Author Topic: Quick TVS diode question. Vclamp < Vbreakdown  (Read 6825 times)

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Offline PsiTopic starter

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Quick TVS diode question. Vclamp < Vbreakdown
« on: July 24, 2022, 12:33:19 am »
I see TVS diodes sometimes where the Imax clamping voltage is lower than the 1mA breakdown voltage.
Which, all things being equal, doesn't seem to make sense unless something else is going on.
I did some googling but didn't find a satisfactory answer as to why this occurs.
Do thermal effects of the power dissipation change its properties and lower the breakdown point?
eg, are Vclamp and Vbreakdown exactly the same property just measured at active vs inactive temperature.
« Last Edit: July 24, 2022, 12:37:06 am by Psi »
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Offline niconiconi

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Re: Quick TVS diode question. Vclamp < Vbreakdown
« Reply #1 on: July 24, 2022, 09:50:45 am »
Some high-end ESD protection diodes for high-speed data lines use the thyristor principle to create negative resistance - once triggered, they remain in conduction until the voltage drops to a level much lower than the initial trigger voltage. These are marketed as "snapback TVS diodes".

There's also a related technology called Thyristor Surge Suppressor (TSS), commonly used for telecom applications to clamp huge surges in telephone lines. Large TSS can compete with Gas Discharge Tubes (GDT).

But if you are talking about ordinary TVS diodes, I don't know the answer.



« Last Edit: July 24, 2022, 09:56:25 am by niconiconi »
 
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Offline PsiTopic starter

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Re: Quick TVS diode question. Vclamp < Vbreakdown
« Reply #2 on: July 24, 2022, 01:05:13 pm »
Here's an example of what i mean. It doesn't seem to be a snapback, or it doesnt say that anywhere.

The graph in fig 3 on page 4 looks normal, ie no inversion/hysteresis like in your example graph.
But the clamp is still less than the breakdown which seems to go against what fig3 shows

https://assets.nexperia.com/documents/data-sheet/PESD5V0C1USF.pdf

« Last Edit: July 24, 2022, 01:07:02 pm by Psi »
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Offline T3sl4co1l

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Re: Quick TVS diode question. Vclamp < Vbreakdown
« Reply #3 on: July 24, 2022, 09:32:50 pm »
Yeah, snapback:
https://assets.nexperia.com/documents/leaflet/Nexperia_TrEOS_ESD_protection_for_USB_Type-C_leaflet.pdf


Some high-end ESD protection diodes for high-speed data lines use the thyristor principle to create negative resistance - once triggered, they remain in conduction until the voltage drops to a level much lower than the initial trigger voltage. These are marketed as "snapback TVS diodes".

Just to clarify -- snapback isn't a thyristor (4-layer) mechanism, it's... punch-through, I believe?  So, a 3-layer structure, like a BJT without a base connection, and with a light enough doped base that it fully depletes at some point, effectively shorting from C to E.  (I might have this wrong, but ESD protection features are notoriously secret-sauce, so that's not necessarily my fault. :P )  When this is tuned for just the right breakdown voltage, it can happen that a fair amount of current is already flowing (avalanche/zener mode) by the time the negative resistance effect pulls in, and the negative resistance effectively compensates for the bulk resistance of the device, hence flattening the curve -- or at least not having such a severe "snapback" as for thyristor devices.

They can be made in much lower voltages than avalanche types (which are zeners below 5V, so, basically useless for surge purposes!), and are the only type that offers effective protection for systems 3.3V and below.

they're also faster, as it only depends on the motion of carriers, rather than diffusion of minority carriers (thyristors are fast enough for surge, which they excel at, but aren't suitable for ESD, AFAIK).

Tim
« Last Edit: July 24, 2022, 09:39:28 pm by T3sl4co1l »
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Offline Benta

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Re: Quick TVS diode question. Vclamp < Vbreakdown
« Reply #4 on: July 24, 2022, 11:19:55 pm »
It seems that the "clamping voltage" is the rise above the "breakdown voltage" related to current.
 

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Re: Quick TVS diode question. Vclamp < Vbreakdown
« Reply #5 on: July 25, 2022, 12:03:40 am »
Yeah, snapback:
Also, Figure 7 of the datasheet.

Here's an example of what i mean.
Max Standoff > Typical Conduction
Nothing fishy about that, even without snapback.
 

Offline jonpaul

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Re: Quick TVS diode question. Vclamp < Vbreakdown
« Reply #6 on: July 25, 2022, 02:01:09 am »
4 layer, snapback, are not TAZ.

