"High" voltage timer

Driving a dumb chopper: 24V supply, NMOS H-bridge, 12V gate drive, 50% duty cycle at 200kHz. All I need is complementary drive to the left and right-hand side gate drivers. Trouble is, my gate drive circuitry needs 12V. Old skool metal gate logic would suffice, but fancy that, no one makes special circuits out of it. :)

Options:

- Use, e.g., LT's TimerBlox, or logic (e.g., Schmitt trigger oscillator at 400kHz, followed by a T flip-flop to generate complementary outputs and 50% duty) and level shifter. Ugly, the level shifter wastes all the components the TimerBlox was supposed to save.

- I *could* use a TimerBlox, *if* I use logic level MOSFETs. But my simulations show this will at least double the losses, based on typical MOSFETs. I might go shopping to find others more suitable but Rds(on) will always be higher than with 10-12V drive; the expense is an overkill transistor with disproportionately higher Qg, which means more drive, more supply consumption...

- Skip the gate drive circuit I have, and use one of those VCOs with gate drives included (e.g., IR27xxx), downside being I need four gate outputs, not two. I don't know of any offhand that do that in one chip; the 8-pin models don't support synchronization so this would be a challenge (even more level shifting? fuggedaboudit).

- Do the Schmitt trigger + T flip-flop, but implement it in CD4000 MOS. Downside: I'm pretty sure no one makes CDTinyLogic4000's, so that's two SO-14's almost entirely wasted.

- Use an SMPS controller. I'm intently eyeballing UC3825, the "improved TL494". It's fast enough and has true complementary outputs, but it has limited duty cycle (dead time), as do most PP/ bridge controllers. I can tolerate plenty of deadtime, but it's not as elegant as having simple complementary outputs. It's also an SO-16 or whatever, when all I really need is an SOT-23-6, but still, it's better than two SO-14s.

- Just make a f****ng astable multivibrator*. We live in a world of everything integrated, but hey, what works, works... Plus, it saves one of those damn $5 chips. Not that the placement of all those resistors is any cheaper.

Optimal solution: a TimerBlox (or CD4000 logic oscillator, or..) that runs from 12V. CMOS outputs drive a nominal amount of current (>5mA) at roughly supply voltages, give or take drop. Heck, crude bipolar outputs would be acceptable too (e.g., a 555 with complementary outputs).

*Last time I had this same problem, I built the multivibrator out of the very power stage itself (2N4401 for the win). And the time before that, I had some FPGA outputs to spare, so I just clocked some square waves out of that. Which still required a level shift, but a transistor per phase happened to be enough in that case.

Tim

Reply to
Tim Williams
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Gee, that's an awfully wordy ramble Tim.

50% duty complementary outputs @ 200Khz? Any CMOS flop will do that. For a driver, IRS2101. Or, translate your logic-level to 12v with a couple 2n7002s, plus pull-ups.

-- Cheers, James Arthur

Reply to
dagmargoodboat

Thanks? I've had a few and I'm tired, conditions under which I tend to get unusually verbose. Sometimes good or bad, I suppose?!

To a point, that's what I've got now (the 2N7002s, for the high sides; high and low actually get to the gates via emitter followers, hence the 12V source requirement), but that doesn't save me oscillator + level shift gibberish. :(

Tim

Reply to
Tim Williams

Well, don't let me get in the way of your progress.

A 555 timer will run off of 12V, if I'm not mistaken.

--
Tim Wescott
Control system and signal processing consulting
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Reply to
Tim Wescott

Use a driver chip with inverters in the input, route the output (clamped) to the input to make a RC osc (won't be 50% duty exactly)

Use the L6569 with internal oscillator (wont be exactly 50% duty)

Use a ballast controller (FAN7544). Has internal osc and F/F, so will be 50% duty

Google other drivers with internal osc, there's plenty

Cheers

Klaus

Reply to
Klaus Kragelund

What James Arthur presumably had in mind was good old 4000 series CMOS. MC14xxxB runs from 3V to 18V.

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You could use a couple of inverters (or 2n7002s) to make a non-50% duty cycle oscillator and divide it with the 14013 to get exactly 50%.

-- Bill Sloman, Nijmegen

Reply to
Bill Sloman

This is exactly what I proposed, and dismissed, in my OP, since it wastes more than half the pins of two SO14 chips. There must be a better way. Like I said, an HV TimerBlox or TinyLogic would be good.

Tim

Reply to
Tim Williams

Has exactly the drive and size I need, but it's just not fast enough

-- 100kHz max, and too much dead time. :(

Yuck! There must be a smaller solution out there.

Tim

Reply to
Tim Williams

yeh hadn't looked too closely

two of these then?

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500KHz, 50-200ns deadtime

it was just a description of what is inside those chips ;)

-Lasse

-Lasse

Reply to
langwadt

Ahh, that looks good! Doesn't mention how to synchronize, but I'd guess RT from one (which comes from an internal flip-flop) could be tied to CT on the other (flipping the thresholds at will), at a few nanoseconds' expense in delay. Hopefully not enough to care.

As for stock, Digikey doesn't have them. Mouser and Newark do, which isn't bad. Hmmmmm...

Tim

Reply to
Tim Williams

digikey has the non-automotive version ir2085

They also have ir2086 a full-bridge version which seem like a perfect fit, but the datasheet says not recommended for new designs

maybe theres a newer replacement for it

-Lasse

Reply to
langwadt

CD4047 has complementary output. VDD upto +15V

--
?? 100% natural
Reply to
Jasen Betts

Beautiful! And TI makes a TSSOP, which won't take up much space at all. Thanks!

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Tim

Reply to
Tim Williams

Make a FF out of two 2n7002s that runs at 12 v. Now you only need an oscillator. One more 2n7002 for a blocking oscillator?

?-)

Reply to
josephkk

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