TPS40303: Regarding buck converter webench simulation and inductor selection

Caliber

Intellectual 810 points

Part Number: TPS40303
Other Parts Discussed in Thread: TPS40305

Dear TI team,

Good evening.

We have done webench simulation for 12V to 5V at 3A(TPS40303DRCR),which will be supporting maximum 15watt load.

We are now checking in PCB practically and facing some issues as below.

Currently we have connected only 6Watt load(1.21A),but the inductor is getting heated up (Temperature crossing 90 degrees when checked via thermal camera) which is having 4.2A DC current capacity.

Kindly check EN and BP pins of TPS40303 in webench simulation and their respective capacitors value.

Hope 4.2A current capacity of inductor can support maximum current above 2.5 A according to design.

Kindly check and suggest us solution.

Thank you,

Caliber

WBDesign46.pdf

over 2 years ago

+1 Peter James Miller over 2 years ago

TI__Mastermind 48200 points

Caliber

Can you double check that your design matches the schematic with the Wurth 18μH inductor and only three 10V, 10μF ceramic output capacitors?

What kind of load is being driven by the 5V?

I don't see any immediate issue with the design, and the inductor should not be rising 65C with only 1.2A of load current, but if there is additional output capacitance, large parasitic inductance from the layout, or a load with a strange dynamic impedance, those could increase the RMS current and thus inductor self-heating.

Also, check to make sure you have a TPS40303 and not a TPS40305, which operates at 1.2MHz instead of 300kHz.

If those check out, can you provide a picture of your board and some oscilloscope waveforms of VIN, VOUT and SW to make sure switching is stable at 300kHz?

0 Caliber over 2 years ago in reply to Peter James Miller

Intellectual 810 points

Dear Miller,

Good morning.

Thank you for your response.

Currently we have connected 100uF/25V electrolytic capacitor,we will replace with 3 numbers 10uF/10V ceramic capacitor and check again.

Also we will check components and come back to you with details of oscilloscope pics of Vin,Vout and switching of mosfet.

Thank you very much for your feedback and suggestions.

Thank you,

Caliber

0 Caliber over 2 years ago in reply to Caliber

Intellectual 810 points

Dear Miller

Good morning.

I have shared the details in a document to you in private message.

kindly check and suggest us solution.

Thank you,

Caliber

0 Peter James Miller over 2 years ago in reply to Caliber

TI__Mastermind 48200 points

Caliber,

I looked through the waveforms which you shared. I do not see evidence of instability which would contribute to excessive inductor current and thus self-heating.

The design appears to be sound.

Are the six 6.2E resistors on the output 6.2Ω resistors? Is that your 1.2A load current?

Can you provide a measure of the 12V input current into the TPS40303 design, and the DC voltage drop across the 18uH inductor?

To measure the drop across the inductor, connect a 1kΩ resistor to a 1μF capacitor, then connect the resistor to the switching node (Source of Q10 / Drain of Q11) and the capacitor to VOUT, then measure the voltage drop across the capacitor with a voltage meter. The 39mΩ DCR of the inductor will serve as a current sense resistance for measuring the current through the inductor.

With the switching node input and output voltages stable, but the inductor getting hotter than expected, we need to check to see if there is additional current flowing in the inductor causing it's self-heating.