Author Topic: Quick TVS diode question. Vclamp < Vbreakdown  (Read 6829 times)

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Offline T3sl4co1l

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Re: Quick TVS diode question. Vclamp < Vbreakdown
« Reply #25 on: September 03, 2022, 05:02:09 pm »
Wow, I need to revisit college notes...

>> then we can take its slope of ~ -0.443

How did you derive this number out of that logarithmic graph in fig.1 ?

Take the rise and run.  Take the log of each, then take the ratio of logs.  That is:

\[ a = \frac{ \ln \frac{y_2}{y_1} }{ \ln \frac{x_2}{x_1} } \]

for pairs of points on the curve (x1, y1), (x2, y2) such that \$y \propto x^a\$.  I noticed that, for 1 decade (10 times) of y, there was about a 180 times span in x -- you don't need absolute points, you can read the ratios off the axes directly.

Then, say, knowing that you need to extrapolate 10 times further than the plot shows, go \$10^a\$ times further along the other axis.


Quote
>>The load dump peak power is Vc * (Us - Vc) / Ri or 4.8kW,

 Of course (2)! Also, reading the note on load dump impulse in my attached picture a few posts back:

Quote
a
 If not otherwise agreed, use the higher voltage level with the higher value for internal resistance, or use the lower voltage level with
he lower value for internal resistance.

Does that mean that under worst circumstances, for the diode in question, P_peak yields about 29 * 50 / 0.5 =~ 3 kW ?

That or 29 * 72 / 4, which is lower, so yeah.  Or inbetween values, which I guess would be at customer request (if someone (OEM, say?) is looking for a particular combination).


Quote
>>probably two 30kW in parallel

Would that not require perfectly matched V_clamp ?

Not so much: Vclamp is moderately resistive, and has a positive tempco.  They might not share evenly at first, but once the die heats up, both will be bearing the load.

Which is why you need extra capacity: they won't track perfectly, so you lose some difference in capacity.  The result is strictly more capacity than a single, but how much more does depend.

I wouldn't have a problem using several in parallel at say... 30% derating?  Or less in greater numbers.


Quote
These 15 kW diodes are over budget anyway, so, if MOVs are that bad... reed switches! Yes I would actually use a comparator to trigger the reed relay(s). Many of them are specc'ed at 1ms max engage time, which would be great I think. Most of the automotive impulses are ~ uS long, so the reed switch would not even engage them - a smaller TVS diode may be employed for them. And when the big one hits, then TVS takes out a bit of their energy and then 1ms later reed switch relays the rest of it into an 1M resistor or such. But... their carry / switching current ratings are quite lousy. What are they going to do, weld into place ?

Engage, sure, what about disengage time?  What's a 1M going to do?  (Did you mean 1m?)  You don't have many ms left before it has to open into the full 100A surge.

Welded?  Melted?  Absolutely.  The contacts are quite small.

If only we had some kind of technology, that could switch faster... ???

Tim
« Last Edit: September 03, 2022, 05:04:35 pm by T3sl4co1l »
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Offline kellogs

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Re: Quick TVS diode question. Vclamp < Vbreakdown
« Reply #26 on: September 04, 2022, 01:35:28 pm »
Got it, thank you! Actually t_d will be 400/2 = 200 ms, not 100 ms. That takes the diode ability further down to 1.45 kW, so 3 of these in parallel is probably what I need. Too expensive...

Reed switches will be cheaper if I can make them equally share current. Is the 1M resistor set up like below not suited for absorbing the load dump energy ?

 

Offline T3sl4co1l

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Re: Quick TVS diode question. Vclamp < Vbreakdown
« Reply #27 on: September 04, 2022, 03:52:57 pm »
What energy, it's a meg?  It doesn't need to go anywhere, you can just open-circuit it...

You know that 100V 5A MOSFETs are cheaper than reed relays..?

Tim
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Offline kellogs

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Re: Quick TVS diode question. Vclamp < Vbreakdown
« Reply #28 on: September 04, 2022, 07:20:25 pm »
Right, of course...

Hmm, there are also (shorter) pulses defined for +150V and -220V... And what about their driving BJTs, these will need some sort of protection as well.
« Last Edit: September 04, 2022, 07:22:42 pm by kellogs »
 

Offline T3sl4co1l

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Re: Quick TVS diode question. Vclamp < Vbreakdown
« Reply #29 on: September 04, 2022, 09:09:38 pm »
What about BJTs?  Like for general purpose load switching (lamp/solenoid/etc. driver)?

A small TVS or MOV handily absorbs the higher voltage, shorter, and importantly: higher impedance, pulses.  With the inductance trick if you like. :)  A typical MOSFET load dump switch won't respond fast enough for those, passing such pulses easily to a TVS.

