Surviving automotive load dump

Transconductance devices, yes, with their operating drain current controlled by their gate voltage. But that's not a current-regulating capability as we think of it, because you have to separately supply a programming voltage. Depletion-mode current-regulating FETs work without an external voltage: A self-biasing gate-drain current-sensing resistor nicely does the job.

It's just a simple matter of power dissipation and thermal resistance vs. your maximum junction temperature spec.

I don't want to dissapate much power in normal use, so I have a SMPS downstream.

How about this:

Supertex DN3525 Ferrite .------------. ----UUU-->|-----+|+---------| | | | |V| | | SMPS | z .-. === | | | A 10k| | | | '------------' | | | | | | 150V TVS '-' | --- | | '------' --- | | | |100uF | | z | | | A 33V | | | | | | ------------------------------------------

I'm not sure. The way I read the data sheet, the device cuts of for VGS = -2.2V, and normal operation should maintain VGS >= 0

And what is the hold-up time for your SMPS? As someone else pointed out, load dump can last over 400ms.

...Jim Thompson

I think you may be reading it wrong (or perhaps the mosfet symbol is wrong). It is supposed to work like an emitter follower. The 33V zener should limit the downstream voltage to ~33V.

The circuit should keep giving out 33V during the load dunp.

I understood that.

I'll give it a whirl on PSpice.

How much current is the SMPS drawing?

...Jim Thompson

I should clarify that the 200mA is the expected *load* current. The supply current to the SMPS obviously vary with supply voltage.

"Winfield Hill" schrieb im Newsbeitrag news: snipped-for-privacy@m58g2000cwm.googlegroups.com... | > Even enhancement-mode MOSFETs have a current-regulating capability. | > See the output diagram in their datasheets. Depends on Vgs and Id. | > I don't know if this mode is effective in every type made. But I've seen | > that in every datasheet I seen. | | Transconductance devices, yes, with their operating drain current | controlled by their gate voltage. But that's not a current-regulating | capability as we think of it, because you have to separately supply a | programming voltage. Depletion-mode current-regulating FETs work | without an external voltage: A self-biasing gate-drain current-sensing | resistor nicely does the job.

Yes.

| | > Maybe there is a power problem inside which could destroy the | > MOSFET. I never thought it to the end. Suggestions here? | | It's just a simple matter of power dissipation and thermal resistance | vs. your maximum junction temperature spec.

Some MOSFET specs specifiy a maximum Id regardless of other values. So I'm not sure.

- Henry

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I'm not quite sure how to interpret the "pulsed" safe operating area chart. I couldn't find the pulse width definition on the data sheet, though I didn't look really hard ;-)

...Jim Thompson

Same thing, except that spec attempts to describe the maximum continuous power dissipation capability of the lead-bonding wires.

Worth playing with on the bench...

It seems to work nicely in simulation. There is a current limit effect too due to the characteristics of the fet. I also added a capacitor across the zener for a slow-start. This reduces the inrush current to "Idss" (fet current at 0V gate drive).

The current is only ~30mA during the load dump, thanks to the SMPS taking a lot less at high input voltage.

The alternative to all this is Joe Legris' suggestion ___ o>|--|___|-o----o----o--- 1N4007 3R | | | z --- --- A === --- | | | o----------o----o----o-- SA28A 100u 100n

I am finding the resistor to be the problem here. High surge resistance seems to imply Wirewound, which are relatively expensive, especially surface mount ones. (While searching for some, I note the Ohmite site says that all their "high energy" ones are wound by hand!)

A 2W or 3W one is quite large too.

per my experience, 27V MOV is right solution and you slo can selsect PTC. load dump is a commom requirement at automotive application. some OEM sugget to use MOV, but i have seen JCI's board that use PTC. you can use surface mounted TVS.

"John Devereux =D0=B4=B5=C0=A3=BA "

Hmm, last time i knew, PTC positive Thermo coefficient devices?

"Winfield Hill" schrieb im Newsbeitrag news: snipped-for-privacy@m58g2000cwm.googlegroups.com... | Henry Kiefer wrote: | > Winfield Hill wrote | >>

| >> It's just a simple matter of power dissipation and thermal resistance | >> vs. your maximum junction temperature spec. | >

| > Some MOSFET specs specifiy a maximum Id regardless of other values. | > So I'm not sure. | | Same thing, except that spec attempts to describe the maximum | continuous power dissipation capability of the lead-bonding wires.

Interesting. The problem is that the wire system is not specified in most datasheets.

Seems, that I must do experiments.

- Henry

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news: snipped-for-privacy@m58g2000cwm.googlegroups.com...

datasheets.

It's usually in footnotes along the lines of "total current limited by package"

Robert

"Robert Adsett" schrieb im Newsbeitrag news:45aa61b1$0$19423$ snipped-for-privacy@free.teranews.com... | In article , Henry | Kiefer says... | > "Winfield Hill" schrieb im Newsbeitrag news: snipped-for-privacy@m58g2000cwm.googlegroups.com... | > | Henry Kiefer wrote: | > | > Winfield Hill wrote | > | >>

| > | >> It's just a simple matter of power dissipation and thermal resistance | > | >> vs. your maximum junction temperature spec. | > | >

| > | > Some MOSFET specs specifiy a maximum Id regardless of other values. | > | > So I'm not sure. | > | | > | Same thing, except that spec attempts to describe the maximum | > | continuous power dissipation capability of the lead-bonding wires. | >

| > Interesting. The problem is that the wire system is not specified in most datasheets. | | It's usually in footnotes along the lines of "total current limited by | package"

Yes. Sometimes.

To place the problem a level higher: If it is so, that MOSFETs cannot be burned if the bond wire is thick enough. Why it is then, that sometimes Microcontroller pins destroy to internal short if overloaded?

- Henry

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FWIW I found the Ford EMC spec here

This is also interesting, a comparison between VM EMC specs

martin

Thanks martin, I hadn't seen these. These car manufactures standards quote a maximum "load dump" spec of 60V (but I have seen 125V mentioned elsewhere).