Hello,

I am building a LED driver based on the MP24894 IC (https://www.monolithicpower.com/en/.../document_type/Datasheet/lang/en/sku/MP24894/)

I built the circuit using the following components (roughly according to the datasheet):
- Rsense 0.2 Ohm,
- Inductor: 68 μH
- Caps: 22 uF electrolytic and 2.2 uF ceramic for everything else
- Diode: B2100-13-F
- MOSFET IRF710PBF

I am currently driving a single blue LED with a Vin = 12 V supply, but will drive 2 in series in the final design.

The circuit works well and is stable. However, at maximum power (EN pin > 2.7 V, current = 1 A), the MOSFET is getting extremely hot quite quickly. The datasheet doesn't say much about the power dissipation of the MOSFET, so I guess it's one off these things that I'm supposed to know.. However I don't

So my questions are:
- Why is the MOSFET getting so hot? Isn't the circuit supposed to be efficiently running current through the LEDs, but not much through the MOSFET?
- Does this depend on some properties of the MOSFET itself, or am I doing something wrong in the choice of my other components?
- How can I calculation the power dissipation of the MOSFET?

Thank you for your time,

Bart

 

You're on the limit with that fet current as the leds are in series with the fet, I would use a IRF540 mosfet on an heatsink ideally.

 

You're on the limit with that fet current as the leds are in series with the fet, I would use a IRF540 mosfet on an heatsink ideally.

Okay, I will change the fet. However, won't the power dissipation still be equal and therefore produce as much heat? How do I know which thermal resistance for the heatsink I should get?

 

You need to measure the current with a dvm to see what's wrong,, it could be ringing on the inductor, a scope would help better.