Researchers in the US have developed a solid-state filter for power converters that does not need a capacitor.

Capacitors are one of the main points of failure in an e-mobility power system, responsible for 30% of failures in power electronics.

Traditional filtering methods involve a dual inductor-capacitor (LC) cell or an inductor-capacitor-inductor (LCL) T-circuit, but capacitors are susceptible to wear-out mechanisms and failure modes. 

The development by researchers at Purdue University-Indianapolis is a single-phase, DC-AC converter with two elements. A low-frequency, H-bridge converter works with a solid-state power filter (SSPF) that is capable of generating a sinusoidal voltage output for the load.

The design is suitable for applications in power-generation units for shipping where the power grid operates at 60 Hz and in aircraft where the power grid operates at 400 Hz.

The power-filter design uses a high-frequency planar transformer to eliminate the need for both the LC filter and the DC-link capacitor, which enhances efficiency and reliability.

Operating at 30 kHz, the H-bridge converter injects voltage harmonics to generate a sinusoidal output voltage.

Theoretical analyses, simulations and experiments conducted on a 60 Hz, 120 V system demonstrated a low total harmonic distortion of 1.29%, meeting the IEEE 519 standard for the design of electrical systems that include both linear and nonlinear loads. This also defines the voltage and current waveforms that may exist throughout the system, and the limits for waveform distortion.

A key focus is to reduce the size of EV chargers by integrating stray capacitance into the operation of various DC-DC converters, effectively reducing or eliminating reliance on external capacitors. To achieve this, the team is introducing a novel material, calcium copper titanate (CCTO), which is expected to enhance the stray capacitance so it can be used.

The proposed transformer measures 11.5 cm long, 15 cm wide and 11.2 cm high, with a total volume of 1.93 litres, representing a significant reduction of 12.2%, compared with conventional designs with a DC-link capacitor.

The research team aims to expand the concept of capacitorless topologies to a wider range of power converters.