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transformer coupled logic isolator (2023 Update)

Version 4 SHEET 1 880 680 WIRE 480 -32 384 -32 WIRE 608 -32 560 -32 WIRE 608 80 608 -32 WIRE 608 80 544 80 WIRE 656 80 608 80 WIRE 688 80 656 80 WIRE -112 96 -160 96 WIRE -64 96 -112 96 WIRE 48 96 16 96 WIRE 80 96 48 96 WIRE 240 96 192 96 WIRE 272 96 240 96 WIRE 384 96 384 -32 WIRE 384 96 336 96 WIRE 432 96 384 96 WIRE 480 96 432 96 WIRE -160 144 -160 96 WIRE 80 144 80 96 WIRE 192 144 192 96 WIRE -160 256 -160 224 WIRE 80 256 80 224 WIRE 192 256 192 224 FLAG 80 256 0 FLAG 192 256 0 FLAG -160 256 0 FLAG 656 80 OUT FLAG 432 96 G FLAG 240 96 S FLAG 48 96 P FLAG -112 96 IN SYMBOL ind2 64 128 R0 WINDOW 0 -53 39 Left 2 WINDOW 3 -60 70 Left 2 SYMATTR InstName L1 SYMATTR Value 10µ SYMATTR Type ind SYMBOL ind2 176 128 R0 WINDOW 0 56 34 Left 2 WINDOW 3 51 66 Left 2 SYMATTR InstName L2 SYMATTR Value 10µ SYMATTR Type ind SYMBOL res 32 80 R90 WINDOW 0 -40 54 VBottom 2 WINDOW 3 -34 56 VTop 2 SYMATTR InstName R1 SYMATTR Value 50 SYMBOL res 464 -16 R270 WINDOW 0 -38 20 VTop 2 WINDOW 3 -10 94 VBottom 2 SYMATTR InstName R2 SYMATTR Value 200 SYMBOL voltage -160 128 R0 WINDOW 0 39 113 Left 2 WINDOW 3 -61 184 Left 2 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value PULSE(0 5 10n 5n 5n 10u 20u 10) SYMBOL Digital\\schmitt 480 32 R0 WINDOW 0 15 116 Left 2 SYMATTR InstName A1 SYMATTR SpiceLine Vhigh=3.3 VT=1.65 VH=0.4 Td=5n SYMBOL cap 336 80 R90 WINDOW 0 -48 30 VBottom 2 WINDOW 3 -39 29 VTop 2 SYMATTR InstName C1 SYMATTR Value 250p TEXT 64 40 Left 2 !K1 L1 L2 0.98 TEXT 352 304 Left 2 !.tran 0 250u 0 1n TEXT 360 208 Left 2 ;Logic Coupler TEXT 352 256 Left 2 ;JL Dec 29 2021

Reply to
jlarkin
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What is the use-case for this that a conventional digital isolator wouldn't be suitable for?

Sylvia.

Reply to
Sylvia Else

Sometimes used in lower frequency isolated gate drive, when minimal magnetics cost is the aim.

RL

Reply to
legg

Or very fast edges are needed, as in the examples JL and I posted in the "CML-CML level shifter" thread. Logic isolators don't go much faster than 40 ns AFAICT.

Cheers

Phil Hobbs

Reply to
Phil Hobbs

It has no internal clocks, so it's fast and has no jitter or EMI sources. It's cheap. Its high-side static power requirement is zero. And it's cute.

Fun circuits don't need use cases.

Reply to
jlarkin

Right. And most modulate/demodulate so have a lot of jitter.

One could make a very fast low-jitter optical isolator, but it would be a big expensive power hog.

Reply to
jlarkin

That circuit was just a doodle. It would need tweaking in real life. It's most elegant when the transformer time constant matches the Schmitt feedback time constant, and when the final RC also damps any leakage inductance ringing. That actually gets complex.

It really doesn't need a Schmitt gate, but it's nice.

Reply to
jlarkin

It's faster than most isolators, and is DC-coupled, after a powerup priming shot.

Reply to
John Larkin

And would have crappy latency compared with a coaxial transformer.

Cheers

Phil Hobbs

Reply to
Phil Hobbs

That's one place where the common-gate GaN FET trick is good--it pretty well ignores reasonable amounts of ringing. I need to try it out to see if the body diode of the EPC parts is quick enough.

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has an interesting discussion of how third-quadrant conduction works in GaN FETs, but doesn't give any numbers.

It does make the point that the reverse conduction mechanism isn't a diode as in vertical MOSFETs, but is just the channel turning on in the reverse direction (drain and source exchanged).

That's all majority-carrier conduction, so there's no reverse recovery issue, and since the channel turns on due to the gate voltage and not diffusion, there should be no forward recovery delay either.

Cheers

Phil Hobbs

Reply to
Phil Hobbs

The drive function allowed low frequency modulation using cheap small pulse transformer with minimal VT.

Many issues with this technique, but it had it's day.

