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Rohm adds isolated DC-DC converters for xEV


Contributes to smaller mounting area in isolation circuit by 30 percent compared with standard products

Rohm has recently developed an isolated flyback DC-DC converter, BD7Fx05EFJ-C (BD7F105EFJ-C, BD7F205EFJ-C) optimised for gate driver power supplies in xEV applications such as electric compressors, and PTC heaters.

xEVs, which primarily use electricity as their energy source, are equipped with unique applications such as, electric compressors for AC, and PTC heaters to raise the cabin temperature. And as these applications are driven by high voltages, it is necessary to isolate the primary circuit that includes the battery from the secondary circuit comprised of the motor and other systems to ensure safety.

However, in addition to issues such as larger mounting area and power consumption, conventional isolated circuit configurations require considerable noise countermeasures for switching frequencies that vary with output current.

Rohm’s new product utilises a circuit configuration that achieves stable switching frequency characteristics without requiring a photocoupler, decreasing application size together with workload for noise design countermeasure. Rohm uses it analogue design technology so the BD7Fx05EFJ-C has no need for photocouplers, transformer auxiliary windings, and peripheral components conventionally necessary to detect secondary-side voltage and current.

On top, next to solving issues with photocouplers, such as large power consumption, fluctuating detection accuracy due to temperature, and deterioration over time, Rohm says it has also achieved greater miniaturisation by reducing the number of components. As a result, it is possible to reduce the number of components – including the photocoupler – used for current detection by 10 (equivalent to a board area of 30 percent) compared to general isolated flyback power supply circuits.

The BD7Fx05EFJ-C is also equipped with an adaptive ON-time control function that fixes the switching ON time, ensuring a stable frequency of around 350kHz regardless of output power. To be compliant with the CISPR25 automotive EMC standard requires considerable noise design in the frequency range from 150kHz to 300kHz, but as Rohm’s new converters do not fall within this band, noise countermeasures can be greatly simplified. This, together with a spread spectrum function that minimises radiated noise, contributes to reducing the number of efforts for noise design.

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