Testing IRFB38N20D mosfets, are they ok?

Discussion in 'General Electronics Chat' started by sdowney717, Apr 1, 2015.

  1. sdowney717

    Thread Starter Member

    Jul 18, 2012
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    http://www.irf.com/product-info/datasheets/data/irfs38n20d.pdf
    I was testing some mosfets from a broken inverter.
    When I put 4.5vdc positive on gate and negative on source to turn on the mosfets, most read from 25 to 55 or more ohms across gate to source. All of them turn off if short gate to drain. A few read low single digit ohms when turned on.

    Comparing to testing IRF1010E mosfets, all those read low single digits. So are the IRFB38N20D ok or are they defective?
     
  2. wayneh

    Expert

    Sep 9, 2010
    12,151
    3,058
    Did you mean drain to source? There should be very high resistance gate-source or gate-drain.
     
  3. alfacliff

    Well-Known Member

    Dec 13, 2013
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    some mosfets have so little internal leakage that if you turn them on from source to gate, they will stay on from source to drain for days, until you short the gate to source, then they turn off from source to drain. takes a whiloe for the gate to source capacitance to discharge out of circuit.
     
  4. ian field

    Distinguished Member

    Oct 27, 2012
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    You can test MOSFETs by charging the gate capacitance to a suitable potential and measure the drain/source condition.

    If leakage dissipates the gate charge, I'd regard the device as suspect.

    I usually charge the gate negative (for N channel) for the drain leakage test - if there's any leakage whatsoever in that condition; its a fail.
     
    Hypatia's Protege likes this.
  5. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
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    Assuming the cited resistances were, in fact, measured drain to source they're probably ok -- Inasmuch as the the bias (as described) was only slightly above threshold, 'discrepancy' between devices of the same type is to be expected --- A proper test will apply a G to S bias of at least 10V...

    Best regards
    HP
     
  6. sdowney717

    Thread Starter Member

    Jul 18, 2012
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    Hi, so your saying they were barely turned on?
    What then means this?

    VGS(th) Gate Threshold Voltage 3.0 ––– 5.0 V

    PDF says min 3.0, max 5.0 v? I thought that was the turn on volt range?

    Then it also says this
    VGS Gate-to-Source Voltage ± 30
    So what do these numbers mean? How much max volts for the gate?

    I have tested some bad mosfets which the gate turns them on, and then they slowly shut off.
    I am turning them on using three 1.5vdc batteries in series.
     
  7. sdowney717

    Thread Starter Member

    Jul 18, 2012
    392
    13
    yes, I meant drain to source.
     
  8. Hypatia's Protege

    Distinguished Member

    Mar 1, 2015
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    Indeed - That is the specified threshold range -- howbeit (gauranteed) saturation requires a bias of Ca 1ov (as per the RDS(on) parameter [q.v.])

    Granted! Your VOM is supplying far less than 26A! Even so, near-threshold bias is productive of 'ambiguous' results at best...

    That would be the 'absolute maximum' specification (that is, in this case, the maximum G-S EMF that is guaranteed not to [instantly] damage the device) -- Note, however, that good design practice will not subject components to absolute maximum stresses...

    BTW how was your inverter damaged?

    Best regards and good luck!:)
    HP
     
    Last edited: Apr 2, 2015
  9. sdowney717

    Thread Starter Member

    Jul 18, 2012
    392
    13
    It is an old inverter I got years ago which I salvaged for parts.
    I had wired in a remote on-off-on DPDT switch and accidently sent 120vac into the switch using a screwdriver with an unsteady hand which energized the 12 vdc control circuitry by shorting across screw terminals.
    I recall wanting one switch to do two things at the same time and I paid the price.

    I bought a 5 pack of broken 750 watt inverters off Ebay and I night be able to use some parts I have been collecting over the years to fix them.
    http://www.ebay.com/itm/311323084551?_trksid=p2057872.m2749.l2649&ssPageName=STRK:MEBIDX:IT

    Can you see the link?

    Got them today.
    One works .
    Others display SC, H.P, bAd, and one has a DC input short.
    Manual describes these, not sure what bAd really means.
    http://www.batterychargers.com/Documents/00-99-000724.pdf
     
    Last edited: Apr 2, 2015
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