Understanding Safe Operating Area

[Riccardo]

Hi,

I just want to check if I am understanding the SOA graphs correctly on a datasheet..

The STP30NF10 MOSFET is rated for 100V and 35A.
http://www.datasheetcatalog.org/datasheet/stmicroelectronics/7817.pdf

The SOA graph shows that the actual current it can pass varies with Vds. Now the graph goes upto 100V, but Vds (when on) should be really small, so is this referring to the Vds voltage when off?

Looking at the graph, and taking a 24V supply as an example, it looks like it could only switch something less than 5A. Is this correct? Is this still true even when on a large heat sink?

It also shows the current handling peaking at around 3.5V. I don't really understand why lower voltages lower the current ability. Its also seems odd that a transistor rated for 100V, 35A should only work at 35A when switching from a 3.5V supply.

Where am I going wrong?

[Hero999]

No,

The graph show how much current the MOSFET can pass when it is not quite fully switched on. If it was switched off then no current would be flowing.

The graph has nothing to do with switching and is about the saturation region when the MOSFET is neither fully on nor fully off and is dissipating the most power.

When the MOSFET is off the V_DS is high but the current is near zero.

When the MOSFET is fully on, the current is high but the V_DS is near zero.

There will be a fraction of a second when the MOSFET is in the process of turning off or on, when both the V_DS and current will be high but it doesn't matter as long as it's switching fast enough for the power dissipation to be negligible.

[Riccardo]

Oh good! I couldn't understand how the physics of what I thought I was reading could work! Thanks for clearing that up  ;D