Has anyone got a low power 5v PIC 240v Switching PSU Design?

[nurquhar]

Has anyone got a low power 5v PIC 240v Switching PSU Design?

Up until now when ever I have needed a small low power (less than 5w) PIC psu I have always gone down the linear route. ie put a small encapsulated 6v Tx, bridge rect, smoothing cap and a 7805 on the PCB. Although the Tx is bit of a lump the design is robust and takes a modest amount of PCB space.

Thinking I may be missing a trick I looked at some switching PSU designs from ST, Power Integrations and National. They all have reference designs for such small PSU's and some have software tools that will design a circuit for you. Great you might think !

However
When you look down the designs parts list to see if you can get all the bits from your favourite supplier such as Farnell or RS everything is ok until you get to T1. At this point you find the part is either not listed or your supposed to wind your own on a std bobbin. Searching the internet may reveal manufactures/stockists of these parts but if you only want 5 parts they are probably not interested or want you to pay a fortune to cover min order charges and shipping, especially if you’re in the UK.

Has anyone cracked this conundrum and can point to a simple generic switching PSU design for a PIC using commonly available transformers? I am thinking of something capable of supplying 50-250mA @ 5v from a 240vac supply.

[adrian]

I have stripped out the internals of a phone charger power supply for a one-off product of mine - obviously not an option if you want a mass production production. Just an option?

[SherpaDoug]

Check out the National Semiconductor "Simple Switcher" line. They use somewhat standardized transformers that are available in small quantities.

Better yet, learn how to wind transformers. As I work in sonar I had to bite this bullet early. Get a couple of spools of wire, a few cores, build some wooden tooling, and you have a very flexible skill. A worthy investment.
_________________
The search for better is endless. Instead simply find very good and get the job done.

[crystallattice]

SherpaDoug do you have the formulas needed for transformer winding calculations??

[treitmey]

My new most fav chip is the MC34063. under $1/one OR $0.17/2500
http://www.onsemi.com/pub_link/Collateral/MC34063A-D.PDF
Most every car cell phone etc charger uses them.

handy web site config
http://www.nomad.ee/micros/mc34063a/index.shtml

I'd use 240AC thru transformer to less then 40V output.
Full rectify.
Then feed that( less than 40V DC) into mc34063 switcher for 5V DC out.

[SherpaDoug]

[Guest]

TI (Unitrode) has for 15 years or more had a yearly seminar on Power Supply Design. They have many good working examples in the handbooks that they have published along with more general design papers. This not only covers their chips, but much information on magnetics design.

Start at ti . com and search down the power applications route.

The books are all up there and are free. 100's of hours of very worthwhile reading.

HTH - Steve H.

[crystal_lattice]

[nurquhar]

I have just been looking at the offerings from Power Int. ie the LinkSwitch-XT http://www.powerint.com/en/products/linkswitch-family/linkswitch-xt.

It seems their reference 2-3w/6v design uses an EE16 core transformer. I think I can get those. If I have to wind my own on a blank bobbin are there any guides as to how to do this. ie is at simple as winding n primary and n secondary coils by hand. Do the coils have to be separated by some insulator, fixed by lacquer or any other such considerations that a novice Tx winder would not be aware of ?