Thin-film High Voltage Capacitors for Hybrid Electric Vehicle Inverter Applications

Abstract

The propulsion system in hybrid electric vehicles (HEVs) requires an alternating current (AC) electric motor in combination with an internal combustion engine. When the HEV is being propelled by the AC motor, the power for the motor is provided by batteries whose direct current (DC) voltage is chopped into an AC waveform via an electronic device called a power inverter. Capacitors known as DC bulk capacitors are placed between the battery and the inverter to “decouple” the AC switching inverter from the power source. Power electronics inverters use several large high voltage discrete DC bulk capacitors, which negatively influence the inverter's size, weight and are a high cost item in the assembly. The use of a high dielectric constant (Dk) ferroelectric material enables smaller, higher temperature capable, lower-cost power capacitors. Ceramic ferroelectrics, such as (Pb,La)(Zr,Ti)O3 [PLZT], offer the highest dielectric constants. Argonne National Laboratory is developing a novel film-on-foil technology for high-power capacitors utilizing PLZT. These capacitors, with an increasing dielectric constant with temperature, low equivalent series resistance and a benign failure mode, are well suited for power applications. The PLZT is deposited onto a metal foil via a chemical deposition process and the top electrode metal (Pt or Al) is then deposited by electron beam evaporation onto the top surface of the dielectric thus creating the capacitor. This project involved the fabrication and electrical evaluation of film-on-foil capacitors for HEV inverter applications. Capacitors utilizing both nickel and platinum-on-silicon as the base substrate, and PLZT as the dielectric material were fabricated. These capacitors were tested for dielectric integrity, capacitance, voltage breakdown, and benign failure mode. Results from these mechanical and electrical evaluation tests will be presented. Key processing challenges and implementation methods will also be described.