PSR High-Speed Square Body Fuses Littelfuse PSR Series High-Speed Square Body Fuses are specially designed for the protection of power semiconductor devices. The fuses from Littelfuse combine extreme current limitation benefits with balanced performance levels to meet the precise requirements for protecting sensitive power electronics devices. These devices include diodes, triacs, IGBTs, SCRs, MOSFETs, and other solid-state devices that are typically designed for power conversion and power conditioning equipment.