Surviving automotive load dump

How about a 200V mosfet as a disconnect switch during the transient?

...Jim Thompson

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|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

Hi Jim,

I did think of that - I seem to recall I couldn't find a suitable p-channel one (surface mount, small, cheap).

I forgot to say that it is feeding a 100uF capacitor, so there will be a bit of turn-on surge too.

I will look again though, I went through a lot of different ideas quite quickly and might have missed it.

--

John Devereux

Probably a depletion-mode power MOSFET as a current-source? See as intro:

- Henry

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"John Devereux" schrieb im Newsbeitrag news: snipped-for-privacy@cordelia.devereux.me.uk...

Well, that's pretty cool. The one in the application note doesn't look easy to get hold of; E.g. Digikey have never heard of them.

In fact I can only find *one* hit there for "depletion fet" - Clare CPC5603C

It is not very well specified, but it looks to me like the operating current is too low.

(I don't think I have ever seen a depletion mode FET - outside of "Art of Electronics" - will have to read up on them again now!).

Thanks,

--

John Devereux

They exist in some ASIC processes. I've used them as start-up devices in micro-current designs (like RFID tags)... get a bandgap started then kill the start-up current.

But I've not seen a discrete one.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

I just use 1N4007 to block -ve transients, a 3 ohm series resistor to limit the current during transients, a SA28A TVS to clamp the peaks,

100uF + .1 uF input filter caps and a 7805 regulator. Cheap and functional. Installed over 20,000 pieces in heavy trucks and have never had a failure in over 10 years. I've often wondered how the 1N4007 and the SA28A manage the peak currents. Working theory: worst-case load dumps are very rare events.

-- Joe Legris

Joe, that is very helpful.

It is the difference between what is needed "in theory" and what works fine in practice.

I've already got most of that in there, in that order. (1N400x, SA28,

100uF, 5V regulator). But I was using my 4-transistor limiter instead of your 3 ohm resistor!

Anything special about the resistor?

Thanks,

--

John Devereux

There are about two dozen types to chose from, made by a half-dozen manufacturers. I've often called the LND150 (the part suggested in the app note) "my favorite" and I've designed it into various ASCII circuits posted here (dunno why Jim doesn't remember that).

Mouser stocks a range of Supertex depletion MOSFETs, like the 1mA LND150, plus higher-current parts (to 800mA), and even one type (DN2540N5) in a TO-220 package.

They're super-easy to use in this application:

HV spikes in ------, | depletion-mode D MOSFET G ---, S | | | __|__ | | | | | |---+--- LV out |_____| | \ gnd LDO regulator

I didn't show bypass caps, protective zener, etc. The LDO regulator runs from the MOSFET's -Vgs gate-bias voltage appropriate for the current drawn by the load.

IXYS has recently introduced a nice assortment of depletion-mode MOSFETs.

Mouser stocks them.

Robert

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"Winfield Hill" schrieb im Newsbeitrag news: snipped-for-privacy@l53g2000cwa.googlegroups.com...

The nice thing about this circuit is the integrated compensation of the variance of Vgs by the LDO. Bad: High drop-out due the MOSFET and not reverse voltage save because of the integrated GS diode. So it needs the 1N400x.

- Henry

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"Winfield Hill" schrieb im Newsbeitrag news: snipped-for-privacy@l53g2000cwa.googlegroups.com...

And the price?

- Henry

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"Robert Adsett" schrieb im Newsbeitrag news:45a80c9e$0$4820$ snipped-for-privacy@free.teranews.com...

intro:

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.

Maybe there is a power problem inside which could destroy the MOSFET. I never thought it to the end.

Suggestions here?

- Henry

--

Supertex has some nice ones. They're popular in some industrial process control equipment.

I have not seen a p-channel discrete though.

Best regards, Spehro Pefhany

--
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

Actually, N-channel might be ideal for such occasions... tie the gate to an R/C filtered version of Vout (A+ under normal conditions). Then the transient will automatically turn off the device.

I downloaded the model and will play around with it... intriguing ;-)

Though I don't know why the model is 4-terminal and the device is

3-terminal :-(

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

We use a 2-Watt wirewound but it's probably overkill.

One thing to keep in mind is that our product is not used on cars or light trucks, where the conditions may be different those in from heavy trucks. Do some tests before commiting to this.

-- Joe Legris

Perhaps a 5W WW cement type?

Best regards, Spehro Pefhany

--
"it\'s the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

I think you are going the wrong way with this. Check the load dump spec and you will find that it specifies a series resistance as well as the peak voltage. In most cars you can get away with a transient suppressor zener diode type device eg Multicomp 1.5SMCJ33A. When I was doing automotive stuff (a few years ago now) each ECU (Electonic Control Unit) had its own trasnient supressor capable of takling the whole load dump so in real life they all got an easy time (there being many ECUs in //). If thsi approach works for you then it's one fairly cheap sm device.

125V is a a very big load dump so you won't easily find integrated regulators that will take it. The usual suspects (ST, Infineon, NS) all make regulators that will take typical automotive "12V" inputs and stand reverese supply, load dump, 24V etc.

Michael Kellett

We used epcos automotive varistor (much energy absorbtion capabilitiy) and series PTC in front of it. Varitor stops initial surge and PTC limit the current before burning of varistor.

Load dump time can be as high as 400 ms. It is a very long time for a peak shaving circutry.

Another important issue is that your circuit shouldnt reset during surge period. In varistor ptc method circuit is supplied by power supply during surge period.

This wouldn't be for cars either - it will mainly be used in industrial systems and fed from a regulated supply. But at some point it could get fitted to some types of commercial vehicles, e.g. diggers).

--

John Devereux