The 42-V Powernet and its Influence on Automotive EMC

Martin O'Hara

A proposed change to the 42-V Powernet has far-reaching EMC implications.

The proposal to change the automotive electrical power network from 12 to 42 V has been “under development" for more than 6 years. It is the most significant change to the electrical power architecture of the automobile for more than 50 years, when the 21-V battery replaced the 6-V battery.The increase in electrical content over the last 15 years to meet exhaust emission (e.g., engine management) and safety requirements (e.g., ABS and airbag deployment) are unlikely to affect EMC as much as this proposed change. The 42-V Powernet affects all electromagnetic compatibility (EMC) standards for road vehicles and their components, including electronic subassemblies (ESAs).

The reasons for the change are well documented.1-3 However, the EMC test implications of the proposed change are much less understood. And, this latest change appears to be still far away from implementation. This article examines the EMC implications of a migration to 42-V Powernet vehicle power architecture. It includes those test issues that might be considered trivial, so that all the consequences of electrostatic discharge (ESD), radiated and conducted emissions, and radiated and conducted immunityi can be referenced from a single source.

Background

Figure 1 (click to enlarge).

The international 42-V Powernet consortium is organized primarily by the Massachusetts Institute of Technology in the United States. Through its 42-V Bordnetz group in Germany, the consortium has considered some of the EMC implications of 42-V Powernet. The scope of the standardization work was relatively limited and primarily targeted at the conducted susceptibility of 42-V systems (see Figure 1).  This organization has so far produced a working group (WG 14) under the International Organization for Standardization (ISO) technical committee 22 umbrella (ISO TC22 N2182). It has produced two working documents on the implications for transient immunity testing.^4,5  Although the working group has developed the standard up to an ISO release, it is still under review (discussion stage; DIS) and not yet published; ISO/DIS 21848-2.

ESD

With HBM unchanged, There is no for any new ESD consideration.  The existing standard ISO 10605, which was updated in 2001 was specifically developed for the automotive environment and provides for adequate ESD protection.

Similarly, there should be no implications for the design of interfaces on a 42-V Powernet ESA. Although the power lines would be at the higher level, the interfaces and communications signaling would be at voltage levels similar to currently available devices (lambda sensor, TMAP, CAN, etc.), which are not part of the proposed change).

Radiated Emissions

There should also be no need to change the standards for radiated emission tests. The levels called for in CISPR-25 and in the EU Automotive Directive (95/54/EC) should still be applicable.  These standards provide sufficient requirements and are intended to protect off-board systems and on-board radio receivers. 

The change could present some challenges for designers. The existing radiated emissions levels as the higher voltage power rail could provide higher-voltage unintentional signals, making radiated emissions compliance more difficult to achieve on a vehicle.  On the positive side, the currents distributed around the vehicle may be lower given the similar power levels per system in use today.  Although the overall electrical power requirement per vehicle is expected to increase—hence the need for a 42-V Powernet—this increase is required only for newer systems and not for existing ESAs.

Radiated Immunity 

As with radiated emissions the immunity requirements are likely to remain as they are today.  Any new system will have to be equally immune to the radiated fields encountered in the automotive environment; 30V/m for 95/54/EC and up to 200V/m for ISO 11452.

The higher voltage power rail of 42V Powernet should help with power line radiated immunity (lower currents being easier and less expensive to filter) and the other ESA interfaces will remain as they are today (e.g. road speed, MAP and Lambda sensor, Squibs etc.).  The high available voltage could even be used to increase immunity be making some functions operable from this high rail, in particular high-sided driving functions may find meeting the immunity requirements of the automotive environment easier and less costly with a 42V supply rail.

Conducted Emissions

The general levels of emissions should still be within existing standards, primarily CISPR-25 for most non-OEM specific standards.  There is an additional limit set within the second working document [5] that restricts maximum conducted emissions to 50V.  This is effectively more stringent that the existing automotive power network requirements where emissions are limited to power levels rather than absolute voltage levels.

The 50V maximum excursion (including ripple) may be a design issue for driving high voltage motors, but it ensures that inductive suppression is provided at point of use rather than at any remote control unit.  This should help with radiated emissions as well as reducing the potential cost of protection on ancillary ESA’s that are connected close to the inductive load.

