On 15/04/2011 16:21, Michael Watterson wrote:
> On 15/04/2011 15:23, Sean Breheny wrote:
>> I've never been able to find a general reference which states that
>> this is always the case for MOSFETs, but have a look at this
>> datasheet:
>>
>> http://www.irf.com/product-info/datasheets/data/irfs4310.pdf
>>
>> Specifically, see fig 1 versus fig 2. The lowest curve is for Vgs=3D4.5
>> Volts. At Vds=3D1 volt, the 25 deg C curve is about 1.5 Amps. the 175
>> deg C curve is about 22 Amps.
>>
>> Compare with figure 4 which shows that Rds ON also increases with temper=
ature.
>>
> Yes, RDS on *INCREASES* with temperature, thus they can be safely
> paralleled without BJT thermal runaway and without source "ballast"
> resistors.
>
>

If I understand Sean correctly, look at the curve Vgs =3D 4.5V pointed out=
=20
above - at 25degC it is definitely lower at ~1.5A than at 175degC and=20
~22A, this would mean Rds decreases quite a bit. This is the case for=20
all Vgs up to ~8V in the linear region - you notice the curves are a lot=20
tighter in figure 2 with the Id scale starting at 10A.
In comparison, look at the corresponding curve for figure 4, which=20
states Vgs =3D 10V and Id =3D 75A. If we pick Vds of 1V for simplicity, for=
=20
175degC you would get Id ~60A, but at 25degC Id >100A, which means in=20
this case Rds increases.


--=20
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist
.