Downsized powder choke coil helps boost battery charging in PEVs/EVs

July 16, 2013

The increasing use of hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles (PHEVs), and electric vehicles (EVs), has made power conversion equipment for these vehicles as important as the electric motors and batteries. The equipment includes boost converters to enhance electric motor output in HEVs, buck converters to step down the main battery voltage to 12 V in HEVs and EVs, and AC-DC converters to charge the main battery in PHEVs and EVs.

The power inductors, also called reactors or choke coils, are used extensively in switching power supply such as voltage conversion and energy transmission, and are made up of magnetic cores with copper wire windings. The magnetic core is said to determine the size and performance of a power inductor and power conversion equipment.

SEI-fig-1

Fig.1 Shapes of E-type choke coils using powder and ferrite core designs. (X. Zheng, etal. SEI Technical Review No,75, 2012, pp55-61)

Sumitomo Electric Industries Ltd in Japan has been developing and producing pure iron based powder cores with an operating frequency range from 10 kHz to 30 kHz for boost converter reactors in hybrid-electric vehicles (HEVs), and also low-loss Fe-Si-Al alloy powder cores with an operation range of several hundred kHz. The alloy powder cores have shown the potential to replace ferrite cores for buck converter choke coils. The company’s low loss alloy powder cores are also said to be a competitive alternative for choke coils in plug-in HEV and EV on-board chargers, which generally operate in a range from 50 kHz to 200 kHz.

A recent development published by Sumitomo Electric researchers in SEI Technical Review (X. Zheng, etal. No,75, 2012, pp55-61) involves downsized low loss alloy powder E-type choke coils for PHEV/EV battery chargers. In one case cited by the researchers, the high saturation flux density of the alloy powder allowed the powder choke coil to be reduced in size by 34% compared with a conventional ferrite E-type choke coil, giving a weight saving of 45% (see Table).

sei-table-1

Table1 Results of ferrite core design and powder core design (X. Zheng, etal. SEI Technical Review No,75, 2012, pp55-61)

However, the downsizing of the conventional E-type choke coil design led to an increase in heat density with poor heat dissipation. Sumitomo Electric researchers overcame the problem by designing a new rectangular shaped choke coil comprising two simple shaped cores as shown in Fig.2. This allows the choke coil to be even smaller than the irregular shape E-type coil, but with better heat dissipation. The simple core shape of the new design choke coils is said to be easy to manufacture by compacting, and the coil can be wound automatically.

SEI-fig-2

Fig.2 Top: New shape choke coil developed by Sumitomo Electric Industries, and Bottom: cross sectional view of new coil shape. (X. Zheng, etal. SEI Technical Review No,75, 2012, pp55-61)

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