I make a led driver to light 3 HPL led using led driver chip sq9910 (datasheet). Each Led is 1watt 3.3v and driving current is 250mA. however the mosfet, diode D2 and inductor L1 gets hot. Inductor is drum 0912 type 3.3mH (not knowing amp rating). I think with 250mA current it will get only warm to touch but after only 2 minutes the mosfet and diode get really hot to touch. I'm new to electronics, please help what is wrong.

 

Check that inductor. The datasheet says that the largest one in that range is 1mH and that saturates at 350mA.
Even if there were a 3.3mH on the same bobbin, it would saturate at about 100mA.

 

So the problem is the inductor gets saturated and the current flow through the mosfet and diode increased causing it to heat. Thanks for pointing it out. then i have to find a bigger core inductor. but i'm afraid there is no bigger drum inductor. i will try toroid ring inductor instead

 

Do you have an oscilloscope?
If so, look at the voltage across R1.
That will show you the current and if the inductor is saturating.

 

Careful, though! Use an isolating transformer.

 

Careful, though! Use an isolating transformer.

Yes, certainly if it's being line powered, which it does appear to be.

 

Do you have an oscilloscope?
If so, look at the voltage across R1.
That will show you the current and if the inductor is saturating.

unfortunately i dont have oscilloscope. but when r1 is 1ohm, using DMM, it measure below 100mA led current. change r1 to 0.5 ohm, led current is 250mA. it is very strange indeed, do not match with datasheet calculation.

 

Careful, though! Use an isolating transformer.

thanks for your concern. i will try to be safe as possible. when the project is finished, it will be in a full covered plastic box