Abstract Innovative test methods are required to keep up with the increased demand for insulation materials which can withstand the high‐frequency square‐wave voltages generated by power‐electronic equipment. Test systems using high‐voltage, high‐frequency transformers have proven versatile and easy to realise. The authors investigate the application of amorphous cut cores in such transformers. Analytical and numerical models for the transformer and its frequency response are developed, aiding the design process. An amorphous core‐based transformer is designed for 8 kVpk output at 100 kHz and compared to two previous designs based on ferrite cores. The frequency and pulse response, as well as the high‐voltage and thermal performance, are evaluated. The comparison shows that while the low parasitics of the amorphous‐based transformer allow for superior frequency response, they are unsuitable for long‐duration tests with high pulse repetition frequencies (25–100 kHz) due to increased core losses. The partial discharge inception and flashover voltage are comparable to the ferrite‐based transformers.