Author Topic: Quick TVS diode question. Vclamp < Vbreakdown  (Read 6826 times)

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Offline T3sl4co1l

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Re: Quick TVS diode question. Vclamp < Vbreakdown
« Reply #50 on: September 17, 2022, 08:04:06 pm »
Hmm, minimum rating is -60V, you'll have to do something about that. Can you not afford a series diode?

Tim
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Offline kellogs

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Re: Quick TVS diode question. Vclamp < Vbreakdown
« Reply #51 on: September 18, 2022, 09:47:37 am »
Is the LT4356 not going to survive -220 shocks because of its GND, TMR and FB connections ?

Series diodes get hot really quickly at my amperage, and device will be mounted in crammed hot place already.
 

Offline T3sl4co1l

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Re: Quick TVS diode question. Vclamp < Vbreakdown
« Reply #52 on: September 18, 2022, 01:24:30 pm »
Maybe just use the "passive" PMOS polarity protection thing in front, not integrating it with the LT chip.

Also, again, you're worrying about 220V like the terminal is going to reach -220.00V with respect to ground? You know what impedance means right?  It's not plugging into mains here.

Tim
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Offline kellogs

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Re: Quick TVS diode question. Vclamp < Vbreakdown
« Reply #53 on: September 18, 2022, 01:41:16 pm »
220V PMOS @8A would be worse than series diodes... best I could find was Rds_on of 0.5 ohm (typical value)

Hmm, impedance, let me check - Ri = 50 ohm and Us = -220V, in series with all of the grounded surge stopper IC pins... no, in series with...  :-//
But why do they say protection up to -80V if the chip itself can only do -60V ? Page 21 here, first diagram titled "Overvoltage Regulator with Reverse Input Protection Up to –80V"
Side note, those IRLR2908 are rated to 80V.



 
 

Offline T3sl4co1l

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Re: Quick TVS diode question. Vclamp < Vbreakdown
« Reply #54 on: September 18, 2022, 01:44:57 pm »
Yeah like that. Note also 2N3904 rating. Or MMBT3904 more likely.  1N914 for that matter.

But why do they say protection up to -80V if the chip itself can only do -60V ?

Vin doesn't directly touch any IC pins...

Tim
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Offline kellogs

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Re: Quick TVS diode question. Vclamp < Vbreakdown
« Reply #55 on: September 18, 2022, 02:14:07 pm »
1n914 ?

Quote
Hmm, minimum rating is -60V, you'll have to do something about that

Ok, so... if chip does not touch Vin, where is the problem ?
 

Offline kellogs

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Re: Quick TVS diode question. Vclamp < Vbreakdown
« Reply #56 on: September 19, 2022, 09:14:47 am »
Well, I ain't hell bent on understanding what's the matter. Would be nice if I would though.

In the meanwhile, how about using the MCU for protecting the bigger load ?



Q1 = 250V N channel MOSFET
Q2 = logic level N MOSFET
D1 = standard variety 0.5 - 5 kW TVS
D2 = probably not needed, some very low value Zener

MCU has a 330 uF reservoir so it will still operate even on a negative spike; its 5V is protected separately. This ought to work, right ?

LE: actually Q1 will be two back to back MOSFETS
« Last Edit: September 19, 2022, 11:27:30 am by kellogs »
 


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