[Solved] IRF3205 has positive temp coefficient, but conducts better as it warms

[ballsystemlord]

Hello,
Baring the possibility I got fake parts, which is possible, but they're labeled correctly, I'm at a loss to describe what's going on here.
I am still rather new to electronics, and I like to test things to make certain that I understand what's going on.
I created a circuit to play with these IRF3205 MOSFETs. (Yes, I know DC isn't the "normal" way to operate these things. It's easy to see what's going on though.)

I've noticed that as the MOSFET warms up, the current in the circuit increases. This tells me that RDS(on) is decreasing. But the datasheet https://www.infineon.com/dgdl/irf3205pbf.pdf?fileId=5546d462533600a4015355def244190a shows an increase in RDS(on) as the MOSFET warms up.

So, any idea as to what's going on?

Thanks!

EDIT: Vds is 12v.

[T3sl4co1l]

What is Vgs in the circuit?

[ballsystemlord]

Umm, I didn't measure that...
I can't be more than Vds, which is 12v.
Vgs max is 20v.

[Vovk_Z]

So, any idea as to what's going on?

Ids rises with a temperature with the same Vgs, if Mosfet is open. Look at Fig.1 and Fig.2 of your datasheet.

[ballsystemlord]

So, any idea as to what's going on?

Ids rises with a temperature with the same Vgs, if Mosfet is open. Look at Fig.1 and Fig.2 of your datasheet.

Ah, good point.
But that still doesn't explain why it would rise if the temperature coefficient is positive.

[magic]

This is not how you test RDS(on). The proper way is to turn the FET hard on (source to -, gate to +) and limit conducted current by means of a suitable power resistor between + and drain.

Your circuit is similar to a source follower, always maintaining Vds > Vgs and Vgs is quite far from zero. What you actually observe is probably the dependence of Vgs on temperature - the hotter the FET, the less Vgs is necessary for any given Id.

[langwadt]

So, any idea as to what's going on?

Ids rises with a temperature with the same Vgs, if Mosfet is open. Look at Fig.1 and Fig.2 of your datasheet.

Ah, good point.
But that still doesn't explain why it would rise if the temperature coefficient is positive.

the temperature coefficient of Rdson only comes into play when the fet is fully on

[Circlotron]

On resistance has pos temp coefficient.
Gate threshold voltage has neg temp coefficient.
So if the mosfet is only partially on it will turn on more as it gets hotter.
If it is fully turned on with say 10-15v on the gate with respect to source then it will conduct a little less as it gets hot.

This is why paralleled mosfet don’t share current if operated in linear mode like in an audio amplifier. The hotter one causes its gate to be more sensitive and so it conducts more and gets hotter still. They will share somewhat only if they are fully turned on like in some kind of switching circuit.