transformers for cheap low current flyback supply, +/- 45V ?

In article , mw wrote: [...]

If it doesn't have to be isolated, consider making it a SEPIC and CUK, perhaps with multiplier outputs. This way you can just use off the shelf power inductors.

---)))))---+----------!!---+--->!---+----- +V ! ) ! ! ) --- ! ) --- ! ! ! ! GND GND ! ! +----------!!---+---))))--+---- -V ! ! ! !!- V --- ---!!- --- --- !!- ! ! ! GND GND GND

---)))))---+----------!!------------+--->!---+----- +V ! ) ! ! ) --- ! ) --- ! ! ! ! ! GND ! ! +----------!!---+--->!---+ ! ) ! ! ) --- ! ) --- ! ! ! ! GND GND ... etc ...

That's pretty clever. It doesn't need to be isolated from the mains, so had thought about using a boost circuit. But I wasn't sure how to derive the negative rail. It looks complicated to get the -45V to be inside the PWM controller's control loop. But really, my requirements aren't too critical (it doesn't matter if it is -42, or -46, as long as it is close to -45V and reasonably stable).

What I could do is regulate using the +45V side as the feedback, then let the negative run open-loop but derived from the same switching waveform. I'd really like to avoid hand-winding of transformers, that's a pain, and this approach would avoid it entirely.

Thanks to everyone that responded. There's lots of ways to get this to work since my requirements are very loose.

mw

In article , mw wrote: [... circuit snipped ..]

You can only regulator one or the other in this case. To regulate the minus supply, you most likely need an inverting op-amp circuit.

If the loads are fairly closely matched, the regulator can be quite good on the open loop output. By its nature, the curcuit makes the drain waveform have quite flat tops and bottoms, if the coupling capacitors are large enough.

Be careful about the placement of the inductors. Chances are, you will be using unshielded ones. If they are close together, there will be some mutual coupling. Ideally, you want the mutual coupling to add to the value not subtract and to be equal between the two supplies.

Looking at your suggestions I found that Coilcraft does have a good assortment of transformers, for example these:

I will have to look closely at them to see if any can be made to work, and whether they can be obtained without a lot of hassle. I don't see any of them listed using findpart.com , so that's not so good, could be that you have to get personal with sales folks. ($$$).

This afternoon I spent some time working on my supply, using an old board of mine (it uses a difficult-to-obtain custom flyback transformer), and was able to get it working a little differently than it did in its original +/-14V output configuration.

Some results:

Input (feeding the UCC PWM chip) : +17V through a 402 Ohm resistor. The PWM chip and the FET together only draw 9.4mA! Input (feeding the transformer) : +43V Output (pos) : +56.2V Output (neg) : -56.9V That's today's set up, it will probably change.

The freq of the PWM is 57kHz. I find that PWM freq isn't too critical. That's one thing nice about closed-loop controllers, they will work reasonably well over a broad range of f, if you don't have tight req'ts. I don't.

I'm getting a nice waveform on the current sense line that shows current ramping up to about 1.36V during a 1.28us interval (while the FET is on). I thought it should turn off at 1V, but maybe not. My current sense resistor is 1.05 Ohms. So let's see, that would be peak current,

ipk = 1.36V/1.05 Ohm = 1.42A.

Let's assume that the current waveform is flat, rather than a ramp, just to make things easier to calculate and to allow some slop for worst case conditions. So current being drawn from the 43V supply is 1.42A but only during the time 1.28 us interval (out of the 17.5 us PWM period), or about 7.3%. So the worst case total power into the transformer is about (43V * 1.42A) * 7.3% = 2.15W. That's not bad, and that's about how hot things seem. Just noticeably warm to the touch.

OK. The "application" circuit that uses this draws about 4mA * (56V *

One good thing is that the PWM isn't creating noise for my "application", although it's probably radiating EMI big time. I have it wired in using various 6" to 18" wires, but both boards (the PWM board and the "application" board) have sweet ground planes.

Thanks to all, mw

The OP will not supply any clue, so has branded himself a TROLL.

......... Phil

look harder Graham. output +/-45V @ 10mA, input 12V(ish)....

Cheers Terry

MPP?

KoolMu?

#52?

oops, Vishay had best remove the numerous product lines they have, closely followed by coilcraft.

Philips (or whoever they became) make an Integrated Inductive Component (IIC) that may also do the trick.

