isolated DC/DC converter

Just squirt a little triangle current into the Vadj pin. That dithers the duty cycle. How much it wobbles the output voltage depends on the triangle frequency and the output filter caps.

This much FM wobbled the DC output about a millivolt p-p.

This was an FTMS spectroscopy app where any birdies in the FFT would be noticed. The dither sawtooth was around 4 KHz, below the range of interest of the physics.

John

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Maybe, maybe not.

What are you willing to pay for a solution?

I have no suggestions for the driver, John, but I would suggest some resistance in series with the supply as the cap will need to charge to half the supply voltage and the core may saturate in the process. Just a precaution, and it may not be necessary.

Looks good to me.

John S

the PMT HV supply.

I need nine separate power supplies, from a prime 24 volts that I have to isolate.

I just breadboarded the HV supply. I can pick off the 24 volts p-p from the input half-bridge and run it through a cheap ISDN transformer to get +110 VDC. Works like a charm, eliminates a boost converter I was going to use. Those ISDN transformers are very cool.

Dumping a full watt in the 2.5 linear regulator is sort of painful, but it sure is cheap.

A sinewave type converter would too inefficient for these sorts of power levels.

John

Heck, the most fun thing I do at work is design circuits. I'm happy to pay people to hack VHDL or write test code, but not to design my circuits!

John

the PMT HV supply.

No smoothing inductors at the rectifiers? The current peaks when the caps get replenished "whambam style" are often not too friendly towards analog stuff in the neighborhood. Like a Harley at night :-)

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Regards, Joerg

http://www.analogconsultants.com/

for the PMT HV supply.

things,

That might cause more problems than it cures. I am resigned (sigh) to having a noisy power supply, and doing PCB tricks to keep it from getting into the front-end amps and the 250 MHz ADC.

John

That's what I suspected, but doesn't that also dither the output voltage?

Impressive.

I would have thought it would take significantly more to change the edges significantly. Neat.

Yes, interesting.

What kind of PCB tricks will keep fast edges out of wideband amps and a

250MHZ ADC?

Mike

On a sunny day (Sat, 05 Nov 2011 16:34:45 -0700) it happened John Larkin wrote in :

the PMT HV supply.

Nice frame around it :-) Why not use different turns and diodes + cap for each voltage?

I think this is not correct. In your sinewave oscillator the transistor actually works as a switch, it is only on for a very short period of time, sort of class C. I have used the sinewave circuit (with feedback in the base) to power relais in industrial applications were we needed isolation. The transistor stays cold, even under heavy load. What got warm was ceramic caps (for tuning), after replacing those with poly caps it all stayed cool. I suspect with your emittor feedback system it will work even better. Last week I replaced an other regulator with a LM2596-33.

This whole thing runs of such an oscillator:

Diagram, see top left:

These guys put a nice transformer together for me rather quickly and for a reasonable price (competing with off-the-shelf transformers):

--
Failure does not prove something is impossible, failure simply
indicates you are not using the right tools...
nico@nctdevpuntnl (punt=.)
--------------------------------------------------------------

No, it's the built-in simplicity that's hard to find, as an integrated feature.

Most IC designers just don't get it.

RL

There are a few, but they're a bit oddball and expensive.

MAX253 MAX845 MAX13256

This one is a bit more complex, but claims to be ultra-low noise: LT3439

Best regards, Spehro Pefhany

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Embedded software/hardware/analog  Info for designers:  http://www.speff.com

It means a lot of decimal places, if you're trying to build a longer pattern within a set period, but I prefer this to some unknown homogenization going on, at the SW writer's half-baked whim.

Realistic rise-times are best developed by identifiable elements in the simulation.

I guess you're wondering, right about now, why there isn't an integrated, self-oscillating transformer driver in SO8 or even SOT23-5.

Rail pins, output, timing and inhibit pins.

No doubt there will be one, someday, but it will require 1-Wire serial programming and run on 2.65 - 3.6V supply rails. It'll only be available in 9-pin (3mm x 2mm) DFN. It will still require external schottkys, but this will not be discovered till 6 months after product release.

RL

I've used UCC3808 in a similar project, SO8, but it's a dual square wave gate driver, so it needs a center-tapped transformer, namely custom magnetics.

John

"Most" IC Designers are laughing up their sleeve ;-) ...Jim Thompson

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| James E.Thompson, CTO                            |    mens     |
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I love to cook with wine.     Sometimes I even put it in the food.

Keep the switcher far from the ADC stuff; avoid circulating currents poked into the ground plane, by slitting the plane if necessary; add ferrite bead+capacitor lowpass filters to the supplies of the sensitive stuff. Keep the analog stuff very tight. And slow down those fast edges with gate resistors.

Compounding the difficulty, there's the thermal design, getting rid of all that heat from the linear regulators. Less noise, more heat. Should be an interesting PCB layout.

John

the PMT HV supply.

I'm trying to use standard magnetics. Two Coiltronics DRQ127's for the

+-10, one Talema ISDN transformer for the 100 volts. Custom magnetics is a pain to develop and tends to be expensive in modest volumes.

Or maybe an ISDN for the negative supplies. I'll have to try that.

OK, but a class-C resonant converter has to circulate a lot of energy in the L-C tank, high currents into custom magnetics. Sloshing all that energy around doesn't seem, to me, to have any real advantage.

I do recall some of the Tek 7000-series scopes having a single-transistor class C resonant main power supply, off the rectified AC line.

We should establish a PayPal fund to buy you some nice gridded vellum and an electric pencil sharpener.

John

When transistors are free, people tend to use lots of them.

John