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A "simple" 5/12VDC regulated switching power supply.

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oeginc

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Well, I woke up about a week ago and had a need for a +5VDC / +12VDC regulated switching power supply. I set out on what I thought was going to be a relatively simple journey (queried Google for a bunch of different terms) only to come up short.

Exactly what I am looking for is this:

* Input voltage will be a battery pack around 14-24 VDC with 125 AMP continuous discharge capacity (250A burst).

* I need 2 outputs: +5 VDC & +12 VDC.

* I would like them to be regulated (+/- 0.5 volts or so).

* Ideally, I'd like to have a +6.0 & +7.2 VDC output as well if that's not terribly complicated to add.

* Loads will vary significantly over time (driving large servos with high current draw).

* I expect maximum current draw to be on the order of around 120 AMPS max (each servo can draw ~5.21 AMPS at stall, we typically run around 24 servos, although it's unlikely that they ALL will hit stall simultaneously)

* I need the circuit to be very efficient, drawing these kinds of loads I need the batteries to last as long as possible.

Any help would be greatly appreciated, even if you could just point me in the right direction.

Thanks!
 
there is actually some computer power supply's out there that run off 12 volts and have 5 and 12 volt output
 
you should look for info on that robot snake recently built and floating around on the intertubes, it won a bunch of awards at some competition

They were using individual 16amp 5v buck converters at each servo, and had additional power supply circuitry to run the computers. I think their power supply was a massive series string of NiMH or Lion batteries.
 
If your servo amplifiers can be designed to use 14-24 volts input, then you don't need a 12V supply.
 
things said:
there is actually some computer power supply's out there that run off 12 volts and have 5 and 12 volt output

Thanks, but I've needed this circuit enough that I don't want to have to be looking for more parts every time I need it again and space is an issue. The idea is to come up with a power supply that handles all of my needs (5, 6, 7 and 12VDC at high amperage) and them up on small circuit boards ready to insert into whatever project I am working on at the moment.

mneary said:
If your servo amplifiers can be designed to use 14-24 volts input, then you don't need a 12V supply.

The servos run at 6.0VDC - 7.2VDC. The +5VDC and +12VDC will be used to power the CPU and related circuitry, the 6.0-7.2 will drive the servos. I may be able to get by without the 12.0VDC (just have the 5/6/7.2), but I need something clean, regulated, and efficient without a doubt. The 12.0 is there for driving larger motors and such.
 
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Thanks for revealing more, and correcting some of your requirements. The servos must be the self-contained high power hobby servos?

So you need 6.0 and/or 7.2 at 120 amps? each?
Do the 6.0 and 7.2 need to be separate, or is it 6.0<V<7.2?
Apparently 5.0 is low current (<2A?) or do you need 120A at 5V also?
12.0 is possibly very high current, but since it's undefined, can you make motor drivers that tolerate the full range of your supply? (14-24).

Why would you even think of operating a servo (or 24 of them) at stall? How long before the smoke starts to pour out?
 
mneary said:
Thanks for revealing more, and correcting some of your requirements. The servos must be the self-contained high power hobby servos?

So you need 6.0 and/or 7.2 at 120 amps? each?
Do the 6.0 and 7.2 need to be separate, or is it 6.0<V<7.2?
Apparently 5.0 is low current (<2A?) or do you need 120A at 5V also?
12.0 is possibly very high current, but since it's undefined, can you make motor drivers that tolerate the full range of your supply? (14-24).

Why would you even think of operating a servo (or 24 of them) at stall? How long before the smoke starts to pour out?

Yes, the 5.0VDC is relatively low current (PIC's, etc), 2 Amps would probably be enough. I will only ever have a total of 24-32 servo's powered off of one board at a time, regardless of whether they are connected to 6.0VDC or 7.2VDC, or a little bit of both - so the total amperage draw from the servo's won't exceed 120A. For the 12.0VDC motors I'm really not concerned with that at the moment, and after thinking about it - those really don't need to be regulated anyways. Originally I wanted a universal power supply, something that I could power PC motherboards all the way down to small electronic circuits, I see now that even if that power supply WAS feasible, it would probably be much too large and cost too much to create one per project.

As far as operating the servo's near stall, as I said - they most likely would never ALL be there simultaneously, and not for long periods of time. Here are some results of tests that have been performed with these hobby servos:

Results near stall:
4.8 Volts - 260 oz/in Torque, 1.3 Amp Draw
6.0 Volts - 336 oz/in Torque, 1.8 Amp Draw
7.2 Volts - 405 oz/in Torque, 2.0 Amp Draw (Getting warm)
8.4 Volts - 495 oz/in Torque, 2.4 Amp Draw (Overheating)
9.6 Volts - 510+ oz/in Torque, 2.7+ Amp Draw (Overheating big time)
12 Volts - 540+ oz/in Torque, 2.8+ Amp Draw (She's toast)

The reason I may place some on the 6.0 VDC, and others on the 7.2VDC is because certain servo's tolerate the 7.2 Volts, and others don't. My guess is that a realistic number of AMPS that will be drawn is somewhere in the 30-40 AMP range. The 5.0VDC should be isolated from the 6.0/7.2VDC so as the servo's turn on/off it doesn't interfere with any micro's running..
 
I've never had very good luck with switching power supplies subject to load transients. They have a tendency to go to their knees when a step command comes in and the motor wants maximum current for maximum torque and maximum acceleration. Some switching supplies don't even bother to stop when they come to their knees they continue on to a supine position. Looking at spec sheets is not very helpful since this behavior is hard to characterize and specify. Most switching PS manufacturers stay away from this area like the plague. Some things that seem to help are stiff input supplies, sufficient energy storage in the magnetics, and fast switching frequencies. Optimizing all of these things at the same time can be incredibly challenging.

I hope you can find what you want, but I'm not very optimistic.
 
I like the suggestion made by Glyph. Maybe not that particular chip, but the idea is to drive your servos in groups from lower power supplies, maybe in 10A groups. I wouldn't suggest trying to make one big supply, considering your application.

I would consider the TI TPS40056, which drives both high side and low side MOSFETs. The advantages here are a smaller inductor and better transient response. For motor loads I would use much bigger output caps. (It seems like you need a lot of components, but remember you're dealing with quite a bit of power.) You can adjust each of them for 6.0 or 7.2V or whatever you need.

Another advantage of local supplies is power distribution. You can distribute 14-24 volts with smaller wire without worrying about voltage drop. (Current is half or less than it would be at 6V.)
 
Very true... Alot of good ideas, I'm going to go back to the drawing board and rethink the whole idea and see what I come up with.

Thanks a bunch for your help! I wasn't aware of a switching PS chip, that's pretty cool. What will they think of next, huh?
 
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