How does the dead time of a frequency converter affect EMC?

The dead time setting of a frequency converter has a certain impact on EMC, requiring a balance between preventing bridge arm shoot-through and optimizing the output waveform.

Dead time is a delay set to prevent simultaneous conduction of the upper and lower bridge arm IGBTs. Impacts include: dead time can cause output voltage waveform distortion, generating low-order harmonics that may fall within the low-frequency test range of conducted emissions. Improper dead time settings can cause slight inconsistencies in switching timing, affecting the symmetry and repeatability of switching noise and potentially complicating the noise spectrum. During the dead time, current freewheels through the anti-parallel diode, and the diode's reverse recovery characteristics may introduce additional noise spikes.

Therefore, a smaller or larger dead time is not necessarily better. The optimization method is to minimize the dead time as much as possible while ensuring safety, which requires precise drive circuitry and fast IGBTs. Adaptive dead time compensation technology, dynamically adjusted according to current direction, can reduce harmonics. The impact of different dead time settings is evaluated by measuring the harmonic spectrum of the output voltage and current, as well as conducting EMI tests. Dead time optimization is usually performed as part of inverter software tuning, in conjunction with hardware EMC design.