Small transformers and LTSpice simulation

Watchdogs don't do all that much in a switcher. By the time that comes on a few clock cycles might have gone by. It can be a matter of microseconds for a switcher to go from fully ok to kablouie. On a current-controlled switcher things have to react within a fraction of one cycle. Once the inductor goes too deep into core saturation all hell breaks loose. Fireworks and all.

The UVLO is a smart thing you added there. Switchers should never run without, else it'll kill the FET some day. All it takes is a slow brown-out.

On a sunny day (Sun, 24 Apr 2011 08:57:59 +1000) it happened Grant wrote in :

Now that is nice :-)

but

Sorry, I never made uC controlled switchers before, just products that had to run 24/7. Switching supplies I worked on decades ago were mains commutated SCR monsters (up to 500A) charging industrial batteries -- so one had a filter inductor about half the size of the mains transformer with an air gap adjusted for no saturation pip at full current, high input and lowest output. Battery model? Short circuit with a voltage offset!

I agree, one must switch off on current or not allow saturation by inductor and cap selection -- what I did with the simulator was dangerous in the sense I built what appears to be a safe power circuit without a current sense.

So I rewired a 100mR into the prototype so I could at least loot at the current in operation -- but this particular whatsit is only a couple amps and polyswitch or fuse protected.

Yeah, not supposed to happen, I had a visitor from interstate yesterday who told of a brown-out lasting two hours! I was surprised, more than a few minutes here and the power comes back or goes out completely.

Grant.

Maybe he is on one of those long SWER lines. Australian line power is the ultimate test for any power supply. On the last one I re-designed for your neck of the woods I made sure that nothing would ever saturate up to 275V/50Hz. In the testing I gave it bursts of 300V for 15min. Then I tested for the "eternal brown-out" where the voltage would hover around where things draw max current and where the UVLO just hasn't come on yet.

The other test would be islands with generators, where they sometimes reach in and goose a recalcitrant engine. Vroom ... vrooooom ... *POOF*

I haven't seen it, apart from the 'that's not a knife' clip that was shown all over the place.

No it's crap during the on time. . . I found it, it's actually the ringing on the secondary while a diode there is reverse biased during the MOSFET on time (flyback). I was simplifying the circuit to have something to post, killed it, started over and found the ringing.

I guess with perfect coupling the ringing was absorbed by the primary's snubber, and that's why it only showed when I set coupling less than unity.

Good way to learn.

I changed snubber to diode feeding an RC to +ve rail to recover some of that energy, much better, and I have just as much control over the peak MOSFET drain voltage as a straight snubber gave. This is where the sim 'what if' is helpful for me.

Flyback converter. I was trying to ignore checking out all those LT parts, I'll look at the LT3757, see what it's like.

I bought a heap of the 3063s for running LEDs, I don't have to use them everywhere. I bought some PWM chips too, but on price, not for ease of use with LTSpice -- this was a fair while back, before I started using LTSpice.

Grant.

On a sunny day (Mon, 25 Apr 2011 11:56:50 +1000) it happened Grant wrote in :

You will find once you solder things together that there are all sorts of ringing from cable inductance too. In some I did, even moving a wire one way or the other way can cause huge oscillations to appear or disappear. That is why I only believe in testing the real circuit. But sure, spice will show you the basic circuit with perfect wiring.

ringing

oscillations to appear or disappear.

Today it showed me enough to learn that the particular approach I was looking at is not worth building :) I've already built part of a more complex thing that I'll continue with, that's a half bridge into the load, more parts, but more likely to work as expected, plus there's a PIC chip in there to help do some power accounting for the up-market version.

But I had to see how good the cheapie idea was for the job. Rather be able to point out its shortcomings if someone asks why didn't I use that particular topology in the future.

Grant.

te:

n.

n.

Crocodile Dundee was always comic fantasy - part of the humour in the original film was the under-clad fair-skinned New York bint trotting around in the tropical sun and not getting sun-burnt. In real life she'd have spent most of the film bright red and blistered.

There were lots of other - more obvious - jokes, and I greatly enjoyed the film, though I did peeve my wife a bit by pointing out that quite a few of the jokes had been piched from some of Dal Stivens more extravagant short stories

-- Bill Sloman, Nijmegen

I sat through it twice just to see the bint.

John

Hi, Grant -

Take a look at the LTSpice non-linear transformer in the educational folder. Probably not what you're looking for, but it may be helpful in some way.

John