Isolated DC/DC converters for xEV slash development effort for noise suppression

Isolated DC/DC converters for xEV slash development effort for noise suppression

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By Christoph Hammerschmidt

Rohm offers the BD7Fx05EFJ-C series of isolated DC/DC flyback converter ICs for gate driver power supplies in electric vehicles that must be isolated from the battery for safety reasons. Examples include electric compressors and PTC heaters.

Battery, hybrid and fuel cell vehicles will (xEVs) primarily use electricity as an energy source. They are therefore equipped with special applications such as electric compressors for air conditioning and PTC heaters for the vehicle interior. These applications are operated at high voltages. To ensure safety, the primary circuit (contains the battery) must be separated from the secondary circuit (includes the engine and other systems). In addition to challenges such as larger mounting area and higher power consumption, conventionally isolated circuit configurations also require extensive noise suppression measures at switching frequencies that vary with output current.

Rohm’s new isolated DC/DC converters use a circuit configuration that achieves stable switching frequencies. This reduces the size of the application and the design effort required for noise suppression. Thanks to Rohm’s proven analogue design technology, the BD7Fx05EFJ-C series eliminates the need for optocouplers, transformer auxiliary windings and peripheral components typically required to detect secondary-side voltage and current. In doing so, the manufacturer has not only solved the various challenges of optocouplers, such as high power consumption, fluctuating detection accuracy due to temperature changes and ageing over time. By reducing the number of components, greater miniaturisation has also been achieved. This makes it possible to reduce the number of components used for current detection – including the optocoupler – by a factor of 10 compared to general isolated flyback converter power supply circuits. This reduces the footprint of such a circuit on the board by 30 %.

Switching frequency is independent from load

The BD7Fx05EFJ-C series is also equipped with adaptive turn-on timing, which ensures a stable frequency of about 350 kHz regardless of the output power. Compliance with the CISPR25 EMC standard for motor vehicles requires extensive noise suppression measures in the frequency range from 150 kHz to 300 kHz. However, since Rohm’s new converters do not fall within this band, noise suppression measures can be greatly simplified. Together with a spread spectrum function that minimises the radiated noise, this reduces the development effort.

The BD7Fx05EFJ-C series ICs incorporate multiple protection circuits as well as an output voltage stabilisation function. This not only reduces the number of components and the amount of work required for noise suppression measures, but also improves the reliability of the application.

The devices are available immediately from series production. They are initially available through Rohms online distributors such as Digi-Key, Mouser and Farnell; distribution through other online channels is planned.

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