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'[EE] Shottky Diode performance below Vf'
So I'm looking for ideas, and information.
I have 0.05 ohm shunt resistor which I need to protect from overcurrent (effectively overvoltage). It's measuring steady-state current, and also being used for overcurrent trip purposes. In certain very-high-current 'dead short' situations, this poor component becomes less of a resistor and more of a fuse - I'd like to prevent that, bypassing some of the current long enough for the trip to occur - a few ms at most.
The obvious cheap solution here is to do some sort of tvss-like device around the unit to bypass some of the current once the voltage across the resistor rises to a certain point. And looking at the curves of the B1100 Schottky diode which is a standard part around here, it looks like I might be able to just put one forward-biased across the resistor, and use it in sort of tvss-mode...
My mental picture of a diode is that below Vf there is minimal if any current conducted. Is this an accurate mental picture? What current will flow if you 'bias' a schottky diode at say 0.2V? I'm not worried about capacitance more than what I'd call 'leakage current' (in the forward direction though).
Is there something else I should be using instead?
Hmmm - beware that the diode may well have an effective internal
resistance of more than 0.05 ohm, in which case, the majority of the
current will flow through your shunt even when the diode is present
during the fault condition. I don't think that there is any
alternative for protection here other than to make your overcurrent
detection faster or add enough mass so that the temperature will not
rise too much before overcurrent is detected and shut-down. By the way
- how are you shutting-down this current when there is a fault?
Schottky diodes are fundamentally different than normal PN junction
diodes. They can have significant leakage current - I know that in the
reverse-bias condition, at high temperature, they can pass 10s or 100s
of microAmps. I would expect even worse in the slightly forward-biased
condition. These current levels, though, do not seem relevant when
your sense resistor is only 0.05 ohm.
I searched for "Schottky diode iv curve" and it seems that they follow
the same exponential diode equation as PN junction diodes, although
with different values for the constants. This means that the forward
current is roughly exponentially-dependent on the forward voltage, so
that a given incremental increase in the forward voltage causes a
multiplicative increase in the forward current. Let's say, for
example, that a particular Schottky diode has a forward drop of 0.2V
at 1mA. Then, at 0.15V it might be 30 microAmps, and then at 0.1V it
might be 1 microAmp (a factor of 30 for every 0.05V change in the
voltage). This relationship no longer holds true, though, once the
current is around the "saturation current", which is highly
On Fri, Dec 16, 2011 at 10:45 PM, Forrest Christian <imach.com> wrote: forrestc
On Sat, 17 Dec 2011 01:27:27 -0500, you wrote:
Schottkys get very leaky at higher temperatures, so probably not a good choice.
Is the current really so high that you can't use a more chunky sensing resistor?
If you really want to bypass the shunt, a MOSFET is probably a better bet - arranged such that it
turns on quickly when the measured current exceeds a threshold, witha monostable to hold it on for a
while, limiting the current duty cycle through the resistor to the monostable time / the turn-on
On 12/17/2011 3:09 AM, Mike Harrison wrote:
> Schottkys get very leaky at higher temperatures, so probably not a
> good choice. Is the current really so high that you can't use a more
> chunky sensing resistor? If you really want to bypass the shunt, a
> MOSFET is probably a better bet - arranged such that it turns on
> quickly when the measured current exceeds a threshold, witha
> monostable to hold it on for a while, limiting the current duty cycle
> through the resistor to the monostable time / the turn-on time. Just to be clear what we're typically talking about here...
The normal load is around 250mA. Inrush currents can be extremely high, in the range of amps, but not close to high enough to exceed the resistor rating.
The issue comes where our customer is using a large battery bank with virtually unlimited current available, and decides to dead short the output. I think we've actually got the firmware in the PIC doing the overcurrent protection tweaked so failures just aren't likely - It's a fine line between being slow enough that the inrush doesn't trip the protection, and a short or long-term overcurrent will. With some creative integration (in the mathematical sense), things seem to be working well - but I still hate not protecting something I know is close to the edge.
At this point, I think I might look at using a mosfet on the input of the whole device to limit current to something reasonable - dozens of amps for instance - probably a more reasonable solution.
And yes, we're already using about as chunky of a sense resistor as is practicable.
> At this point, I think I might look at using a mosfet on the input of
> the whole device to limit current to something reasonable - dozens of
> amps for instance - probably a more reasonable solution.
You can get MOSFETs which have zero Vgth or very small and
"programmable" Vgth. They have charge injected into a floating gate to
trim them to whatever threshold is required on the externally
accessible gate. One of these driving a clamo or one of these driving
itself as a clamp MAY meet your need.
The Analog Devices ALD11... are examples
10 mV nominal Vgth
A glance through the spec sheet suggests it lacks brawn and has
significant Vds drop at low gate voltages at low Ids - so use as a
trigger and not a clamp by itself, is indicated.
