LM3478: Heating issue

Hi Saiprakash,

thank you for using the E2E forum.

In general some heating needs to be expected as there is always some losses in a SMPS.

Can you check your efficiency - this would give some more details.

How much is the heating?

Is your output voltage stable?

Also check if the input voltage is stable - so your supply can properly provide the required current.

C26 should be on the other side of R14.

Best regards,

 Stefan

Hi stefan,

thanks for your reply,

Input voltage - 15v constant and same with 10.5V

output voltage - 11.9 constant in both the inputs.

my board is heating up to 60 and my heat sink is heating up to 70 Celsius.

how can i check my efficiency? This is my first time in designing a power supply. 

Is the layout oky?

Regards,

Saiprakash

Hi Seiprakash,

efficiency = (I_OUT * V_OUT) / (I_IN * V_IN)

To check the layout it would help to also have an image with the reference designators to identify the location of the components.

There is a quite large hole in the ground area which might influence the current return path in a negative way.

Best regards,

 Stefan

Hi Stefan,

this following are the efficiency of different modes,

buck mode

Vin was 10.5 and Vout was 11.9 with 92%

Boost mode

Vian was 15.04 and Vout was 11.9 with 87%

is this efficacy fine for a SEPIC.

Please find the Layout images attached,

Regards,

Saiprakash.C.J

Hi Saiprakash,

this efficiency is in a expected range (as you can also see this in this reference design: TIDT346.PDF)

I think the layout looks OK.

Best regards,

 Stefan

Hi Stefan,

Good day,

just wanted to know something, I am putting 2 amps of load now. and the board is heating up to 80 and the junction is reaching up to 103 degree. Is this expected.

Regards,

Saipraksh.C.J

Hello Saiprakash,

With an output power of 24W and an efficiency of 87%, the system (mainly the FET) will burn more than 3W.
In that case I would expect a temperature rise of about 80 degrees or even more.

Best regards,
Harry

Hi Harry,

good day,

thanks for your reply,

So the heat is expected and i have to figure out a way to dissipate the heat right?

Regards,

Saiprakash.C.J

And is there a possibility to reduce this heat by choosing an even more better mosfet if yes can you suggest me a better one?

Hello Saiprakash,

There are mainly two types of losses in the FET: Switching losses and Conducting losses.

To reduce the switching losses, you can reduce the switching frequency, if possible. 
This may result in a higher ripple of the output voltage.
So you will need to find a good compromise there.

To reduce the conducting losses, please select a FET with a very low RDSon.
Please understand that I cannot give a recommendation of a particular device or supplier.

 Best regards,
Harry

Hi

I also have another concern where the Inductor is heating up a lot than the mosfet. inductor is the one who is heating up  the board than the mosfet.inductor part(DRQ-127-6R8-R)

Regards,

Saiprakash

Hi Saiprakash,

Thanks for the update.
If your inductor is heating up a lot, this may be related to a very large current ripple, resulting in larger inductor losses.
You can calculate your current ripple using our power stage designer tool. Please choose the according topology, enter all parameters and double click on the inductor within the schematic to see the current waveforms. The tool also recommends you a suitable inductance for you application. If your chosen inductance is much lower than the recommendation, please consider replacing this part.
https://www.ti.com/tool/POWERSTAGE-DESIGNER

Best regards,
Niklas