Improved Spice Models of Aluminum Electrolytic Capacitors for Inverter Applications.pdf

Improved Spice Models of Aluminum Electrolytic Capacitors
for Inverter Applications
Sam G. Parler, Jr.
Cornell Dubilier
140 Technology Place
Liberty, SC 29657
Abstract — Impedance modeling of aluminum electrolytic capaci- scopic-to-macroscopic surface area enhancement of a factor
tors presents a challenge to design engineers, due to plex on the order of 100. For the purposes of this paper it will be
nature of the capacitor construction. Unlike an electrostatic capaci- assumed that the anode etch pattern is cylindrical. See Figure
tor, an electrolytic capacitor behaves like a lossy coaxial distributed 3. The anode foil is electrochemically anodized in a bath of
RC circuit element whose series and distributed resistances are hot electrolyte. This process grows alumina (aluminum oxide)
strong functions of temperature and frequency. This behavior gives onto the surface of the pits at a ratio of about nm/V. The
rise to values of capacitance, ESR (effective series resistance), and anodization voltage is generally 20 to 50% higher than the
impedance that vary by several orders of magnitude over the typi- rated voltage, depending on the temperature and life rating.
cal frequency and temperature range of power inverter applica- Since for a plate capacitor the capacitance
tions. Existing public-domain Spice models do not accurately ac-
C = εε A /d (1)
count fo