See extensive papers and app notes from TAZ Mfg like MicroSemi, Claire, General Semiconductors from 1970s..1980s.

I gave a paper on this tipoc at PCIM 1983 Paris with a TAZ mfg.

Indeed  there's a tempco.

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« Last Edit: July 25, 2022, 02:23:26 am by jonpaul »
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Offline mag_therm

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Re: Quick TVS diode question. Vclamp < Vbreakdown
« Reply #7 on: July 27, 2022, 03:57:36 pm »
Last month a FET here failed during a thunderstorm.
It was on a HF antenna longwire, downstream of a toroidal isolating transformer followed by resonant LC.
I think I need  OVP with capacitance  > 20 pF and fast acting  ~ 50 nsec hopefully to minimise ringing  and Hi Z below -10dBm,   to be placed across antenna input terminal. Already have a 1 A 3AG fuse which did not fail.

Any suggestion for a device  would be appreciated. I know various PL 259 insertion OVP are available from ham suppliers but I want to do it at component level.
 

Offline Benta

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Re: Quick TVS diode question. Vclamp < Vbreakdown
« Reply #8 on: July 27, 2022, 09:23:30 pm »
Last month a FET here failed during a thunderstorm.
It was on a HF antenna longwire, downstream of a toroidal isolating transformer followed by resonant LC.
I think I need  OVP with capacitance  > 20 pF and fast acting  ~ 50 nsec hopefully to minimise ringing  and Hi Z below -10dBm,   to be placed across antenna input terminal. Already have a 1 A 3AG fuse which did not fail.

Any suggestion for a device  would be appreciated. I know various PL 259 insertion OVP are available from ham suppliers but I want to do it at component level.

The HP engineers solved this brilliantly in the legendary 34401 DMM.
A TVS in series with a GDT suppressor works great. It has extremely low capacitance due to the GDT and will not interfere with your signal.
But the combination will take a lot of abuse.
 
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Offline thm_w

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Re: Quick TVS diode question. Vclamp < Vbreakdown
« Reply #9 on: July 27, 2022, 10:17:09 pm »
Last month a FET here failed during a thunderstorm.
It was on a HF antenna longwire, downstream of a toroidal isolating transformer followed by resonant LC.
I think I need  OVP with capacitance  > 20 pF and fast acting  ~ 50 nsec hopefully to minimise ringing  and Hi Z below -10dBm,   to be placed across antenna input terminal. Already have a 1 A 3AG fuse which did not fail.

Any suggestion for a device  would be appreciated. I know various PL 259 insertion OVP are available from ham suppliers but I want to do it at component level.

The HP engineers solved this brilliantly in the legendary 34401 DMM.
A TVS in series with a GDT suppressor works great. It has extremely low capacitance due to the GDT and will not interfere with your signal.
But the combination will take a lot of abuse.

Varistor and GDT:

"RV100-RV102 Diode-Varistor 1.1KV"
"E100-E101 Tube Electron Surge Arrestor 1500V/10A"
https://xdevs.com/doc/HP_Agilent_Keysight/service/34401A/34401A%20service.pdf

I guess the reason is only to limit the clamping voltage (glow voltage) of the GDT, as that can be too low to the point of blowing a fuse somewhere (<150V).
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Offline mag_therm

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Re: Quick TVS diode question. Vclamp < Vbreakdown
« Reply #10 on: July 27, 2022, 11:59:46 pm »
Thanks Benta and Thm_w

I had been thinking of GDT and TVS in parallel, but now I will do series. I can get some GDT from Surplus Sales.

I have put in a station ground close by, per ARRL recommendation consisting of two driven copper plated rods 2.5 m (8ft) long spaced 1200 mm.
 Before brazing on the 4AWG leads, I measured the DC resistance between them with tractor battery, at 36 Ohm.
Then when trying later with a Ohm meter, the ground had charged up between them to 0.4 V DC and stayed like that.
So I don't know if the grounds will conduct a surge.
 

Offline T3sl4co1l

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Re: Quick TVS diode question. Vclamp < Vbreakdown
« Reply #11 on: July 28, 2022, 12:25:00 am »
Series is when you need a high turn-off voltage, such as for mains voltage protection.  For signal purposes such as telephone (POTS), antennas probably, Ethernet, etc., a raw GDT or thyristor will do.

