Design of a Medium-Frequency Electrical Stimulation System Based on FPGA

Abstract

Medium-frequency electrotherapy, owing to its non-invasive nature and deep tissue stimulation, has become an essential physical rehabilitation modality for chronic pain. However, long-term hospitalization often fails to meet the demands of extended treatment courses. To this end, a portable medium-frequency electrical stimulation system was developed. The system adopted a field-programmable gate array (FPGA) as the controller, generated sinusoidal pulse-width modulation (SPWM) signals through a look-up table method, and produced a sinusoidal carrier via an active second-order low-pass filter and a direct current (DC)-blocking capacitor. Based on the direct digital synthesis (DDS) principle, the system outputted modulating waveform digital values by traversing read-only memory (ROM) data, and employed the carrier as the digital-to-analog converter (DAC) reference voltage, multiplying it with the modulation wave to achieve amplitude modulation. To increase the output power, a Class-D amplifier was used to amplify the amplitude-modulated signal, and a transformer provided impedance matching with the load. The system constructed pulse and output current detection circuits using optocouplers and current transformers, incorporating a software threshold algorithm to detect electrode open-circuit and short-circuit faults. Experimental results demonstrate that, with a $500~\Omega $ simulated human-body load, the output current remains below 45 mA, and the system can stably deliver a 1–10 kHz sinusoidal carrier and a 1–150 Hz modulation wave with deviations within 20%. The system supports five modulation waveforms—sine, sawtooth, triangular, square, and trapezoidal—offering a practical reference for both home-use and clinical medium-frequency electrical stimulation systems.