smps design

He's been growing encrustations from his bottom apparently. I wouldn't advise meeting him at all !

Graham

I would suspect so. I think you have it backwards; the maximum inductive current is equal to saturation current, which depends on turns and core material. The current is, of course, recycled each cycle, being purely inductive, so you don't lose anything*.

You can, of course, reduce magnetizing current as much as you want, by using a hugely oversized transformer, so that resistance is low and inductance is high.

In flyback or buck (half wave I suppose) designs, the peak transformer amp-turns must, of course, be less than saturation. There is no minimum limit, though.

Tim

(snip)

No direct connection. The magnetizing current is an artifact of having a non infinite permeability in the core.

If you don't drive the primary through a capacitor or otherwise force the average current to zero, you may have to gap the core and that lowers its effective permeability more, raising the magnetizing current.

If you can afford a larger core, you can raise the winding inductance and lower the magnetizing current.

So the magnetizing current is one factor among many that you compromise when designing a transformer.

John Popelish skrev:

Ok, so the best thing would be to have a very high inductance primary for extremely low magnetizing current to lower dissipation in switches. This will appearently have no effect upon maximum output power level? May I ask you another thing: In buck derived converters, the primary current is a result of secondary loading + the magnetizing current. To keep secondary voltages constant you need a certain duty cycle, but that has nothing to do with the primary current?? If this doesn't make sense, it's probably because of my bad explanation. I try this one also: Imagine a full-bridge design outputting 100VDC on secondary. Load is very low. This results in a 50% duty-cycle. Primary current is low. Suddenly load increases sharply. Duty-cycle goes up to 75% to compensate for drops. Primary current is high. What I want to ask is there are no direct relationship between primary current and output voltage regulation, so how would a current mode full bridge work?? Best regards, Stefan

That is the general idea. In fact, if you lower the magnetizing current by using a larger core, the power output capability will go up while the magnetizing current goes down. The magnetizing current is essentially a process going on in parallel to energy transfer between windings. It is just necessary to produce volt seconds across each turn (even at zero power transfer).

That's right. In a buck derived converter, the load current determines the primary current except for magnetizing current) and the primary voltage and the duty cycle determine the average voltage per turn.

By this, I assume that you mean that the primary switches are supplying voltage to the winding only half of each half cycle, and zero the other half.

That is a lot of drops, but okay. Usually, the drops need a much smaller correction, assuming the output filter has continuous current. Most of the duty cycle variation is usually reserved for changes in primary voltage.

There has to be a monotonic relation between primary current and output voltage or current mode control will not work. But they don't have to be proportional under all conditions, with a fixed proportionality factor. But at any output load current, increasing the primary current must increase the output voltage.

You promised me a good face sitting session with Agnetha.

I don't know what went wrong but perhaps you might write back to her and explain that it involved her buttocks and my face rather than the other way around. Obviously I shall buy some more Haddock and Chips with an extra portion of scraps along with mushy peas and curry sauce. I have learnt how to do gravy as well, ask her what flavour she wants from the Bisto range, I might be able to stretch to a bit of best if she wants to be picky.

Thanks

DNA

Genome skrev:

Ok...things seems to have collapsed for you... I remember you as a competent smps designer but obviously you've gone to other business. Good luck with Agnetha!

--- Stefan

Sigh, I suppose you are right.

It's a trade off between core losses and winding losses. The number of turns on your primary, and hence secondary, are set by the peak (or peak to peak) flux excursion you can accept.

N = VIN.TON/Bp.Ae

The greater the flux excursion the fewer turns you need so the lower the winding losses. Unfortunately the flux excursion causes core losses and the greater the flux excursion the greater the core losses. Of course operating frequency also affects core and winding losses.

With fewer turns the primary magnetising inductance is smaller so the magnetising current is larger so that implies a higher magnetising current is associated with a higher throughput power. That's really a secondary effect though. The higher magnetising current also results in greater switch losses but the relative levels mean it's not so important.

You can work out core losses by referring to the graphs given by manufacturers. Winding losses are a little bit harder but here's a program that works out what the AC impedance of your wires might be......

Not gauranteed to give the right answer......

So, yes, higher magnetising current implies greater power throughput. However it's all a trade off with other things.

DNA

Genome skrev:

Hi! That's the 'DNA' I know of... Nice to hear from you again, I'm still reading your docs about slope matching and error amp compensation, subharmonic oscillation and how big waste the voltage mode topology is. Thanks for your reply. It indicates that I'm not that far out in space....

Regards, Stefan