My first flyback, Help!

Your dropbox link works for me. But when your circuit is simulated on my ancient version of LTSpice IV, it displays an output voltage of

+21VDC.

73,

--
Don Kuenz, KB7RPU 

Genius is nothing but a greater aptitude for patience. - de Buffon

On Wednesday, April 18, 2018 at 1:00:56 PM UTC-4, snipped-for-privacy@googlegroups.com w rote:

on in

Hmm well with a 24V supply 21V is pretty wimpy. Does this work any better? I cleaned up the ringing by playing around with an RC across the coil. (a bit of hunt and peck.)

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SYMATTR Type ind SYMATTR SpiceLine Ipk=1 Rser=0.07 Cpar=1pF SYMBOL ind2 688 32 R180 WINDOW 0 -22 77 Right 2 WINDOW 3 -12 42 Right 2 SYMATTR InstName L2

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SYMBOL cap 464 304 R0 WINDOW 0 51 26 Left 2 WINDOW 3 52 59 Left 2 SYMATTR InstName C16 SYMATTR Value 1p SYMBOL res 624 240 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R11 SYMATTR Value 100 SYMBOL cap 368 -80 R0 SYMATTR InstName C3 SYMATTR Value 1n SYMBOL res 560 48 R90 WINDOW 0 0 56 VBottom 2 WINDOW 3 32 56 VTop 2 SYMATTR InstName R6 SYMATTR Value 500 TEXT 632 -160 Bottom 2 !K1 L1 L2 1 TEXT -472 -480 Left 2 !.tran 0 30m 0m startup TEXT 568 -224 Left 2 ;VPH5-0155-R 1:5 TEXT 960 240 Left 2 ;0.8V TEXT 168 -536 Left 2 ;HV SUPPLY J LARKIN Oct 11, 2015 TEXT 192 -488 Left 2 ;Severly modified by G Herold TEXT 1216 -80 Left 2 ;+300 TEXT -504 -432 Left 2 !.model HVD D(Vfwd=0.7) TEXT 288 -352 Left 2 ;Eff ~~ unknown

Oh dear, I'm afraid I made a large error. I was looking at the boundary between CCM and DCM the wrong way. DCM is for large load resistance. I'm very much on the DCM side of things. (sorry) George H.

It looks fine to me George.

Miscellaneous comments: The inductor ringing is normal. Putting a snubber or a damping resistor across it speeds up the simulation--I started with 50 ohms and 1nF across L1.

I find it also runs well at +12V with R2=1k, and that 220pF feed-forward across the feedback divider's 3M tames your initial Vout overshoot (LTC3803,

100pF for the LTC3803-3).

You could reduce ipk(L1) (operating) by using the LTC3803-3 version (50% faster), reducing the time spent in the "no energy delivery" part of the discharge cycle. That would allow you to provide the same energy with smaller pulses, delivered more often.

You can reduce ipk(L1) (startup) by increasing your sense resistor.

Another option: making L1 large enough to run continuous-mode drops Ipk dramatically.

I prefer grounding the cold end of the output inductor, making the supply short-circuit proof.

Cheers, James Arthur

Yep, you're stuck with that. For your 300V load and 300kHz, you'd need ~100mH output inductance (a 4mH-per-coil VersaPak) for CCM @ 1mA i.ripple.

Faster switching would help a bit, but probably not worth sweating.

Cheers, James Arthur

VPH5 seems awfully big, for only wanting 3mA out of the thing?

Fixed frequency on the controller sucks, when you don't have many options for the inductor. It may be helpful to use a smaller inductance, and a lower ratio, to get a modestly higher primary inductance. This approximates the VP5-0155-R:

Also added plausible diode parameters, a couple parasitics, and damping. (Hm, a ferrite bead on the diode might do a similar job with less power loss. It'll need to be a fairly high Z chip type, mind.)

There are also coupled inductors of various ratios on DK, and from Coilcraft and others. Or SMPS transformers, which may be cheaper than a VPH thingy (or even just Coilcraft's version, is cheaper than the original).

Tim

-- Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website:

ed

There's the Vlad memorial 'antiseptic' converter, which grabs the flyback k ick and puts it in series with the output.

Cheers

Phil Hobbs

need

e.

kick and puts it in series with the output.

Hmm. I remember there was such a thing, but not what it actually entails.

A voltage multiplier stage would move the SMPS' duty cycle considerably in George's favor--that might help if one ever really needed CCM.

Cheers, James Arthur

Hi James, Thanks... Yeah I somehow figured a bunch of that out today. I have to look more closely at some of your comments. (I mostly just copied JL's circuit and haven't tried to understand most of the values.)

(It was a rare day at work, I sat down at my 'puter with book, notebook scratch pad and calc. and no one bothered me, (except for once) and when I pulled my head up it was 2:00 and I had to hurry down to the cafe, to get some lunch.

So here was the idea I had driving home today. If I didn't have the transformer boost (1:5) I could use the same inductor array, as a 1:1 (3:3) with (22x9) ~200uH of inductance, I then have a short boost ~1/12.. ~0.4usec... and I should be able to get it into CCM. Hey, that Larkin guy is no dummy. :^) I wonder if a 2:4 might be better?

Thanks again James.

George H. ps. the 12V is good to know, the same box will have a little TEC in it, not sure exactly how much but a few volts at a few amps. My second smps circuit will be a buck.

GH

need

e.

kick and puts it in series with the output.

Oh boy, I remember Vlad, can I do it with this six inductor array?

George H.

looked at this:

To get to CCM you need to do two things: 1) have an output inductance large enough so that it won't fully discharge during the switch OFF time, while 2) having a primary inductance low enough that your supply voltage can charge it during the available switch ON time.

Reconfiguring your VersaPak as you describe works against both of those goals.

To get closer to CCM you have a few options, including reducing the output voltage the output inductance discharges into (such as by putting a C-W after it), or switching the PWM faster, or finding a different flyback transformer with a higher winding ratio. (I recently used a 12:1, Pulse p/n PA0367A.)

Cheers, James Arthur

I didn't examine any of your files, etc., but I'd like to warn you that the bifilar-wound transformer types have extremely-high winding-to-winding capacitance. In many configurations this highly degrades performance.

--
 Thanks, 
    - Win

Hmm OK, It's my first so I'm most likely be confused... I'll have to spice it. There's a turn's ratio in there somewhere. At 1:1 I'm going to bang on hard, an amp.. then pull out slow at 10 mA... right there's a voltage gain in there too (In my sim I was surprised how fast the output current decayed.)

George H.

Cheers

Phil Hobbs

Thanks Phil.

Cheers, James

Right now your output inductor is fully discharging in about 300nS. To get to CCM you'd need it to be, say, not more than 50% discharged in maybe 1.5uS (assuming a primary inductance that charges in 1.5uS, just for a starting point).

So, you're short of CCM by a factor of roughly 10. You could get there by combination of transformer ratio, increased switching speed, or lower output inductor voltage drop, but DCM is fine.

It's probably not worth changing.

Cheers, James Arthur

Hi James, I realized last night that I had this whole thing backwards in my head. I was picturing a short buck, and then a long boost. That just doesn't work. I want a really long buck... which means a big inductor.

Anyway thanks again for your help. I think I now have it mostly right. And it's time to order some parts... let the solder meet the road.

George H.

Thanks Win, I'm looking at these Versa-Pac inductor arrays. (I have no idea of the capacitance of these arrays.. the spec sheet is rather minimal.) They do quote leakage inductance... ~10% of the coil L.

I'm mostly a virgin when it comes to inducotrs, so I expect I'll learn a little once I try using them for real.

George H.