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Low Leakage Power Supply

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jnnewton

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Hello,
I have a need for a power supply for a medical application. Currently we either buy an off-the-shelf supply (expensive) or put a UL approved transformer before a rectifier and then a smps for an on-the-pcb solution. This setup has worked well and always passes UL inspections. The problem is, transformers are big, heavy and I need 6A (24VDC) for a new application. I would like to know where I can find information on building a supply similar to those from condor, xpower, etc. If anyone has any experience with this and could point me in the right direction, I would be greatful.

A little background, the extent of my power supply usage design is simply getting information from the manufacturers of linear and smps regulators and running through their calculations to select the external components, test and tweak, and it works. Most of my time is spent on networking, motor control and user interface electronics.

My input voltage is always 110/220 VAC and the output is always 24VDC, and for this one I will need 6A, but would like information regarding general designs, so that I can adapt for new applications (5V, 12V, 1A, 10A, etc)

Thanks for the help.
 
Mike, thanks for the reply. I believe you are asking what i normally refer to as constant or peak supply. In this case, the requirement is 6A constant, with the peak being the inrush from a couple relay coils, of which i have measured at about 300-600mA x 3 or 900-1800mA (I can't seem to get a very consistant reading on the inrush, even with a scope, they vary from one coil to another).
 
Actually what I was asking was; is the load constant or changing and with the relay coils coming in and out it's what I would refer to as a dynamic (changing) load.

If it had been a static load you could have used a capacitive reactance type supply, which is perfectly acceptable in medical equipment. I know how expensive medical grade transformers can be.

Have to think on that one...
 
You would have to design a line-operated, high-isolation, switching power supply to allow the use of high-frequency, low-weight magnetics, and those designs are far from trivial. If you need to build your own to minimize costs I would suggest getting an expert consultant for the design.

But that will only pay off if you are looking at a high manufacturing volume to amortize the design cost. For low volume production I would buy from a manufacturer of such units. I imagine there are a number of such manufacturers. A Google search turned up REL-150-1006, IPD REL-150-1006 Integrated Power Designs for example, which you are perhaps already familiar with. At a unit price of $130 and even less in quantity, you'd be hard pressed to build your own for less.
 
I can take 3 transformers 3 rectifiers and 3 smps and make the system work. this will cost $90-100 which is still cheaper than the one you showed, however, the cost of assembling them probably makes yours win out. I now need to weigh the costs, and choose the best of those two. However, this area of electronics is the one I would like to learn about. Do you know of any reference material or example designs that I could begin to look at?
 
I can take 3 transformers 3 rectifiers and 3 smps and make the system work. this will cost $90-100 which is still cheaper than the one you showed, however, the cost of assembling them probably makes yours win out. I now need to weigh the costs, and choose the best of those two. However, this area of electronics is the one I would like to learn about. Do you know of any reference material or example designs that I could begin to look at?
I don't quite follow. Don't the 3 transformers make the unit big and heavy which you wanted to avoid? Or is reducing the weight and size not a large consideration as compared to cost?

Designing smps is quite specialized as I noted. A good place to check is for tutorials from websites such as TI, National Semi, or Linear Technology. Wikipedia has a good basic tutorial at Switched-mode power supply - Wikipedia, the free encyclopedia. A Google search of smps will turn up other resources.
 
You are following perfectly. Both are large considerations. this is the way it is done now. If we can make it smaller / lighter, the enclosure gets smaller and cheaper, hence the desire to make it smaller and lighter while keeping the ability to pass UL inspections of the product. A smps is fairly simple to put together using a reference design. I have used 3 or 4 from national semiconductor. The problem is that I don't know of any reference designs for a medical grade one.

Also, if it is small and light, i can think of other places that it may be very useful that it be smaller and lighter than the current design due to physical (as opposed to monetary) constraints. The nice part about the current setup is that it is a single board solution, the transformers are pcb mount ul606061 units.

On a completely separate system, i use an xp xpower supply (3A) and have a separate control board, which keeps the size / weight down, but it is now two parts, that have to be wired together and this takes time and looks messy.

The ideal solution would be to have a medical grade power supply reference design that i could use as i do the reference designs on data sheets, making a single board solution, with no wiring and no heavy transformers. I would also venture to say that this would be the smallest, lightest and cheapest of my choices.
 
There are two further issues to consider:

1. Do you have any input power factor requirements? For example, the European Union now require that supplies over 75W have power factor correction. This adds complexity to the input circuit.

2. A medical supply would require high isolation in the switching transformer. The primary task in making a SMPS meet medical UL requirements would be to find a source for such isolated UL rated switching transformers. You may have to buy a custom unit or make you own (which is a whole specialty unto itself).
 
The NCP1217 IN A 24 Volt, Off-line Flyback
100W OUTPUT NOMINAL,230W PEAK CAPABILITY.

https://www.onsemi.com/pub_link/Collateral/DN06049-D.PDF

Sounds like what your after?

I've used the NCP12XX series they are reliable,this one (1217)has internal slope comp so you can save a few components.

More related datasheets are here

https://www.onsemi.com/PowerSolutions/search.do?searchString=ncp1217&searchType=others&tabbed=Y&clearFilters=Y

They also have some ready made application examples for spice you can play with.

If your lucky coilcraft may have a suitable off the shelf transformer for you.

But for medical I'm not sure what the requirements are but I'm sure you would Know that.
 
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