Active common-mode filter for capacitive-coupled isolated signalling

Capacitive coupling is a well-established principle in isolated signalling. A major challenge esp. in power electronics is common-mode transient immunity caused by the displacement currents at the coupling capacitors. Digital isolator integrated circuits (ICs) employ modulated signalling, the authors propose an active filter circuit to remove common-mode components from baseband signals. This allows for reliable data transmission during transients in power semiconductor control, AC-coupled RS-485 or transformer-less Ethernet. The filter structure is simple and consists of two current mirrors, one for positive transients, and one for the negative. A current on one differential signal line is ‘mirrored’ onto the other, common-mode currents are conducted to ground. Differential signals are minimally affected. The proposed filter is verified in simulation and experiment: baseband signalling with 50 Mbps in presence of 30 kV/$\,{\rm \mu s}$μs burst transients is achieved using discrete semiconductors. Even better results are expected when monolithically embedding the filter in an integrated circuit.