VUE32_PWR_OUT
VUE32 Power Output
[-img src=Vue32 sch power.png: missing =-]
Schematic and maths
Q4 and Q6 form a push-pull driver, driven by the PIC through Q5. At start up the load isn't powered. Q3 was chosen for its low series resistance (RDSon), allowing minimal power dissipation. The four power outputs are built with the same components but have different current ratings due to the differences in PCB layout. OUT1 and OUT2 are sized for 10A continuous, assuming the minimal footprint:
TJ = TA + (RthJA)(RDSon * I²)
TJ = 35C + (62C/W)(4.1mOhm * (10A)²)
TJ = 35C + (62C/W)(410mW) = 60.42C
The formula doesn't take the dynamic behavior into account as this circuit isn't intended for PWM usage.
The main limiting factor is the PCB trace resistance. OUT3 and OUT4 are rated for 4A continuous.
To drive higher power loads, use the power outputs as relay drivers.
Experimental results
Two experiments were done on a v1.0 board:
[-img src=Vue32 pwrtst 1.png: missing =-]
[-img src=Vue32 pwrtst 2.png: missing =-]
Without the cover, the temperature never reached higher than 49C (24C over the ambient). With the cover, on OUT4 (the most heat sensitive output (smallest dissipation pad)) the maximum after an hour was 58C (36C over the ambient).
PWM shouldn't be used, except for a light load. See TI's AN-558 for more details: http://www.ti.com/lit/an/snva008/snva008.pdf....
