The typical linear driver has an MCU and some 7135’s.
Which produce the most heat? MCU or 7135 regulator?
Does the 7135 produce more heat when limiting the current or when passing the maximum rated current?
Is the PCB material the components are mounted to very efficient at transfering the heat to the metal casing? Is any appreciable amount of heat transfered through air cooling?
Thanks to all the more technically minded.
My (limited) understanding… The MCU produces little, if any, heat. The 7135 produces heat because it’s “burning off” the excess voltage that’s not being dropped by the LED (s).
Example: you have a 2.8A (8x 7135 at 350mA each) driver and a LED with a vF of 3.2V at 2.8A. With a full cell (4.2V), there’s 1.0V excess that has to go somewhere. That gets absorbed by the 7135’s. At 1.0V * 2.8A, that’s 2.8 watts being essentially turned into heat by the 7135’s.
Granted there’s voltage sag at the battery and voltage drop across the springs and wires, but that’s the general idea.
Oh and as far as heat transfer goes, I don’t think PCB’s transfers heat very well. However, the wide center pad of 7135’s is usually connected directly to the grounding ring of the driver. So as long as the grounding ring has a reasonable heat transfer path (eg, soldered to a pill or firmly pressed against the body) then the heat from the 7135 has a decent escape path to the body.
7135 heat in watts = (Vin minus Vf)×current (Amps). Very simple.
Mosfet heat in watts = fixed voltage drop (0.1…0.2V) × current.
Both of them can transfer heat to pcb, you can use aluminium core pcb for driver, works very well.
7135 chips can limit current if they overheat. But usually driver components critical temperature is competitive big and simple filling driver cavity with thermal glue fixes this problems even with simple fr4 pcb.