[SOLVED] MOSFET saturation - voltage drop

[NevskyProspect]

Hello,

Before posting I have checked several posts but was unable to collect infromation solving my problem.
It seems to be simple, sadly it is not

Circuit:


IRL6372 Datasheet

Issue: IRL6372 (MOS type N) is having too big voltage drop, 200mV instead of 20 mV.
Conditions: Vgs = 11.0 V Vds = 5.0V Load = 10Ω

On the paper the voltage drop is about 20mV, in real conditions it is 200mV which leads to the lower current and higher power dissipated on my mosfet.
The Rdson with such Vgs should be about 15-20 mΩ, I believe it is saturated, why then so big voltage drop in real circuit?

Regards,
NevskyProspect

 

[godfreyl]

Conditions: Vgs = 11.0 V Vds = 5.0V

Nope. Source is at +4.8V. Thus:
Vgs = 11V - 4.8V = 6.2 V
Vds = 5V - 4.8V = 0.2V

 

[dick_freebird]

In saturation MOS has a "voltage drop" D-S that is greater
than Vgs. This is certainly not the case. Perhaps you are
confusing it with BJT saturation which is sort of the opposite.

What you believe about MOSFET on-resistance can easily
be put false by realities of packaging and PCB design.
20mOhms pad-pad on the die could easily become
a hundred or more if packaged using inferior bond wire,
leadframe, cold solder joints or too-thin PCB traces.

At high current and low voltage your measurement
methods also matter. You want a 4-wire (Kelvin)
resistance measurement for sure.

MOSFET on-resistance also increases with temperature
and there may be a self-reinforcing on resistance creep
if you measure it at power, especially with less than
proper cooling.

 

[NevskyProspect]

In saturation MOS has a "voltage drop" D-S that is greater
than Vgs. This is certainly not the case. Perhaps you are
confusing it with BJT saturation which is sort of the opposite.

What you believe about MOSFET on-resistance can easily
be put false by realities of packaging and PCB design.
20mOhms pad-pad on the die could easily become
a hundred or more if packaged using inferior bond wire,
leadframe, cold solder joints or too-thin PCB traces.

At high current and low voltage your measurement
methods also matter. You want a 4-wire (Kelvin)
resistance measurement for sure.

MOSFET on-resistance also increases with temperature
and there may be a self-reinforcing on resistance creep
if you measure it at power, especially with less than
proper cooling.

I was connecting the voltage for drain via chineese breadboard cable and measuring in the wrong place.
The mosfet is working correctly - the voltage drop is as it should be and equals Rds_on * I.

Be aware of the chineese breadboard cables - resistance about 0.5Ohm for male-male, and only 20mOhms for female-female (both bought from same supplier)
Anyway, thanks for your replies - especially with the 4 wire resistance measurements.