MOSFET ambiguous drain current - Page 1
Forum section: electronics / beginners

author: Serg65536
author_rank: Regular Contributor
timestamp: January 12, 2024, 10:50:05 am
content: Eg: Nch MOSFETAO3400At p. 2 it says:Condition: 25°C1) BVDSS Drain-Source Breakdown Voltage atID=250µA,VGS=0V is 30V2) IDSS Zero Gate Voltage Drain CurrentVDS=30V, VGS=0Vis1µASo what drain current should be at shorted gate to source and 30V drain-source voltage250µA or 1µA!!!??NB: it's NOT the problem of this datasheet only, but same ambiguous info could be found on other transistors.

author: Kleinstein
author_rank: Super Contributor
timestamp: January 12, 2024, 11:00:31 am
content: These number are min max limites for the test. The first test is for more than 30 V with 250 µA of current on breakdown.The second test is applying 30 V and checking that the current is < 1 µA .The 2nd test looks like it is a bit strickter, but there could be some heating effect or negative differential resistance, so that after brakedown the voltage may drop initially. Also different engeniers may look for the information in either format. So it makes sense to have it both ways, even if one value is stricter than the other.

author: ArdWar
author_rank: Frequent Contributor
timestamp: January 12, 2024, 11:09:43 am
content: Those are qualification limit parameters. It effectively says that BVDSSisat least30V and IDSSisat most1uA when operatingbelowBVDSS. The two statement can be true since BVDSSwill always higher than 30V. No manufacturer actually characterize their MOSFET at exactly BVDSSsimply because it isn't a static value, and honestly no one expect to actually use MOSFET at breakdown region.Anyway 99% of MOSFET datasheets are autogenerated and those two limits are industry standard, so expect to see the exact same situation on almost all MOSFET datasheet from almost all manufacturers.

author: RoGeorge
author_rank: Super Contributor
timestamp: January 12, 2024, 11:11:56 am
content: 1uAor lower. 1uA is the max.The other current is when a voltage breakdown occurs, which is not at 30V. 30V is theminimumguaranteed voltage such that the breakdown doesn't occur. But at more than 30V (say for example at 45V, when the breakdown eventually occurs) the current will be higher, and they consider 250uA as the current that will determine the breakdown.Exact voltage breakdown is not given, but it is guaranteed to be higher than 30V.

author: Serg65536
author_rank: Regular Contributor
timestamp: January 12, 2024, 03:05:14 pm
content: Does it mean that MOSFET is within specs even if at 30V the transistor has 250uA current and is entering breakdown curve?It's some sort ofSchrodinger's drain current, that is in the IDENTICAL CONDITIONScould be maximum 1uA AND 250uA at the same time.

author: RoGeorge
author_rank: Super Contributor
timestamp: January 12, 2024, 03:25:07 pm
content: No, the conditions are not identical.That datasheet, translated in words, means the following:- we, the transistor's manufacturers, consider 250uA as the breakdown condition.- and, at what voltage would that happen?- well, depends on many factors, we can't say the exact voltage corresponding to those 250uA, all we can tell is that voltage will be somewhere above the 30V- at 30V or less, we guarantee you, there will be no breakdown problems, and at less than 30V the current will be smaller than 1uA

author: Terry Bites
author_rank: Super Contributor
timestamp: January 12, 2024, 04:00:26 pm
content: Page 1.Its not ambiguous, its just poorly defined and temperature dependent.