Mosfet Rdson

Thread Starter

joemmech

Joined Jan 22, 2010
32
Hi to all. I have a question about my Rdson testing. I have been reading in the internet but until now I haven't found the answer. We have a device tested 4 times for Rdson. Below are the test configurations:

1st test: Vgs = 10 and Id = 30
2nd test: Vgs = 10 and Id = 15
3rd test: Vgs = 4.5 and Id = 30
4th test: Vgs = 4.5 and Id = 15

The difference of the Rdson reading for the 1st and 2nd test is less than 1mohm but for the 3rd and 4th test they have greater than 1mohm difference. I also noticed that the measured Vds for the 1st and 2nd test has a ratio of less than 1, but for the 3rd and 4th test it has a ratio of greater than 2. Can somebody explain to me why the device is having this kind of response? What I am expecting is that the 3rd and 4th Rdson test will also have a less than 1mohm and a Vds ratio of less than 1, just the same with the 1st and 2nd test. Please advise.

Thanks and Best Regards
 

thatoneguy

Joined Feb 19, 2009
6,359
Unless you are using Logic Level MOSFETs, 4.5V isn't enough to run the MOSFET as a switch. Vgs needs to be rather high, near Vgs (10V+).

For this, there are several solutions for "High Side Drivers" to get Vgs to the required level from the output of a microcontroller. A search for High Side Driver on this forum will show many results, some discrete component, and some using HSD IC's.
 

eblc1388

Joined Nov 28, 2008
1,542
The FDS6680A is a logic level MOSFET, usable at gate voltage below 5V.

However, it is still better to drive it with 10V gate voltage if one wants to get the lowest Rds ON.
 
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