Question:
Can we use discrete capacitor for ADM2687E to reduce emissions?
Answer:
When choosing the discrete capacitor for part like ADM2687 which utilizes isoPower technology, the capacitor needs to meet the isolation/safety requirements of the isolation barrier; that implies an X/Y cap with suitable voltage rating. Also, the capacitor’s Self-Resonant frequency should be as high as possible, and it needs to be above the ~360 MHz (IsoPower Secondary Side rectifying frequency). Above the Self-Resonant frequency, the capacitor acts more like an inductor. Given the dimensions needed for a High Voltage capacitor, they typically have a lower Self-Resonant frequency. A PCB stitching capacitor is preferred instead because it has a very low Equivalent Series Inductance and therefore a high Self-Resonant frequency.