Tim
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Offline kellogs

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Re: Quick TVS diode question. Vclamp < Vbreakdown
« Reply #30 on: September 06, 2022, 11:05:07 pm »
Hmm... perhaps that could work. How about adding an TVS diode (+ maybe inductor & MOV trick) to this diagram



With some caveats:

- substitute U1 and U6 with at least 101V able P-mosfets. Actually just U1 I think, U6 should be disengaging -220V spikes, I wonder if fast enough...
- parallelize U1 and U6 into a number of P-mosfets each in order to avoid heat issues and voltage drop
- work these resistors to adjust for +150V spikes on the small Zeners + fat one  (-220V too?), and maybe not screw up whatever balance the current values achieve

What do you think ?
 

Offline T3sl4co1l

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Re: Quick TVS diode question. Vclamp < Vbreakdown
« Reply #31 on: September 07, 2022, 02:09:51 am »
Something like that.

Note that TL431 can't saturate less than a Vbe below VREF, so won't turn off a 2N7002.  Some voltage offset could be added to M1 source, or a P-type inverter used.

Also note you want hysteresis to prevent chatter as it goes on and off.  Your load is going to be pretty significant after being off for a few 10s-100s ms. :)

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Offline kellogs

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Re: Quick TVS diode question. Vclamp < Vbreakdown
« Reply #32 on: September 07, 2022, 09:23:30 pm »
Your load is going to be pretty significant after being off for a few 10s-100s ms. :)

Tim

Could you elaborate on this ? Not sure what to make of it.

Also, does a 1uH inductor look right for the clamping voltages involved in 12V automotive ?
 

Offline T3sl4co1l

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Re: Quick TVS diode question. Vclamp < Vbreakdown
« Reply #33 on: September 07, 2022, 09:47:50 pm »
I mean when it switches back on, charging your input caps will surely make it chatter.

1uH is probably fine, also helps with general filtering, put some small caps around it (couple uF?).

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Offline kellogs

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Re: Quick TVS diode question. Vclamp < Vbreakdown
« Reply #34 on: September 07, 2022, 10:23:27 pm »
I mean when it switches back on, charging your input caps will surely make it chatter.
Tim

Ok... but why ? And which part is going the chatter to form at, the fat Zener ?
 

Offline T3sl4co1l

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Re: Quick TVS diode question. Vclamp < Vbreakdown
« Reply #35 on: September 07, 2022, 10:39:55 pm »
The... TL431 and switch, it's being used as a comparator?
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Offline kellogs

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Re: Quick TVS diode question. Vclamp < Vbreakdown
« Reply #36 on: September 10, 2022, 02:35:00 pm »
Err... right! Already over my head. How about this other one - trying to protect the FETs Q2 and Q3 with a kit for reverse spikes made of

Q1 (a and b) R1 (a and b) R1 and D1. Normally each of the Q1 should carry around 1A.

A positive spike (up to 150V per ISO) will either pass through Q1 and Q3 ending up at R2 and at the lamp if it happens when it is already lit
A positive spike (up to 150V per ISO) will either pass through Q1 ending up at Q2 ad Q3 when the lamp is not already lit

A negative spike (up to 220V per ISO) will always be clamped by the Q1 D1 R1 kit in some, perhaps, 10us then be kept at Q1 drains.

MCU branch of the circuit, due to its low current draw, has its own protective circuitry consisting of fast TVS and rectifier diodes + limiting resistor.

Taking care at the P-FETs for rds_on and current, does this stand a chance in real life ?

Thanks!


« Last Edit: September 10, 2022, 02:38:17 pm by kellogs »
 

Offline T3sl4co1l

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Re: Quick TVS diode question. Vclamp < Vbreakdown
« Reply #37 on: September 10, 2022, 09:50:03 pm »
Not sure about your definition of "spike" here: those PMOS might be 10nF+ equivalent, so that 47k gives a time constant of some 470us, not 10!  But that's easily solved with a diode clamp from GND to G.  Maybe with a clamp diode to S as well, to avoid dumping surge current through the 6.8V zener there, or a (much smaller) limiting resistor on said clamp diode (>10 ohm?).

Likewise Q3 seems to be exceeding Vgs(max).  Just an oversight I guess.

Adding a separate path for an MCU is a poor idea for both efficiency and EMC, but I suppose can be done.

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Offline kellogs

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Re: Quick TVS diode question. Vclamp < Vbreakdown
« Reply #38 on: September 11, 2022, 06:29:30 pm »

Adding a separate path for an MCU is a poor idea for both efficiency and EMC, but I suppose can be done.

Tim

I'll deal with that later :)

I so wanna use one N channel  MOSFET instead of these parallel high Rds_on P-MOSFETs for the negative spikes protection

They say:



These gate driver ICs seem to all have this drawback - they are tied to V_in = 12V, the one I am trying to protect my circuitry from. As far as I can tell, not an option without a never ending rabbit hole.

