Only one module making noise?

After visiting the customer and checking the circuit diagram, I found something suspicious.

However, the frequency of occurrence is low as it only occurred on one car, and questions remain.

I immediately moved to the laboratory and started verification by arranging multiple boards with normal and problematic boards.

Then...

The board that I thought was normal also made a whining sound!

Sounds like it's coming from here...

This is not a problem because the sound is small.

！？

Upon closer inspection, I found that all the boards I tested had sound.

There is a high possibility that there is a problem with the peripheral circuitry of the power supply, so let's check the waveform just in case.

When we asked the customer to check the input and output waveforms of the power supply, they confirmed that the voltage fluctuated at a low frequency, apart from the switching frequency.

(…This is the cause!)

I looked into the circuit diagram including the filter related to the power supply circuit.

As a result, it was discovered that there was a problem with the design of the input filter.

cause of noise

Approximately 1200Hz (0.8msec) low frequency oscillation of approximately 200mVp-p

Normally, when designing an input filter, the power supply behaves as a negative resistance, so |ZS|<|ZI| must be maintained (a condition for stabilizing the power supply system).

In this customer's design, there was a problem with the filter's inductance value and the capacitor's ESR value, and the stabilization conditions were not met, causing vibration.

When we asked the customer for more details, we discovered that the DC/DC converter on the prototype board was also broken!

After all, this oscillation was the cause of the destruction.

We redesigned the input filter based on our suggestions, and were able to prevent oscillation and resolve problems with the DC/DC converter.

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The point of this time

Power input filter design is very important

application note

You can download an application note that explains in more detail the oscillation of switching power supplies due to negative resistance operation.