Hmm, I wonder if parallel connection is reasonable anyway.  Let's see, they break down at a minimum voltage, but it takes time (some µs), and it's faster as you go up from there, I think up to an upper limit where breakdown is very fast.  Presumably, you could clamp that initial excess, given a TVS beefy enough (which isn't too bad, it's only a few µs), then the GDT kicks in and clamps the rest.

Looks like at least a few others have thought so:
https://itecnotes.com/electrical/electronic-two-gas-discharge-tube-in-parallel-a-fast-acting-gas-acting-tube-and-a-gas-discharge-tube/
so if that's a good enough reference, I guess that's something.  The case here with a series inductor between them, is nice: under the surge's dI/dt, the GDT gets much more voltage, triggering sooner and dumping less energy into the TVS.

Note if this is on something like feedline, treat the shield (ignore the signal inside it) as a single-ended line with respect to earth.  Mind you only get as good suppression as you have ground impedance.

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Offline mag_therm

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Re: Quick TVS diode question. Vclamp < Vbreakdown
« Reply #12 on: July 28, 2022, 01:59:12 pm »
Thanks Tim,
I will get some devices and test with a 200 V DC supply.

However, I ran some qucs on the circuit. If a pulse of only 50 V  and 30 ns on antenna  gets into the tuned circuit,
it rings, being high Q and  is all over for the fet which is rated 35 V between all.
If a TVS on input fires some time later to an ideal short it (depending on timing) increases the ring up voltage as Q of tank is raised by shorting the antenna Z.

So I will also add  back to back zeners at output of tuned circuit before fet.

Later Edit: For HF radio application, the MicroSemi  SA5.0CA Bi-Directional series might be suitable.
No snapback, < 5 nanosec, with a nominal 500 Watt at 1 millisec. in DO-41

SA14CA 14 V standoff for receivers and SA150CA for up to 100 Watt transceivers.

https://download.datasheets.com/pdfs/diode/mcs/msc1400.pdf
« Last Edit: July 28, 2022, 02:54:15 pm by mag_therm »
 

Offline Tomorokoshi

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Re: Quick TVS diode question. Vclamp < Vbreakdown
« Reply #13 on: July 29, 2022, 02:44:32 am »
I've been replacing damaged MOVs in Tripp-Lite Isobar power strips with the GMOV series from Bournes:

https://media.digikey.com/pdf/Data%20Sheets/Bourns%20PDFs/GMOV_Series_DS.pdf
 

Offline kellogs

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Re: Quick TVS diode question. Vclamp < Vbreakdown
« Reply #14 on: August 30, 2022, 05:56:04 pm »
Good thread!
And I am about to make a bunch of mistakes. Can you spot them ? :D

So I'd like to perhaps implement the solution from the link below:


https://itecnotes.com/electrical/electronic-two-gas-discharge-tube-in-parallel-a-fast-acting-gas-acting-tube-and-a-gas-discharge-tube/
so if that's a good enough reference, I guess that's something.  The case here with a series inductor between them, is nice: under the surge's dI/dt, the GDT gets much more voltage, triggering sooner and dumping less energy into the TVS.

for automotive device protection. I am going to take a nasty load dump for the first impulse to mitigate. Attached its specs. Instead of a GDT I shall use this guy, an GMOV-20D450K - which by the way is a 49J part so I see no way for it to be able to absorb the load dump energy; let me assume I am wrong here, moving on:

I've been replacing damaged MOVs in Tripp-Lite Isobar power strips with the GMOV series from Bournes:

https://media.digikey.com/pdf/Data%20Sheets/Bourns%20PDFs/GMOV_Series_DS.pdf


Does V_c in the datasheet table stand for V_clamp ? I am going to assume that as well. For the GMOV to *only* begin conducting the inductor should build up a voltage spike of 150V

150V = U_ind < L * di/dt

For the impulse rise, worst case: di = (U_s - U_alternator) / R_i = (101 - 14) / 0.5 = 174A and dt = 5ms

150 < L * 174 / 5
L > 150 * 5 / 174 mH
L_min = 4.45 mH

For the impulse fall, worst case:

U_ind = 4.45 * 174 / 40 = 19.36V

 - which, with a proper TVS, will not cause problems. But I would probably want more L so the GMOV will actually have time to conduct that energy away, perhaps a tenfold value, 44.5 mH? Any possible issues with the TVS or the rest of the circuitry to be protected?

Now, please correct me. Thank you!
 

