Electromagnetic Emissions in High Density and Fast GaN Switched Half Bridges with Resonance Filter Structures

The wide bandgap semiconductor GaN is a promising material for future extended use in high power electronic applications due its fast and low loss switching. For high-density power electronic systems with miniaturized system size, a balancing of the semiconductor switching speed, duty cycles ranges, frequency of switching, current density, ripple and overshoot voltages are necessary. In order to assess the design rules of GaN switch circuits in connection with the supporting passive components, in particular the DC-link and filter capacitors, simultaneous measurements of the electromagnetic emissions and the waveforms of the voltage and currents in the capacitors and also in the switched node in a 650V GaN half bridge followed by a resonant filter structure were made. Results are obtained for different switching frequency ranges, mainly between 50 kHz and 1000 kHz, different interlock delay and capacitor technologies. The results are interpreted in terms of the correlation between the voltage and current waveforms and the emitted radiation for varying input voltage levels. It was found that for the half bridge with RC filter - circuit without load, that film capacitors show higher amplitudes of voltage and current transients at high switching frequency, resulting in increased levels of EMI, than the ceramic ones. Also the interlock delay time setting in conjunction with duty cycle parameters and the rise time of the gate pulse showed considerable influence on EMI levels. For a LC combination in a buck converter structure it has been established that no optimized interlock delay time could be found where ripple currents and EMI emissions could be reduced while maintaining a fast rise at the GaN, independent of load. The results from a GaN - capacitor combination test circuit showed that clamping of gate voltage either passively, actively, or by topology is necessary for avoiding unwanted switching actions in fast GaN circuits.