MOSFET SOA - Page 1
Forum section: electronics / general

author: zanzeoo
author_rank: Contributor
timestamp: October 17, 2020, 10:00:52 am
content: HelloCan someone help me to understand the SOA of this MOSFET Si4288DYI dont understand how to match the absolute maximum rating ( ID =9.2 A) with the SOA curve page 4.Suppose i want a max current of 6A at DC and mosfet is fully turned on so Rdson is Max i suppose ( 0.02 ohms ) Is it still in the SOA ?thx

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 17, 2020, 11:33:03 am
content: Ref:https://www.vishay.com/docs/67078/si4288dy.pdfSaturated DC rating is all about how much cooling you can provide.Evidently, they give the most pessimistic curve for DC SOA, around 5A. The ratings say you can go higher, assuming different cooling options.What footprint are you using? Thermal vias, heatsinking?Tim

author: zanzeoo
author_rank: Contributor
timestamp: October 17, 2020, 12:06:28 pm
content: thx for your answer ...so 9.2A if for 100ms ?It's a little bit confusing beacause it doesnot match the curve of VDS....The more is VDS the more you can have current in a short time ?thx

author: Siwastaja
author_rank: Super Contributor
timestamp: October 17, 2020, 12:26:57 pm
content: No, obviously the more Vds, the less current, because power dissipation in the die at any given time is Vds*Id.

author: langwadt
author_rank: Super Contributor
timestamp: October 17, 2020, 12:34:12 pm
content: Quote from: Siwastaja on October 17, 2020, 12:26:57 pmbut Id is also limited by Vds/Rdson

author: Siwastaja
author_rank: Super Contributor
timestamp: October 17, 2020, 12:49:51 pm
content: What's the application? Are you going to run the FET in partially conducting ("linear") state, or switch slowly on purpose? Or are you just going to use the FET in a switching application?If switching application, what will be the switching frequency - is it an on-off power switch or slow PWM (such as heater control), or are you doing a switch mode converter, motor inverter or similar?You would typically aim to switch in less than a microsecond in which case the SOA curve provides no information you would be interested in; instead, you'd calculate the junction temperature from the worst case Rds(on) at your target junction temperature (it will be some 50% more than the room temp Rds(on)) from P = I^2*R and Tj = Tamb + Rthj-a * P. If switching frequently, i.e., switch mode converter, you need to assess switching loss as well.

author: zanzeoo
author_rank: Contributor
timestamp: October 17, 2020, 01:15:09 pm
content: Just trying to understand the SOA curveFor exemple in DC condition suppose the VGS is good enough to have RDSON min , then what is the process to check if the mosfet is in the SOA ?Power dissipated into the mosfet is Rdson²*I so VDS is very low ?how do you match these information with the SOA curve ?

author: Siwastaja
author_rank: Super Contributor
timestamp: October 17, 2020, 01:31:32 pm
content: The idea of SOA curve is thatyouknow both IdandVds. This means, MOSFET isnoton, and it isnotoff, but in a middle state defined by you: for example, you use it as a pass transistor for linear regulator, dropping 7 volts over the MOSFET at a load current of say 5A. Then you can look at the SOA curve to see how long the MOSFET can take that said combination of Vds and Id, without blowing up.It appears you are considering switching MOSFET fully on, or fully off. In fully on case, Vds = Rds_on * Id. In fully off case, Id = 0. In neither case is SOA curve useful.On the left-hand side, SOA curves show the degenerate/trivial line labeled "limited by Rds(on)" which is highly misleading to beginners, sometimes even to more experienced designers. The futility of that part of the curve is easiest demonstrated by an example: say our imaginary MOSFET have Rds(on) of 0.01 ohms and Id=10A; as a result, Vds simplycannotbe lower than 0.01 ohms * 10 A = 0.1V. Just looking at the curve, itlookslike having even lower Vds would be "outside" of safe area, but this is a wrong interpretation. Such condition would appear if the MOSFET wasbetterthan specified and would then obviously be totally fine.

author: Siwastaja
author_rank: Super Contributor
timestamp: October 17, 2020, 01:52:25 pm
content: To rephrase and explain the SOA curve in general:Area at the bottom, inside the lines is safe operating area.Downward lines on the upper right corner are safe operating limits due to thermal dissipation within the die: don't go beyond these lines.Vertical line on the right is safe operating limit due to maximum voltage rating: don't go beyond this.Horizontal line on the top is safe operating limit due to maximum current: don't go beyond this.Upward line on the left, often labeled "limited by Rds(on)", is not a safe operating limit: it's just showing the area you won't be able to enter due to ohm's law, so area outside of the curve on the left isirrelevant, notunsafe.

author: zanzeoo
author_rank: Contributor
timestamp: October 17, 2020, 02:20:26 pm
content: thx ...I had the wrong interpretation of this curve