Offline T3sl4co1l

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Re: Quick TVS diode question. Vclamp < Vbreakdown
« Reply #15 on: August 30, 2022, 08:52:04 pm »
L what?  Show an equivalent circuit.

If you're thinking about the source: load dump isn't an RLC equivalent circuit -- well, maybe you could design a pulse generator that way, but the real deal is an equivalent due to a positive feedback effect in the alternator; there will be some ESL still (winding inductance) but it isn't dominant, and it's modeled as a waveform from an ideal Thevenin source.

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Offline kellogs

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Re: Quick TVS diode question. Vclamp < Vbreakdown
« Reply #16 on: September 01, 2022, 10:56:25 am »
Ugh, L is the inductor in between the two protection devices in https://itecnotes.com/electrical/electronic-two-gas-discharge-tube-in-parallel-a-fast-acting-gas-acting-tube-and-a-gas-discharge-tube/

L_min would be my calculated minimum value for it.

Thanks
 

Offline T3sl4co1l

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Re: Quick TVS diode question. Vclamp < Vbreakdown
« Reply #17 on: September 01, 2022, 05:33:08 pm »
Ah, that way.

That's only needed for fast-ish rise times, where the GDT breakdown delay matters.  You're well past that here.  Just pick a GDT low enough to work.

More important is the GDT needs a TVS in series with it, so it doesn't short out the supply after the load dump.  It's a discharge, it stays latched on until current falls below holding.

Which is a TVS that still needs to handle the full current of said dump, so, doesn't save much compared to a full size TVS just clamping the whole thing.  The GDT also needs a quite generous energy rating, as its voltage drop during breakdown is some 10-20V or whatever.  I haven't checked but I'm guessing they're neither recommended for, nor available for, load dump application.

...You're also spark-discharging any capacitors on the bus, which, whatever, it's going to be less energy than load dump itself, but still, it's more stuff going on, and that extra backwards current might do weird things to attached devices, who knows.

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Offline kellogs

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Re: Quick TVS diode question. Vclamp < Vbreakdown
« Reply #18 on: September 01, 2022, 06:11:23 pm »
GDT in series with TVS would clamp the voltage way to high for automotive.

Back to the linked diagram, I would not use a GDT, but, as said, a MOV (more precisely the GMOV that @Tomorokoshi has mentioned - https://media.digikey.com/pdf/Data%20Sheets/Bourns%20PDFs/GMOV_Series_DS.pdf)

Which is a TVS that still needs to handle the full current of said dump, so, doesn't save much compared to a full size TVS just clamping the whole thing.

Huh? Tim sir, can I possibly get a practically sized TVS diode that would actually handle that whole blast of energy by itself? Oh, and within budget too would be nice :D - say max $10 a pop.
 

Offline T3sl4co1l

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Re: Quick TVS diode question. Vclamp < Vbreakdown
« Reply #19 on: September 01, 2022, 07:40:23 pm »
Just use the MOV straight up?  MOVs are just sloppier TVSs...

Huh? Tim sir, can I possibly get a practically sized TVS diode that would actually handle that whole blast of energy by itself? Oh, and within budget too would be nice :D - say max $10 a pop.

I'm sure the name brands get theirs for less; but then, I suppose your production quantity is somewhat less than 10 million/year, too, huh? ::)

:-DD

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Offline kellogs

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Re: Quick TVS diode question. Vclamp < Vbreakdown
« Reply #20 on: September 02, 2022, 07:41:59 am »
I shall start with, uhm, 10 per year :)

Which diode would that be anyway ? I am calculating 4 kJ of energy in that impulse; approximating rise and fall curves with straight lines

E = (U^2 / R_i) * 1/2 *(t_r +t_d)

Worst case: U = 101V, R_i = 0.5 \$\Omega\$, t_r + t_d ~= t_d = 400ms

Now, there is a bit of resistive wiring before 12V hit my PCB as well, and the TVS diode would present some resistance too. I should factor those in too, not sure what values to estimate them at. Does my calculation look right ?
« Last Edit: September 02, 2022, 02:48:40 pm by kellogs »
 

Offline T3sl4co1l

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Re: Quick TVS diode question. Vclamp < Vbreakdown
« Reply #21 on: September 02, 2022, 04:20:42 pm »
That would be the energy dissipated by the source, assuming it has real resistance, into a short circuit; at most your power is 1/4 that, because of power transfer theorem.  But your load or TVS won't be 0.5 ohm: the fact that Vc < Us/2 means E is less as well.

