LM5176EVM-HP: Output voltage error

Liu Yang

Part Number: LM5176EVM-HP

Dear All

My Japanese customer has a question about LM5176EVM-HP. The question is below:

In the LM5176EVM-HP, when VOUT is changed from the standard 12V output to 5V output, ripple occurs at VOUT. The ripple increases in proportion to the input voltage and output current, and a ripple of about 700mVp-p occurs at an input voltage of 20V and an output voltage of 2A. (Vout-5V_Iout-2A_Vin-20V.png)
When changing VOUT, only R14 is changed from 280kΩ to 105kΩ. Are there any parts other than R14 that require adjustment? Also, please tell us how to remove the ripple.

Using the WEBENCH, Cout is 220uFx1 and 22uFx3 even when the condition such as switching frequency is the same as the LM5176EVM-HP. (WBDesign 274.pdf)

WBDesign274.pdf

The operation has been confirmed by matching peripheral components to the value of WEBENCH. The output voltage results did not change (WBDesign275 (1). pdf). When the signal of SW1 is checked, an undershoot at no load and an overshoot at 2A load occur. The above undershoot/overshoot seems to be output as ripple. If you know the cause, please tell me.

WBDesign275 (1).pdf

The measuring instrument uses the following.
Oscilloscope: DPO5204B
Probe: TPP1000
A photo of TPP1000 measurement is attached.
Even when the output was returned from 5V to 12V, overshoot/undershoot occurred.
(The LM5176EVM-HP is in the condition at the time of purchase.)
The measurement waveform is also attached.

WBDesign34-5V.pdf

WBDesign34-12V.pdf

Pls help me

Tom.Liu

over 5 years ago

0 Garrett Roecker over 5 years ago

TI__Mastermind 21720 points

Hi Liu,

The easiest way to reduce the output noise is to add more ceramic capacitors to the output. These values need to be different values (100pF to 100nF) to help dampen the high frequency ringing.

Another suggestion is to slow down the switch nodes to help reduce the ringing. This can be done by adding gate resistors to the MOSFETs or by adding snubbers to the switch nodes.

Please let me know if you have any questions.

Thanks,

Garrett

0 Liu Yang over 5 years ago in reply to Garrett Roecker

TI__Expert 5915 points

Hi Garrett,

Please help me answer the further question from my customer.

How can I add the gate resistance of the MOSFETs?
Please tell me the detail about adding gate resistance of the MOSFETs.

BR

Tom.Liu

0 EricLee over 5 years ago in reply to Liu Yang

TI__Mastermind 46061 points

Hi
The responsible engineer is out of office now. He will reply by 4/23
Please see www.ti.com/.../slua618a.pdf
Regards,
EL

0 Liu Yang over 4 years ago in reply to EricLee

TI__Expert 5915 points

Hi Eric,

Is the responsible engineer in the office new?
Can anybody help me answer the question above?

BR
Tom.Liu

0 Garrett Roecker over 4 years ago in reply to Liu Yang

TI__Mastermind 21720 points

Liu,

The gate resistor just needs to be increased. on the EVM these are resistors R2 to R5 and R24 to R27. I suggest increase R2, R3, R26 and R27 to 1Ohm and seeing if there is any improvement.

The other option to improve the output ripple is to add an LC filter on the output.

Thanks,

Garrett

0 Liu Yang over 4 years ago in reply to Garrett Roecker

TI__Expert 5915 points

Hi Garrett,

My customer conducted an experiment according to your suggestion and asked another question.
Please help me answer the further question.

experimental results are following.
① Before change
② Ceramic capacitor 0.1uF is added to outputs
③ Add Snubber circuit (1.5 ohm + 2200 pF) ※ All gates are added.
④ Add Snubber circuit (1.5 ohm + 2200 pF) and gate resistance ※ R2, R3, R26, R27 changed to 1Ω
We measured output 12V / 2A when input voltage is 6V and 24V. The measurement points are output (Vout), SW1 and SW2.
The addition of the snubber circuit seems to be the most effective. However, I did not make a complete improvement. Will it be effective to increase the gate resistance more?

BR
Tom.Liu

0 Garrett Roecker over 4 years ago in reply to Liu Yang

TI__Mastermind 21720 points

Hi Liu,

Please attached the waveforms. Adding more gate resistance will help.

If the customer is wants a clean out put signal it is possilbe to add and LC filter on the output to help reduce any high frequency noise.

Thanks,

Garrett

0 Liu Yang over 4 years ago in reply to Garrett Roecker

TI__Expert 5915 points

Hi Garrett,

I attached the waveforms.

Please see if you can give me some advice.

Measurement results.zip

Tom.Liu

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LM5176EVM-HP: Output voltage error