Safe operating area of a MOSFET? - Page 1
Forum section: electronics / beginners

author: Artlav
author_rank: Frequent Contributor
timestamp: December 28, 2014, 06:37:06 pm
content: I was looking at the datasheet for STP14NM50N, and found a mysterious graph in there.The FET is 550V, 0.32? Rdson, claimed to be 12A rated and 90W dissipation capable.However, according to that graph it can't do even 1 A at mains voltage (320VDC).On the other hand, the test conditions for gate charge, for example, list VDD = 400 V, ID = 12 A, which implies it can actually do more.What does this graph mean, exactly?

author: Simon
author_rank: Unknown
timestamp: December 28, 2014, 06:41:08 pm
content: I think the graph click post may be referring to the actual voltage across the source and drain with the given current. If you work the figures out it comes to approximately the wattage that the package is limited by. It's not trying to tell you the maximum voltage allowed in the circuit but the maximum voltage that can fall across the source and drain before the MOSFET will overheat.

author: c4757p
author_rank: Super Contributor
timestamp: December 28, 2014, 06:41:37 pm
content: You're right, it can't do 1A at 320VDC, that's insane (320W!). The voltage the MOSFET sees in the voltageacrossit, and hopefully it won't have 320V across it while it's switched on...

author: Simon
author_rank: Unknown
timestamp: December 28, 2014, 06:46:35 pm
content: The section of the graph under the dotted lines shows you how much overload the MOSFET will withstand for those brief periods shown. The whole purpose of the graph is to show you how much heat and for how long the MOSFET can withstand it. Usually most of the voltage will be taken up by the load so only a small amount of voltage falls across the MOSFET and causes heating.

author: rob77
author_rank: Super Contributor
timestamp: December 28, 2014, 08:11:15 pm
content: when switching with a mosfets, you go from max voltage and zero current (no power dissipated) when OFF, to "zero" (small voltage drop accorss DS) voltage and max current when ON (dissipating some little heat because of Rdson and the current)...both ON and OFF states are the low power states, the states "in between" might be (and usually are) the source of overheating - and that chart is showing you where is the safe area for various pulse durations - you have to make sure that the conditions in your circuit are well inside that area all the time.

author: Artlav
author_rank: Frequent Contributor
timestamp: December 28, 2014, 08:49:52 pm
content: Ah, i get it. Thanks.So it is for the cases when the FET is not completely open - the linear region.When it is used as a regulator, or if the switching is too slow.The voltage shown there is the voltage due to it's resistance, not the voltage between the rails.While in the regular switched mode it should work fine with as many amps as it can dissipate the heat of.Right?

author: Simon
author_rank: Unknown
timestamp: December 28, 2014, 08:56:29 pm
content: Yes pretty much but still be careful any switching operation must complete within the times shown for the amount of current being carried.Don't assume if you are switching at frequency that it will just turn off and on. All switching operations take time, and MOSFET gates have a capacitance. The more amps the MOSFET can carry the larger the gate capacitance in general although there will be other determining factors. So you need to look at the driving power of your circuit and work out the time constant that the resistance of your driving circuit and the gate capacitance will have two ensure that that gate capacitance is charged and discharged fast enough to enable switching within the specified time at the current required.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 28, 2014, 10:26:45 pm
content: The numbers are "what's applied to the device". In switching, you spend most of your time at e.g. 300V ~0A, or 5A ~3V (in Rds(on) mode), with only a few short instants at e.g. 300V 5A, or ~3V ~0A.You're also looking at the 10ms curve, which isn't DC (this device is not rated for linear DC operation -- don't use it in a HV current sink!). The 10us curve would likely be more representative, but even then, since only the switching edges are dissipating much power, an even shorter time curve would be relevant, like 100ns or less! At these time scales, heat doesn't even leave the junction, and the die thermal mass can be treated strictly as a capacitor (see also the transient thermal graph for t < 20us). So, no worries.Tim