Utilizing overlooked characteristics of ferrites for improved printed circuit board EMI suppression

While nickel-zinc composition soft ferrites are routinely used for PCB EMI suppression, the consequences of DC and low frequency AC bias on a device's high frequency impedance has not been widely discussed in the EMC and magnetic component literature. Virtually all EMI suppression applications in DC and AC power filter circuits place two-terminal ferrites under bias. This paper discusses the impact of low frequency bias on high frequency device performance and suggests several methods to address these impacts while improving the performance of EMI filters containing nickel-zinc ferrites.