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Find a substitude mosfet

Discussion in 'Datasheets, Manuals and Component Identification' started by Mosfat, Jul 21, 2019.

  1. Mosfat

    Mosfat

    4
    0
    Jul 21, 2019
    Im new here so hey guys first of all.
    Ive been working lately on building a quadcopter, with unfortunatly ended up with 2 ruined mosfets.
    Now im trying to find an equivalent fets that will go with my flight controller and im not really sure about what specs i should regard to
    This is the datasheet for my ruined fets:
    https://www.google.com/url?sa=t&sou...FjAAegQIAxAB&usg=AOvVaw0R8PWy9m7YrUYh1PHnavat
    Model is sis476dn
    If anyone can please help me ill be thankful, have a nice day
     
  2. hevans1944

    hevans1944 Hop - AC8NS

    4,214
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    Jun 21, 2012
    Why not order this part and replace the two ruined MOSFETs? It appears to still be in production with lots of vendors. See here.

    How did you manage to ruin two MOSFETs? If you don't know what you are doing, why are you doing it? Letting the smoke out of components is a time-honored way of testing and troubleshooting a design, but it can be a bit expensive. Why don't you tell us what you are trying to DO. Provide some details that will perhaps allow someone to explain why what you tried didn't work.
     
  3. Mosfat

    Mosfat

    4
    0
    Jul 21, 2019
    Hey thanks for replying, I havnt seen the smoke actually, i ruind them by soldering and shortening their legs by mistake and improffesionality.
    Ill deffinatly try to use some flux and save them but i wanted to have some extra just in case.
    Basically those are connected to my drone's motors and they sit on the flight controller.
    The problem is that i live in Israel and shipping from those sites you mentioned will cost me 100$, so i thought on buying fets that can fit properlly but i dont know what specs to look at.
    Is the only options is to buy the same model?
    There is a pic of the fets what do you think? [​IMG]
     
  4. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    These MOSFETs have extremely low RDS(on) resistance at reasonable Vgs drive levels. Take a look at this DigiKey link for some possible substitutions with different VDS(max) and ID(max) specifications. Note that DigiKey offers FREE shipping to Israel for orders totaling more than $100... presumably that's US dollars. You might want to budget a couple hundred dollars for replacements and/or substitutes. Or try surfing the Asian market (AliBaba, etc.) sellers, but beware that there are a lot of counterfeit devices being sold there... caveat emptor.
     
  5. Mosfat

    Mosfat

    4
    0
    Jul 21, 2019
    Hey hevans and thanks again for your detailed and helpful reply.
    Unfortunatly couple hundred are way beyond my budget (poor engineering student lol) as a new controller would probably cost me around 60$ and I can keep mine for replacement.
    but I was searching at the Asian market as you mentioned and i actually found the same model on ebay:
    https://rover.ebay.com/rover/0/0/0?mpre=https://www.ebay.com/ulk/itm/192833548552

    Although it says unbranded specs and model are same as mine, also total price for 5 pcs is quoted for around 10$ which is not bad so I might go with this one.
    Just for some general knowledge and curiosity, did you linked those fets based only for the fact that they are low resistance on the ds junction?
    Does it mean it allows more current flow?
    And why Vds and Id do not have to be exactly same value? Are those just limits?
    Again thanks for all really appriciate it.
     
  6. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    Yeah, at that price I would take the risk, for a buy of a few of them, that they are in fact "the real deal" and will work as advertised. But they are probably "clones," perhaps counterfeits if not licensed to be manufactured by a foreign fab, but not likely not. Wouldn't matter to me, if I were to purchase for hobby use, as long as they worked to specification. Not everything manufactured in China is junk parts with zero quality control, but you can only find that out by trial and error buying.

    As a former student at university in the late 1960s and throughout the 1970s I learned to appreciate just how precious a dollar was when it came to purchasing parts for an electronics project. This was before the Internet and eBay, but we were fortunate in our city to have surplus stores that sold military electronics items. If you knew what you wanted, and what equipment contained your part, you could sometimes bargain for an entire chassis just to obtain that one part. Some of the electronics stuff we bought was sold to us by the pound, because the owner didn't realize the value of those itty bitty parts or anything about electronics. So he priced them as he would nuts, bolts and washers.Of course you took whatever else looked useful from your chassis or whatever, but most of the time it was just the one part that was needed. Today, there are much larger and longer threads in the marketplace supply line and much more to choose from. The "old man" had sons, who he sent off to college from the profits of his salvage business. They soon learned about electronics and came home and began sorting parts and placing them in plastic bags for individual sale... for like thirty or forty cents each instead of thirty or forty cents a pound. Still a pretty good bargain most of the time.

