small low-capacitance Schottky diodes

Winfield Hill · 2017-12-08T04:32:44+00:00

Some here may remember my big RIS-767 project, the "highly-efficient low-voltage boost-converter testbed", with free testbed PCBs offered up for experimenting. I claimed we could create 98% efficient converters. The RIS-767 testbed allows creating custom converter circuits, as well as using the same platform to test the best commercial boost-converter ICs we could find. Rob Legg has been investing his consider talents in using these testbed PCBs to test conventional boost converters and creating new ones that meet and exceed the 98% criteria. His work uncovered deficiencies in my PCB design, and even as he was spending untold hours at the bench, I've been spending untold hours taking his observations and advice to improve and fix the RIS-767 testbed. He's been working with rev 3, and meanwhile I'm well along on changes with rev 4. But I digress. About small diodes. We're talking about voltages from 0.6 to 2.4 volts, and this means exclusively using a synchronous converter topology. Rob Legg discovered that the LT3400 and LT3402 boost- converter families require external Schottky diodes across the internal synchronous-rectifier P-channel MOSFETs, as suggested in their datasheets, so I'm cramming the parts onboard. Sorry, digressing again. Any parallel diode capacitance adds to the MOSFET switch Coss capacitance, creating P = C V^2 f loss. So we really need to use low-capacitance diodes. Small low-capacitance diodes. Most diode footprints are rather large**, for example the common SOD-123 is substantially-bigger than the already-large 0805 footprint. In this context, the next-size smaller SOD-323 is welcome. Hence my enthusiasm for ST's '323 part, the bat54j*, compared to its larger -123 brother, the bat54z. Hmm, I previously encountered, with shock, that ST's bat46z was OK, but the smaller bat46j had disappeared from distributors. (The scene might be different for an alternate manufacturer, like Diodes, Inc., etc., but...

Dec 08, 2017 28 Replies

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piglet 8 years ago

Getting rid of dead time sounds promising. Instead of using the MOSFETs common source like nearly everyone else what about using them as source followers? Link the P and N gates and drive from something that swings Vgs(on) volts above and below the rails. Topologically is the same but only one gate drive is needed with no dead time issues and as John Larkin once pointed out shoot-through current is eliminated.

piglet

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Tim Williams 8 years ago

At turn-off, I mean.

Depends on carrier lifetime in the material, which maybe isn't so bad. In which case maybe it's kind of a shame that no one makes GaAs or GaN BJTs (or HBTs), because storage time depends on that.

Tim

Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website: https://www.seventransistorlabs.com/

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Tim Williams 8 years ago

Indeed!

Downsides:

- You need gate drive beyond the rails (including transients)

- PCH suck

Bootstrap NCH drivers are pervasive because PCH is about 2-3 times worse performance than NCH. If you can solve a problem with a slightly more complicated IC and no PCH, you're very likely to come out ahead in cost, size, efficiency and so on.

Tim

Seven Transistor Labs, LLC Electrical Engineering Consultation and Contract Design Website: https://www.seventransistorlabs.com/

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pcdhobbs 8 years ago

?In

So does beta, which is why gold-doped transistors (as used in regular TTL) suck lemons.

Plus hole mobility in III-V materials is less than in silicon, so you don't gain anything.

Cheers

Phil Hobbs

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John Larkin 8 years ago

I haven't seen that, either. No sign of storage.

Microwave MMICS are usually HBT Darlingtons, InGaP or something. I've used them as pulse amplifiers and they are pretty fast.

Most of the common MMICS are nothing but a Darlington with a collector-base resistor inside. That's usable as a transistor.

John Larkin Highland Technology, Inc lunatic fringe electronics

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John Larkin 8 years ago

Please abide by group standards and remain rigidly on topic.

John Larkin Highland Technology, Inc lunatic fringe electronics

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Phil Hobbs 8 years ago

High level injection comes to the rescue. ;)

Yeah. He wrote an almost-unreadable book on safe driving, then (iirc) crashed into a tree with no seat belt on. A pity--we lost a lot of good tech people in 2011: Jim Williams, Bob Pease, and my old friend and mentor Tony Siegman.

Cheers

Phil Hobbs

Dr Philip C D Hobbs Principal Consultant ElectroOptical Innovations LLC / Hobbs ElectroOptics Optics, Electro-optics, Photonics, Analog Electronics Briarcliff Manor NY 10510 http://electrooptical.net http://hobbs-eo.com

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Winfield Hill 8 years ago

I discovered the BAT165 in a tiny SOD-363 package. 0.44V at 750mA and only 8.4 pF capacitance at 10V. Tried it on the 1W bipolar converter (5V to +/-15V) in my MPX-16H today, worked OK. Will include some in my next shipment to Rob for his RIS-767 tests.

Thanks, - Win

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John Larkin 8 years ago

One of the (soon EOL) Avago phemts can be a 1 amp, 1 pF diode.

John Larkin Highland Technology, Inc picosecond timing precision measurement jlarkin att highlandtechnology dott com http://www.highlandtechnology.com

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