Doctorbass said:
<snip>
since we are many to use the controlelr with 24s of 4.2V lithium cells ( 100.8V) ... and some that want to do 30s with lifepo4 (111V).. I wonder how it can be risky when using these mosfets at so high voltage since they are 100V rated...
How their life can be affected if overvolted to 10%... Does using a 18 mosfet controller instead of 12 mosfet could be better if every mosfet would drive less current and btw be less stressed with current to compensdate the stress of HV ?.. in other words, does overvolting them and to compensate this stress by driving less current can decrease the risk of blowing?
<snip>
Doc
Any time you operate a MOSFET at greater than about 80% of any of its ratings by, you start to significantly increase the chance that MOSFET will fail. It can be a gradual degradation of their specs or the FET can suddenly blow. Operating a 100V MOSFET at 100V (or even higher) places that MOSFET under severe stress.
Yes, they can operate for years that way. And it's also just as likely that they'll blow quickly too. Unfortunately, it's all about curves and average specs.
For example, the 100V max Vds rating of the IRFB4110 is just the voltage at which the vast majority of 4110's can operate at with a reasonable expectation of not failing (if kept cool, the gate voltage isn't too high, etc.). Because it would cost a lot of money to absolutely guarantee that every 4110 could operate at 100V, the manufacturer gambles a bit and manufactures the MOSFETs so that they "should" survive at that voltage. But they all recommend operating a voltage level way below the max. That rating really and truly is a maximum rating.
Because of these life expectancy curves, if the 100V rating was a "guarantee to fail" spec, then many MOSFETs would fail at way below that. If the 100V rating was a "guaranteed operate forever" rating then they would cost a fortune because the manufacturer would have to label 150V-200V MOSFETs as only 100V ones.
Back to the curves....
A couple of 4110's will fail at below 100V but most will survive. As the voltage increases, more and more of them will fail. There's no way to predict which ones will fail at which voltage though. Extensive testing of a single batch can tell you but that takes a lot of time and money. That's exactly why they have a max Vds rating. No one needs to do any testing.
Overvolting a MOSFET by 10% beyond its max Vds rating is begging for trouble. At 80% of its rating, the reliability starts to drop significantly. Now, since it can operate (not must operate) at up to 100V, you can have MOSFETs survive for years at 110V since they would survive for decades at below the rated max. Yes, the life has been severely shortened but the life was so long before that the shortened life is still long enough for our uses. There's no way to predict how short the life is though. It could be hours, all depending on the tiny differences between each MOSFET and how hard you push them.
Operating at high temperatures reduces it life too and also increases the chance of hot-spotting and thermal runaway (destroying the MOSFET) during turn-on and turn-off times. The MOSFET operates in its linear region for a short period during this time and is very susceptible to these problems then. And this damage is cumulative. Everything can seem great for hours, days, months or even years and suddenly the unit fails at significantly below its rated max specs because of this damage building up. That's another reason to not operate anywhere near the MOSFET's rated maximums. It adds a buffer to "absorb" damage and extend the MOSFET's life.
Even worse, you can operate the MOSFETs within all their ratings but still blow them because of thermal fatigue. The different coefficients of thermal expansion for the materials in the MOSFET cause them to expand and contract differently as the MOSFET heats up and cools down. Each time this happens, a little more physical damage can occur. This can damage the bond wires inside, allow moisture in, etc. Eventually the MOSFET fails by overheating due to increased internal resistance or cracking of the die or bond wires. Each time the MOSFET heats up and cools down, its life is shortened. Reducing the temperature the MOSFET heats up to increased its life. Even if that temperature is below its rated maximum.
To answer your question...
Overvolting damages the MOSFET. Operating at high temperature damages them. Thermally cycling a MOSFET damages it. Reducing the current can reduce the temperature but the lower current level itself will not extend the life (unless you were exceeding one of the current-related specs) of the MOSFET. The farther you get above that 80%-of-spec level, the faster the MOSFET will fail. Not "may fail"..."will fail". But, since we can't predict the life of any particular MOSFET (just a statistical average for the life of the batch based on educated guesses after testing), you have to decide for yourself how much it wold affect you if the MOSFETs failed. Operating at below the spec (especially the 80% level) means that you have a good chance of it never failing. Operating above that level just means you're rolling the dice...no way to tell.
These mosfets are costing me a lot!! 
:x 
