www.vicorpower.com CONDUCTED EMISSION COMPLIANCE 9 www.power-mag.com Issue 3 2024 Power Electronics Europe Conventional challenges facing highfrequency DC-DC power conversion Although moving to higher-frequency DCDC conversion can yield many tangible benefits, a number of technical challenges have historically prevented this pursuit. First, moving to higher frequency operation may present a barrier to achieving EMC compliance. For conducted emissions standards such as CISPR32 (required for V2G applications), the frequency range evaluated by the standard is from 150kHz to 30MHz. Operating at a higher fundamental frequency, such as above 1MHz, creates the largest harmonics within the frequency range of interest, running the risk of compliance failure. For this reason, many power converter designers choose to operate at lower frequencies, such as 100kHz, ensuring that their first harmonic falls below the frequency range of interest. Same issues can found if the power stage is called to be compliant with the CISPR25 reference standard. Also, fear of increased losses is another potential drawback when using higherfrequency switching converters. Switching losses occur when a switch, such as a MOSFET, transitions from its on-state to its off-state and vice versa. These losses are significant because both the voltage across the switch and the current through the switch are non-zero during the transition period. (Figure 2) All else being equal, higher switching frequencies result in more frequent transitions per unit of time, leading to increased switching losses. Since the energy dissipated per switching event is proportional to the crossover time and the product of the voltage and current, increasing the frequency means that these energy losses accumulate more quickly. Therefore, the total power loss due to switching is directly proportional to the switching frequency, making higherfrequency operation associated with higher switching losses. Finally, issues concerning the selfresonance of passive components occur during high-frequency operations. Selfresonance is a phenomenon in which electrical components exhibit resonant behavior due to their parasitic properties. This leads to unpredictable behavior, impedance peaks, efficiency losses and signal integrity issues. Self-resonance becomes a significant problem at higher switching frequencies as these frequencies approach the self-resonant frequencies of components, amplifying noise and EMI and complicating circuit design. Also working beyond the self resonant frequency an inductor exhibits a capacitor behavior and vice-versa a capacitor Figure 2: Switching losses occur during “hard switching”, where the MOSFET transitions while voltage and current waveforms are both non-zero. Figure 3: Zero-current switching is achieved through a set of dedicated circuitry, which avoids high-frequency switching losses through specially timed MOSFET transitions.
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