E-Mobility Engineering | January/February 2024 29 H3X | Dossier and marine applications due to the higher EMI and reliability requirements specified by aviation authorities. While many regulations are published by the FAA and EASA, aerospace EMI limits are set by the standard DO-160, published by the Radio Technical Commission for Aeronautics in the US. “DO-160 was made many years ago for lower-voltage electronics, not high-voltage power systems, and its interference requirements stemmed from aircraft communicating over sensitive AM radios. Because of SiC’s high switching speeds, it’s very hard to get EMI levels down to a value that passes DO-160,” says Sylvestre. Integrating the inverter with the motor has been key to H3X realising SiC’s full potential while achieving compliant EMI performance. Mounting the inverter as a separate but still regulation-compliant device would have mandated heavy and loss-prone filters on the system’s input and output, Sylvestre explains. One reason for this is that the separated motors and inverters are often connected using long, shielded phase cables. “Those cables end up working like a transmission line; they can induce substantial ringing [oscillation] in the motor winding, leading to insulation degradation and additional EMI. They also provide increased surface area for noise currents to potentially escape. The phase cables can carry a very high common-mode voltage, and when combined with the capacitance of the shielding, this can result in more difficulty preventing commonmode current from egressing on to other circuits within and beyond the powertrain,” Sylvestre says. Integrating the motor with the inverter reduces the phase-connection distance to a few centimetres, while minimising the engineer’s EMI filtering and containment burden. It also enables lightweight EMI filters to be integrated in a very tight manner between the motor and the inverter. As a result, so far, H3X’s dyno, iron bird and integrated vehicle tests have not turned up any EMI-related functional issues. “That might not sound like a big deal, but it’s really challenging to get SiC inverters to not wreak havoc on neighbouring systems,” Liben explains. “Some of our customers have tested other SiC inverters in their platforms with disastrous results and sometimes weeks of troubleshooting to even get a working powertrain out of them.” Mitigating SiC EMI issues Also important is the management of sources of parasitic impedance across the e-powertrain. “For instance, the DC link capacitor and busbar inductance directly influence the switching overshoots and ringing in your SiC devices, which in turn lead to EMI, so we’ve designed a very low-inductance bus structure, allowing us to operate with very low overshoot and ringing,” Sylvestre says. “That overshoot reduction allows us to push our bus voltage up to 900 V while still using 1200 V switching devices. As another example, parasitic capacitance between the switch node and the grounded low-voltage [GLV] system has to be managed, so we install tightly coupled filters – which is possible thanks to our integrated approach – to create a very low-inductance pathway for common-mode current to travel back to its source. That helps prevent situations where common-mode currents egress into the GLV and control circuitry.” The most common strategy for mitigating SiC EMI issues is to intentionally reduce switching speeds by increasing gate resistance. This scales back MOSFET-level problems such as overshoot and ringing, but it eats away at the efficiency and power-density benefits of using SiC in the first place. H3X claims it can often run its systems with zero external gate resistance, meaning the transistors switch as fast as physics allow. H3X also claims it has observed dv/dt (the rate at which voltage changes over time) and di/dt (the rate at which current changes over time) as high as 40 V/ns and 35 A/ns, respectively, during double-pulse testing. “Our grounding and shielding strategies are very meticulous and The gearbox in the HPDM-30 and -250 is an epicyclic (or planetary) gear system, sealed with oil for its lubrication and cooling
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