How accurately do you need to sense current?
I am concerned about the ability of your MOSFET switch to stop the
short-circuit current from a large battery bank. Remember, during the
time that the FET is switching off, there is a huge power dissipation
because the voltage is rising at the same time that the current is
falling. I don't know what kind of battery bank you are talking about,
but let's say it's a 24V 50 Amp-Hour lead-acid battery. The short
circuit current could be 5000 Amps. At the worst point, there will be
about half voltage (12V) across the FET and about half current (2500
Amps) through it. That's 30kW. Let's also say that the FET will switch
off in 500 nanoseconds and that the overall average power dissipation
is half of the worst case point. So, 15kW for 500 nanoseconds - that's
7.5 milliJoules. Might not seem like much but given that the actual
die is quite tiny, and the bond wires are even smaller, it might well
be enough to destroy them.
This is ignoring the additional heat which will come from just I^2 R
heating during the time when the current is ramping up before your
system trips and decides to turn the FET off. It is also ignoring the
inductive energy in the leads which will show up as a huge voltage
pulse across the FET, increasing the power dissipation and possibly
exceeding Vds max. Finally, it is also ignoring the difficulty in
keeping a FET turned on fully during the very high current conduction
- the voltage drop across the FET (drain-source) will likely exceed
Vgs max which will prevent you from applying enough gate drive to keep
the FET in the ohmic region.
What I am saying is that a FET swtich that is designed to catch and
stop the short circuit current from a large battery is NOT a small or
inexpensive thing :)
On Sat, Dec 17, 2011 at 6:40 AM, Forrest Christian <imach.com> wrote: forrestc
|These are typically very large, very deep battery banks. 50AH is wimpy. In my other life, I have a location which has an array running at 24VDC, 480AH. There is a *lot* of current there, admittedly.
Fortunately there are also a lot of connectors, small-guage wires, fuses, and the like typically between my gear and the array, all which serve to limit the instantaneous current.
In this application FQB22P10's are pretty much jellybean parts. -88A pulsed drain current. Eas of 710Mj, Ear of 12.5Mj.
Admittedly it's all a race of 'what will melt first', or maybe better 'will we get it all shut down before something exceeds a rating'. Fortunately, this is also a very rare occurance, and so statistcally we're just trying to make the product more bulletproof in those cases - I.E. adding a lot of cost isn't in the cards in this case.
I'm actually eyeing the 22P10 I'm using on the front end of this for reverse polarity protection and thinking that with a few truly jellybean parts it could become not only reverse polarity protection, but also a rough current limiter to a few amps. If I can get the impulse energy available to the downstream shunts and FETS down a ways, then things become much less critical, and the 22P10 only needs to dissipate at worse case that initial (admittedly huge) pulse while it adjusts, and then a bit of additional power at the lower amperage for however long it takes the circuitry to decide there's a true overcurrent/short going on here and shut things down. But as is mentioned below the challenge might be really in the realm of not worth the effort.
On 12/17/2011 10:05 AM, Sean Breheny wrote:
Forrest Christian <forrestc <at> imach.com> writes:
> So I'm looking for ideas, and information.
> I have 0.05 ohm shunt resistor which I need to protect from overcurrent
> (effectively overvoltage). It's measuring steady-state current, and
> also being used for overcurrent trip purposes. In certain
> very-high-current 'dead short' situations, this poor component becomes
> less of a resistor and more of a fuse - I'd like to prevent that,
> bypassing some of the current long enough for the trip to occur - a few
> ms at most.
Taking it on another angle from what's being posted so far:
Where the in-rush current go (I presume it's not cold filaments of vacuum
tubes?). If it's filter capacitor, does the current sensor have to be before it?
Maybe put a current-limiting resistor in series with the battery, and shunt it
off with MOSFET once current settles close to "normal" range?
On 18-Dec-11 03:53, Forrest Christian wrote:
> These are typically very large, very deep battery banks. 50AH is
> wimpy. In my other life, I have a location which has an array running
> at 24VDC, 480AH. There is a *lot* of current there, admittedly.
> Fortunately there are also a lot of connectors, small-guage wires,
> fuses, and the like typically between my gear and the array, all which
> serve to limit the instantaneous current.
> In this application FQB22P10's are pretty much jellybean parts. -88A
> pulsed drain current. Eas of 710Mj, Ear of 12.5Mj.
I would add an inductor and a flyback diode, a few uH of inductance should get you enough time to turn the MOSFET off.
Your previous idea of a Schottky parallel to the sense resistor should also work, at <100mV the Shottky will pass little current compared to the shunt, even at high temperatures. However, the next weakest component may fail - it's better to prevent huge short-circuit currents from happening.
Why wouldn't a varistor or thermistor perform an anti inrush current check satisfactorily?
cdb, btech-online.co.uk on 27/12/2011 colin
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