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    SOA of MOSFET....the meaning of "DC" in the graph

    Hi

    In the graph of SOA in the SPB07N60C3 FET datasheet (pg 5), i presume the "DC" means "duty cycled", or "Direct Current"...in other words "pulse trains" or continuous DC?

    The "tp" obviously stands for one-off pulses....its surprising how low these are....you wouldnt think the FET (especially if well heat-sunk) could suffer damage in such a short space of time especially if well heat-sunk so its temperature did not rise significantly during the one-off pulse?

    SPB07N60C3 FET datasheet
    https://www.infineon.com/dgdl/SPB07N...12b42df065491f

    Alos, why is the SOA graph only given for T(case) = 25degC?....this is surely unlikely, do you know how we can get a SOA graph for T(case) = 80degC?
    Last edited by treez; 1st May 2019 at 10:05.

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    Re: SOA of MOSFET....the meaning of "DC" in the graph

    You can safely refer to the commonly understood meaning of the term DC (continuous) direct current…

    For a purely energy/power limited SOA, the characteristic can be easily constructed from the transient thermal impedance curves, respectively a distributed thermal model.

    I don't see a reason to doubt the underlying thermal chip parameters.

    - - - Updated - - -

    In a first order, I would apply linear derating to the SOA power/energy values.


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    Re: SOA of MOSFET....the meaning of "DC" in the graph

    Thanks,
    We need MOSFETSs that are at least 650V rated and for continuous linear mode operation.
    Therefore we need mosfets with a low ZTC point. Most mosfets are for switching mode service, and so have high ZTC points.
    Do you know how we can search for mosfets with low ZTC point?
    Its is not possible to sort and search for this parameter on digikey etc.

    Do you know any vendors which specialise in mosfets for linear mode?..most of them seem to have gone obsolete it seems.



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    Re: SOA of MOSFET....the meaning of "DC" in the graph

    note that the graphs are given assuming the device is on an infinite 25 deg C heatsink.


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    Re: SOA of MOSFET....the meaning of "DC" in the graph

    Thanks yes, and i must admit it is a problem that there is no way of knowing whether or not a particular mosfet will have a bigger SOA than any other fet...the SOA is never a search parameter on digikey etc.
    We are wondering if there are any tell-tale signs of a mosfet with bigger SOA?
    Eg, it seems that mosfets with high rdson, or high vds voltage ratings, appear to have a bigger SOA.
    Would you agree?
    I started pasting SOA graphs of different fets next to each other in an excel docuemnt yesterday, (so as to compare them) but then many of the infineon ones wont allow "take snapshot" from their mosfet datasheet........the datasheet opens up and says something like "secured" on it. So its awkward to even try and compare SOA graphs of different FETs.
    Do you know a way round this?



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    Re: SOA of MOSFET....the meaning of "DC" in the graph

    the larger the die size the bigger the SOA, the higher the power rating for the device - the lower the Rj-c you can search these last two



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    Re: SOA of MOSFET....the meaning of "DC" in the graph

    Your requirements are not completely clear. In post #3 you are asking for MOSFET optimized for linear operation, no hot spots at lower currents. That's different from "bigger SOA" which simply means large die.

    IXYS has a series of "linear MOSFET" with extended FBSOA:
    When Power MOSFETs are used in the linear-mode operation, as opposed to their conventional switch-mode one, they are required to endure substantially high thermal and electrical stresses due to the simultaneous occurrence of high drain voltages and currents; these extreme stresses can cause typical devices to fail. IXYS Linear MOSFETs have been designed to address these kinds of device failures – the FBSOAs are “extended” when the positive feedback of electro-thermal instability is suppressed, giving rise to larger “operating windows.” The FBSOAs are guaranteed at 75°C.
    Alternatively small discrete IGBT might be considered for linear operation.


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    Re: SOA of MOSFET....the meaning of "DC" in the graph

    respectfully - small lGBT's do not do well in linear applications - due to the very small die size compared to mosfet equivalent.


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    Re: SOA of MOSFET....the meaning of "DC" in the graph

    Treez for very short pulses the heat sink is irrelevant.

    Infineon can’t stop you from taking a screenshot or using windows built in “snipping” tool.

    I was also going to mention IXYS linear parts also. Possibly look at depletion mosfets as they’re commonly used for linear applications and may have good SOA.



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    Re: SOA of MOSFET....the meaning of "DC" in the graph

    I wonder why the SOA for the D2PAK and DPAK versions of the STB6N65M2 (STD6N65M2 = DPAK) FET are the same?
    You would have thought the SOA for the D2PAK would have been bigger?
    But on pg 5 of the datasheet they are just using the same SOA graph

    STB6N65M2 (STD6N65M2) datasheet
    https://www.st.com/resource/en/datasheet/stb6n65m2.pdf



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    Re: SOA of MOSFET....the meaning of "DC" in the graph

    laziness / mistake at ST - or the die is the same - and the package is so little difference in size they decided one was as good as another


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    Re: SOA of MOSFET....the meaning of "DC" in the graph

    Quote Originally Posted by treez View Post
    I wonder why the SOA for the D2PAK and DPAK versions of the STB6N65M2 (STD6N65M2 = DPAK) FET are the same?
    There is no difference in rdson, currents, gate charges so its the same die in dif packages.
    bigger package is just a possibility to put bigger die, not obligation


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    Re: SOA of MOSFET....the meaning of "DC" in the graph

    Power MOSFETS are optimized for switching and high-power use in their linear region is fraught with peril.
    It's like walking into a cas ino, the datasheet SOA curves could be conservative... or hyped due to pressure to compete with other manufacturers. MOSFET specs can be deceptive.

    For a figure of merit on die size (and related higher SOA) , I look for a high repetitive avalanche energy-rating. SPB07N60C3 is terrible at 0.5mJ and is a 14 year old part.

    Using a HV MOSFET to ride-through transients and overvoltages is fraught with peril, as energy is E^2. You might bite the bullet and try a MOV instead.


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