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UCC21750-Q1: Power dissipation inconsistency (maybe?)

Part Number: UCC21750-Q1

Hello,

From the datasheet table 6.5 the power dissipation is limited to 985mW. Whereas from table 6.4, the R_th,j-a is 68.3K/W which at a power dissipation of 985mW yields a rise of less than 70 Celsius. I can understand that for a general specification where the ambient could be high, the power dissipation would be limited to 985mW. However, if I could ensure a low ambient (laboratory use only) and/or a lower thermal resistance (airflow), is it reasonable to assume that the driver can dissipate more power?

For instance, if the ambient was always 30 Celsius or lower, would it be alright to dissipate (150 - 30)/68.3 = 1.76W?

Thank you!

• Hi, Saurabh,

As you mentioned, the power dissipation is limited by the maximum junction temperature which is listed as 150C in the Absolute Maximum Ratings table.

Power dissipation is discussed in section 9.2.2.5 Turn on and turn off gate resistors of the data sheet.

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• Hi Don,

Thank you. Can you confirm that there is nothing special about the 985mW and I only need to ensure that Tj remains under 150 C?

• Hi, Saurabh,

That's odd, I guess I shouldn't answer e2e questions before I've had my coffee... I just looked at the d/s section 6.4 again, and that's not a maximum rating there, that is the power dissipated under the conditions in the middle column.

So, you are correct, there is nothing magical about 985 mW.

• Hi Don, in that column the ambient is 25 Celsius. Although I did not calculate whether the dissipation is 985mW, for a 985mW dissipation, the temperature rise should be under 70C (68.3K/W). Which is why I am wondering what is special about 985mW.

Thank you

• Hi, Saurabh,

If you go through the actual calculations for power dissipation, under the conditions given in the middle, that is exactly how much power the chip will be dissipating. This is NOT the maximum power dissipation, but the actual power dissipation given those conditions.

• Thanks Don.