Integrating coupled magnetics in a two-stage LC low pass filter for low ripple power supply applications

Electrical noise sensitive systems using mixed signal devices such as data converters and transceivers are prone to performance degradation due to switching regulator output ripple. Often switching regulator is used as power supply due to high efficiency performance it can offer. However, due to its undesirable output artefact, problems arise on powering these analog mixed signal devices directly from this regulator. Reducing the fundamental output ripple to a miniscule level is the main objective of this research. A novel concept using coupled magnetics or coupled inductors incorporated in a two-stage LC low pass filter was developed. This provides significant reduction on the fundamental frequency of switching regulator output ripple through the formation of a notch filter. However, to fully maximize the effect of the notch filter, the notch frequency should be tuned near the fundamental frequency location of the output ripple. The notch filter is created through resonance that provides better reduction of the fundamental output ripple in comparison to a conventional two-stage LC low pass filter with less volume and component count. A single package off the shelf coupled inductor can be used representing the inductors in the two-stage LC low pass filter. The resonance is formed by the mutual inductance created by the two coupled inductance and the series capacitor. Design imperfections such as the component tolerances, capacitance drift due to temperature and part-to-part coupling variations of the magnetic components are some factors that make the notch frequency hard to control. A phase-lock feedback circuit has been developed to overcome these design imperfections wherein it provides the loop correction and tuning control. A variable capacitor is used as a tuning element to align the resonant frequency with the switching frequency of the switching power supply therefore maximizing the notch filter effect to the fundamental frequency of the output ripple. Barium Strontium Titanate (BST) dielectric capacitor and off the shelf ceramic capacitor are the variable capacitors that can be used to make resonant tuning achievable. The coupled magnetics in a two stage LC low pass filter in combination with the automatic resonant tuning concept was filed as a patent last November 6, 2017 and was issued last June 11, 2019 entitled “LC filters including coupled inductors for reducing ripple in switching power supplies” with patent no. US 10,320,280 B2. The author of this thesis is the co-inventor of the issued patent with Analog Devices Global Unlimited Co. as patent assignee.