Created by
Created June 09, 2019
Last modified 5 days, 11 hours ago
Tags mosfet   power   switching  

Summary

An INCORRECT application of an IRF540 MOSFET to switch a 12V SMD5050 LED strip load.


Description

Example written for this stackexchange question, showing an incorrect application of a IRF540 MOSFET to switch a 12V load.

The problem here is that the gate is driven VG at the supply voltage which is also supplied directly to the drain VD. So the source voltage VS when the MOSFET is on is close to the gate voltage and VGS is nowhere near the full-on voltage needed to achieve its specified RDS(on).

This MOSFET has a test RDS(on) of 0.077Ω at VGS 10V but this circuit delivers a VGS of only about 4V.

Plot P(M_Load), V(M_G) - V(M_S) and I(M_Load.nS) in DC Solver to see what I mean. The solver reports a 3.6V VGS and a PM_Load of ~ 1.8W @ an IDS of 500mA. I measure a much lower DS voltage drop in my real circuit and I haven't tried to measure dissipation, but it's in vaguely the right territory.

Corrected version can be found here.


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