    I ran across that linked page by accident. I was searching for something else, but forget exactly what my search string was and that page popped up. I didn't want to confuse you, but this looked like a reasonable selection to pick from, all the same series from the same manufacturer. That way you get to compare apples to apples and oranges to oranges.

    And, yes, I was looking for MOSFETs with very low RDS(on). This is a parameter that it is absolutely essential to minimize for quadcopter and other battery-operated brushless DC motor applications. Lower RDS(on) does mean more current flow is allowed. And, just so you know, there is no such thing as a "ds junction" in a MOSFET. There is a parasitic "body diode" present in every MOSFET manufactured, but it is an inescapable consequence of how the MOSFET is made. Suggest you Google this to come up to speed. This body diode is often used in lieu of an external diode to help suppress back emf generated from collapsing magnetic fields in motors.

    VDS(max) and ID(max) cannot ever be exactly the same value because they are measurements of two different things. VDS(max) is the maximum voltage you can apply between the drain and source of a MOSFET without electrical breakdown occurring, leading usually to instant "death" of the device. ID(max) is the maximum current you can draw through the drain terminal from the source terminal. This produces heating in the conduction channel equal to ID2 multiplied by RDS(on). The MOSFET usually fails slowly as the current approaches the maximum, and may recover and operate normally if the current in the conduction channel is reduced.

    I try to avoid operating semiconductor devices close to the their published maximum values for any parameter. The hassle of replacing a damaged part, and whatever else it took with it to the grave, requires way more time to troubleshoot, find, replace, and test the repair than the cost of the part... usually. I don't want to have to go through that process more than once because the replacement part was in any way marginal in its performance, and itself required replacement. However, "basket cases" either go directly to the re-cycle bin or are returned, un-diagnosed and un-repaired, to the customer.
     
  7. Mosfat

    Mosfat

    4
    0
    Jul 21, 2019
    Hevans, I bet living at those times was really authentic and pure, nowadays when everything is posted online and you just need to use your finger in order to get things, a little bit of the magic part of the proccess is ruined, but that is a different story.

    As you suggested, I ended up buying 5 pcs of this clone Mosfets hoping that they will be as advertised.

    My apologies for my poor knowledge on semi-conductors, this subject is really not intuitive for me, but thanks to you things are way more clear to me.

    As much as I know a MOSFET contains a bulk which we also refer as the Body of the fet, suppose the bulk is a p-type it must derive that the source and the drain areas must be n-types, that creates two PN juctions, one is forward biased and the other one is backward biased.
    When voltage is applied, the forward biased juction runs electrons through a channel and the backward biased is "collecting" those electrons (that is why this kind of mosfet will be refered as nmos).
    I do have some other questions if you'd have some free time.
    -What is the purpose 8 pins mosfets? As much as I know they have source drain and gate.
    -I can understand why not working near to the Vds max but isnt it pretty much safe and desirable working at Id, max since the current reaches saturation pretty fast?
    -Is there any industrial benefits on making lets say nmoses rather that pmoses? Are those electrically equivalent?

    Wish you a wonderful day and again I thank you a lot for your big effort and time!
     
  8. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    This is wrong in so many ways. Go read some articles devoted to MOSFET construction.

    [​IMG]
    The body of the MOSFET is used to define, in conjunction with the gate electrode, a conduction channel, which can be either N-type or P-type. The width of this channel is influenced by the electrical field between the gate electrode and the body. Ohmic contacts, not PN junctions, are made between the drain electrode and the body, and between the source electrode and the body. Current flowing between the source and the drain DOES NOT flow through any PN junctions. The parasitic diode formed between the substrate and the channel serves to electrically isolate (insulate) the channel from the outside world. This diode is normally reverse-biased and does not conduct during normal MOSFET switch operation.

    Extra pins are there to help more evenly distribute the current passing through the MOSFET.

    I guess that depends on how comfortable you are pushing the power dissipation to the point where the silicon begins to melt. How fast (or slow) the current increases to a value you call saturation is a design factor that you control. However, "saturation" only occurs during collector-emitter conduction of bi-polar junction transistors (BJTs), not during channel conduction of MOSFETs.

    An n-mos fet is not electrically equivalent to a p-mos fet. AFAIK, the process steps to manufacture either type are similar. An n-mos fet stacked in series above a p-mos fet forms a classical "totem pole" output that is the basis for CMOS (Complimentary Metal Oxide Semiconductor) integrated circuit design.
     
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