# Power dissipated across transistors

#### nathan.derringer

Joined Apr 17, 2019
2
Hi there,

I am trying to find out how to work out the power dissipated across transistors.
In particular:
• A Darlington transistor with a saturation voltage of 1.8V at 5A collector current
• an n-channel MOSFET with RDS (on) = 0.4 Ohms
These 2 devices are considered for switching a load that required 5A at 48V.

I need to work out the power dissipated by each device when it is switched on.

Any help would be greatly appreciated!

#### AlbertHall

Joined Jun 4, 2014
12,347
Power is voltage multiplied by current.
Don't forget that for transistors you should also consider the power dissipated by the base current.

#### nathan.derringer

Joined Apr 17, 2019
2
Thanks for that Albert but I'm still not sure from the info I have how to calculate the power dissipated by the base current?

#### kubeek

Joined Sep 20, 2005
5,795
That darlington will dissipate 1.8V*5A=9W. Base dissipation here is hard to tell, but you could guesstimate 10% of collector current times Vbe, so 0.5A*0.7V=0,35W more.
Mosfet will have voltage drop of 0.4*5A=2V, so the dissipation will be 10W.

#### BobaMosfet

Joined Jul 1, 2009
2,113
Hi there,

I am trying to find out how to work out the power dissipated across transistors.
In particular:
• A Darlington transistor with a saturation voltage of 1.8V at 5A collector current
• an n-channel MOSFET with RDS (on) = 0.4 Ohms
These 2 devices are considered for switching a load that required 5A at 48V.

I need to work out the power dissipated by each device when it is switched on.

Any help would be greatly appreciated!
Whenever you are discussing power dissipation in a transistor, what you're really discussing is _junction_ dissipation. Your TJW value on your datasheet will tell you what your dissipation is. There are equations out there for doing the rest of the calculations- including a great book called The Art of Electronics by Horwitz & Hill (I think I spelled that right?) - Which can show you how to calculate the dissipation curve of a transistor (or any component, really) and find the operating sweet spot.

#### ScottWang

Joined Aug 23, 2012
7,409
You don't have too much choice for the Darlington transistoring, and the base current is not that much comparing with the collector current, sometimes we will ignore it, you can choose a lower Rds for the MOSFET to decrease the power dissipation, when Rds is lower then the Vds will be lower, so the Wds will be lower.

You could refer to -- N, P type MOSFET Components Simple Data.
You can choose a VDSS = Vcc * 1.5 or higher.
Id = Ids * 5 or higher. (At least Id=Ids*3, Ids is your real current flows through the Rds and load)
Rds = 2.0 ~ 20 mΩ