If we take more like Vc * (Us/Ri) * td, that's a better approximation of the energy absorbed by the load.  So, you can see TVS are superior for having Vc ~ 2 Vnom, whereas MOVs can be more like 3-4x (and they're worse in low voltages, besides).

If you're still not convinced that a voltage limiter (switch or regulator type) is viable in many situations... :)

Tim
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Offline kellogs

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Re: Quick TVS diode question. Vclamp < Vbreakdown
« Reply #22 on: September 02, 2022, 06:00:23 pm »
Of course! it is V_c * I, not U_s * I...

Ok I have found one that I guess would work, although marginally, perhaps not at all if I get unlucky - https://www.mccsemi.com/pdf/Products/15KP17(C)(A)-15KP280(C)(A)(R-6).pdf

Extending the P_pp - t_d graph yields some 1kW+ able to withstand over 200ms.

Do you know how would a 1A reed switch behave if battered with 2A? Will its life just shorten (to whatever, even 1% will be ok) or will it fail short circuit , or something else?
 

Offline T3sl4co1l

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Re: Quick TVS diode question. Vclamp < Vbreakdown
« Reply #23 on: September 02, 2022, 09:18:28 pm »
IIRC, 15kW parts are okay for lower levels of load dump, maybe not the worst case you've selected.  There are 30kW parts out there too.  The lower voltage rating helps.

Note that Fig.1 must eventually flatten out: the 8W continuous rating will take some 10s of s to stabilize at.  The sub-sqrt slope of the plot suggests limited thermal diffusion (exactly, diffusion would go as t^(-1/2)).  It's not clear if that would get steeper over package-scale time scales (i.e., it takes ~10s ms for heat to diffuse from die to lead frame and package).  There's nowhere to go beyond the package, so the curve should steepen at that point, until lead conduction and convection dominate, then finally flatten out at the 8W figure (in the ~10s s range).

If we assume the curve remains flat, then we can take its slope of ~ -0.443, and... let's see, the rated pulse is a 10/1000us waveform, but that's the 50% figure; helpfully, the curve shows it reaching 10% at about 4ms.  The load dump waveform takes 400ms to reach 10%.  If we simply equate these, then we need a 100x time figure, which would be 100^(-0.443) = 7.7 times lower power, or 1.95kW.

The load dump peak power is Vc * (Us - Vc) / Ri or 4.8kW, so it seems this part is undersized by ~2.5 times.

Which hey, that meshes with my expectation that it's fine for lower ratings, but not the full thing.  And in fact an even bigger one is needed then (probably two 30kW in parallel would be safe enough? -- three 15kW would be too marginal I would say, but four or more is probably fine).

Contrast with MOVs, for example:
https://www.yageo.com/upload/media/product/productsearch/datasheet/cpc/mov/20D_1.pdf
Nothing particular about this brand, I think they're all the same materials; ratings/specs are more or less industry standard, curves always look the same.

Notice 18-68V types are only rated 20Apk, and the 82V part clamps at the same voltage as the 68V but at 5x the current!  (LV MOVs rather stink.)

Looking at the clamping curves, the 18V and 22V parts clamp at 100A and 40/50V respectively, which is... useful enough, but the energy ratings are tiny.  (A much bigger disc would afford more energy, but likely not enough.)  The derating curves suggest for N = 2 about a t^(-1/2) slope, and the pulse is the same type so let's assume 10 times the highest (10ms) point or sqrt(10) less current, or about 13A.  Or 22A for the J version ("high surge").

They don't say here BTW, but the maximum current spec I believe is for 8/20us, which, let's see... lines up on the derating plots for single event, yeah.  So that's why that figure is so high.

So, for about a 100A surge at this level, you'd need more like a stack of 5 of these in parallel, preferably 7-10 for better sharing/reliability.  Or larger discs, but you'll still need multiple in parallel I think.

Note they aren't rated for leakage; that's probably a bad thing for automotive.

As for relays, you're not thinking of using one to disconnect the load, are you?  They don't respond fast enough (~1 to 10s of ms).

Tim
« Last Edit: September 02, 2022, 09:20:44 pm by T3sl4co1l »
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Offline kellogs

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Re: Quick TVS diode question. Vclamp < Vbreakdown
« Reply #24 on: September 03, 2022, 10:09:15 am »
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 ?

>>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 ?

 >>probably two 30kW in parallel

Would that not require perfectly matched V_clamp ?

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 ?
 


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