Connect with us

mosfet, not fully on. losses?

Discussion in 'General Electronics Discussion' started by newuser, Mar 26, 2016.

Scroll to continue with content
  1. newuser


    Dec 16, 2012

    ive read that this is a formula to calculate turn on losses for a mosfet:

    2* Vcc * I_start * t_rise * pwm frequency

    (2* because the mosfet goes on and off...yes trise and tfall are usually different. this is just a estimate)
    this of course does not take into account that voltage and current change over time, but its a worst estimate.
    losses could be as far as 1/3 of the above.
    can somebody confirm this formula?

    and now to the main question. what happens when a mosfet is not fully turned on because of to low gate voltage
    for example this one:

    as far as i understand the mosfet can conduct ~10a with 4v at the gate..but what happens to Rds in this it hugh?
  2. BobK


    Jan 5, 2010
    Don't know where you get 10A at 4V. The characteristic curves start at 4.5V, and it conducts over 20A at that voltage.

    To find the power loss anywhere on the characteristic curve, find the point on the on curve for Vgs and then multiply the Id by Vds. For example, on the datasheet you linked.

    At Vgs = 4.5V, Id = 20A and Vds = 0.6V so the power dissipated is 12W

    In any case, you should avoid using Vgs less that what is required to turn on fully for switching applications. In this case, 10V.

Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day