General > General Technical Chat

Quick TVS diode question. Vclamp < Vbreakdown

<< < (9/12) > >>

Psi:

--- Quote from: kellogs on September 11, 2022, 06:29:30 pm ---I don't get it... is the pump above supposed to charge C20 to, what, 3x Vcc - 6x Vf ? I think it does not. How do I charge the final cap to 4 or 5 times Vcc ?

--- End quote ---

The diode Vdrop is proportional to current and usually you would use schottky diodes for this circuit to minimize loss.
Something like 1n5819.
The Vf would then be more like 0.2V per diode, especially once everything has charged and current is small.
Also, 10uF caps is too large. The MCU GPIO wont be able to handle that except at very slow switching speed which will make it perform very bad.
I would run between 1u to 100n caps and use a much faster speed, like 10-100khz.
Then you'd probably get around 25V from that.   ( 4.8 + 4.6 + 4.4 + 4.2 + 4.0 + 3.8 )

Or even better, just get one of those little isolated 5V to 5V DCDC brick for $2 and you can have 5V referenced to whatever point you want. https://www.digikey.co.nz/en/products/detail/gaptec-electronic/1S4E-0505S1-5U/13691708

kellogs:


Lower caps, Schotkies, got it.


--- Quote from: Psi on September 12, 2022, 12:40:20 pm ---
Then you'd probably get around 25V from that.   ( 4.8 + 4.6 + 4.4 + 4.2 + 4.0 + 3.8 )

Or even better, just get one of those little isolated 5V to 5V DCDC brick for $2 and you can have 5V referenced to whatever point you want. https://www.digikey.co.nz/en/products/detail/gaptec-electronic/1S4E-0505S1-5U/13691708

--- End quote ---

So that pump does go beyond 2x Vcc, interesting.

Why are those ethernet bricks better ?

Psi:

--- Quote from: kellogs on September 12, 2022, 01:10:46 pm ---
So that pump does go beyond 2x Vcc, interesting.


--- End quote ---

Ops, sorry. I'm wrong.  it's only 3 stage not 6 stage.
(The 3 stages will be around 8.5V then 11.5V then output 15V with maybe 1-2mA loading.   100khz 1uF caps)
I should go to bed instead of posting on here.



--- Quote from: kellogs on September 12, 2022, 01:10:46 pm ---Why are those ethernet bricks better ?

--- End quote ---
Because they are isolated so you can generate 5V relative to any point you want. Excellent for generating some amount of voltage above the voltage at the source pin of an n-channel mosfet (high side switch)

kellogs:

--- Quote from: T3sl4co1l on September 12, 2022, 01:21:41 am ---
But this is a lot of bother when you can just get an LT surge stopper chip ready to go or whatever.  They're spendy but that's a lot of faffing around you save.


--- End quote ---

Ok, having read through their datasheets only reaffirmed what I already knew. I am way to clueless to make this work. So yeah, a surge stopper, totally!

BUT!

Best there is: "Reverse Input Protection to –60V". In the meanwhile, ISO 7637-2-2011 talks about -220V spikes...

BUT(2)!

Such bidirectional TVS on input makes me think the thing can actually do -220V as well, as long as the TVS diode can handle the energy left after spike has been clamped:





--- Quote ---Oh, and... how to protect Q3's GS junction ?  :-[

The... same way Q1 is??

Tim

--- End quote ---

Sort of like this ? I think 47k resistance should be fine for Q3. V_z = 20V as Q3 will likely have Vgs_max at that point.



Thanks a bunch!

T3sl4co1l:

--- Quote from: kellogs on September 12, 2022, 04:34:05 pm ---Best there is: "Reverse Input Protection to –60V". In the meanwhile, ISO 7637-2-2011 talks about -220V spikes...

BUT(2)!

Such bidirectional TVS on input makes me think the thing can actually do -220V as well, as long as the TVS diode can handle the energy left after spike has been clamped:

--- End quote ---

I already?... :palm: It's a low energy pulse!  It's not plugging into the wall, a TVS is fine!



--- Quote ---Sort of like this ? I think 47k resistance should be fine for Q3. V_z = 20V as Q3 will likely have Vgs_max at that point.

--- End quote ---

Uh, check the drain current before you wire that up...

Also, you've already got a 6.8V zener in there, why not like...

...Or you didn't understand the choice of that, before, either, but, why not ask instead of, I guess copy-pasting something else--?

Perhaps, I've overestimated your level; these examples seem roughly reasonable, but then your understanding of them seems quite lacking.  Should this [subthread] actually be in the beginner section, say?

Not to belittle you, mind -- well, admittedly, maybe a little bit, in as much as giving a bite for this apparent discrepancy :P -- but still not much, after all, fake it long enough, you will eventually make it! :D -- but just from the point of: if you have a particular, immediate need, then please lay out those requirements, and let someone solve it for you, don't worry about understanding it; or if you have a general need for which understanding and knowledge is required, then show us where to start from and we can build upon that.

And these are not exclusive options [practical vs. theoretical], the one can motivate the other; but make it clear what your aim is, and be open to examples alongside, but different from, your particular need, as a means of illustrating how the outputs (component choice, values, topologies) can vary with the inputs (requirements).

Or more generally still: learning how to learn.  Use your existing knowledge to guide the acquisition of new knowledge.  This circuit could be discussed top-down (it has to do X, Y and Z, using T, U and V components in the process; which in turn operate as....), or bottom-up (we're inevitably going to need diodes, transistors, etc., and they operate in this and that way....), or anything inbetween.  Figure out what works for you and guide the conversation as best suits you. :-+

Cheers!

Tim

Navigation

[0] Message Index

[#] Next page

[*] Previous page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod