An input filter is often required for the on-board battery chargers to meet the stringent electromagnetic interference (EMI) requirements. This creates a high-Q resonant tank and may cause input current oscillation if not properly designed. The adapter with high-Q resonant tank generates a peak output impedance at resonant frequency. The charger is a current-limiting DCDC converter with a large duty cycle, it creates a very low input impedance. When the peak output impedance adapter meets the low input impedance battery charger, it forms an unstable region, and induces input current oscillation. This paper studies a practical 4-cell 130W notebook charger, uses impedance analysis to illustrate the input filter interactions with the battery charger, discusses the design trade-offs of the input filters, and proposes three system solutions to avoid the input current oscillation. Experimental results show the input current oscillation is avoided. Furthermore, new dual random spread spectrum (DRSS) technique mitigate the EMI noise without the input filters. This provides an alternative and cost-effective path to address the EMI requirements without the input filter interactions.
