maximum power rating (jfet) - Page 1
Forum section: electronics / beginners

author: metebalci
author_rank: Frequent Contributor
timestamp: July 04, 2024, 10:27:11 am
content: Looking at J212, P_D=Continuous Device Power Dissipation is rated 360mW. Also, it is written "Pulsed" in the conditions column for I_DSS. I guess because it exceeds P_D.If P_D is continuous, what is the not-continuous P_D value ? or in other words, is the actual maximum rating (other than breakdown voltage etc.) that matters junction temperature ? Thus, P_D a derived quantity based on the thermal properties of the package. In other words, is it enough to monitor the temperature of the device, stay below 100C or so, and ignore P_D to not permanently damage it ?

author: David Hess
author_rank: Super Contributor
timestamp: July 04, 2024, 01:15:01 pm
content: The pulsed power dissipation depends on the thermal mass of the silicon die and close to the silicon die.For a JFET, Idss is close because not much further enhancement is possible without forward biasing the gate junction.

author: metebalci
author_rank: Frequent Contributor
timestamp: July 04, 2024, 01:47:02 pm
content: Quote from: David Hess on July 04, 2024, 01:15:01 pmDo you mean the pulsed power dissipation would be close to power dissipation at Idss (and rated breakdown voltages) ?Edit: For example, J212 PD is 360mW. The IDSS measured with pulsed is [15, 40] mA and reverse voltage is -25V. So the pulsed power dissipation would be something like (assuming actual IDSS of a particular device is 30mA) 30mA*25V ~ 750mW ?

author: David Hess
author_rank: Super Contributor
timestamp: July 04, 2024, 02:11:43 pm
content: Quote from: metebalci on July 04, 2024, 01:47:02 pmYes, because Id cannot be raised much above Idss. Most designs never raise Id above Idss because it means approaching the point where the gate becomes forward biased.Lots of JFETs however have an Idss which will support a dissipation higher than the maximum continuous dissipation, so Idss for them is specified under pulsed conditions.QuoteYes.

author: metebalci
author_rank: Frequent Contributor
timestamp: July 04, 2024, 04:40:47 pm
content: Quote from: edavid on July 04, 2024, 02:43:40 pmThat is correct, the test V_DS is 15V, but does that mean the test is done at the thermal limit ? InterFET J212 datasheet does not mention the pulse width but Fairchild says it is less than 300uS. Assuming the actual pulse used is 200-300us, I guess it would mean a higher V_DS could be used with a shorter pulse.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 04, 2024, 07:47:40 pm
content: Pulsed measurements are standard, regardless of Pd, because it reduces drift due to temp rise, and is -- I assume -- standard practice on test fixtures. It may also be specified in a JEDEC or other industry standard regarding semiconductor testing and specification.Tim

author: metebalci
author_rank: Frequent Contributor
timestamp: July 05, 2024, 10:19:48 am
content: Quote from: T3sl4co1l on July 04, 2024, 07:47:40 pmI didnt see any standard yet, nor a document from a manufacturer. I usually see a range is given in I-V measurement devices, so I guess the test parameters is up to the semiconductor manufacturer.I had a second part in my question, which was actually the reason I asked this. The failure of the JFET can happen either due to breakdown voltage or from high temperature, is that all ? So the power is actually indirectly related (to temperature) and that is why there are different continuous and pulsed values etc.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 05, 2024, 11:35:27 am
content: Not sure what all mechanisms are likely to occur, but probably localized overheating (subsequent melting and destruction), general overheating (package warping and decomposition, bondwire failure, chip destruction), and electromigration (failure of interconnects or junctions due to extreme current density) are the top candidates.General overheating is simply when the chip gets too hot, from power dissipation applied over time. This is specified by the transient impedance curves (if provided). Peak handling for short duration is always higher than continuous, because the heat capacity of the chip, lead frame, etc. are applicable. The thing has to be heated up first, for heat to conduct out of it into the next thing, and so on.Localized heating can occur due to 2nd breakdown (part of the die gets hotter than the rest, raising current flow in that area, getting it hotter...), but this is rarely applicable for signal JFETs. Note that 2nd breakdown requires some degree of power dissipation: there has to be enough power flowing to get some temperature drop across the die, and the die is normally one of the most (thermally) conductive parts of the device (Si ~150 W/(m.K), vs. Cu ~400). Trapped inside a plastic case, signal transistors rarely suffer 2nd breakdown.It can also be due to breakdown effects like avalanche, where a bit of breakdown frees charge carriers, which frees more, and a cascade (avalanche current filament) ensues. The effect varies with type; in BJTs, the high localized current density, plus temperature rise, can cause punch-through (Si heated so much it becomes intrinsic across the base region, and the N-type C and E regions join momentarily). In diodes, this usually happens without ill effect (myriad discharges occur across the junction, more or less all at once, or at least given a uniform fabrication process; avalanche also has a positive tempco, enforcing balance across the die). In MOSFETs, it's mostly safe, but there is a side-effect: carriers generated by avalanche can become lodged in the gate oxide, leading to eventual failure; older families (planar e.g. HEXFET and friends) had adequate distance to confer extreme robustness, but modern types are more susceptible to failure (lower peak avalanche current ratings, rarely if ever repetitive rated). JFETs I don't know about offhand, but would assume something along the diode to BJT range of effects. In short: a JFET might avalanche in a local spot, causing melting and destruction, but it might also distribute across the junction and handle it effectively.Electromigration occurs when current density is so high that atoms are dislodged by sheer current flow. It's not a thermal effect, though it is exacerbated by temperature, and use of low-melting materials (more generally: binding energy). Currents of say ESD magnitude, applied to a tiny JFET junction, might not cause immediate destruction, but the device might only survive dozens or hundreds of strikes, not an unlimited number.So, you have an overall operating area of, at very long time scales, the steady-state power limit applies; for intermediate scales, thermal mass applies; and even at the shortest time scales, there are limits due to breakdown, spot heating; or wear effects due to migration, where you might be able to exceed some nominal peak limit for some number of events, but not forever.Tim