LM25118-Q1: Thermal problem of buck-boost converter using LM25118.

SUYEONG KIM

Part Number: LM25118-Q1

Hello.

I designed the buck-boost converter of LM25118-Q1 and made a product using 'WEBENCH' provided by TI.

Although parts proposed by 'WEBENCH' could not be used due to parts supply and demand problems, parts with similar specifications were used as much as possible.

The design specification is 10A with an output of 11V, but the heat of the components exceeds the allowable range even if only 6 amps flow from the actual board. (PCB is 10 layer board)

The figure shown in the actual circuit diagram is the heat measured when 6 amps were spilled for 5 minutes. When two 3.3uH inductors were connected in series (6.6uH), the surrounding parts decreased slightly, but the inductor became 30℃ hotter.

/resized-image/__size/8800x4342/__key/communityserver-discussions-components-files/196/FC_2D00_Main_5F00_BD.jpg

/cfs-file/__key/communityserver-discussions-components-files/196/LM25118Q1_5F00_11V10A.pdf

I don't think lowering the switching frequency is the answer. I need your help. Please tell me how to lower the fever.

Thank you.

over 2 years ago

0 Harry Hofner over 2 years ago

TI__Genius 15665 points

Hello Suyeong,

Thanks for reaching out to us via e2e.

I would assume that your inductor goes into saturration.
Please doublecheck if the inductor that you are using has especially been built for being the energy storing element in a switch mode power supply.

You also might face some ringing on the FETs.
So you could try putting some RC snubbers across the FETs and rectifier diodes to dampen those.
Using the gate resistors that you have already prepared for can also help to reduce spikes and unnecessary power losses.

All information in this correspondence and in any related correspondence is provided “AS IS” and “with all faults”, and is subject to TI’s Important Notice (http://www.ti.com/corp/docs/legal/important-notice.shtml).

Best regards,
Harry

0 SUYEONG KIM over 2 years ago in reply to Harry Hofner

Prodigy 20 points

Hello, Harry.
Thank you for your reply.

The inductor used 'XAL1010-332ME' proposed by the design tool, but the FET and inductor are still hot. (higher than 90°C)

In general, gate resistance causes an LPF effect, distorting PWM and reducing Vgs by voltage distribution, resulting in a larger heat generation of FET. (We confirm that there is no significant difference when actually applying 10 ohms.)

Can you show me the design of RC snubber between FET and rectifying diode that you suggested?

My circuit now has a switching frequency of 197 kHz (RT:29.4kΩ) and a power inductor of 6.6uH (3.3uH serial connection).

In this case, the FET is 72°C, the inductor is 95°C, and the rectification diode is 86°C.

I look forward to hearing from you.
Thank you.

0 Harry Hofner over 2 years ago in reply to SUYEONG KIM

TI__Genius 15665 points

Hello Suyeong,

I am sorry, my wording must have been misleading.
I did not mean to say that you should add a snubber between FED and diode.

Instead I wanted to propose adding RC snubbers in parallel to the rectifying diodes.
Also, you can add RC elements across the FETs, to reduce ringing, as described here:

https://www.ti.com/lit/an/slva255/slva255.pdf?ts=1660066058248&ref_url=https%253A%252F%252Fwww.ti.com%252Fsitesearch%252Fen-us%252Fdocs%252Funiversalsearch.tsp%253FlangPref%253Den-US%2526searchTerm%253Dfet%2Bringing%2526nr%253D3507

https://www.ti.com/lit/an/slvaee3/slvaee3.pdf?ts=1660064208000&ref_url=https%253A%252F%252Fwww.ti.com%252Fsitesearch%252Fen-us%252Fdocs%252Funiversalsearch.tsp%253FlangPref%253Den-US%2526searchTerm%253Dmultiphase%2526nr%253D1325
https://www.ti.com/lit/an/slvaee6/slvaee6.pdf?ts=1660064214005&ref_url=https%253A%252F%252Fwww.ti.com%252Fsitesearch%252Fen-us%252Fdocs%252Funiversalsearch.tsp%253FlangPref%253Den-US%2526searchTerm%253Dmultiphase%2526nr%253D1325

Best regards,
Harry

0 Harry Hofner over 1 year ago in reply to Harry Hofner

TI__Genius 15665 points

Hello Suyeong,

I have not heard from you for two weeks now, so I assume your problem has been solved and I will close this thread.
You can re-open it or simply start a new one if this one got locked.

Best regards,
Harry

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LM25118-Q1: Thermal problem of buck-boost converter using LM25118.