LM3478: Mosfet getting heated

HI Saiprakash,

thank you for using the E2E forum.

Can you try to attach the schematic with more resolution.

What is the saturation current of your inductor?

Can you also add some scope plots of:

- Vout

- Comp

- Switch node

- Gate signal

Best regards,

 Stefan

Hi Stefan,

Good day

thanks for your reply,

Please find the schematics attached,

6724.LM3478.pdf

Vout:

comp:

at DR pin:

At the gate of mosfet:

This is the inductor that is in the schematic at L4 and L5 - ECS-MPIL0603-6R8MC

And at present L4 is replaced with the following part - 10uH and 7A

Regards,

Saiprakash

Hi Saiprakash,

i just checked the MOSFET you are using and this is just rated for 30V. This to close to the peak voltage of 29.5 V which will be seen at the MOSFET  during operation.

So the MOSFET might be already damage or have lost performance.

Please replace this first with one with higher VDS rating, e.g. 40V.

Best regards,

 Stefan

Hi stefan,

Thanks for your reply,

will try with other mosfet , do you recommend any part,

is the schematic fine other than the mosfet?

Regards,

Saiprakash   

Hi Saiprakash,

please check the connection of the current sense resistior. This looks not to be OK.

Best regards, 

 Stefan

Hi stefan,

Good day,

R52 is mounted with 9mOhms resistor.

And do you think CSD18543Q3a will be a prefect fit mosfet for design.

Regards,

Saiprakash.

Hi Saiprakash,

R52 is mounted between the MOSFET and a cap to GND but should be between MSOFET and GND

Please check this in your schematic and then proper connect ISNS input of the LM3478 to the right node.

Best regards,

 Stefan

Hi stefan,

Got it wit take care of it, Thank you very much.

we have order the component CSD18543Q3a will let you know when we test it with the new mosfet and will update you with the resut.

Regards,

Saiprakash.C.J

Hi Stefan,

Good day,

How are you.

I did test the circuit with the new MOSFET CSD18543Q3a , Still it is heating up the only difference is this mosfet is lasting for 10-15 mins where as the old one will go off in 4-5 mins.

so what do you think could be the reason.

Regards,

Saiprakash.C.J

Hi Sai,

can you please share your updated schematic.

Best regards,

 Stefan

Hi stefan,

The schematic is same I didn't change anything except replacing the  MOSFET (SIS412DN) with (CSD18543Q3a) and shorted the C50 capacitor. Other than this everything remains the same.

Regards,

Saiprakash.C.J

Hi Sai,

this means your schematic is still not OK - ISEN is shorted to GND.

Please check your schematic against the reference in the datasheet:

Best regards,

 Stefan

Hi Stefan,

I did realize it and this time i made the whole circuit on the on a general purpose PCB, This time my heating issue was resolved only after changing the diode(D11), from my calculation's for power diode I got 9.8 Amp's. So i changed it with a 16 Amp diode and my heating issue was resolved. But my voltage is dropping after 1Amp of load. My Vout should be 12V but it drops to 10 at 1Amp of load. the circuit is designed for 3.5Amps of load.The schematic is from webench, I have attached the schematic for your reference. Any specific reason for the voltage drop.

Regards,

Saiprakash.C.J

Hi Sai,

do I understand right - you have done the circuit an standard through hole PCB.

This might give very bad performance you you might pick up a lot of noise, esp. in the current sensor.

So for a SMPS you always need proper layout.

Best regards,

 Stefan

Hi Stefan,

What you have understood is right, will work with the layout and let you know.

Regards,

Saiprakash.C.J

Hi Sai,

thanks, for the update.

~ Stefan

Hi Stefan,

Good day,

just a small doubt,

Do you feel that this inductor DRQ127-6R8-R

And i will be using this diode MBRB1645-E3/81 instead of MBRS540LT3G.

MOSFET - IRFR3806TRPBF

Do you think the following parts will be an will suitable for the design?

Regards, 

Saiprakash.C.J

Hi Sai,

The FET and Diode seems to be ok as far as the rating is concerned, as for the inductor why are you using dual coiled inductor?

BR,

Haroon

Hi wardak,

I am just trying to use the part that is suggested in webench.

regards,

Sai