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  1. #1
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    Coils question and mosfet's base

    Hello guys,

    I am a little bit confuse, i have two questions:
    1) Every kind of coil have a core? even smt coil? if all the coils have cores so all the coils can store energy? air coil can store energy?
    2) Why the mosfet's base of n ch connected to the source?

    Thank you guys

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  2. #2
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    Re: Coils question and mosfet's base

    1. Inductors can be designed without or with a ferromagnetic core, either with closed or open magnetic path. That's also the case with SMT inductors. Energy is stored in the magnetic field, not the core.

    2. Presume you are talking about the MOSFET's substrate or body. In lateral (small signal) MOSFET, it's not necessarily connected to the source terminal. In IC design, it's usually connected to the most negative circuit node(respectively most positive for p type), e.g. a power supply rail. Power MOSFET have a vertical structure and don't expose a separate substrate terminal. The source is connected to the substrate by design.

    See below drawing from an IRF/Infineon technical paper. Source metal connects n+ source terminal and p+ body.

    Click image for larger version. 

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  3. #3
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    Re: Coils question and mosfet's base

    2. If the BULK of the FET is connected to more positive voltage than the source, the source-bulk diode won't be reverse biased, and the FET won't operate normally.
    If the BULK voltage is more negative, the threshold voltage of the device will increase by "body-effect".
    If you connect source and bulk together above issues won't occour, it will always operate and the threshold-voltage can't change with source voltage.

    What FvM said is also true, however I think not the vertical structure is the point why they connect source and bulk, in above structure the bias of the p+ body region could be also solvable.
    "Try SCE to AUX." /John Aaron/


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    Re: Coils question and mosfet's base

    I think not the vertical structure is the point why they connect source and bulk, in above structure the bias of the p+ body region could be also solvable.
    I assume that it's theoretically possible, but the real power MOSFET is a large array of transistor cells, it won't be easy to connect a fourth terminal.



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    Re: Coils question and mosfet's base

    I don't know, but I don't think so. I could draw the wiring on 1st metal layer, and bias of the BULK requires only voltage, wide wires are not necessary for it. And actually it is easier with lot of fingered transistor cells. Imagine 2 combs in the face of each other from metal, one is the source and one is the bulk terminal. Gate is under the source comb, drain is on the backplate, ready.
    "Try SCE to AUX." /John Aaron/



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