guard ring?

I'm still iterating on my high-voltage pulse generator. Rev C looks very good. I've been pushing it up voltage and frequency (like, 1200 volts and 4 MHz) and seeing what parts fry, and working on them.

I have a dpak schottky diode in a sort of snubber place. According to Spice, it shouldn't get too hot, and it's soldered to all sorts of copper pours and thermal vias and stuff. It's an SBRD10200 and it's frying. I happened to have some smaller, lower current diodes around, UPS5100E3, which is a sawed-off sub-dpak thing. I figured I might squeeze in two of those in parallel... but just one runs cool.

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Weird. I'm guessing that the big one has a p-n guard ring that makes it have reverse recovery current right after a big forward current spike. Data sheets never talk about that.

I heat sinked things too well, so it's a bear to desolder/resolder.

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John Larkin
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Heat gun/ + heat pad under...

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Reply to
TTman

I don't want to melt everything else, and there's stuff on the back side of the board, too. My manufacturing people could do it for me, but they're busy. I applied my biggest Metcal tip and waited about 5 minutes for the big diode to finally let loose. I suspect that the solder didn't fully flow under the new tiny diode, but it runs cool.

Anyhow, that's a cute little schottky without a guard ring problem.

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John Larkin         Highland Technology, Inc 
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John Larkin

TTman wrote in news:ppr5sh$3mq$ snipped-for-privacy@dont-email.me:

snip

I agree. Even a chamber is a good idea. Bring the assembly UP in temperature so that the transition to reflow the part is less of a jump.

starts adding the reflow heat source (hot air, etc.). A 200 degree transition on a hot part is far easier to impart than a 330 degree jump from ambient.

Reply to
DecadentLinuxUserNumeroUno

Schottky can be pretty bad for snubbing, depending. The peak current can hit the guard ring diodes and you get big recovery loss. The capacitance is huge.

You'd have to add a shunt to be sure what was going on with the other one.

Regular diodes can have faster recovery than schottky-guard-ring recovery, but they also cost forward as well as reverse recovery.

Tim

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Reply to
Tim Williams

I was going to suggest a good size iron, and solder sucker. Then an exacto, under the tab.

5 min is much too long.

George H.

Reply to
George Herold

Hot air machines are super cheap, and don't melt anything but what they're pointed at. Assuming the only thing melting is solder. You probably shouldn't have melty things like electrolytics and film caps nearby on an SMT board, anyway, but a heat shield (aluminum foil) keeps that feasible.

Tim

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Reply to
Tim Williams

John Larkin wrote

4 MHz at 1000V, I did higher frequencies with tubes in the sixties, with 1000V supply at .5 A peak. Without seeing the circuit it is hard to tell, but Schottky diodes would not be my choice, there are nice fast si diodes or at least there were. Schottkys leak, get very hot because current flows both directions that way. Thermal runaway results. A few pF from a fast si diode .. helps tune the circuit ;-)

Anyways no way to tell without seeing what that diode is doing. Snubber = flyback diode here?

Reply to
<698839253X6D445TD

"Tim Williams" wrote in news:pprker$6d5$ snipped-for-privacy@dont-email.me:

With PCBs that have heavy via and high via counts near heavy parts even a single hot air jet can be detrimental to a board.

Bringing the entire assembly up in temperature OVER ambient means less thermal WORK has to be done to do the job, AND less overall stress occuurs on the whole thing.

One brings the entire assembly temp up (at least the area surrounding the rework location), THEN use a direct hot air jet to reflow the part.

They have benchtop setups for this.

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Reply to
DecadentLinuxUserNumeroUno

Well, you set it too high then (somewhere north of 260C).

And yeah, if you have too much thermal mass for this to work, you need a preheater.

Tim

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Tim Williams

"Tim Williams" wrote in news:ppsf6u$970$1 @dont-email.me:

That was his entire beef. Hard to desolder... preheat was the suggestion a couple of us gave.

Reply to
DecadentLinuxUserNumeroUno

Error (500) Something went wrong. Don't worry, your files are still safe and the Dropboxers have been notified. Check out our Help Center and forums for help, or head back to home.

Reply to
Robert Baer

Looks fine to me. Anybody else having problems seeing my links?

Some companies don't allow their employees to see Dropbox, or to receive verious email attatchments.

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John Larkin         Highland Technology, Inc 

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John Larkin

No problem at all here. Nice photos.

Reply to
John S

Same here. No problems.

Reply to
krw

Sometimes they don't work. Retrying later usually works. Those are working now.

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Reply to
Jasen Betts

Jasen Betts wrote in news:pq05cm$lfo$ snipped-for-privacy@gonzo.alcatraz:

Are you still on a POTS connection and a modem somewhere in backwoods hellville?

Reply to
DecadentLinuxUserNumeroUno

Replacing three schottky diodes with the cute little Microsemi parts helped my pulse generator efficiency a lot. They also solder into an SMB diode footprint, with some care to not short out.

My circuit still isn't as efficient as the Spice sim, so I've been prowling around with our FLIR imager to see where the heat is. Surprisingly (to me) at 1KV and 4 MHz pulse rate, the Phoenix output terminal block is hot.

I ripped out the metal bits from the unused center terminal, and it really cooled off.

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Power consumption dropped by 3.4 watts.

I suppose the ham radio RF power guys already knew about stuff like this; I'm still learning.

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John Larkin         Highland Technology, Inc 
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Reply to
John Larkin

Dremel?

Might have been the Great Internet Reset:

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It was announced that some ISPs might have issues.

Mark L. Fergerson

Reply to
alien8752

No. And how the f*ck would a POTS connection cause dropbox to fail anyway? sometimes drop box is drop ball.

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Reply to
Jasen Betts

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