Cheers Terry

isolation is really the important part, followed by turns ratio.

For a DCM flyback, Pin = (Vinmin*Dmax)^2/[2*Lmag*Fsmps]

so for a given Vinmin, Lmag you can twiddle Dmax and Fsmps to get Pin. until things like minimum pulse widths and saturation rear up.

Cheers Terry

"Terry Given" Phil Allison

It is NOT enough info for me to recommend a suitable transformer.

Hiding the app plus not knowing that an app is not the same as a raw spec makes the OP look like a time wasting troll and a fake.

........ Phil

Maybe it top secret 007 James Bond type stuff ???

Give even a tiny hoot ??

No need to reply.

.......... Phil

Hi Phil,

its enough info to design a power supply....

Cheers Terry

Perhaps. The app may be confidential, who knows. More info on the requirements would help optimise the xfmr, for sure.

I once got asked to peer-review a guys smps "design." His output 0V was at the same potential as the -ve DC bus (rectified mains), so about

-160V. it was a battery charger.... he also used IF transformers as little smps xfmrs for gatedrives :)

needless to say the review was less than favourable, and the word "lethal" featured prominently in the first sentence.

Cheers Terry

In article , mw wrote: [...]

Have you considered this:

---------+------->!----- +V ! ! )!!( --- )!!( --- )!!( ! )!! ! ! )!! ---------------------+---- GND )!! ! ! )!! ---------+------->!-- )!! ! )!!( )!!( )!!( ! -------------------------- -V

You can get light load to full load tracking that is well under a diode drop this way. It does increase the ripple current in the -V's filter cap a bit but usually not enough to matter.

Coilcraft will sample parts. I believe RencoUSA does too.

J.W. Miller is known mostly for their RF parts but they also make power inductors. These are carried by digikey.

... for some values of "nice". There is a spike at the turn on due to interwinding capacitances etc. Sometimes the spike will have a bit of ringing after it.

If the slope decreases with time(enough to see on a scope), you have way too much resistance in the primary side unless:

Many years back I did a supply that made about 1KW running at 4KHz. On one prototype inductor, the slope slightly decreased. Grabbing the transformer with my hand changed the shape. We just happened to line a mechanical resonance up with the running frequency. Doing up the bolt fixed it.

I second the vote for bifilar if the option is open to you.

It's mostly like a straight line. Not perfect, but I think my probe is picking up a lot of EMI.

It's in discontinuous mode.

Thanks for the math. I don't have the Lmag available to me here at home, but I will keep this in mind when I look at the other transformers. I know that this transformer works well in its original application (it supplies about 8W, and doesn't saturate).

I am not sure how it is wound (don't have the data sheet here at home), but it's a recent design by a custom house, and they do pretty thorough work.

Thanks for your help Terry, mw

I'll take a look using my simulator. It would be nice to avoid regulating the neg rail.

Thanks, mw

try digikey or mouser, ISTR one of them does coilcraft.

it really should. you should have a nice triangular waveform, the slope of which should be Rsense*[Vin/Lmag].

if the "slopey bit" isnt a straight line, and curves up, the core is saturating. usually thats what causes Vsense to rise above 1V.

dreadful approximation.

hows about this:

firstly, the average input current * Vin = input power.

energy stored in core at end of on-time is:

Ecore_end = 0.5*Lmag*Ipeak^2

if the current sense waveform is a triangle (complete energy transfer, aka Discontinuous Conduction Mode), then the energy stored in the core at the beginning of the on time is:

Ecore_start = 0.5*Lmag*0^2 = 0

otherwise the waveform is a trapezoid, so

Ecore_start = 0.5*Lmag*I_min^2

energy transferred to load is Eload = Ecore_end - Ecore_start

power transferred to load is Eload*Fsmps.

For a DCM flyback, if you do the maths you get:

Pin = (Vinmin*Dmax)^2/[2*Lmag*Fsmps] Ipeak = (Vinmin*Dmax)/[Lmag*Fsmps]

its pretty good. did you bifilar wind the two secondaries - that'll help the cross-regulation.

urgh!

NWD

Cheers Terry

In article , mw wrote: [.. circuit ...]

What simulator do you use? What do you do about the AC resistance of the windings?

It is Tina Pro (Designsoft). I will use one of their transformer generic models and change the parameters to get "close". I don't expect anything highly accurate from it, I just want to understand your circuit better.

mw