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

you need to think about the effect of rectifier drop to answer that question yourself. for output voltages >> Vdiode, it should be fairly easy to achieve much, much better efficiency than 68%, but for low output voltages its hard without sync. rectifiers.

yep.

that sounds like closed-loop oscillations. do you have voltage feedback before or after the LC filter? the LC adds a pair of poles to the transfer function, making the converter much more prone to oscillation. Look at the output of your error amplifier, if it has said LF oscillation on it, congratulations - you just built a practical example of satisfying nyquist's criterion.

say 70uH.

Ae = 0.25*pi*d^2 = 126mm^2

lg = 2.8mm (a big gap)

L = 26^2*Uo*Ae/lg = 40uH so that sounds about right.

your 390T is likely the problem, due to skin and proximity effect. skin depth in Cu at 75kHz is about 0.24mm.

Assuming 6 layers, what you are seeing is an AC resistance much higher than you expect, due to proximity effect. convert the round wire to equivalent square wire gives a wire height of about 0.253mm, so phi =

for 6 layers, this would give you an Fr ( = Rac/Rdc) of about 5, so the AC resistance is about 5x the DC resistance.

for 10 layers, Fr = 12 (ish).

if you split the secondary into 2 halves, and bung the primary in between them, you halve the effective number of layers (there is a line of symmetry running thru the centre of the primary).

at 3 effective layers (6/2), Fr drops to 2, so 60% reduction in Rac and hence Pcu; 5 eff. layers gives Fr = 3.5, about 3.5x less Rac and hence Pcu.

the next step would be to use even thinner wire. if you tell me the no. of layers, I'll tell you what wire to use.

alas, blah-blah-crap.

thats the important part.

HELL YES.

its not DC resistance you are having problems with. Try getting your xfmr, and bunging an equivalent amount of DC current thru it (eg DC psu), watch it get nowhere near as hot. that'll prove its an AC phenomenon.

less turns is usually better, unless you have a real core loss problem, which will get worse :(

can u email it to me? I cant see abse.

jolly good, I thought.

you may also be having problems with copper loss caused by fringing flux from the air gap hitting your winding - much like prox. effect. the easiest thing to do is re-design the xfmr to minimise prox. effect, then see how well it works.

do you need such a large gap? measure the saturation current of the primary, if its much greater than the peak current you run at, reduce the gap (Lmag increases, so you can then reduce the turns).

I re-designed a xfmr last year that had Isat = 3.5A in an 0.5A peak-current-mode-controlled flyback. I shrunk the gap a lot (Isat =

Cheers Terry

.....Clip a ton of great stuff! Terry, I can keep you busy if you ever come to the US. Regards, Harry

I'll be trying to digest all that for awhile. It sounds like I really don't have very good core for this application. The gap is too large requiring too many turns in the secondary and some major fringing effects. I have a fairly large pot core, but it didn't have a gap at all. The pot core is 1.4" OD and the center posts are 0.62" dia. The bobbin has 0.5" height for a layer of wire and can accomadate a winding depth of about 0.2". The only markings on the core are FX2242 and 8328K I managed to grind one of the center posts using a dremel mounted on a drill press stand. It's hard to measure, but it looks like the gap is about 0.01". Before grinding it I wound 5T of #22 on it and measured

Mike

ye gods, DIY ferrite grinding - you are keen.

I once tried to gap a toroid, and destroyed a wide variety of cutting implements. surprised the hell out of the guy from trade tools when I brought the diamond- and tungsten-carbide-tipped hacksaw blades back with all the teeth gone.

looking at the schematic, there are a couple of other points:

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.

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.

again, beware resonances. in this case Zo = 10.8R, Fo = 491Hz. betcha that rings like an SOB :(

Oh, just looked at the LM3578 datasheet. 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 :)

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.

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.

Cheers Terry

I used to just put in a few layers of high-grade paper until I got what I wanted.

...Jim Thompson

yep. hard to do with a toroid though ;)

I have a small planar flyback sitting on soak test at the moment, that is gapped with a piece cut from an anti-static bag - which just happens to be 75um thick, and I needed a 150um gap.... it hasnt melted, which is a good thing.

I must say I was quite surprised when a single swipe with a diamond hacksaw blade ripped the teeth off, and barely scratched the ferrite.

That sent me in search of people who gap ferrites, and I found a company in Christchurch (By-Ray products, products done by Ray) who did a great job, quickly and cheaply.

Cheers Terry

Probably "sand" blasting.

I bought a small pencil blast cabinet last year for my wife's crazy Girl Scout craft projects.

Amazing what you can do with SiC "sand".

...Jim Thompson

Hi Jim,

Shot glasses with your favourite insignia?

speaking of which, I had a night out on the fine whiskeys last week. absolutely fabulous, live music (rock & roll, perhaps a dozen singers, and fifty or so rock & roll dancers), and 30m from the hotel. Laphroiag is still my favourite tipple though :)

Cheers Terry

Of course ;-)

...Jim Thompson

Hello Terry,

I use a tile saw. The cheap $88 Home Depot edition but with a top notch blade. The blade is a "Mad Dog" MK225 (seriously, that's its name). Then a bit heavy on the water and it cuts like butter. The challenge is to hold it in a way that minimizes vibration, to avoid chipping the corner that goes through last. Hasn't caused any visible wear on the blade. All you hear while cutting is a whooshing sound, no grinding. NEVER hold the core with your hand, got to make a custom holder.

