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Thread: MOSFETs for Advanced Power Switching and Polarity Protection article at GeoFex?

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    Supporting Member Steve A.'s Avatar
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    MOSFETs for Advanced Power Switching and Polarity Protection article at GeoFex?

    R.G. Keen's article from 1999 and updated in 2005 listed several different MOSFETs that could be used in his advanced polarity protection scheme. Mouser only stocks the IRFD9024PBF (4 pin DIP) and FQPF11P06 (TO-220F) MOSFETS with 10 lot prices of $1.35 and $0.93, respectively, with USPS economy shipping for $4.99.

    https://www.mouser.com/ProductDetail...EBtZ2OJQ%3D%3D

    https://www.vishay.com/docs/91137/sihfd902.pdf



    https://www.mouser.com/ProductDetail...gskh9yOVeWA%3D

    http://www.onsemi.com/pub/Collateral/FQPF11P06-D.pdf

    I was wondering if there are better options available today, preferably in a TO-92 package that I think would be easier to add to an existing project. Or with better specs (i.e., a lower forward voltage drop.)

    Thanks!

    Steve A.

    P.S. I ran across a reference to R.G.'s highly-recommended article while browsing the DIY FX forums yesterday. A typical diode protection circuit using a 1N4148 has a voltage drop of ~650mv ( or ~300-400 mv for one with a Schottsky) while R.G.'s MOSFET circuit had a voltage drop less than 1mv. Wow!

    Advanced Power Switching and Polarity Protection for Effects

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    MOSFETS don't have the same voltage drop characteristics as a P-N junction device such a diode (or BJT) and the rough equivalent is the resistance between source and drain when the device is 'on', given as Rds in the spec sheets. Because the drain-source acts like a resistor the voltage drop is dependent upon the current flowing through it. In the case of guitar FX that's pretty low.

    One problem with TO-92 packages is the resistance is usually quite high (several ohms) compared to a medium-power device (much less than 1 ohm). Plus, SMD has taken over, though a medium-power SMD MOSFET is fairly easy to handle.

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    Last edited by Mick Bailey; 02-22-2018 at 08:04 AM.

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    All mosfets have body diode. If you want polarity protection you must swap drain and source wrt. what is written in the article.

    It is even mentioned there as the 'backwards' connection.

    Mosfets: you can try some new TO-92 mosfets from Supertex, like TP0604, but the ones already mentioned above are great for the application

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    Last edited by darkfenriz; 02-22-2018 at 09:10 AM.

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    The body diode voltage drop is only significant at the instant power is applied. Once the MOSFET conducts the channel is effectively in parallel with the diode and the Rds becomes the important factor in the effective voltage drop. Agreed, the body diode direction is important for the circuit to work (for a P-channel device this is in a forward direction from drain to source).

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    The idea was described in an Application Note from Texas Instruments. I use it in all my effects/preamps. But I use SMD transistor because they have lower resistance. If you want to use TO-92, you may try MICROCHIP (SUPERTEX) VP3203N3-G. It has 0.6 Ohms resistance. SMD transistors are much better. Eg. ALPHA & OMEGA SEMICONDUCTOR AO3401A only 50 miliOhms.

    Mark

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    Check Vgs max too.
    It is no longer wise to assume it is always +-20V.

    AO3401A suggested above has only +-12V, which is marginal...

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    I listed this transistor as an example of P-channel MOSFET transistors with resistance in a range of miliohms. Why Vgs max is important? I assume that we are talking here about guitar effects supplied form 9V battery. Or, this is not the case?

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