RL

Reply to
legg

you hope. . . . In circuits that can go boom, the issues aren't just speed and data integrity.

In a two-transistor forward converter, there's a second series switch that can police the circuit, without magnetics and restoration issues. Bridge circuits not so lucky.

RL

Reply to
legg

Did you check out the LTspice file I posted with the common-gate GaN FET? With a better transformer that looks like a pretty sweet solution.

Cheers

Phil Hobbs

Reply to
Phil Hobbs

2 ns maybe could be done with a laser diode and a fast photodiode.

I was idly musing about making a dc-coupled optical link from SFP modules. The trick would be to get the jitter below the data clock period limit. FM or PWM or something.

Reply to
jlarkin

On Thu, 30 Dec 2021 13:54:42 -0500, Phil Hobbs snipped-for-privacy@electrooptical.net wrote:

Our lamented ATF-50189 made a fabulous diode in a couple of modes. D+G against source, S+G against drain. 400 mV drop, 1 amp, 1 pF.

formatting link
The SAV parts could be useful as diodes. Sims look promising, but I haven't tried it for real.

(fix the wrap!)

Version 4 SHEET 1 1220 680 WIRE 224 -96 -96 -96 WIRE 528 -96 224 -96 WIRE -96 -64 -96 -96 WIRE 224 -48 224 -96 WIRE 528 -48 528 -96 WIRE -96 64 -96 16 WIRE 224 80 224 32 WIRE 272 80 224 80 WIRE 304 80 272 80 WIRE 528 80 528 32 WIRE 576 80 528 80 WIRE 608 80 576 80 WIRE 224 128 224 80 WIRE 528 128 528 80 WIRE 176 208 144 208 WIRE 480 208 432 208 WIRE 144 256 144 208 WIRE 224 256 224 224 WIRE 224 256 144 256 WIRE 432 256 432 208 WIRE 528 256 528 224 WIRE 528 256 432 256 WIRE 224 272 224 256 WIRE 528 272 528 256 FLAG 224 272 0 FLAG -96 64 0 FLAG 528 272 0 FLAG 272 80 D1 FLAG 576 80 D2 SYMBOL mesfet 176 128 R0 WINDOW 0 90 32 Left 2 WINDOW 3 67 62 Left 2 SYMATTR InstName Z1 SYMATTR Value SAV551 SYMBOL voltage -96 -80 R0 WINDOW 0 68 58 Left 2 WINDOW 3 59 97 Left 2 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName Vd SYMATTR Value SINE(0 20 1 0 0 8) SYMBOL res 208 -64 R0 WINDOW 0 54 46 Left 2 WINDOW 3 53 77 Left 2 SYMATTR InstName R1 SYMATTR Value 5K SYMBOL res 512 -64 R0 WINDOW 0 55 44 Left 2 WINDOW 3 56 73 Left 2 SYMATTR InstName R2 SYMATTR Value 5K SYMBOL mesfet 480 128 R0 WINDOW 0 86 36 Left 2 WINDOW 3 62 64 Left 2 SYMATTR InstName Z2 SYMATTR Value SAV541 TEXT -168 328 Left 2 !.MODEL SAV551 NMF(vto=0.08, Beta=0.3,\n+ Lambda=0.07, Alpha=4 B=0.8, Pb=0.7,\n+ Cgs=0.997E-12, Cgd=0.176E-12, Rd=0.084,\n+ Rs=0.054, Kf=5e-11, Af=2) TEXT -48 272 Left 2 !.tran 5 TEXT 344 328 Left 2 !.MODEL SAV541 NMF(vto=0.08, Beta=0.6,\n+ Lambda=0.07, Alpha=4 B=0.8, Pb=0.7,\n+ Cgs=0.997E-12, Cgd=0.176E-12, Rd=0.084,\n+ Rs=0.054, Kf=5e-11, Af=2) TEXT -120 144 Left 2 ;SAV-541 and SAV-551 TEXT -64 176 Left 2 ;As Diodes TEXT -104 224 Left 2 ;JL May 10 2021

Reply to
jlarkin

Snipped the .asc file. I could fix the wrap, but whatever symbol John inserted for his SAV parts didn't make it. The models were there - as strings of text - but not as any kind of manipulable component, even after I'd let LT Spice update itself.

I suppose I could try to generate symbols to latch on to the SAV541 and SAV541 data, but it doesn't strike me as a sensible use for my time.

-- Bill Sloman, Sydney

Reply to
Anthony William Sloman

NVE GMR isolators do rather better, eg...

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...but their WebSite is down ATM.

Reply to
Clive Arthur

Interesting. Quiescent power is low and dynamic power is high. They must be making a high current driver coil shot on every data edge.

18 ns max at 3.3 volts. It's expensive!
Reply to
jlarkin

Interesting, thanks. 10 ns prop delay eats my whole (aspirational) recovery time budget, but it's much better than 40 ns.

Cheers

Phil Hobbs

Reply to
Phil Hobbs

This would work too:

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Reply to
John Larkin

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