SMPS Transformer Equations

[jackinnj]

It's a little bit circular, but I put the push pull transformer equations (along with capacitor value calculator) for push-pull topology at www.tech-diy.com/smps.htm
These are borrowed from Linear Tech's pdf on the LT1683 gate driver. Click on the link to the Excel Spreadsheet within the web page.

I welcome comments and suggestions -- this is going to be a work in progress.

I am trying to eliminate some of the mystery of switching magnetics for DIY'rs

[ashok]

This is a great idea.
Thanks.

[BeanZ]

Designing the magnetics is one thing, and designing the circuit to control them is another. Make sure that push-pull magnetics do not suffer from flux imbalance. The core must be subjected to equal and opposite Volt-second products. If not, the flux will "walk" up the hysteresis loop and cause a circuit failure. Current-mode control should also be investigated with any push-pull topology. I did not see these very, very important points addressed with the link.

BeanZ

[jackinnj]

Beanz, I agree. I wrote the page from the equations given by Linear Tech. If you have looked at the other SMPS pages you will note that I don't particularly like the SG3524 chip which is voltage mode only. (I have about 100 of these which I am going to unload on EBay if anyone cares.)

Of course, current mode isn't without it's warts, not the least of which is the easy way in which they can go into oscillation.

[JBL]

This site might help:


http://henry.fbe.fh-darmstadt.de/smps_e/smps_e.asp

[jackinnj]

N is the ratio of primary to secondary turns, I just had it as "turns", ESR should have been in milli-ohms.

Thanks to Jan in Finland for the corrections.

He also points out a couple links which might be helpful:
http://www.hut.fi/~jwagner/tesla/SST...recreading.htm

[BeanZ]

Current-mode is a necessity in switching topologies which use the upper and lower portions of the B-H curve. These topologies include half-bridge, full-bridge, push-pull, and most forward converter topologies. As far as oscillations are concerned, they only rise when the closed loop is unstable or conditionally stable. Engineers (like me) get paid to design these control loops to be unconditionally stable. If you have any questions about switching supplies or power electronics...that's what I do....feel free to ask.

BeanZ

[jackinnj]

as Pressman explains it, the reason that slope compensation is necessary is that the loop becomes unstable even with small changes in current.

my concern remains noise, although the little bit of the work that I have done -- essentially measuring the THD of opamps with various linear and switching supplies -- leads me to conclude that there is a place for switchers in high quality audio applications.