# FET device parameters explaination

I'm looking for a FET device to use as ON/OFF switch.

I've found for example this device: http://www.vishay.com/docs/62903/sis990dn.pdf

I need to find a device that can handle on Source-Drain up to 5V-5A and a gate where is enought 3.3v or 5v to enable it.

But I looked at this datasheet and I don't know which values I need to looking for. Can you indicate me which parameters I should look at?

It can be a minefield. Take a look at this graph: -

This graph tells me it is the wrong device for your application. With an applied gate-source voltage of 5V and 5A being taken from drain to source, the volt drop from drain to source is 2V i.e. a power dissipation of 10 watts.

At higher gate drive voltages (say 6V) the characteristic improves; for 5A, the volt drop is probably about 0.5V i.e. a dissipation of 2.5 watts (getting better).

You don't have a 6V (or higher) gate drive so it's no-good for your use. As a starter tip, search for devices that are maybe rated no-more than that 30V Vds. The Si990 is 100 v rated and immediately I saw that I thought it might be be tailored for your application.

All FETs have this graph in their data sheet.

• Can you check this? irf.com/product-info/datasheets/data/irf7530pbf.pdf Also, I see in this datasheet gate-source/current go up to 4v in graph, if I apply 5v does change something? Max. rating is +12v – Yaro Jun 2 '15 at 9:03
• Rds(on) is 0.03 ohms (front page) and this looks good. Rds is the steep slope of the line in the graph before it turns flat. I don't understand "gate-source/current go up to 4v in graph"? Which graph, which current? – Andy aka Jun 2 '15 at 9:12

You're looking for the "Output Characteristics" graph. With a VGS of 5V the drain current barely reaches 5A, and at 4V it's practically 0; find a component with a much lower VGS requirement, using the gate threshold parameter (VGS(th)) as a guide.

You need to look @ 3 parameters. Drain Source Voltage i.e. Vds / Gate Source Voltage Vgs & continous drain current Id.