Sure looks to me that if you apply voltage and measure current, the current increases about exponentially on applied voltage. PN diodes don't do that.
Voltage is more like the log of current, negative exponential.
In a series schottky string, the diode that has the most voltage drop will tend to draw more current than the others, so its voltage drop will decrease. Then the voltage across the others will increase, which moves them up their current curve. The feedbacks are negative and the string is stable.
It would take a negative resistance behavior to make the series string unstable. In olden times, some non-avalanche PN diodes apparently did this, hence the old parallel RCs across each diode.
Imagine putting five 12-volt zeners in series, across a 50 volt supply. The voltage drops may not be equal, but nothing much happens.
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John Larkin Highland Technology, Inc
lunatic fringe electronics
pendent on current in reverse region, not vice versa.
That's forward operating region, not reverse, which ideally is constant cur rent.
In the series string the diodes all pass *the same* current which is the pr oblem. A current that may produce a modest reverse voltage in the on diode could result in a much much larger reverse voltage another. The reverse cur rent characteristic shows has this happens.
Right, and that's because as the weak link diode approaches avalanche it on ly takes a minuscule increase in current to force the remaining diodes to s tand off the remainder of the applied voltage. So the worst that happens if the diodes have wildly different reverse IV is that one diode may avalanch e very weakly.
Why should it? Avalance breakdown involves generating pair of charge carrie rs in the reverse biased region of the diode.
The "soft" characteristic means that there's more reverse current at high v oltage, and quite a lot more heat deposited in the reverse biased region, s o it might make hot spot generation more likely, but that's not an avalanch e.
The whole point of the "soft" characterisitic is the leakage through a bunc h of reversed biased diodes in series has to be the same, and the voltages across each diode can adjust to make the true without dropping the voltage across any one diode enough to drive any of the others into breakdown.
You wouldn't get away with 210V across three 70V diodes, but they might sur vive 150V if the leakage currents weren't wildly different
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For this part the typical leakage current at 50V reverse voltage is 25nA an d the worst case is 90nA. At 70V the worst case is up to 3uA, which is a lo t higher.
In a PN diode, that's mostly true. It's not for a schottky. Just look at the curves in your Fig 4, which is the *reverse* bias curve. Nobody would apply 70 volts in the forward direction.
Of course the steady-state currents are the same. It helps to think of stuff happening in an incremental cause-and-effect way, so you can envision the stability situation. Stuff actually does happen sequentially, because of delays from capacitance and thermals.
The situation that you describe just above is true, but it's not a problem. The feedbacks are negative.
No harm done. The series string works.
You could Spice it, except that the LT Spice schottky diodes, the ones in the standard list, get it all wrong.
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John Larkin Highland Technology, Inc
lunatic fringe electronics
There was a time when people put a resistor and a capacitor across every diode in a series string, to protect the diodes from an avalanche catastrophe. Heck, some audiophools still do. Tradition is a wonderful thing.
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It's impressive how many people don't understand electronics.
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John Larkin Highland Technology, Inc
lunatic fringe electronics
dependent on current in reverse region, not vice versa.
ts.
The point was that non-avalanche diodes avalanche through a very narrow pat h, and that path can form a hot spot that gets hot enough to suffer permane nt damage. Avalanche diodes are built in a way that gurantees that the aval anche spreads out from the initial path and doesn't generate a fatal hot sp ot.
It can pay to give the customer what they think they need. Educating them i s difficult, and some people don't like getting educated - they like to thi nk that thye know it all already.
Most people don't undertand it any better than they have to.
An expert is somebody who has had to do job at some time in the past, and c an remember what had to be done to get a device that worked and kept on wor king.
Some people are prone to trying to generalise a rather narrow expertise bey ond their actual experience.
Leakage dissipates power, heating increases leakage and leakage increases heating. But that lowers the voltage so leakage comes down. So isn't the V/I curve what stabilizes it?
I wouldn't call self-heating a stabilising effect. The diode with the highe st voltage gets the most self-heating, which - in the first instance - incr eases its leakage current, and consequently increases the voltage over all the other diodes, lowering the leakage current to something only slightly h igher than the value it started off at, but that's going to mean more self- heating in all the other diodes, which could eventually run away.
hest voltage gets the most self-heating, which - in the first instance - in creases its leakage current, and consequently increases the voltage over al l the other diodes, lowering the leakage current to something only slightly higher than the value it started off at, but that's going to mean more sel f-heating in all the other diodes, which could eventually run away.
That seems like a fine description Bill, and exactly what I'd want and desc ribe as stabilizing... the voltages will tend to balance. It's my job as a designer to make sure that doesn't lead to the whole strin g overheating.
dependent on current in reverse region, not vice versa.
urrent.
Not for Schottky diodes - leakage current in reverse bias is generated by t unneling through a potential barrier, and that does go up eponentially with voltage.
problem. A current that may produce a modest reverse voltage in the one dio de could result in a much much larger reverse voltage another. The reverse current characteristic shows has this happens.
