Primary Supply Circuit

[PsychoMaster]

Hello All
I have built the output stages of a switching psu but now I need to power it from 24-30volts 16-20 amp primary.I could use two ATX units in series but I do not want all the extra voltages and would like to build from scratch.I can build from single chip low power devices but do not have the know how for beefier builds .I need a circuit I can use or adapt or help with information .
    Thanks

[Whales]

You have a couple of overall design options:  wound transformers or switch-mode. 

Wound transformers are heavier, bulkier and more expensive, but are much simpler and galvanically isolate your power supply (safety!).    Switch-mode supplies are much more complicated but in general are cheaper and lighter for the power they produce.  I presume you are going for the second option.

Designing and making a switch-mode power supply that can deliver ~25V at ~18A (450watts) is not a trivial task and there is no "single-chip" solution for this power level.  You'll be dealing with mains voltages, complex switching circuits with large, external switches and possibly extra layers of transformers and rectifiers depending on your design.  All of these things have complexities you don't realise at first -- and 450 watts is not a tiny amount of power.

For example: an ATX power supply schematic.  You would be able to cut out about a third of this as you don't need the extra voltage rails, but that's about it.  These are designed the way they are to be efficient enough to avoid catching fire (at-least  about 65% for even the worst PSUs), small and simple/cheap as they possibly can.  You could get away with not meeting regulatory specs (eg line filtering, power phase correction) but that only chops away a few components at best. 

If you don't want to do this your best options are to mod an existing PSU, join two as you have suggested or use a large toroidal wound transformer and regulate from there.  Notes for these options:

• Modding an exiting ATX PSU -- If you open up several PSUs that can output your desired wattage on their 12V rail, lookup the part numbers on their main switching controller ICs.  Modding the 12V rail to output 24V or above may be as simple as changing a couple of resistors and making sure other parts (eg capacitors) are rated to go that high.
• Joining ATX PSUs -- Make sure to isolate (atleast) one power supply from ground.  Commonly the DC GND is attached to the case/AC ground, so if you hook two ATX PSUs in series then you are just shorting 12V directly to GND.  To isolate a PSU you need to cut traces/parts that you believe are cuplrit and then test to see if this has worked with a DMM.
• Using a wound transformer -- a 1/2 kW one will weigh quite a bit.  If you are lucky an old piece of equipment will have one you can steal, otherwise they are pretty expensive, but also the simplest and safest option.
  

Overall: don't forget that the primary side of power supplies is dangerous.  Take your time and think things through.

[PsychoMaster]

I am lookiing at a old ATX unit now and I am sort of breaking it down into stages like incoming filter,rectification ,power factors ,inverter, opto coupled feedback regulation. It is quite a mess on these ATX boards ,I'm only learning but some of this stuff looks well dodgy.(less than a millimetre between high voltages) they have left themselves no room for anything with this layout , how would you design a new product? .Going this route is going to take some study (high frequency transformers wow) but using big wire wound transformers doesn't really fit in with a switching front end which is a waste. I'll save using a transformer for a low noise linear design.I am taking most of the electrical s in the house apart and having a look at the power supplies .I have not really found a good source on this on the internet.
  Thanks for replying

[RR]

I would not bother with modding ATX psu you would need to change all caps etc on secondary

probably right choice will be industrial switching psu like this

http://www.ebay.com/itm/Universal-5-12-24-48V-1-2-3-10-15-20-30-50A-Switch-Power-Supply-Driver-LED-Strip-/351105796408?pt=LH_DefaultDomain_0&var=&hash=item51bf894938
http://www.tme.eu/en/katalog/#id_category=112529&page=1&s_field=artykul&s_order=ASC

[Zero999]

• Joining ATX PSUs -- Make sure to isolate (atleast) one power supply from ground.  Commonly the DC GND is attached to the case/AC ground, so if you hook two ATX PSUs in series then you are just shorting 12V directly to GND.  To isolate a PSU you need to cut traces/parts that you believe are cuplrit and then test to see if this has worked with a DMM.

That will work but beware that the isolation between the primary and secondary will not be enough to leave the secondary floating. If it's designed to have the secondary earthed then it only needs to have class 1 insulation between the secondary and primary.

Only isolate one power supply from earth and make sure it's permanently connected to the other isolated supply.

I am lookiing at a old ATX unit now and I am sort of breaking it down into stages like incoming filter,rectification ,power factors ,inverter, opto coupled feedback regulation. It is quite a mess on these ATX boards ,I'm only learning but some of this stuff looks well dodgy.(less than a millimetre between high voltages) they have left themselves no room for anything with this layout , how would you design a new product? .Going this route is going to take some study (high frequency transformers wow) but using big wire wound transformers doesn't really fit in with a switching front end which is a waste. I'll save using a transformer for a low noise linear design.I am taking most of the electrical s in the house apart and having a look at the power supplies .I have not really found a good source on this on the internet.
  Thanks for replying

The minimum clearance between tracks doesn't just depend on the voltage. The gap between two live conductors depends on the class of insulation and the environment i.e. whether it's indoors or outside.

http://frontdoor.biz/HowToPCB/HowToPCB-extra/CreepageandClearance.pdf
http://www.silabs.com/Support%20Documents/TechnicalDocs/AN583.pdf
http://www.smpspowersupply.com/ipc2221pcbclearance.html

[Whales]

The minimum clearance between tracks doesn't just depend on the voltage. The gap between two live conductors depends on the class of insulation and the environment i.e. whether it's indoors or outside.

True, but still be wary.  There have been a lot of power-supply products with lackluster safety design.

[PsychoMaster]

Some on the web suggest cutting the earth wire to the board and leaving the case earthed  but on the unit I am looking at the board ground connects directly to the chassis through two mounting screws also.Also would any one know does the psu units sold on Ebay make more ripple than ATX types?