SLYY243 February   2025

 

  1.   1
  2.   Overview
  3.   At a glance
  4.   Introduction
  5.   48V in MHEVs vs. BEVs
  6.   Reducing the wire harness
  7.   48V architectures
  8.   48V design challenges
  9.   Conclusion

48V design challenges

The design challenges when adopting a 48V low-voltage rail include transient voltages, creepage and clearance requirements, electromagnetic compatibility (EMC) standards, and integrated circuit (IC) costs.

Transient voltages are the main topic of conversation in 48V systems. Today, 12V systems are well known, with standards such as International Organization for Standardization (ISO) 16750-2 specifying voltage transient curves for worst-case events such as load dumps. For 48V systems, the standards available today (ISO 21780 and Liefervorschriften [LV] 148) were written for MHEVs requiring overvoltage points up to 70V. But when factoring for switching transients or component margin, it results in component ratings much greater than 70V.

The standards for MHEVs, while useful as a starting point, are not necessarily valid for an electric or hybrid system generating 48V off the high-voltage battery without a high-power starter-generator system. The exact standards around a BEV 48V low-voltage net are still being defined, but OEMs may begin defining their own standards to contain line transients below 70V. Figure 8 compares a potential BEV standard to the existing ISO 21780 standard.

Potential BEV standard and ISO 21780 transient voltage comparison.Figure 8 Potential BEV standard and ISO 21780 transient voltage comparison.

While the difference between 60V and 70V may seem small, the cost of ICs to accommodate higher voltages doesn’t necessarily scale linearly. Also, even if it were possible to contain the supply ranges, it’s important to consider the potential for harness failure-mode events, which current standards such as ISO 7637-2 do address.

Creepage and clearance requirements are industry-standard measurements of the shortest distance between all conductive parts on the PCB. They are critical design parameters for preventing arcing, which occurs when the voltage between two points exceeds the breakdown voltage. There are many different standards for creepage and clearance (International Electrotechnical Commission 60664-1 and Institute of Printed Circuits 2221A) and OEMs may even have their own internal guidance. Moving from 12V to 48V increases creepage and clearance requirements, directly impacting IC packages, PCB layout, wire harness connectors and more.

A subtler impact of 48V systems is that while they help reduce conduction losses, switching losses increase. This becomes relevant in EMC testing for switching power converters such as DC/DC converters and motor drives. Increasing the voltage (VDS) from 12V to 48V allows the current (IDS) to reduce. However, if the slew rate (tR + tF) in 48V systems remains the same as 12V systems, then the power switching losses (PSW) quadruple.

While there are more factors that impact switching losses, Figure 9 illustrates how slew rate impacts switching losses in 48V systems. For more information on reducing conducted emissions in DC/DCs, see the application note, “Reducing Conducted EMI in a Buck Converter for 48V Automotive Applications.”

Switching-loss impact on EMC.Figure 9 Switching-loss impact on EMC.