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FET Killer

Discussion in 'General Electronics Chat' started by kinarfi, Nov 20, 2009.

  1. Roff

    Roff Well-Known Member

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    Oscillation due to long wires can kill a MOSFET. A resistor in series with the gate (as close to the gate as possible) will help supress oscillations, but you still need to keep all wires as short as possible, and bypass the supplies, as recommended by others. A good ground plane is also recommended.
    With all due respect, kinarfi, if gate dv/dt were a problem, it would have been discovered a long time ago. Do you know how long MOSFETs have been around, and how many man-hours have been spent studying them? Your long wire and switch may have killed your MOSFET, but it was not due to gate dv/dt.
    And I wasn't referring to you when I mentioned the ignored "challenge". I was referring to unclejed613.
     
  2. kinarfi

    kinarfi Well-Known Member

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    I was just looking for some capacitors to add to my PCB and notice they spec a shelf life of 1000 hours and a load life of 2000 hrs at 85c.
    How do you interpret this? do they die quickly when that hot, do they only last for 2000 hours, that's only 83 days, I have caps well over 10 years old and still seem fine, even though I wasn't using them until I just used them for bypass filter caps. Here's where I was looking at them. http://www.electro-tech-online.com/...CAPACITOR_ACTIVES-AND-PASSIVES_6130356PDF.pdf
    an off subject question, maybe, is what fails in them, My TV, 35" Hitachi is going to have to back into the shop again, first time, it was about 8 yrs old and now 2 years later, it's starting to roll again. last time it was a cap gone bad.
    Thanks
    kinarfi
     
  3. unclejed613

    unclejed613 Well-Known Member

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    no that was the length of the test, and the amount of capacitance, leakage and dissipation factor change that takes place between the start and end of the test.
     
  4. dave

    Dave New Member

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  5. kinarfi

    kinarfi Well-Known Member

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    Thanks Unclejed, just looking at the spec sheet, how would someone know this, I was starting think I'd be changing caps every 83 day:eek:but I knew that couldn't be right.
    Just realized something, If I protect my FETs in the H bridge, then there is no way the motor can "free wheel" because any regeneration will be shorted by the diodes and I can't have that. Back to back Zeners would limit the spikes, Right???? or do the zener in the FETs provide enough protection, my guess is no. I'm going through my books to see if the is already a specialize device like this, but if you already know what I need, please post.
    Thanks
    Kinarfi
     
  6. mneary

    mneary New Member

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    When the motor is free wheeling, it only generates voltage proportional to its speed. If the motor isn't going faster than it did when powered, it won't generate a voltage greater than the supply.

    The diodes would only turn on due to spikes caused by stray inductance.
     
  7. kinarfi

    kinarfi Well-Known Member

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    Thanks Mneary, you got me to go back and look at what I thinking,:rolleyes: I would be correct IF the battery was removed and the battery terminals shorted together. Worst case scenario now is it may become a battery charger, Ha :eek:
    Thanks Again, I appreciate your correction.
    Kinarfi
     
  8. vlad777

    vlad777 Member

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    Sorry for this off topic question.

    But does anybady have
    panasonic stereo SA-AK25:
    service manual,
    power circuit shematic or
    rsn3502a datasheet ?

    Or can y please tell me where to
    download it for free?

    Many thanks and apologizes.
     
  9. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Please do not hyjack this thread. Instead you should start your own thread.
     
  10. kinarfi

    kinarfi Well-Known Member

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    The Item I was looking is a Diac, but probably not strong enough for this application.
    kinarfi
     
  11. unclejed613

    unclejed613 Well-Known Member

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    see if you can find any info on the web for "energy harvesting" that's done by motor controllers. the braking is better when you place a short across the motor. you would do this with another MOSFET across the motor. if you're wanting to use the braking current to recharge the battery, you will probably need a pair of MOSFETs to divert the current to a DC-DC converter that feeds back to the battery. the back EMF of the motor during braking is usually less than the battery voltage, and needs to be stepped up to the battery voltage. you will need a boost or flyback converter. you can find a lot of this info at Linear Technology - Linear Home Page

    another thing you might want to consider is the use of an ultracapacitor in conjunction with the battery as part of the energy harvesting system. you can find more information about ultracapacitors here (the picture shows a 6000F cap about the same size as a 30000uf/50V cap... amazing) http://nichicon-us.com/english/products/evercap.html

    after reading some of nichicon's material, it looks like one method of regenerative braking would be to switch the motor to the charging circuit of an ultracap bank, the charging current of the caps would provide enough of a load on the motor to brake effectively. the ultracap powers a DC-DC converter to charge the battery.
     
    Last edited: Dec 13, 2009
  12. kinarfi

    kinarfi Well-Known Member

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    Thanks Unclejed, but not looking to harvest, Just dynamic braking, and to accomplish this I'm turning on the two P FETs until I need power, at which time I turn one off and then modulate the N FET. I was thinking the bridge rectifier was going to be a problem, but I was wrong, as Mneary explained.

    Thanks,
    Kinarfi
     
  13. unclejed613

    unclejed613 Well-Known Member

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    i've seen something similar in pro audio speakers as a protection device. there's a bridge rectifier across the speaker terminals with a zener and MOSFET on the rectified side. when the zener conducts, it drives the MOSFET gate, and the MOSFET clamps the audio, protecting the speakers. the clamping level is preset, and because of the bridge rectifier, works on positive and negative peaks.
     
  14. kinarfi

    kinarfi Well-Known Member

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    http://www.electro-tech-online.com/custompdfs/2010/02/AN-9010-2.pdf I think this may be the printed material you asking about In the pdf search box type parasitic diode and parasitic bipolar transistor
     

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