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Subject:	Re: [geda-user] [off-topic] 24V automotive (truck) design issues
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From:	"Richard Rasker (rasker AT linetec DOT nl) [via geda-user AT delorie DOT com]" <geda-user AT delorie DOT com>
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Date:	Sun, 2 Dec 2018 15:52:44 +0100
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Hi Luis,

Op 29-11-18 om 16:18 schreef Luis de Arquer (ldearquer AT gmail DOT com) [via 
geda-user AT delorie DOT com]:
> Richard,
>
> Your proposed circuit seems OK to me. Just make sure the feedback is
> strong enough so the MOSFET never stays in active zone (especially
> since it has low-ish gate threshold voltage).

Well, it turns out that the circuit was spot-on with the values given: 
disconnect at 37 volts, and reconnect at 34 volts.

> We considered using this from Texas Instruments in some project -we
> didn't need to eventually:
> http://www.ti.com/lit/an/snva190b/snva190b.pdf
>
> Maybe your problem was, after all, with the negative 1ms - 2ms pulse.
> That one is pretty bad, and TVS energy spec is not too far from it
> (600W 1 ms 25deg IIRC, worse with higher temperature).

Yes, that could very well have been the cause. Unfortunately, someone at 
the truck company messed up last Friday, as they had the wrong truck for 
me to examine -- this one had an almost perfect onboard supply that 
showed no peak voltages at all, no matter what we tried, and my PCB's 
were just fine on this one.

The truck that blew up my boards was somewhere in Denmark, some 500 
miles away, so I'll have to arrange another measurement session with 
that one.

> Also, since your current consumption is not too high, maybe you could
> consider adding some line resistance, if cost is an issue for you.
>
> Keep us posted :)

After deliberations with the customer, we decided that I create a 
separate supply unit, designed to handle everything outlined in the ISO 
documents.

The main reason was that I didn't have room on the LED PCB's for large 
components (3 mm maximum height), precluding the use of largish 
capacitors for uninterrupted operation during longer voltage peaks. And 
because they also wanted to power multiple LED boards, I chose the 
TPS54260Q buck regulator, protected by the disconnect circuit (with a 
far heavier SQJ431EJ PMOS device).

So alas, I still don't know the exact reason why the first boards blew 
up, other than that it must have been a voltage peak (> 40 volts) with 
sufficient I*t to blast the TVS diode. If and when I do find out, I'll 
be sure to let you know.

Anyway, thanks again for your interest in and input on this matter!

Best regards,

Richard Rasker

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