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Date: Tue, 09 Aug 2011 07:55:38 -0400
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Subject: Re: [LML] Re: Fw: Re: Re-doing my panel - carefully thinking through failures
From: Colyn Case <colyncase@earthlink.net>
In-Reply-To: <list-5086133@logan.com>
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References: <list-5086133@logan.com>

Hamid,

that is very helpful.  I can see lots of ways that the e bus would lose =
power.

The particular scenario I was worried about was an event on the A bus =
reaching the B bus.   This would require probably that both diodes that =
feed the e bus fail closed with little or no reverse current protection =
remaining on the B leg.

Colyn

On Aug 8, 2011, at 9:40 PM, Hamid Wasti wrote:

> Colyn Case wrote:
>> At the risk of topic drift.....I put in those big fat diodes to my =
essential bus also.
>> Since them I'm wondering if there's a failure mode on one bus that =
crosses the diodes and takes out the other bus.
> A diode can fail open, shorted or "out of spec" meaning that one or =
more parameters (voltage drop, reverse current, series resistance etc) =
is very high.  According to various documents on Failure Mode =
Distribution, about 50% of the failure modes in power diodes result in a =
shorted diode, about 30% in open diode and about 20% in out of spec =
failure. Different sources use different numbers and they vary for =
different components, but that is a ball park to get a sense of things.
>=20
> A shorted diode is basically a direct connection, tying the two busses =
together and essentially making them one. An open diode prevents power =
from going through the diode to the destination buss, leaving it =
permanently isolated from one of its sources. Parameter change is an =
unsustainable state. It may allow the diode to function briefly, but =
with use under load, it will invariably fail either open or shorted.
>=20
> Diodes can fail for many reasons, among them: Over-heating, =
Over-voltage, Over-current. If you have under-sized the diode or not =
properly heat sunk it, it will die after some time in "normal" use.
>=20
> Heat is generated in a diode's die and is a function of the current =
and the diode's voltage drop (voltage in volts multiplied by current in =
amps =3D power in watts). That heat needs to be conducted to the outside =
of the case, then through some mechanical interface to the heat sink and =
then to the ambient air. If the case to heat sink interface is not =
properly designed and properly implemented, the die can be considerably =
hotter than the heat sink. If the heat sink is not receiving cooling =
air, it can be considerably hotter than the ambient air around it. If =
the air is circulating in a closed environment it may be a lot warmer =
than the outside. If the air happens to be low on air molecules (flying =
at FL280) it may not be taking away as much heat as you had tested at =
sea level. All of this can conspire to over-heat the die and lead to =
failure.
>=20
> Surely the easiest thing in the world is to make sure that you use a =
diode with a higher current rating than the highest possible current in =
your system. Right? Actually, no!  The important part is to use the =
correct current rating, which is not always obvious. The headline =
current rating is valid at a certain temperature, which is often (but =
not always) 25C die temperature. In real life, the die is going to be a =
lot warmer than that, maybe as much as 100C warmer. Buried in the =
datasheet is a graph indicating the maximum current at higher =
temperatures, or a footnote along the lines of "Maximum current =
decreases by  .....A per degree C for higher temperatures"  To know if =
the diode is going to work for you, you need to figure out the maximum =
power dissipation, figure out the temperature increase due to that much =
power, add that to the maximum heat sink temperature and then make sure =
that it can handle the current at that die temperature. The resulting =
current limit is invariably going to be a lot lower than the headline =
number and if you are exceeding that, your diode is under-sized.
>=20
> If you have battery disconnection on one buss, the alternator can =
generate a voltage spike of several 10's of volts. DO-160 calls for 28V =
certified systems to be able to survive up to a 100mS wide 80V spike, =
followed by 48V for 1 second. If there is an 80V spike on the A Buss, =
while the B Buss stays at 28V, the diode between the essential buss and =
the B Buss will see a voltage of close to 50V. Is it rated for that? If =
you B Buss happens to be off due to a failure, the diode is going to see =
almost the full 80V. Will it survive that? If it fails shorted, you just =
lost your essential buss. Unlike over-current, there is no transient =
specification for over-voltage. Even a momentary over-voltage can damage =
a diode.
>=20
> Turning off switches and hot-unplugging a high current load can cause =
a flyback voltage due to the inductance of the power wire. Unless this =
is anticipated and protected against, it can kill an isolation diode. An =
intermittent power connection in a tray is the same as repeatedly =
hot-plugging/hot-unplugging.
>=20
> Finally, a word about the worst kind of failure: The out-of-spec =
failure. Lets say due to one of the aforementioned events, you have an =
out-of-spec failure where the diode's internal resistance increases an =
order of magnitude or more from the original value of a few mili-ohms. =
Lets say you have a system where the "A Buss" and "B Buss" feed an =
essential buss and the diode on the A Buss side has failed with high =
resistance. If you do a system check at every startup where you =
sequentially shut down both busses and make sure that the essential buss =
can run from the remaining one, you are likely to find that the =
essential buss works. The failed diode will be able to operate the load =
for a little bit while it over-heats. During normal operation, the diode =
on the A Buss will take all the load. But if you have a failure of the A =
Buss and all the current starts going through this high resistance =
diode, it is quickly going to fail and as Murphy's Law states, there is =
100% likelihood that this will be one of the 30% of times where it fails =
open.
>=20
> I am sure I can think of a few more scenarios where a failure can go =
undetected by typical checks. The bottom line is that unless you are =
willing and able to get into it a lot deeper, a "simple and reliable" =
system may only be half so.
>=20
> Regards,
>=20
> Hamid
>=20
> --
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