WEBENCH® Tools/LM25085: mosfet power loss calculation

HARINI KRISHNA

Expert 1905 points

Part Number: LM25085
Other Parts Discussed in Thread: BQ24715

Tool/software: WEBENCH® Design Tools

I used webench tool to design a buck converter with the following specifications

Input:

DC 10.1 V - 12.6 V

Output:

9.8 V at 8 A

I calculated the mosfet power loss manually by using eq8 from the datasheet of bq24715

While comparing the design report power loss to that of calculated one, the values are no where matching.

Can you please help in me in calculating power loss of the mosfet?

i want to use a different mosfet in order to bring down the cost (DMP26M7UFG-7 from diode incorporated).

Could you also tell me the change in efficiency if i go for the mosfet from diode incorporated?

I have attached the design report

Thankyou

Regards

Harini Krishna

WBDesign6.pdf

over 5 years ago

0 Iven Su over 5 years ago

TI__Expert 8245 points

Hi,

You can use its quick start tool to calculate the loss:

www.ti.com/.../lm25085-5085quick-calc

0 HARINI KRISHNA over 5 years ago in reply to Iven Su

Expert 1905 points

Hi Iven,

Thankyou for the reply

How do I calculate the power dissipation of the mosfet.

I tried calculating it manually using eq8 from the datasheet of bq24715 and the calculated value does not match for CSD25402Q3A to that of design report.

I have few mosfets as options and i need to know the power dissipation for different mosfets and choose the best out of it.

Could you help me in this

Thankyou

Regards

Harini Krishna

0 Iven Su over 5 years ago in reply to HARINI KRISHNA

TI__Expert 8245 points

Hi,

Please refer to this application note to calculate the power dissipation of the MOSFET:

http://www.ti.com/lit/an/slpa009a/slpa009a.pdf

0 HARINI KRISHNA over 5 years ago in reply to Iven Su

Expert 1905 points

Hi Iven,

Thankyou for the reference.

The sense resistor used in the converter has a power dissipation of 0.64W (8^2*0.01) but the design report has a sense resistor of 500mW Pd ( power dissipation)

Can you please tell me if I can use 500mW Pd sense resistor?

The web design PDF is attached to the first post.

Thankyou

Regards

Harini Krishna

0 Iven Su over 5 years ago in reply to HARINI KRISHNA

TI__Expert 8245 points

Hi,

Webench result is based on more ideal situation and just for reference. If you calculate a high power dissipation on actual system, you should use one with higher Pd.

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WEBENCH® Tools/LM25085: mosfet power loss calculation