Hey everyone! I'm diving into the world of electronics, specifically looking into how to calculate the power consumption of a transistor. I've been watching videos that cover concepts like junction temperature and heatsink requirements based on thermal resistance. I get the analogy of thermal Ohms law, but what I'm stuck on is figuring out how to calculate the power loss across the drain-source. For example, the Rds of the IRF1405 is about 4.6-5.3 milliOhm at a gate voltage of 10V and a drain current of 101 amps. How can I use this info to calculate watts at different values? Say I want to run 30 amps with a 5V gate for a 360W load at 12V. What's the method to calculate the power consumption in this scenario? Thanks for your help!
3 Answers
To find power dissipation, just multiply the voltage across the drain-source by the current flowing through it: P = Vds * I. For MOSFETs, apply the formula P = I² * Rds with the Rds value. In your case, with Rds at around 5 mΩ and I at 30A, you'll end up finding that Pds is about 4.5W. If you're pushing it near max current, keep an eye on your heatsink because that’s going to be crucial!
For your setup with 360W at 12V (which gives you a nice 30A load), I noticed your gate voltage of 5V may not be ideal for the IRF1405. From the datasheet, it looks like that would start saturating at around 20-30A at typical conditions. Try pushing it to a 10V gate drive, and then you should see it comfortably handle the 30A load. Just keep in mind that at higher temperatures, Rds can increase, which will raise your power dissipation. Proper cooling is key to avoiding thermal issues!
When you're driving your MOSFET, keep tabs on your junction temperature. If you're targeting around 140°C, this changes how you calculate Rds. The specs could double under load, so expect about 9W dissipation at those levels, which means checking your heatsink's thermal resistance is a must. Having an Rja of 10°C/W or less can help manage that heat effectively.
Definitely! Also remember, if you’re using AC, make sure to factor in RMS values for your voltage and current as well.