Losing top brightness when using tip120 and PWM with 70w LED panel

Question

I have been experimenting with pwm dimming using TIP120 on various high powered LED's, and have noticed a marked drop in the top level brightness compared to original connection without the tip 120 in line. For instance a 70w panel connected with heavier gauge cable (not using jumper cables) no longer reaches full brightness with pwm set too full. im using tasmota on a nodemc8266. Is there something inherent the hardware set up causing the loss that I am missing or is there a software limitation with PWM? curious

thanks

• Juraj: don't know if it is the cause, but PWM in arduino esp8266 core is software PWM: pin is toggled in timer interrupt
• Gerben: Remove the PWM and just set the pin to HIGH. Then see if there is still a difference. My guess is that there is. I think the TIP120 will have a relatively high voltage drop across it. Making the leds less bright. If that's the case I'd switch to a MOSFET instead of a bipolar (darlington) transistor
• VE7JRO: One of the main disadvantage of a Darlington transistor pair is the minimum voltage drop between the base and emitter when fully saturated. Unlike a single transistor which has a saturated voltage drop of between 0.3v and 0.7v when fully-ON, a Darlington device has twice the base-emitter voltage drop (1.2 V instead of 0.6 V) as the base-emitter voltage drop is the sum of the base-emitter diode drops of the two individual transistors which can be between 0.6v to 1.5v depending on the current through the transistor.
• OTTO: thank you, all of these comments make such sense and i will experiment with each so I methodically understand the process, and then finally follow up with the specific mosfets as described by @crossroads.

Answer 1

Replace the TIP120 with a Low Rds N-channel MOSFET. Look at AOD508 or AOD514 for example, very low Rds, so it will develop very little voltage drop as the current increases.