20Khz+ PWM high-wattage LED producing too much heat

[Richard Mansell]

I'm working on dimming a high-wattage LED (50W--12V/4.17amps) with a 3.3V micro-controller. The solution that has worked so far is by using a low-side (PNP) MOSFET to drive the LED and a NPN transistor to drive the MOSFET via the 3.3V signal from the MCU. At 488hz PWM I can dim the LED light bar just fine and the temperature produced by the MOSFET is minimal. Below is a picture of the circuit. The transistor is a PBSS4032NT and the MOSFET is a PSMN1R0-30YLDX. (Links are to the Mouser.com catalog.)

That works well until I up the PWM frequency. At the lower frequencies a buzzing sound can be heard so I want to drive the MOSFET at a higher (20Khz+) frequency to get rid of the sound. That is where I'm running into much higher temperatures. I suspect that the temperature increase is due to the amount of time the MOSFET is in transition (turning on or turning off) and figured that the best way to bring down the temperature is to open and close the MOSFET faster. I've tried several arrangements including the following:

The Gate driver is a MIC4420 with the same MOSFET. I've also tried the circuits suggested on Tahmid's Blog here. Specifically Figures 4, 5 & 7. None of these have brought the temperature down to where I would like it to be (20C or less--above ambient temperature).

If there is a more efficient way to do this, I would like to hear some suggestions as my goal is to do it without a heatsink if possible. I will resort to a heatsink if absolutely needed. Thanks for the help!

 

[audioguru]

Your first circuit feeds the gate with 3.3V - 0.7V= 2.6V which is much too low because the datasheet for your tiny Mosfet says 4.5V (the lower voltage graphs are for a "typical" one but yours might be a 4.5V maximum one).
The second circuit with the gate driver powered from 12V should work fine if you added a 10 ohm resistor in series with the gate to prevent the Mosfet from oscillating.

Or maybe your tiny Mosfet is too small to cool off.

 

[zahwi]

I'm working on dimming a high-wattage LED (50W--12V/4.17amps) with a 3.3V micro-controller. The solution that has worked so far is by using a low-side (PNP) MOSFET to drive the LED and a NPN transistor to drive the MOSFET via the 3.3V signal from the MCU. At 488hz PWM I can dim the LED light bar just fine and the temperature produced by the MOSFET is minimal. Below is a picture of the circuit. The transistor is a PBSS4032NT and the MOSFET is a PSMN1R0-30YLDX. (Links are to the Mouser.com catalog.)

View attachment 96766

That works well until I up the PWM frequency. At the lower frequencies a buzzing sound can be heard so I want to drive the MOSFET at a higher (20Khz+) frequency to get rid of the sound. That is where I'm running into much higher temperatures. I suspect that the temperature increase is due to the amount of time the MOSFET is in transition (turning on or turning off) and figured that the best way to bring down the temperature is to open and close the MOSFET faster. I've tried several arrangements including the following:

View attachment 96767
The Gate driver is a MIC4420 with the same MOSFET. I've also tried the circuits suggested on Tahmid's Blog here. Specifically Figures 4, 5 & 7. None of these have brought the temperature down to where I would like it to be (20C or less--above ambient temperature).

If there is a more efficient way to do this, I would like to hear some suggestions as my goal is to do it without a heatsink if possible. I will resort to a heatsink if absolutely needed. Thanks for the help!

You are doing it ok. At 20KHz the switching loses are 40 times bigger than at 488Hz. If you only need a dimmer switch at 100Hz.

 

[Richard Mansell]

Thank you for the quick feedback. It is encouraging to know that overall I've got the circuits right. I guess I will start working on some parallel MOSFETs and heatsinks.