Regards, Joerg

Thanks Joerg, I never thought of that. blindingly obvious really....what with tiles being ceramics and all. doh.

Haere Mai Terry

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

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

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.

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

WOW! You have your very own kingdom!

It looks like it's at about 1KHz.

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

Mike

Oops, that should be 2-2200uf caps on the input.

OK, then its not oscillating.

if it only doubled the Fsw noise, its a pretty crappy filter. with a

1) very low self-resonant frequency of coil.

with 3.5mH, 1.2nF will give an SRF of 75kHz.... depending how its wound this wont be hard to do.

I downloaded the KMX spec, its 3 Ohms ESR at 100kHz. ouch. with a DCM flyback if you know the duty cycle and DC output current, trig gives you the secondary currents. 220V, 2.2k = 100mA DC. assuming 50% duty cycle, the avg sec. current during (1-D) = Idc/(1-D) = 200mA, its a triangle so peak current = 2*Iavg = 400mA peak. that'll give you 1.2Vpp ripple for a single cap....

with so much ESR, you will get a significant benefit from paralleling a big fat film cap - I happen to have a whole bunch of 3u3 400V MKPs, I'd use one of them :) do an impedance plot of your KMF cap, it'll look like

yep. All I need now is Serfs.

that makes it a closed-loop oscillation. that chip is kinda bodgy really, no proper error amp.

:( worth a try.

do a splat test. charge up a big cap. then short it across the inductor and measure the current (I have 10 1/4W 1R resistors I use as a current sense resistor). if Ecap >> Eind, then the cap voltage wont change much, so use V = LdI/dt, knowing V, dI and dt (digital scope very handy here) to calculate L. you should get a straight line (constant L) until the L saturates, at which point the slope will increase quite a bit (such that L becomes the air-cored value).

Cheers Terry

It's wound on a 1/2" torroid core and I just measured SFR to be

Arrgh, I looked at the wrong column and thought the impedance was

I found a tube of 3842s, but I'm not quite ready to scrap what I have and start over just yet.

I'm not sure there is enough room for 3 shields and the 130T secondary. What's the effect of the shield between windings?

I used an 1800uf cap charged to 5.45v and a Dale/Vishay LVR3 low inductance 0.05ohm resistor and it worked like a champ! I got a nice linear ramp up to 315mV in 144us (about 6.3A and 125uH) then the waveform changed to a sinewave. It sure doesn't look like it should be saturating, but what else could cause the input current to rise out of control like that?

Oh yeah, the MUR4100 shorted out, and that's a 4A 1000v "Ultra Fast" diode.

I looked at the voltage across the .086ohm current sense resistor and could barely make out a ramp. It's mostly ringing starting at transistor turn on lasting most(maybe 3/4) of the on time, and the ringing freq is at about 1.8MHz. Nasty looking stuff!!

It looks like I have a long way to go get full power out of this thing.

Thanks again, lots more good info. Who knows, I may just end up learning something yet.

Mike

that cap has 0.5*1800uf*(5.45V)^2 = 26.7mJ

the inductor, at 6.3A, 125uH, has 0.5*125uH*(6.3A)^2 = 2.48mJ

thats a reasonably chunk of 26.7mJ, so the cap voltage isnt really constant, and the L,C are going to start to resonate at about 335Hz.

Also, the current wont want to get bigger than Vdc/Zo = 5.45V/0.26R = 20.7A

crank up the cap voltage a bit more - 30V would be lovely. Then Eind

your current sense resistor needs to be low inductance, as does the associated the wiring. ditto for gate drive - low L is required.

its usually pretty easy to get the controller doing something so I leave that bit til last.

I do a lot of work sans controller, just using a pulse generator to drive my gatedrivers/switches. That way I can measure things like Isat, Lmag in situ (I just tack on a whopping great cap), ensure turns ratios and dotting is correct, measure thermal resistance, prove gatedrives working OK, etc. before attempting to run it as a power supply.

Cheers Terry

Oh yeah, I see, Duh! The cap just ran out too soon. Increasing the cap voltage to 30v made quite a difference. The math came out to 74uH and

Good idea, That's what I'm going to do.

It'll have to wait though. Squirrels have invaded my attic again and I have to try and deal with that first.

Last year those darn things chewed 2 holes through the roof and took up residence in the attic. I put a motion detector in the attic that set off a big-ole alarm bell and strobe light. They quit coming around, so I got the roof fixed. This year they tore out one of slats in one of the attic vents and chewed a hole in one of the soffits. Grrr!

Mike

Better saw off some of those overhanging tree limbs.

yeah, that sounds pretty good. for gapped ferrite, saturation really sticks out. with iron powder though, its much more gradual, and defining "saturation" becomes quite arbitrary.

with a large (typ. > 0.1mm )airgap, L ~ UoN^2Ae/gap, Ae in m^2, gap in m, so you can cross-check your results that way too.

And here I was thinking squirrels were cute. I've spent today fixing 3 leaks in the water trough piping (grr, alkathene piping is shit, and grr grr, the idiot who buried the pipe less than the recommended 12" ought to be horse-whipped - a push-in electric fence stanchion punctured one hose)

and I had to fix the letterbox, which got blown down, and a window, which got a stick thru it. we had 100 - 150km/hr winds yesterday.

Cheers Terry