I am using the LM3409 to power a single LED with a 12V input source. I had purchased the EVAL board and repopulated it with the design resistors and components, except the PFet and diode, in order to simulate my design specs of a 3.1Vout @ 1.5A. The Eval board ran well and the components stayed at a temperature i would expect at 7-12°C rise base on switching freq, current and Rds_on of the EVAL PFET. Final measured value was about 31°C
When I received my prototype boards, the circuit operated as intended with regards to output current and dimming function using PWM. When I compare the waveforms between the EVAL board and my PWA, they are basically identical. The only discrepancies are in the switching noise. The EVAL board has higher switching peaks at turn on/off.
The main issue i am having is the heat being generated from PFET and the recirculating diode. They are getting extremely hot compared to the EVAL board. I will be ordering the same exact PFET and recirculating diode used on the eval board to see if that is the issue. But i would like to ensure that is the problem or maybe there is something else, because in the end i cannot use the PFET on the EVAL board as i need one rated for 100V. The PFET is reaching temperatures of 110°C during operation. When i used the formulas in the datasheet I was calculating a 20-25°C rise. which seems about in line with what it should be based on the variance between the EVAL board PFET and my PFET Rds_on. the EVAL board has an Rds_on of 200mOhms and mine is 500mOhms so i figure 2.5 times the rise of EVAL board. I should be seeing roughly 60-70°C on the PFET for this design calculated. And since i used my calculated values on the Eval board, I would have to say my calculations were correct.
Is there a reason why i am burning up my PFET?
