MOSFETs - Safe Operating Area - Page 1
Forum section: electronics / beginners

author: joeyjoejoe
author_rank: Frequent Contributor
timestamp: September 26, 2018, 10:16:13 pm
content: Saw a nice video explaining this, so I get the concept.However, some I notice some SOA charts do not have a DC rating.For instance, the IRF 5305PbF doesn't.https://www.infineon.com/dgdl/irf5305pbf.pdf?fileId=5546d462533600a4015355e370101993What does this mean? It's not rated/certified/intended to be used for constant linear operation? Or they just haven't bothered?(An aside : I notice International Rectifier/Infineon doesn't seem to have DC on any of the datasheets for the (very small sample set) 3 MOSFET's I chose. I went and picked 4 ON Semiconductor datasheets and they all had DC specified in the safe operating area.)

author: thm_w
author_rank: Super Contributor
timestamp: September 26, 2018, 10:31:24 pm
content: Its not rated to be used in that way. You can find some good discussion here:https://www.eevblog.com/forum/projects/igbt-vs-mosfet-for-dummy-load/http://www.digikey.com.au/Web%20Export/Supplier%20Content/microsemi_278/pdf/microsemi-power-an-make-linear-mode-work.pdf?redirected=1https://www.infineon.com/dgdl/Infineon-ApplicationNote_Linear_Mode_Operation_Safe_Operation_Diagram_MOSFETs-AN-v01_00-EN.pdf?fileId=db3a30433e30e4bf013e3646e9381200It could fail at less than 10x its rated current for example.But some loads are using IRFP250, which has no DC SOA rating as well. So they have either done a lot of internal testing or discussed with the manufacturer.

author: joeyjoejoe
author_rank: Frequent Contributor
timestamp: September 26, 2018, 10:40:37 pm
content: Awesome, thanks - that first one sure is lengthy!So, is there a way to find mosfet's that are good for linear operation, short of reading tons of data sheets? I feel like I know the answer but might as well ask

author: thm_w
author_rank: Super Contributor
timestamp: September 26, 2018, 10:58:19 pm
content: The way is to use the same parts Agilent/Rigol/BK have, and derate them in the same way (IRFP250, IRF540, etc. run at <30W each?)Or buy purposefully linear rated FETs,someare not too expensive if you need guaranteed operation:http://ixapps.ixys.com/family.aspx?i=5https://www.microsemi.com/product-directory/mosfet/732-linear

author: joeyjoejoe
author_rank: Frequent Contributor
timestamp: September 26, 2018, 11:29:05 pm
content: Gotcha.Looks like ON Semi has some nice ones for linear use.2SJ652 (P Channel)NTP5864N (N Channel)Kind of nice as a hobbyist, I don't have to sweat the difference between a MOSFET that's 1.40$ versus 2$. (Or 3.25CAD$ in this case!) Just derate to make up for my lack of the ability or willingness to test.

author: digsys
author_rank: Unknown
timestamp: September 26, 2018, 11:50:45 pm
content: A few years back I spent weeks on blowing up dozens of different FETs, creating my own DC SOA curves. It was for a new generation UL drop-out linearP/Supply series I was making. Comparing my curves against manufacturers - it was clear that MOST the SOA curves are either approximate, or total guesswork.The INXS were pretty close, maybe a bit over-pec'd. Others were either better or worse, sometimes by a large margin. I did find some white papers on thewhys/hows etc. I'll dig them up and post them when I have a min. I settled for ~40V @ 5A per device as my guaranteed standard.Hint: The MIC5156 / MIC5158 is the ideal device for driving several in parallel :-)

author: Zero999
author_rank: Super Contributor
timestamp: September 27, 2018, 08:43:34 am
content: Quote from: joeyjoejoe on September 26, 2018, 10:16:13 pmWhat voltage and current do you need?If it's under 30V or so, a BJT might be better.

author: MiDi
author_rank: Frequent Contributor
timestamp: September 27, 2018, 11:31:13 am
content: Keep in mind that the given SOAs are @25°C die temperature, you hardly could estimate the SOAs for higher temperatures.IXYS gives for their linear mode rated MOSFETS another SOA for higher temperature, 75°C if I remind correctly.

author: BravoV
author_rank: Super Contributor
timestamp: September 27, 2018, 11:53:24 am
content: An example from IXYS linear fet, IXTN46N50L datasheet, at the SOA chart, sometimes they called "Extended" FBSOA, watch carefully the highlighted areas I made, most common switching fets (or almost all) didn't make this kind of details.Also watch the bold DC line as they deliberately made it.

author: aheid
author_rank: Regular Contributor
timestamp: September 27, 2018, 01:04:31 pm
content: I can't recall in which article I read this, but it basically said that if a SOA graph for a FET only has a straight line between R_DS(On) and V_(BR)DSS area for DC, then the SOA graph should not be trusted for DC operation.Attached is the SOA for theIPP65R095C7, which has the characteristic kink in the graph the article talked about (and also at a more representative Tc).Now, I'm no expert and I haven't had a chance to test this in practice yet unfortunately so can't say if the above FET is capable of linear loads. But compared to many other SOA graphs, it at leastlooksmuch better than many others.

author: joeyjoejoe
author_rank: Frequent Contributor
timestamp: September 27, 2018, 01:07:49 pm
content: Quote from: Hero999 on September 27, 2018, 08:43:34 amYup under 30V for sure - "true" current would be around 4A.

author: MiDi
author_rank: Frequent Contributor
timestamp: September 27, 2018, 10:11:28 pm
content: Quote from: BravoV on September 27, 2018, 11:53:24 amBit confused both SOAs are for 150°C Tj (junction/die), what has the Tc (case) to do with the SOA?Edit: Oh, they are doing it on others the same way

author: aheid
author_rank: Regular Contributor
timestamp: September 27, 2018, 10:50:52 pm
content: Quote from: MiDi on September 27, 2018, 10:11:28 pmIf Tj is the same for two different Tc's then, assuming the systems are in equilibrium, the lower Tc system must have a better cooling solution and hence is able to dissipate more power as heat.Alternatively, the absolute temperature of Tj depends on Tc. Higher Tc means less current to reach the same Tj.

author: MiDi
author_rank: Frequent Contributor
timestamp: September 27, 2018, 11:11:18 pm
content: Quote from: aheid on September 27, 2018, 10:50:52 pmtoo late here, I should goIt is the Rthjc which defines how much power at given dTjc...

author: KE5FX
author_rank: Super Contributor
timestamp: September 27, 2018, 11:20:53 pm
content: Relying on Tj is problematic for power FETs that are constructed by paralleling multiple dies internally. In the region below the point where the transfer curves cross over, a given drain current will be achieved at a much lower Vgs as temperature goes up. The dies are never 100% identical, so thermal runaway becomes a possibility as the hotter dies start to carry more drain current.To paraphrase Dr. Bronner, "Derate! Derate! OK!"

author: aheid
author_rank: Regular Contributor
timestamp: September 28, 2018, 06:31:41 am
content: Quote from: MiDi on September 27, 2018, 11:11:18 pmNp, was at the same stage, you might have noticed the edit