mosfet, not fully on. losses?

[newuser]

hi,


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:
http://www.irf.com/product-info/datasheets/data/irf3205.pdf

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

 

[BobK]

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.

Bob