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This is Sasaki in charge of power supply dock technology.
In this column, I would like to introduce some examples of problems related to power supplies that I encountered multiple times during various customer support sessions.
We received the following inquiry via the power dock inquiry form.
Need a power circuit filter?
I am using a DC/DC converter to power my FPGA, but I am having trouble with the large switching noise.
I decided to try to get more information by phone and email.
Could you tell me more about what your symptoms are?
The switching noise of the DC/DC converter was louder than when I checked it on the evaluation board, so I got worried and contacted them.
Is that so.
Are there any differences between the evaluation board and the current board?
The only difference from the evaluation board is the addition of a bead filter to strengthen the output stage filter of the DC/DC converter.
Is there a large fluctuation in the current in the power supply section where the bead filter is installed?
I don't know because I didn't measure it, but I think it's probably big.
I sent you the voltage waveform of the noise, so could you check it?
I see, I understand the cause.
I think that by inserting a bead filter with a large Q, an anti-resonance point was created in the power line impedance profile.
Could you analyze the PDN (Power Distribution Network) and check the impedance profile of the board?
I understand. I will check it.
After checking, I found the point of anti-resonance as pointed out.
The problem was resolved by correcting the problem.
If you use bead filters carelessly, such problems may occur, so please be careful when using them.
Is that so.
I will be careful from next time.
Cause of increased power noise
Inserting a bead filter created a large anti-resonance point.
The point of this time
Don't use filters lightly, check to see if there are any anti-resonance points in the power supply impedance profile.
(Bead filters are by no means a panacea.)