The temperature of the sink generally has only a small or no effect on efficiency, although it may be an indicator of poor efficiency.Another thing of interest might be the maximum temperature of the heatsink. If an accessible heatsink can burn some unsuspecting user if touched, that's a problem (maybe a legal one).. so sometimes there are other things of interest.. not to mention the terrible electrical efficiency one probably has with a "hot" heat sink.
20W out of 3500W total ain't bad.The temperature of the sink generally has only a small or no effect on efficiency, although it may be an indicator of poor efficiency.
I'm using worst case everything in my calculations. Maximum MOSFET resistance at maximum temperature, 50C ambient, 50% slower switching time, zero airflow.Switching regulator efficiency may be slightly effected since inductor and MOSFET resistance will go up some with temperature.
The temperature of the sink generally has only a small or no effect on efficiency, although it may be an indicator of poor efficiency.
For example, if it's sinking the power from a linear regulator, temperature has no effect on the regulator efficiency. That's determined simply by the ratio of the output to input voltages.
Switching regulator efficiency may be slightly effected since inductor and MOSFET resistance will go up some with temperature.
The main reason to keep the heatsink temperature down, besides the possibility of burning someone, is to keep the device temperature below it's rated limit and/or to minimize the failure rate.
You are talking about the measured heatsink temperature vs the junction temperature right? So far this is all just calculations and I've derated most things by 50% because I just am not sure how it will all end up. The onyl way for me to really know is to fire it up and measure the increase in temperature of the heatsink.
How does a heatsink temperature of 100C work out to be 270C inside the chip (what chip withstands that in the first place?) unless the power dissipation was really realy high to begin with...like 1W/C for 170W or 1.7W/C for 100W, or 17C/W for 10W, etc.
I would stick big fans and copper, but this is a helicopter and I am limited to about 200 grams of heatsink right now. I'm not sure about the airflow exactly since it's not a plane.
You should measure lots of stuff. heatsink temp is probably the least useful to know. Case temp of the parts and the delta across theinterface are probably the most revealing pieces of information.
So should I be measuring the temperature of the plastic part of the case? Or of the contact area that is supposed to touch the heatsink (which I guess would be close to heatsink temperature if the interface was good)? I could make a test plate and drill small hole through the center of a heatsink and the plate to get a thermocouple to contact the underside of the module.
Your rig sounds like a lot of weight in the electronics + motor + batt. How big is this thing going to be? I am sure the rotor span will have to be quite large, something like 90 cm diameter maybe?
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