LMR36015-Q1: Regulator too hot

Eugen Hildermann

Part Number: LMR36015-Q1
Other Parts Discussed in Thread: LMR36015

Hello,

To fulfill the EMC CISPR25 class 5 requirements three of the LMR36015-Q1 DCDC regulators were desined in our ECU.

During our thermal inspection (IR camera, themal sensors) we determined that two of three regulator are running hot. Tcase is appox. 80°C and 77°C at 25°C amb! The input voltage is 28,5V; output voltages are 13V and 6V. The loads are amount 300mA. The third regulator has Vin 6V Vout 1,2V and is still cool at 300mA load. The regulator goes hot at voltage gaps (vin to vout) higher then 10V and at higher Vin.

The efficiency of the regulator is not the best but i think it isnt the problem. The package has no exposed pad! The thermal resistance junction to case (top) is 36K/W and no thermal conection to board bottom side. The TPS57140 regulator is regarding themal behaviour much better, but its not low-EMI related!

What is the problem? Any idea? Should I test it with the LMR36015AQRNXRQ1 400 kHz derivate?

Thanks in advance.

Eugen

over 6 years ago

0 Frank De Stasi over 6 years ago

TI__Guru 73655 points

Using the 400kHz option should reduce the power
dissipation and help you thermals.

For this package the heat is removed mostly through the pins.

If you can show us a snapshot of your top layer PCB, we may be able to make
some helpful suggestions.

0 Frank De Stasi over 6 years ago in reply to Frank De Stasi

TI__Guru 73655 points

I am closing this due to inactivity.

0 Eugen Hildermann over 6 years ago in reply to Frank De Stasi

Prodigy 100 points

Hi,

thanks for your help. Sorry, I was not in house last week.

We replace the regulators with the 400kHz derivate on two of our boards. It will be done at the moment. I will come back to you with the results.

Attached you find the short report with layout screenshot of one regulator circuit.

To fulfill the automotive requirements specs the capacitors are placed 90 degrees to each other. Therefore the suggested layout layout approach of TI sheet is not the same.

Thank you and regards

Eugen.

hqr_ECU_Thermal_HotSpot.pdf

0 Frank De Stasi over 6 years ago in reply to Eugen Hildermann

TI__Guru 73655 points

The only thing I could see on the layout is to maybe add some more ground vias.
This may help to get the top and bottom layers to radiate some heat.

0 Eugen Hildermann over 6 years ago in reply to Frank De Stasi

Prodigy 100 points

I think we have enogh grond via on power and ground. It will not solve the problem.

Here top and bottom view. Additional we have one inner layer groung plane.

TI layout example:

0 Eugen Hildermann over 6 years ago in reply to Eugen Hildermann

Prodigy 100 points

lmr36015_layout.pdfthe screenshots can't added here. So, please see attachments.

0 Frank De Stasi over 6 years ago in reply to Eugen Hildermann

TI__Guru 73655 points

As you mentioned you may need to use a regulator with an exposed pad to help get the heat out.
I would measure the efficiency and calculate the ThetaJA of your solution and compare with the
data sheet values to be sure that it is not way off.

0 Eugen Hildermann over 6 years ago in reply to Frank De Stasi

Prodigy 100 points

no, what I mean is that this regulator has too high fix clock rate and works inefficient at high voltage gap (vin to vout) and at lower loads. The wasted power in the package in this application is approx. 0,6W! The regulator is almoust monolytic. Its not possible to eliminate this effect.

0 Eugen Hildermann over 6 years ago in reply to Eugen Hildermann

Prodigy 100 points

so, we have replaced the LMR36015 2100kHz regulator with the 400kHz derivative. Same board and same FW config.

The case temperature is decreased from 80°C to approx. 38°C! The system power consumption drops from 3,8W to 2,5W. See thermal camera pic. Also at higher loads up to 700mA works this regulator well.

So, I would like to say, that the 2100kHz regulator has definitely a problem.

0 Frank De Stasi over 6 years ago in reply to Eugen Hildermann

TI__Guru 73655 points

I am glad that you have resolved your issue.
Thank you for choosing TI.

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LMR36015-Q1: Regulator too hot