Solution: a charge pump which has V_in = 5V already protected from MCU side of circuit. PWM will come form MCU as well, +5 / 0V



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 ?

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

Offline T3sl4co1l

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Re: Quick TVS diode question. Vclamp < Vbreakdown
« Reply #39 on: September 12, 2022, 01:21:41 am »
Don't make an absolute rail, make a relative rail.

For example, use a gate driver supplied from +12V (or since not much current should be required here, a CD4xxx gate would also do), make a square wave say of some 10s to 100s kHz, and connect a capacitor in series from its output to the middle of a series diode pair, anode to source, cathode to +12BS (bootstrap).  Also put a zener from S to +12BS to drain extra current in case the charge pump is doing "too well" (mainly due to voltage swing on the source).

Level shifting is required, for which a regular bootstrap type driver (IR2101 etc.) is fine, leave the low-side channel pulled to GND (or, use it to power the charge pump!) and drive the high side from logic.

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.


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

The... same way Q1 is??

Tim
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Offline PsiTopic starter

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Re: Quick TVS diode question. Vclamp < Vbreakdown
« Reply #40 on: September 12, 2022, 12:40:20 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 ?

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
« Last Edit: September 12, 2022, 01:35:47 pm by Psi »
Greek letter 'Psi' (not Pounds per Square Inch)
 
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Offline kellogs

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Re: Quick TVS diode question. Vclamp < Vbreakdown
« Reply #41 on: September 12, 2022, 01:10:46 pm »


Lower caps, Schotkies, got it.


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

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

Why are those ethernet bricks better ?
 

Offline PsiTopic starter

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Re: Quick TVS diode question. Vclamp < Vbreakdown
« Reply #42 on: September 12, 2022, 01:25:27 pm »

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


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.


Why are those ethernet bricks better ?
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)
« Last Edit: September 12, 2022, 01:43:31 pm by Psi »
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Offline kellogs

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Re: Quick TVS diode question. Vclamp < Vbreakdown
« Reply #43 on: September 12, 2022, 04:34:05 pm »

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.


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:

1588345-0


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

The... same way Q1 is??

Tim

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.

1588351-1

Thanks a bunch!
 

Offline T3sl4co1l

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Re: Quick TVS diode question. Vclamp < Vbreakdown
« Reply #44 on: September 12, 2022, 08:01:02 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:

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.

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
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Offline kellogs

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Re: Quick TVS diode question. Vclamp < Vbreakdown
« Reply #45 on: September 12, 2022, 11:29:05 pm »
So the thing will protect against negative 220 spikes as well, looks like a winner then! Why don't they make it more clear in the datasheet... :D
Just one more question about these surge stoppers, will they also protect fast Schotky diodes in series with them, when such diodes are placed before the protector IC, in line with the two power mosfets ?

Sir, copy pasting is what all great engineers do, not only the rookies; perhaps the source of confusion. I thought it was pretty obvious my level was not that high with all these questions.

P.S. - would have never thought to arrange Q3's Zener that way  :-+
 

Offline T3sl4co1l

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Re: Quick TVS diode question. Vclamp < Vbreakdown
« Reply #46 on: September 13, 2022, 05:26:51 am »
What do you need a schottky for if you have back-to-back MOSFETs?

Tim
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Offline kellogs

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Re: Quick TVS diode question. Vclamp < Vbreakdown
« Reply #47 on: September 13, 2022, 11:23:14 am »
I want some energy to run the MCU a bit after 12V went away. I guess I can put the storage cap + diode after surge stopper too, but probably same deal since they are in series.
« Last Edit: September 13, 2022, 01:45:41 pm by kellogs »
 

Offline T3sl4co1l

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Re: Quick TVS diode question. Vclamp < Vbreakdown
« Reply #48 on: September 13, 2022, 05:42:06 pm »
Oh in a separate path, that's fine.  The diodes won't care, but you get the full load dump out there so probably need a depletion MOS style voltage limiter.

Tim
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Offline kellogs

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Re: Quick TVS diode question. Vclamp < Vbreakdown
« Reply #49 on: September 17, 2022, 07:36:49 pm »
Hello,
Tim, sir, can i have a bit more help please ?

I want to take this diagram:

1592947-0

And adapt it to also guard against +100V load dumps besides negative -220 spikes. Peeking at:

1592953-1

i would make the following changes:

- replace FETs with some 250V capable ones

- keep the SMAJ58CA but use it like the SMAT70A from the second diagram, together with an 1W resistor

- replace Rsns with a 6 mohm value and a max current of 50 mV / 6 mohm ~= 8.33 A

Any obvious faults ?
Thanks
 


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