Hello everyone, I want to calculate heatsink for my mosfet. I made the calculations according to the circuit below. But I'm getting wrong thermal resistance value. Can you tell me where I made mistake?
It says that in order to find Rds on the Rds(on) graph, VGS=10V and Id=49A should be these values. How can I find Rds according to the circuit values that I designed?
Let's say I found Rds. I proceed by finding the power from the formula Pd=I^2xRds. (I=4,3A according to the circuit)
When I put any Rds value into the formula below, I get meaningless RθSA value.
It is a complete waste of time to do the calculation with 5V on the gate, with the MOSFET only partially turned on. The channel resistance cannot be predicted with any useful accuracy.
Wrong mosfet for 5volt-logic.
You should have bought the IRLZ44 (note the "L" for logic-level).
Then you might not need a heatsink for that load.
Leo..
It's been a LONG time since I've worked with this but it's possible, under certain conditions, that an infinite heatsink (holding the case to ambient temperature) isn't enough, and that could give you an "impossible" result.
In a real situation you would not run the junction at 175 °C. This is a max limit. To use this number it would be like making a swing for a 60 LB child using rope rated for 60 LB.
You would not calculate dissipated power by using current and Rds resistance. You would use current and voltage across the device. See Fig 2. Typical Output Characteristics in the device datasheet.
In a real design approach you would need to define:
The power dissipated in the MosFet (of course)
Your maximum ambient temperature.
You best guess at derating the max junction temperature. A typical number is in the 140 °C range.
With these you can find the heatsink you need to give you the required Rcs.
