LED Driver basewd on MP24894 thermal dissipation

Thread Starter

Blimb

Joined Oct 19, 2017
12
Hello,

I am building a LED driver based on the MP24894 IC (https://www.monolithicpower.com/en/documentview/productdocument/index/version/2/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
 

Dodgydave

Joined Jun 22, 2012
8,469
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.
 

Thread Starter

Blimb

Joined Oct 19, 2017
12
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?
 

Dodgydave

Joined Jun 22, 2012
8,469
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.
 
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