How does the power supply ripple of a smart controller (PAC) affect EMC?

The power supply ripple of a power supply in a PAC (Power Amplifier Controller) is not only a noise source but also modulates the clock and signals, leading to sideband radiation and bit errors. The ripple is mainly composed of the switching frequency of the power supply and its harmonics, affecting frequencies from tens of kHz to tens of MHz. Excessive ripple can cause conducted emissions to exceed limits and can also couple to sensitive circuits through the power network, reducing immunity. To assess the impact, an oscilloscope and a spectrum analyzer must be used to simultaneously measure the ripple's time-domain waveform (target less than 50mVpp) and frequency-domain spectrum (target in the low 150kHz-30MHz range).

Suppression measures include: optimizing the feedback loop of the switching power supply and adopting synchronous rectification technology; adding a PWRA6045 power inductor and a low ESR capacitor at the output; and using LDO secondary regulation for sensitive circuits. Experiments show that after reducing the 24V power supply ripple from 100mVpp to 20mVpp, the radiated emission of the PAC at 30MHz can be reduced by 12dB, and the bit error rate can be increased from 10^-5 to 10^-8, meeting the voltage fluctuation immunity requirements of IEC61000-4-17.