So the voltage drops across the various diodes in the reversed bias string aren't equal, but they aren't wildly different either. If you try to at a r everse voltage that is very close the the sum of the rated reverse voltages across each diode, you can end up with enough volts across one diode to bl ow it up, or at least to get it to pass the reverse current through all the other diodes at an inconveniently low voltage drop.
The one Schottky diode data sheet that I've bothered to look at showed a ty pical reverse curren of 25nA at 50V (1.25uW), worst case of 90nA (which imp lies lower limit of about 7nA.The worst case leakage at the rated reverse v oltage of 70V was a lot higher at 3uA, which suggests that coping with the worst leakage difference would require a 12V difference in reverse voltage.
One best case diode in series with a bunch of worst case diodes would opera te with 12V more across it than the rest, so George's string of three 70V d iodes should be okay up to about 210V - 12V or 198V.
The "negative resistance" egffect in "non-avalance" diodes is that the aval anche stay restricted to one very narrow path, which gets hot, forming a ho t-spot, and can rapidly get hot enough to produce irreversible changes in t he semiconducting junction in the diode. Leakage current goes up quite rapi dly with temperature, so it's more thermal runaway than anything else.
only takes a minuscule increase in current to force the remaining diodes to stand off the remainder of the applied voltage. So the worst that happens if the diodes have wildly different reverse IV is that one diode may avalan che very weakly.
Wrong. Schottky diode reverse current is tunnelling current - avalanching d oesn't come into it. Self-heating in a diode could run away, which would ev entually destroy the diode and the string in which is was embedded. It migh t llok like a sort of avalanche, but that probably isn't the right word to use in this context.
The maximum dissipation for that diode is 400 mW, so at 70 V, the reverse leakage could be 5.7 mA. From published data, I do not understand how you could get such leakage even at elevated temperatures.
If the typical leakage is 60 uA at 70 V and 150 C ambient, the dissipation would be 4.2 mW and with 400 C/W, the junction would be at 152 C.
Where is the exponential growth ? Only at low temperatures and voltages below 10 V, the growth us fast, but above that case, the leakage only increase by a decade up to 70 V ?
Yes, the v-i curve keeps any diode from having too much voltage drop. The series string equalizes nicely. Any self-heating is a slow negative feedback, not a thermal runaway hazard or anything like that.
Transiently, the junction capacitances equalize the reverse voltages. C drops with V so that is theoretically a destabilizing feedback, but not a problem in real life.
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John Larkin Highland Technology, Inc
lunatic fringe electronics
dependent on current in reverse region, not vice versa.
Looking at the Figure 3 on the 1N5711 data sheet, the typical leakage curre nt gets progressively clser to the worst case leakage current as the temper ature goes up - 30uA typical at 50V, 35uA worst case, so there does seem to be some kind limiting process involved.
The typical leakage current at 70V and 150C is 40uA - from figure 4 - and t he worst case - extrapolating from Fg.3 would be closer to 50uA, which - as you say - is only 1.2C of self-heating, if you can put the diode on 4mm le ads on an infinite heat-sink. Real printed circuit boards are finite.
The trouble with diode strings is that if the voltage being stood off doesn 't divide equally across the diode string, the peak voltage across one diod e could end up being more than 70V and the leakage current is going to go u p exponentially with voltage.
You can cruch through the numbers and work out when the process will run aw ay, as it eventually must. I'm not going to bother.
highest voltage gets the most self-heating, which - in the first instance - increases its leakage current, and consequently increases the voltage over all the other diodes, lowering the leakage current to something only sligh tly higher than the value it started off at, but that's going to mean more self-heating in all the other diodes, which could eventually run away.
escribe as stabilizing... the voltages will tend to balance.
ring
John Larkin can extract conclusions about the behaviour of a single diode f rom a discussion of what happens when a couple of diodes are connected in s eries.
If would be nice if he elaborated on the "reasoning" that lead him to that bizarre conclusion, but that isn't going to happen - he's much to vain to a dmit that he screwed up.
Right, it's going to be hard to fry it with too much current. which is fine.
Oh.. I didn't say anything about exponential. That was JL. But if you look at fig 4. (A semi log plot.. log current vs voltage) you see sorta a straight line (say from 10 - 60V) for any of the curves. The straight line means some exponential behavior. I'm not sure what the mechanism is. (Tunneling??) I'd have to dig out Sze's book.... I think I'm at the age where I forget stuff faster than I learn new stuff.
highest voltage gets the most self-heating, which - in the first instance - increases its leakage current, and consequently increases the voltage over all the other diodes, lowering the leakage current to something only sligh tly higher than the value it started off at, but that's going to mean more self-heating in all the other diodes, which could eventually run away.
escribe as stabilizing... the voltages will tend to balance.
ring
I get the feeling that lot's of time's he just wants to say you are wrong.
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