so big tall transformers are out of the question.

Has to use off-the-shelf components. When other hobbiests go to build this, they should be able to order all of the parts from a digikey/mouser/etc type of place.

Any help anyone can provide in pointing me in the right direction would be hugely appreciated.

As for the output of the supply.. I will be honest, it doesn't have to be the most stable thing in the whole world.. a little ripple or sag is not going to hurt this application at all. Ripples that would normaly be considered huge and rediculous are acceptable here.. 1-2V is even ok, so hopefully that might ease the cost and size a bit.

Thanks again!
Ben

 

[gootee]

You could start with a dual-primary transformer, with a switch to configure the primaries either in series (for 230VAC nominal in) or parallel (for 115VAC nominal in). Then you can assume that the primary winding's AC voltage might vary from 85VAC to 145VAC. Then select a transformer that will give less than 48V after rectification and smoothing, even for 145VAC in, and design a boost converter.

If you used a transformer with a nominal 24V output, the output would range from about 17.4VAC to about 29.7VAC, which would give roughly 23.2 to 39.2 volts peak after rectification. Subtract half of the ripple voltage's p-p amplitude to get the average VDC.

The ripple voltage's p-p amplitude would be approximately I/2fC, where I = load current, f = AC mains frequency, and C is the value of the smoothing capacitance. With a 2200uF smoothing cap, 60 Hz supply, and 2 A load current, the p-p ripple voltage amplitude would be 7.58 V p-p. The 50 Hz worst case would have 9.1 V p-p ripple amplitude. Half of that would be about 4.55V.

So, with a 24VAC nominal transformer output, the nominal average DC output, after rectification and smoothing, would range from (23.2 - 4.55 =) 18.65 to (39.2 - 4.55 =) 34.65 VDC.

That's probably a bit too low for comfort. And it looks like maybe a 30VAC nominal transformer output might work.

With a 30VAC nominal transformer output: For 85VAC to 145VAC in, output would be about 21.8VAC to about 37.2VAC, giving about 29.4 to 51.2 volts peak after rectification. Subtracting half the ripple amplitude (assuming 2200 uF smoothing cap) gives an average output (i.e. SMPS input) of from 25.61 VDC to 47.4 VDC for 60 Hz mains, or 24.9 VDC to 46.7 VDC for 50 Hz mains.

So, for the SMPS design, you could use Vin = 36V nominal, with Vinmin=25V and Vinmax=48V, and Vout=48V @ 2A. Go to http://www.linear.com and download the LTspice software. After installing and running it, select File --> Switch Selector Guide. Enter the numbers just given. Select the LT1070HV and click OK. You will see the resulting circuit, and a simulation will automatically run until approx steady state, for the nominal input voltage. It looks like it would be 97.7% efficient. 40 kHz output ripple is less than 15 mV p-p. Scrolling down shows the dissipation for each component. There are only eight external components. Nothing dissipates more than about 0.9 Watt, @ nominal. Nice.

The transformer could be Mouser.com # 546-182N15, a Hammond 182N15 120VA toroidal with dual primaries and secondaries. With 117VAC input, the secondaries, if wired in series, are rated at 30VAC @ 4A. That transformer is 1.68 inches tall and 3.98 inches diameter. You can probably find a similar but cheaper transformer, somewhere.

In place of the 85uH inductor (which has an average current of about 2.7 Amps), you can substitute a Bourns/J.W. Miller 2100HT series 100uH 4.6A toroidal choke (DCR=0.053 Ohms). Those are about 0.86 inch diameter and 0.45 inch thick, and come in both horizontal and vertical mount versions. Mouser part numbers are 542-2100HT-101H-RC (Horiz) and 542-2100HT-101V-RC (Vert). Digikey.com also carries them. Pricing at mouser.com is about $2.44 each qty 1 or $1.69 each qty 10.

If you end up with too much switching noise at the output, you could always add another 1000 uF in parallel with the original 1000 uF output cap, and put one of 2100HT-series toroids between them, to form a lowpass Pi filter.

It looks like the SMPS 120 Hz (or 100 Hz) output ripple might be about 1.7 V p-p when the input is from a 30VAC 60 Hz transfomer output. So you might need to increase the value of the smoothing cap, or try 2x 2200uF.

OK: it looks like adding another 100uH choke and then 1000 uF to gnd, to form a Pi filter at the SMPS output, keeps the output's AC mains ripple amplitude at less than 0.9 V p-p (for 30VAC@60Hz input, at least).

Note that I have not included any transformer regulation effects in any of the above calculations.

- Tom Gootee

**broken link removed**

 

[mneary]

You're going to have a really hard time beating $19.95 at http://www.mpja.com/prodinfo.asp?number=13652+PS .

 

[felis]

Select the LT1070HV and click OK.

I'm just curious, why do you want to specify HV variety? Max.Vout for boost is switch breakdown voltage which is 65 Volts for non-HV, far enough from 48V. Non-HV is also ~3 bucks cheaper.

 

[PilotPTk]

Thank you for all of your suggestions. I'll be comparing the reccomendations, and may even end up going with the reccomended off the shelf PS. It's a bit bigger (physically) than I had hoped, but for the price, I'm not really sure it can be beat.

Again, Thank you. All of the suggestions were great!

Ben

 

[gootee]

I'm just curious, why do you want to specify HV variety? Max.Vout for boost is switch breakdown voltage which is 65 Volts for non-HV, far enough from 48V. Non-HV is also ~3 bucks cheaper.

The only reason I suggested the HV version was because the non-HV version was not offered as a choice, in LTspice's "Switch Selector Guide" (My LTspice version is old, though.), and I didn't do any other research.

The OP should probably also check to see what the Webench PSU designer comes up with, at national.com .

 

[gootee]

Thank you for all of your suggestions. I'll be comparing the reccomendations, and may even end up going with the reccomended off the shelf PS. It's a bit bigger (physically) than I had hoped, but for the price, I'm not really sure it can be beat.

Again, Thank you. All of the suggestions were great!

Ben

Hi Ben,

"Just in case:" Rather than the 2100HT-series inductor that I mentioned, it would probably be a good idea to use a higher-current version, since, in the simulation, I was able to get the inductor to have an average current of about 5.7 Amps, by including a second 2200uF smoothing cap, and the Pi filter for the output, when a 22VAC input was used. The 2300-series 100uH inductor is rated at 7 Amps (i.e. to cause a 30degC temp rise). Part numbers would be 2312-H-RC (horiz mount) or 2312-V-RC (vert mount). Mouser price is $2.78 ea qty 1 or $2.08 ea qty 10.

- Tom Gootee

**broken link removed**

 

[felis]

You may also want to look at 1170 part. It's 100KHz variant of (40KHz)1070.