On 27/04/06, Picdude <picdude@narwani.net> wrote:
> Looked it up, but it only has 170mA continuous drain current compared to the BSH111's 335mA.
>
> But that brings up a question ... for a pulsed application (max 33% duty-cycle at ~120hz multiplexing in this case), how does one calculate the proper current/power rating required of the FET?  For the BSH123 for example, Rds(On) = 10ohms max at Vgs=4.5V.  With 250mA (max drawn by LEDs), that means .625W dissipated.  This violates the BSH123's max power rating of .36W.  But at 33% duty-cycle, average current dissipated = .208W.  So would this be acceptable?  Also, Id (pulsed) = 0.68A on the datasheet.
>
> The confusing part here (for me) is that they don't specify the freq & duty-cycle that defines "pulsed".
>
> Cheers,
> -Neil.
>

I think you have to work out the max die temperature from the
transient thermal impedance plot for the max temperature,  timing &
load you're using.
Then, as long as it's less than 150C, you should be OK. (preferably
significantly less).

For your frequency & pulse rate the transient thermal impedance looks
to be about 40K / Watt.
At 250mA & 10 ohms the max power is 625mW which correspnds to a
temperature rise of 0.625 * 40 = 25K (or C)
Therefore at an operating ambient temperature of 50C the die
temperature will be 50+25 = 75C which is much less than the 150C
maximum. You should be OK unless I've stuffed up the calculation
somewhere.

Note also the plot is for a device mounted on a metal clad substate -
For a pcb the continuous typical value is 350k/W cp 150 for the metal
clad.
There should be enough safety marging to cover this however.

RP

-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist