Mosfet swapping

Discussion in 'General Electronics Chat' started by bytraper, Feb 19, 2011.

  1. bytraper

    Thread Starter Active Member

    Sep 28, 2010
    126
    4
    Hi Everyone,

    I've got a PWM controller that uses an irf2804 (n-channel mosfet), can I replace the mosfets in it with a IRF3205' (also n-channel) for higher voltage applications? what do I need to watch out for ?

    Also, I'm using Bill's 555 PWM/Frequency generator which is a <snip> fine circuit (cheers Bill), I want to hook up a Hall effect throttle to it, to replace the potentiometer! How would I go about this? the source (Vcc) voltage is 12v to bills circuit.

    Thanks for any and all help!
     
    Last edited by a moderator: Feb 19, 2011
  2. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    The IRF3205 seems to have about the same gate charge, but the Rds(on) is 8m vs the 2m of the IRF2804. This means your MOSFET(s) will have 4x the power dissipation of the IRF3205, so you will need to improve your heat sinking/cooling considerably.

    If you find another MOSFET with a higher voltage rating and an Rds(on) of 2m, it's a good bet that the gate charge will be considerably higher than the IRF2804. Increasing the gate charge requirement means that the MOSFET will spend more time in the linear region dissipating power as heat. At that point, you'd need a better gate driver circuit.

    What do you mean, a Hall-effect throttle? You have an existing Hall-effect sensor that is a throttle? What is the Hall-effect throttle part number or specification? Or what are you talking about?
     
  3. bytraper

    Thread Starter Active Member

    Sep 28, 2010
    126
    4
    Hmm, thanks sarge, you always make complicated things easier to understand.

    What about a HRF3205?, the specs are roughly the same but the RDS(on) is 0.008 Ohms, so I assume this would spend even less time in the linear region and require less heat sinking than I already have ? Am I understanding this correctly?

    The throttle is a standard Twist grap hall effect throttle (Motorcycle style)

    [​IMG]

    It's a 3 wire unit that works from 0.7v-4.8v.
    The input power to bills PWM is 12v exactly (regulated), and I want it to replace the 10k pot that's currently there!

    Cheers!
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    It seemed complicated when I was first learning it, too. I try to explain things in terms that makes stuff easier to understand.

    Most of what you need to consider about MOSFETs is:
    1) Polarity; are they N-channel or P-channel.
    2) Are they standard or logic level? Standard level MOSFETs require a Vgs of 10v (-10v for P-ch) to turn fully ON, logic level are generally specified for operation at Vgs of 4.5v.
    After you've established whether you need N-ch (preferred) or P-ch, and logic level or standard, then you go for:
    3) Vdss - the maximum voltage from drain to source terminal
    4) Rds(on) - the resistance from drain to source
    5) Id - the drain current rating.
    6) Qg - the total gate charge.

    With P-channel MOSFETs, you're going to wind up with a gate charge that's roughly 2.5 times higher than an equivalent N-channel MOSFET, which is why they have fallen out of favor.

    Vdss, Rds(on), Id, and Qg are all inter-related. As Vdss goes up, either Rds(on) or Qg goes up, or Id goes down.

    No, you'd have the same as with the IRF3205, the Rds(on) is the same. 0.008 Ohms = 8m Ohms (milli-Ohms)

    What's this for, anyway? An electric skateboard?
     
  5. bytraper

    Thread Starter Active Member

    Sep 28, 2010
    126
    4
    Thanks Sarge,

    Its for a electric quad bike. I put a bigger motor in, but i kept blowing the chinese controllers, and since I played a bit with speed controllers I thought I might try and just use a standard pwm controller I built with the hall effect throttle.

    But I am not sure how to hook it up to bills 555 pwm/freq circuit (where the 10k pot is). I think the 12v rail will be too high, so Im not sure what i need to add to make the hall effect throttle work.
     
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