MOSFET Circuit

Discussion in 'General Electronics Chat' started by codyspraker, Dec 13, 2012.

  1. codyspraker

    Thread Starter New Member

    Dec 14, 2011
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    I am repairing a circuit board from an electric motor that recently burned one if it's MOSFETs. Attached are two pictures of the board with and without the MOSFET attached. This motor is capable of driving a 600 watt motor and claims to be able to act as a regenerative brake that charges the battery. The MOSFETs have the same labeling on both. The first link the datasheet for the MOSFETs. What I don't know is if this part can simply be replaced for the old one. Also can someone explain this configuration? I don't understand why the middle leg of one connects to the outer leg of the other. These two legs are connected to the positive lead of the motor. Thanks

    http://www.irf.com/product-info/datasheets/data/irfs3306pbf.pdf
    http://i553.photobucket.com/albums/jj381/Sprakinator/photo2.jpeg
    http://i553.photobucket.com/albums/jj381/Sprakinator/phto1.jpeg
     
  2. Dodgydave

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    Jun 22, 2012
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  3. Ron H

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    Class AB is an analog output stage which is push-pull, with both push and pull devices conducting for a short time during the crossover from push to pull and vice-versa.
    It the link you posted,most of the MOSFETs are drawn with source and drain swapped from their correct orientation.
     
  4. codyspraker

    Thread Starter New Member

    Dec 14, 2011
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    Does this mean that both MOSFETs are used to drive the motor AND act as a brake? I was confused why individually each MOSFET peaks at 230w and the motor itself is rated for 600w. I would have assumed the two MOSFETs work together to reach 460w which is a lot closer to the rated power of the motor. Is is what you are saying? I'm a little confused by what you mean with push pull and some of the other terms.
     
  5. Ron H

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    All I was saying is that the circuit Dave referenced is NOT a class AB stage.
    If you go to the web site that Dave pulled the schematic from, it explains the operation in section 4, Regeneration.
    Where did you get the 230W and 460W numbers?
     
  6. codyspraker

    Thread Starter New Member

    Dec 14, 2011
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    The motor the circuit is connected to is rated for 600w. The individual MOSFETs are rated for 230w max. I would assume the two MOSFETs worked in parallel to drive the 600w motor with 460w (230w + 230w). It made sense to me since 460w is closer to the rated current of the motor than just one MOSFET at 230w.
     
  7. antonv

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    Nov 27, 2012
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    The MOSFET power rating (230 W) refers to how much power you can dissipate in the transistor; it does not say much about the amount of power used by the motor.

    Look at the data sheet: the RDSon is 3 to 4 mΩ when the transistor is turned on all the way. This is the resistance that will dissipate power when current flows through the transistor.
     
  8. Ron H

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    Apr 14, 2005
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    The 230W number has almost nothing to do with the power dissipated by your MOSFETs. I suppose you are thinking that is a spec relating to the power delivered to the load, but it is not. The power delivered to the load is a function of the rest of the circuit.
    MOSFETs are rated for maximum power dissipation with an infinite heat sink (let me know if you find one of those :D). When a MOSFET is acting as a switch, the static dissipation in the ON state is Id (drain current) squared, multiplied by Rds(on) (the static ON resistance).
    Judging by the size of your heat sinks, your MOSFETs' power dissipation is probably a few watts at most. This is possible because the datasheet says Rds(on) is only 4.2 milliohms max (3.3mΩ typical). A current of 20 amps or so would be reasonable, maybe a little more.
    MOSFETs also dissipate power when they switch, but efficiently designd circuits will minimize the transition times, which will usually keep this component pretty low, unless the switching frequency is high.
     
  9. codyspraker

    Thread Starter New Member

    Dec 14, 2011
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    Okay so I'm going to try to follow you. Are you saying the 230w is the amount of energy that can be sunk as heat by the MOSFET? So one of these could drive the 600w motor? Also what is the max current that can go into one of these (assuming same heatsink and constant on without switching)? Sorry to make you keep going over this, I haven't used MOSFETs yet and I'm trying to get a feel for what they can do.
     
  10. Ron H

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    230W is the amount of POWER (energy is power * time) that can be dissipated by the transistor if you have an infinite heat sink that holds the package to 25°C. You don't have those.
    As I said, 22 amps (I actually said 20) would cause the MOSFET to dissipate about 2 Watts (22A *22A*.0042Ω=2.03W) maximum. Those wimpy little heat sinks don't look like they can handle much more than 2W, although I am no expert on heat sinks.
     
  11. codyspraker

    Thread Starter New Member

    Dec 14, 2011
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    So at 20 Amps, If I am using a 36v battery to drive the motor using the MOSFET, would that give me 720w at the peak of this MOSFETs ability before the heatsink would not be able to dissipate enough heat?
     
  12. Ron H

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    If your power supply can deliver 20A@36V, and my guess about the heat sink's capability is correct, then you are correct.
     
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