Two-stage switching converter solutions are widely used in automotive LED driver applications. This architecture consists of a first stage BOOST voltage regulator (CV) with input connected to the battery and output connected to one or multiple second stage BUCK Constant Current (CC) mode LED drivers. The second stage CC-Mode Buck converters behave as a Constant Power Load (CPL) to the first stage BOOST [3]. In industrial or automotive applications, the BOOST converter output capacitor is normally a combination of ceramic and electrolytic capacitors connected in parallel to optimize the size and cost of the system. The presence of the electrolytic capacitor significantly alters the outer voltage loop gain of the system by shifting the pole locations and adding extra poles and zeros to the closed loop gain. This paper provides an analytical understanding of the impact of paralleling ceramic and electrolytic capacitors on the outer voltage loop gain using the extra-element theorem. The application of extra-element theorem takes the electrolytic capacitor as an extra element in the simpler open loop transfer functions instead of taking the entire closed loop system altogether.
