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EETE NOV 2015

Automotive Electronics Fig. 3: Full-hybrid vs. mild-hybrid systems. The battery size is one main difference. On the other hand, the heavier plug-in hybrid solution will remain to offer the opportunity of a drive by electricity only on some 10 to 50km at a moderate speed with the best CO2 reduction (globally up to -45%). This is enough for most of the short trips around home. For such apparatus, different power electronic topologies like ZVS (zero voltage switching) or LLC (inductor-inductor-capacitor tank) resonant half- or full-bridge can be used. Both enable the development of high efficiency converters (> 90%). The switching frequency is commonly set in the 70-350kHz range to optimize also the size and losses in semiconductors. Every power supply of this kind requires a set of magnetic components dedicated to power conversion, filtering, EMI cleaning, feedback and control loop… It appears that their total weight is approximately one-third of the final electronic equipment (also in volume and cost). For instance, a 3kW onboard battery charger is already something like an A4 page by 50-60mm in height with approximately 1kg of magnetics inside. Thus, more powerful systems can become an issue because of their additional weight to the already heavy HV batteries set... A compromise has permanently to be found for global pollution reduction. Inverters and DC/DC battery chargers for electric and hybrid vehicles require minimum space and maximum power handling per cubic centimeters. The size ratio of every electronic component has consequently to be well optimized for an as efficient as Fig. 5: Example of customized magnetic power components technology and mounting onto a cold-plate heatsink. possible cooling when thermally connected to a cold-plate normally made of a heatsink with a coolant flowing inside the outer metallic case (its maximum temperature is generally stated at +85°C). This is particularly true for the magnetic components which show relatively high power losses (10-50W) in a reduced volume (close to only 15-100cm3). Their temperature rise is sometimes monitored to secure the equipment by a reduction of the output power not to exceed the maximum allowed operating temperature (for example 150°C with a possible ambient temperature up to +125°C for electronics mounted close to the thermal engine). Moreover at the voltage level conversion, transformers also have to provide isolation from hazardous voltages (mains network, high-voltage from the battery) to prevent any damage to the end-user that can be directly in contact with many metallic parts inside the cars (the chassis acts like the ground or earth at 0V reference). Inductors are used in a wide range of topologies from storage to filtering operations. The most powerful ones have to withstand a current as high as 300Adc at the output of a DC/DC 3.5kW 12V converter. In addition to the main power transformer and inductors which are the heart of the topology, we find additional magnetic components of importance like the PFC (power factor correction) chokes that are mandatory for quasi-sinusoidal and in-phase current absorption from the mains, CMCs (common mode chokes) and DMCs (differential mode chokes) which complete the EMI filter, current measurement transformers for regulation or safety purpose, gate-drive transformers to pilot the MOS or IGBT transistor. In conclusion, so many magnetic components are involved to fulfill the required power electronic module performances that it cannot be achieved without a good optimization of every design brick. Experience in the field and accurate knowledge not only in the raw materials behavior (ferrite, powder or tape wound cores, conductors, insulators…) versus frequency, temperature, mechanical stress… but also in the way to combine and assemble them together is necessary to come up with reliable solutions. Fig. 4: Example of a ZVS phase-shift full-bride topology that involves three power magnetics in a DC/DC 4kW converter. 24 Electronic Engineering Times Europe November 2015 www.electronics-eetimes.com


EETE NOV 2015
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