It looks like a very useful part for a lot of things so it's been obsoleted, natch.
It looks like a very useful part for a lot of things so it's been obsoleted, natch.
Don't scare me like that. Digikey has the Onsemi versions.
You can also cascode a low voltage optocoupler with a depletion mosfet for more voltage and current. That sort of thing saves a lot of level-shifting junk.
A PV optocoupler can drive a gigantic mosfet, sort of a linear SSR.
Too bad SSRs don't work well in linear mode.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement
So I drew this pic with a fet on the high side and a PV coupler and resistor in parallel between source and gate. I drew it is as an Nfet, I guess a Pfet is better. Is there a 'standard' PV optocoupler? The one with a long product lifetime. Speaking of lifetime, I hoping that as long as I'm nice to the LED, (10-20 mA max.) it'll last a lifetime.
George H. Hey, here's a crazy idea, two PV opto's back to back, anode to anode, drive charge in and out of the gate. (maybe someone's tried that already.)
A series pass transistor is a less heat-producing option: the level translator Q1 needs HV rating, but the pass transistor Q2 is just to take off the ripple, so 100V should be plenty:
You can embellish it, of course; a collector series resistor//capacitor will share the heatload with Q1, and a source series resistor will limit shortcircuit current. Vbb* (R2/R1) is the gate drive limit.
Optos are slow, it's better to draw the milliamp or so from the HV, at 300V. I think. Different, though, if it were 3 kV.
Oops, scratch the last... I realized my brain fart in bed last night. GH
CW multipliers are slow reacting. Easier to drive them digitally. You can get HV zeners fwiw.
NT
OK thanks, I'll have to look at that some more, I half understand it.
Right (slow) I was thinking I could clean up the 300 V before I attenuate it. (if I use an opto) But faster is almost always better.
George H.
My brain does that too, subconsciously checks stuff in background, That saves a lot of mistakes, but it usually takes me about two days.
This works, linear or switching.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
I'm puzzled. How is it isolated when the 2 grounds are connected? Does it do something that 2 capacitors don't? What does it do?
NT
You're only looking at the secondary, the primary circuitry is implied by the three LED symbols.
Basically a big fat LED or something at left is optically coupled to a PV cell to provide isolated power, and isolated switching is provided by driving the two totem-poled optocouplers at right
I often get the current direction wrong with photodiodes... I also have a hard time with right and left... it may be related.
OK, I was thinking of two of the PV mosfet drivers, side by side now.. but looking at some spec sheets it seems there faster turning off than on, so maybe doesn't matter.
I've got a scribble of the circuit whit3rd started. That looks like it will work. Without any opto's.
George H.
The thing on the left (checks right and left to be sure.) is a photovoltaic mosfet driver. And as you say I figure the whole thing is up near the HV rail.
GH
There are no grounds on that sketch.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
Some people are more symmetric, so confuse left and right. Those same people sometimes get logic senses, active high/low, wrong. And tend to be left-handed. No big deal if they make themselves be careful.
My little circuit is faster than a PV, but will run out of current at high rep rates. PVs don't make much power. At higher rates, use a DC/DC converter instead of the PV.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
Ya as I understand it I'm looking at the stack of diodes and cap is a "model" of a PV cell. it's illuminated by something bright enough so it acts like a voltage source good for idk maybe a couple hundred uA of current at say 4 volts before it starts to sag. That's the supply voltage for the upper totem pole switching transistor, which is actually internal to an optocoupler. The lower one's "supply" is the MOSFET's charged gate capaitance
So long a your power requirements are small visible or infrared light is a perfectly cromulent way to transfer energy across an isolation barrier. Or maybe in some cases not even that small, I recall reading about some fashion of nuclear reactor that converts all the photons blasting out directly into a non-negligible amount of electricity via wrapping the thing in some kind of photovoltaics. "nuclear photoelectric battery" or somesuch. If all goes well this paragraph will trigger Neon John and he'll arise from his lead-encased crypt to correct the pertinent details in his usual salubrious fashion.
I got it! Ta.
NT
Cromulent?? No fair using words made up after I was born. :^)
Do you mean the TEG (thermo-electric generators) that they use in space craft.
George H. If all goes well this paragraph will trigger Neon
A depletion fet simplifies things a bit, and adds current limiting.
Some gate protection might be prudent.
-- John Larkin Highland Technology, Inc picosecond timing precision measurement
Any old dc/dc converter (or a battery!) can power the totem-pole optocoupler, instead of the PV.
The PV alone, with a gate discharge resistor, is all you need for a slow loop. The totem-pole opto speeds things up.
For digital apps, just buy one of the Analog Devices or TI logic isolators that has a built-in power isolator. My circuit is only worth the hassle for linear applications, HV power supplies or amplifiers.
Transformer coupled gate drivers can be cheap and useful, too.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
I'm driving some big 1200 volt Cree SiC fets, and they work a lot better if swung to the gate abs-max limits, +20 and -10 volts. The internal gate resistance makes them like being over-driven to get fast turn-on and turn-off edges. I'm using those horrible little EPC GaN fets as the gate drivers into the SiC fet.
Have you looked at SiC instead of IGBTs? They switch fast. Cree has some 1700 volt parts.
-- John Larkin Highland Technology, Inc lunatic fringe electronics
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