Re: Help improving eff of a 24v -> 350v flyback converter



>ye gods, DIY ferrite grinding - you are keen.

Naa, It was actually very easy. Much easier than winding the bobbin.


>1) input filter:

It's gone now, Only 2-220uf caps in parallel on the input,
but it made no difference in the output noise or efficiency.

>an input filter can make a smps oscillate. a smps is a constant power
>device - as Vin increases, Iin DECREASES. so it looks like a negative
>resistance, |R| = Pin/Vin^2.
>
>bung a negative resistor acros the C of an LC LPF, and voila - oscillator!
>
>
>you could read middlebrooks seminal paper, or just do this:
>- ensure the impedance looking up the filter (2200uF C5 then 100uH L2,
>with C13 pretty much shorted by the "ideal" power supply) is less than
>|R|. for a low power supply this is usually automatic, as |R| is very
>high. for higher power supplies, check. spice is your friend here. get a
>decent model of L (include R and use its SRF to caluclate a shunt C),
>and also the cap (inc. ESR). pop these in parallel, excite with 1A
>current source frequency sweep. plot voltage, its actually impedance.
>
>you will pretty soon see that filter resonance can be a real pain.
>100uH, 2200uF has Fo = 340Hz, Zo = 0.213R so the cap ESR probably damps
>the filter (its a HUGE cap), but maybe not - ESR could be as low as
>0.02R. in which case use a pair of 2200uF's in parallel, with an added
>0.213R resistor, as an R-C damper to be applied across C5. or pick a C5
>with enough ESR to do the job. Actually, Cdamp = 3Cout, Rdamp = Zo is
>best - see unitrode app note on snubber design, or just take my proof by
>blatant assertion :)
>
>the general ROT is to use lots of C and bugger all L, that helps
>decrease Zo (making the filter more likely to damp itself with ESR), and
>greatly reduces the filter impedance seen by the smps.
>
>
>2) output filter:

I shorted across the output filter inductor. All that did was double
the noise at the switching frequency. Hard to tell what it did to the
low frequency noise since it got buried in the other noise.

>again, beware resonances. in this case Zo = 10.8R, Fo = 491Hz. betcha
>that rings like an SOB :(
>
>3) R1, C13 (the other C13)

Ok, ok, I fixed the part numbering. :)

> - thats an LPF, Fo = 946Hz. congratulations,
>you just added a 2nd pole to your single-pole smps transfer function,
>thereby making it possible to build an oscillator.
>
>Oh, just looked at the LM3578 data***. error amp? I dont see no
>stinking error amp.... C11 couples a ramp into the comparator input,
>which your voltage feedback signal gets compared against. yuck. but the
>comments re. R1,C13 still apply.
>
>A UC3842, a UC3842, my kingdom for a UC3842 :)

WOW! You have your very own kingdom!
>
>
>measure the LF ringing - if its about 300Hz, then its an input filter
>oscillation - short L2 and it will go away. if its about 500Hz then its
>just the output LC filter oscillating all by itself - short L1 and it
>will go away. If its about 1kHz, its a closed-loop oscillation caused by
>C13 (the other one). remove the 330p and again, it will go away.

It looks like it's at about 1KHz.

>I see what you are attmpting with C13, just use a smaller cap (and/or
>split R1) to keep the frequency well above your closed-loop frequency.
>33pF would be a good start, 10pF even better.

I swapped the 330pf with a 10pf and again no difference.

I have had some success. I used the pot core with it's new found gap
and wound half the 130T sec with 7/37 Litz wire(maybe 7/36) then wound
the 10T pri with #22 solid magnet wire, then finished off the sec.
With no other changes from the original circuit the eff jumped to
76%(from 68%) and the low freq output noise dropped to around 20mV and
is much closer to sine wave. I donno, but this may be because I
grounded the transformer core. The noise at the switching frequency
dropped to around 60mv. The core temperature increased by 18deg C
after an hour of operation.
At this point I tried the things above, but no further improvments.
All testing was done at 200v out and a 2.2k load, or about half the
power I wanted to get from it.
I then adjusted the output voltage up to 220v with the same 2.2k load.
The input current was 1.18A, but after just a couple minutes the
current had increased to 1.32A and the longer it ran the faster the
current increased. The core temperature seemed to follow the input
current. Core starting to saturate?

Mike

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