Measuring gate capacitance of any MOSFET: Basic

Discussion in 'General Electronics Chat' started by Willen, Feb 9, 2016.

  1. Willen

    Thread Starter Member

    Nov 13, 2015
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    Hi,
    Once I bought IRF840 which was 2.5 times expensive than IRFz44. And its manufacture is unknown like it's marked as "IRF840 Y64K DK". IRF840 is expensive because it's very faster like 10MHz or less.

    But seeing its unusual marking, I suspect that it might be a counterfeit part (maybe have slower chip inside instead of real chip of IRF840) to earn 2.5 times more.

    I have one genuine IRF830 (almost same feature as IRF840) made by ST semiconductor. So to test the genuineness of the mosfet I bought with unknown marking, I want to check its gate capacitance (I cannot apply the mosfet in the device because I have not made the device, just planning) and want to compare with genuine IRF830 fast MOSFET and with IRFz44 slower too.

    Can I check its gate capacitance by using a capacitance meter to Gate and Source pin simple as measuring any discrete capacitor? (maybe asking silly but I want to know about the unknown IRF840, I bought)
     
  2. Papabravo

    Expert

    Feb 24, 2006
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    I don't think that will yield the result you are looking for. Most Z5U capacitors have a pretty wide tolerance (+20%, -80%) unless they are graded for tighter tolerance. With the gate of a MOSFET the capacitance is a dynamic thing, it actually changes in operation. The best insurance is to buy from a reliable supplier. If you are uncertain about the quality, just return them and ask for a refund. Refusing to take them back would be a pretty good indication.
     
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  3. Willen

    Thread Starter Member

    Nov 13, 2015
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    I bought from a small local shop, and they think me like a terrorist or criminal or their biggest enemy if I went there to returned back the purchased parts. There's just few shop of consumer electronics parts so they are safe even if the behave cruel to customers. I am 100 years back from advanced world, maybe, in the case of electronics. (Online shopping is out of my capacity and hobby parts are no-where.)
     
  4. DickCappels

    Moderator

    Aug 21, 2008
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    Most likely the extra letters and numbers "Y64K DK" are there to indicate features or packaging options, the factory, and/or the date of manufacturer.

    You can measure the input capacitance if you do so in accordance with the conditions specified on the datasheet.

    For example on the Vishay datasheet (www.vishay.com/docs/91070/91070.pdf) the conditions given are:

    VGS = 0 V,
    VDS = 25 V,
    f = 1.0 MHz, see fig. 5
    [​IMG]
     
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  5. Willen

    Thread Starter Member

    Nov 13, 2015
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    Maybe it's more complicated because of my knowledge limit and equipment limit. For simple comparison, I will just measure the GS capacitance of genuine IRF830 and unknown IRF840 and IRFz44, lets see if I can get some clue or not. :)
     
  6. crutschow

    Expert

    Mar 14, 2008
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    Certainly you can compare capacitances, no matter how you measure them, to determine if there is a significant difference (say >20%).
    If you measure both the same way, then it should be a valid comparison.
     
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  7. Papabravo

    Expert

    Feb 24, 2006
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    I would caution that low cost meters might have the capability of damaging the gate oxide. If you have no other way to go then that's what you are left with.
     
  8. Willen

    Thread Starter Member

    Nov 13, 2015
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    Did you mean the if I connected the Gate and Source pins to the capacitor testing shockets (two pins too) which measures capacitance migh generate high spikes voltage and might damage the Gate of IRF840?

    Really I have a $5 DMM and it has a 'Cx' range to measure capacitance from 2nF to 200uF and has a two hole as shocket for capacitor insertion.
     
  9. #12

    Expert

    Nov 30, 2010
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    no
     
  10. #12

    Expert

    Nov 30, 2010
    16,278
    6,789
    no
     
  11. #12

    Expert

    Nov 30, 2010
    16,278
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    no
     
  12. #12

    Expert

    Nov 30, 2010
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    The gate of most mosfets can survive 20 volts. The battery in most cheap meters is 9 volts. I don't understand why 9 volts is more than 20 volts for this measurement.
     
  13. Willen

    Thread Starter Member

    Nov 13, 2015
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    For capacitance measurement? Amazing the testing the gate with 9V DMM damages the gate! Being worried! I have tested some 2N7000, BS170 small fets.
     
    Last edited: Feb 9, 2016
  14. #12

    Expert

    Nov 30, 2010
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    I just told you a 9 volt battery can't destroy a 20 volt gate, so now you're worried?
     
  15. cmartinez

    AAC Fanatic!

    Jan 17, 2007
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    Maybe Willen hasn't realized that the difference between a traditional bjt transistor and a mosfet is that the former is current-drive, while the latter is voltage-driven
     
  16. Willen

    Thread Starter Member

    Nov 13, 2015
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    I tested them with cheap DMM (diode test) which is powered by 9V battery. I think the probe has just few volts.

    What you think about static charges in the case of such damage? Are you sure the damage is due to DMM?
     
  17. Willen

    Thread Starter Member

    Nov 13, 2015
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    The result about gate capacitance of my unknown IRF840 made me frustrate!

    IRF830 genuine fast FET= 700pf
    IRFz44 slow FET= 1.8nF
    IRF840 unknown FET= 1.7nF
    7N60 power FET= 1.5nF

    the IRF840 seems counterfeit part! What the h___! IRF830 and IRF840 should have almost similar gate capacitance, should't it?
     
    Last edited by a moderator: Feb 10, 2016
  18. dannyf

    Well-Known Member

    Sep 13, 2015
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    Power MOSFETs typically have gate capacitance around 2000 to 3000pf. So to get it to work at 10Mhz, you hsvd to deliver loads of current.

    On that from, check out irf510.
     
  19. dannyf

    Well-Known Member

    Sep 13, 2015
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    Checjbthe data sheet.
     
  20. DickCappels

    Moderator

    Aug 21, 2008
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    Not necessarily. If you don't have the datasheet for the suspected part you can't really know -except in cases like the lot of 2N2222 NPN transistors I bough through the mail from a surplus dealer. Not only did they have the wrong pin-out, but they were PNP.
     
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