Convert AC to DC

Asking a question multiple times rarely results in the answer you want, if it doesn't come forth on the first query.

Your thinking standard linear power supply with large steel core 50 or 60Hz transformer, what you want is a smaller switch mode power supply design using a smaller ferrite core transformer operating at a much higher frequency.

High input voltage switching regulators with small external ferrite core transformers are available, potentially even with 24V 3A outputs (Linear Technology, Onsemi, TI, Maxim Integrated, etc).

What exactly are you trying to power?

Would alternative transformer-less solutions such as a simple Zener diode rectifier (very load dependent however) be suitable?

What is the application?

I am having a linear power supply with large steel core transformer like you mentioned, I want to use smaller transformer on board with PCB to reduce the cost. I thought of the switching power supply too but then the increase in the amount of components is probably not worth pursuing due to cost.

Stumble across the flyback converter, it seems to require less components. You reckon that would do the job?

PS: I would prefer to have a transformer to have electrical isolation. By high frequency you mean the switching frequency yeh? not the frequency of the AC signal I assume (remain at 50Hz as usual)

You can build it lighter:

Net - rectifier - capacitor - HF chopper - HF transformer - rectifier - capacitor.

For a straight forward supply without regulation. With e.g. 15 kHz chopper, you might be able to reduce the weight by 25 if you use a ferrite core transformer.

However, I don't know how this higher freq. might interfere with your application.

I thought of the switching power supply too but then the increase in the amount of components is probably not worth pursuing due to cost.

Component quantity and complexity (generally but not always) increases when comparing a SMPS to a linear power supply but this is offset against reduced overall costs, weight, size and efficiency (50Hz transformers are big heavy, expensive and lossy). Linear power supplys will likely be cheaper for small production runs however, so it depends on what you want to do.

Stumble across the flyback converter, it seems to require less components. You reckon that would do the job?

It should do, although your power levels may be a bit high for modern standard low power SMPS regulator designs which include the bulk of the electronics INSIDE the chip itself GREATLY decreasing the number of additional components required and hence complexity and cost. Just have a look at the IC manufacturer websites mentioned in the above post, you may find something that is both suitable and simple.

If it is all too complex, just mount the transformer separately from the circuit board (a common industry practice).

By high frequency you mean the switching frequency yeh?

Yes.

That is exactly what we did with our latest PCB design! It even comes with UL,CE, CSA, etc stickers!

We used a Lambda supply, but there are lots out there.

A switch mode power supply is what you should just purchase. You are choosing for unknown reasons an atypical output voltage. 24V is much more common. So your choice of suitable suppliers will be limited.

Your numbers also do not add up. 100VA/(3A)=33.3V not 28. Unless you expect the power supply to inefficiently dissipate 16 watts. I doubt that your circuit board can safely handle 16 watts of heat. What cooling do you expect to use, convection in a sealed box? I also dislike having main power voltages on the same circuit board that has the custom electronics. 240VAC leakage current can easily interfere with so many things.

I would rethink your design requirements and see if you can do this in a more "standard" method. If really you must use this voltage and current then buy a COTS power supply that meets your local codes and be done with it. You can choose either an open frame or enclosed product.

28 VDC is a common voltage on military aircraft. 240 VAC is not and so the OP's application may possibly have something to do with AGE (Avionics Ground Equipment) or the OP is looking to power military surplus gear?

100 VA, 3 A may be a maximum or absolute maximum spec for a power supply which in practise draws significantly less.

Search on GlobalSpec for suitable power supplies. Switchmode power supplies will be much smaller for a given power rating. Does it have to be Mil-Spec (28 VDC is commonly found in military hardware, especially aircraft, hence the question)? Given the rating you may need to heat-sink the supply. You will need to carefully consider the PCB's placement in the enclosure, cooling and other thermal-management issues. As the supply is on the circuit board, connection to the mains brings its own issues. Surge protection, leakage. Will the board be conformal coated? Potted? Will it be subject to moisture? Salt spray? Dirt? Grime? All of these contribute to leakage and other problems. What is the source of the input power? A GPU? Mains? How much variance in frequency & voltage must it tolerate and still remain in-spec?

Least expensive way to go would be to purchase a 28v switching power supply. Their quality control is much better than an amateur trying to design and build one. I say that from personal experience. It can be modified to reduce the dimensions. It will also cost you less than the components necessary to build your own. Getting one to fit on the main PVB may be tough but most likely can be done with some effort.

I usually order these type items from MPJA, Lake Worth, FL (NO COMERCIAL ENDORSEMENT OR BIAS). They have a 28vdc @ 5a unit for less than $30.

Not much else you can do for a reduced size without deep pockets.

Good Luck, Old Salt

Buy one of these and copy it:-

http://uk.farnell.com/xp-power/ecs100us28/psu-open-pcb-100w-28v/dp/1738300

But it will cost you a lot more than buying them (~$70) and mounting them on your PCB. And, you won't get it right.