Surviving automotive load dump

[John Devereux]

Hi,

As well as the usual spikes and brownouts, automotive systems have a
"load dump" condition, where the supply can e.g. rise to ~125 volts
for ~100ms.

What is the best way to protect against this? I need fairly low cost,
and small space (SMT). It must be a very common requirement, but all
I can find are some giant MOVs.

My best effort so far is a 4-transistor discrete "pre-regulator" to
stand-off the pulse (made from high voltage BJTs). Cheap in parts but
seems overly complicated.

Current would only be ~200mA, & must not drop more than a volt or two
during normal operation.

Any ideas?

 

[Jim Thompson]

Hi,

As well as the usual spikes and brownouts, automotive systems have a
"load dump" condition, where the supply can e.g. rise to ~125 volts
for ~100ms.

What is the best way to protect against this? I need fairly low cost,
and small space (SMT). It must be a very common requirement, but all
I can find are some giant MOVs.

My best effort so far is a 4-transistor discrete "pre-regulator" to
stand-off the pulse (made from high voltage BJTs). Cheap in parts but
seems overly complicated.

Current would only be ~200mA, & must not drop more than a volt or two
during normal operation.

Any ideas?

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

...Jim Thompson

 

[John Devereux]

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

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.

 

[Henry Kiefer]

Probably a depletion-mode power MOSFET as a current-source? See as intro:
http://www.ehydra.dyndns.info/pdf/app_notes/AN-D11.pdf

- Henry

 

[John Devereux]

Probably a depletion-mode power MOSFET as a current-source? See as intro:
http://www.ehydra.dyndns.info/pdf/app_notes/AN-D11.pdf

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

<http://catalog.digikey.com/scripts/partsearch.dll?PName?name=CLA220-ND>

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,

 

[Jim Thompson]

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

<http://catalog.digikey.com/scripts/partsearch.dll?PName?name=CLA220-ND>

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,

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

 

[J.A. Legris]

Hi,

As well as the usual spikes and brownouts, automotive systems have a
"load dump" condition, where the supply can e.g. rise to ~125 volts
for ~100ms.

What is the best way to protect against this? I need fairly low cost,
and small space (SMT). It must be a very common requirement, but all
I can find are some giant MOVs.

My best effort so far is a 4-transistor discrete "pre-regulator" to
stand-off the pulse (made from high voltage BJTs). Cheap in parts but
seems overly complicated.

Current would only be ~200mA, & must not drop more than a volt or two
during normal operation.

Any ideas?

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.

 

[John Devereux]

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, 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,

 

[Winfield Hill]

Probably a depletion-mode power MOSFET as a current-source? See
as intro: http://www.ehydra.dyndns.info/pdf/app_notes/AN-D11.pdf

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. [ snip ]

(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!).

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.

 

[Robert Adsett]

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.

Mouser stocks them.

http://www.mouser.com/search/refine.aspx?Mpc=Supertex+MOSFETs+-+Depletion+Mode+N-Channel&Mpcn=68902

Robert

 

[Henry Kiefer]

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

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

 

[Henry Kiefer]

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

And the price?

- Henry

 

[Henry Kiefer]

Mouser stocks them.

http://www.mouser.com/search/refine.aspx?Mpc=Supertex+MOSFETs+-+Depletion+Mode+N-Channel&Mpcn=68902

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

 

[Spehro Pefhany]

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

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

http://www.supertex.com/products/selector_guides/102

I have not seen a p-channel discrete though.


Best regards,
Spehro Pefhany

 

[Jim Thompson]

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

http://www.supertex.com/products/selector_guides/102

I have not seen a p-channel discrete though.


Best regards,
Spehro Pefhany

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

 

[J.A. 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?

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.

 

[Spehro Pefhany]

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,

Perhaps a 5W WW cement type?


Best regards,
Spehro Pefhany

 

[MK]

Hi,

As well as the usual spikes and brownouts, automotive systems have a
"load dump" condition, where the supply can e.g. rise to ~125 volts
for ~100ms.

What is the best way to protect against this? I need fairly low cost,
and small space (SMT). It must be a very common requirement, but all
I can find are some giant MOVs.

My best effort so far is a 4-transistor discrete "pre-regulator" to
stand-off the pulse (made from high voltage BJTs). Cheap in parts but
seems overly complicated.

Current would only be ~200mA, & must not drop more than a volt or two
during normal operation.

Any ideas?

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

www.mkesc.co.uk

 

[EVOS]

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.

 

[John Devereux]

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.

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).