The most likely potential design problem for conducted emissions will be the increase in switching supplies used around the 42V power network.  Presently low power systems operating under say 100mA (under 0.5W load consumption) will utilise linear regulators from the existing 14V power network to provide the 5V or 3V their internal logic requires, the linear regulator dissipating between 0.7W and 0.9W of wasted power as heat.  With a 42V Powernet the waste power for such linear regulators becomes close to 4W for equivalent systems and linear regulation is no longer an attractive option, a consequence of which is that switching regulator use will be increase even for low power ESA’s.  The increased use of switching regulators will not only increase switching noise into the network per se, but there could be issues of harmonic content (especially for loads relying on the battery to provide a sink for the noise) and new standards may be required to limit low frequency harmonic content due to beat frequencies between systems as well as the base switching frequency noise levels.

Figure 2 (click to enlarge).

There is already evidence that some of the highest load examples can be designed to meet the requirements of CISPR-25 at levels 2 and 3 without onerous filtering.  There have been technology demonstrators at most of the larger vehicle OEM’s using high power (500W or higher) DC-DC converters to supply 42V Powernet circuits (figure 2).  The results of these tests are published in the 42V Powernet archives and at some SAE conferences [7].  Simulations with additional filtering suggest CISPR 25:class 4 can be achieved at the DC-DC converter for 1kW loading and with switching frequencies close to 1MHz.

Consequently existing conducted emissions standards need not require revising for 42V Powernet systems.  It may be advisable to use more stringent classes of CISPR 25 for 42V Powernet  (e.g. class 3 or above only) and a new harmonic specification may be required for a full 42V Powernet network, but more development work on the full 42V implementation is needed before any new harmonic standard should be considered.

Conducted Immunity

This is the area of EMC standardisation that will obviously have significant implications with a change in the power network and the area that has received the most attention from the 42V Bordnetz working group.  The working group has taken the pragmatic approach of adopting existing standards, mainly ISO 7637 for transient immunity and ISO 16750 for electrical interruptions, and applied relative scaling factors where applicable [5].

Figure 3 (click to enlarge).

One major change that makes the ESA designers life easier is the use of a centralised load dump protection at the generator for the 42V Powernet system.  This means load dump is not scaled up relative to existing 12V and 24V system tests (re. ISO 7637 pulse 5), instead the “dynamic overload” pulse is limited to 58V on the power network (figure 3).  The testing of load dump (dynamic overload) on 42V systems is much less severe than some 12V and 24V tests where the transient can reach 86.5V and 200V respectively for severity level IV.  Another difference is that there is not a series of test severity levels; the test level is a single fixed waveform for all ESA’s on the 42V power network.

Figure 4 (click to enlarge).

Cranking profile (test pulse 4 in ISO 7637) is scaled as might be expected, with the lowest voltage point set at 18V (figure 4).  As with dynamic overload and other 42V Powernet specific pulses, the pulse is not specified for a series of severity levels but given a single fixed severity for all ESA’s on the network.  Even the dwell at low voltage (21V) after the starter current inrush is being considered as a fixed period of 10s.

Pulses 1 to 3 of ISO 7637 are not covered in the 42V working documents.  These signal types are unlikely to be present on the 42V supply network due to the more stringent requirement for supply conditioning at source (generator) and point-of-use for inductive loads. 

Table 1:  42V Powernet Ripple Test Specification

Figure 5 (click to enlarge).

The 42V working document also covers transient events more commonly associated with ISO 16750-2 for 12V and 24V systems, not always considered EMC by some designers and test houses, but electrical disturbances non the less.  Again the majority of these are scaled versions of the signals contained in ISO 16750-2 and covers ripple amplitude, immunity to sinusoidal ripple over 1kHz to 20kHz (table 1 and figure 5), fuse failure (figure 6) and micro-interruptions to the supply (figure 7).

Figure 6 (click to enlarge).

As already stated, the signal line levels for a 42V Powernet ESA are unlikely to be different from today’s’ vehicle signalling levels.  Consequently the existing signal line immunity test standards (ISO 7637-3) should still be a valid and applicable test.

Whole Vehicle Testing

As can easily be assumed from the above, there is no reason to change any of the standards applicable to whole vehicle testing. Hence ISO 10605 for ESD testing, CISPR-12 and CISPR-25 for radiated emissions, 95/54/EC for European legislative requirements for radiated emissions and immunity and ISO 11451 for immunity standards within OEM specifications. 

Figure 7 (click to enlarge).

Other Implications

The existing CISPR 25 automotive LISN/AMN (50Ω/5uH) should be suitable for 42V Powernet.  Although CISPR-25 does not specify voltage ratings explicitly, an automotive LISN is usually also designed for ISO 7637 where 1500Vdc for the capacitor is explicitly stated.  Consequently running most CISPR-25 LISN’s from a 36V battery for 42V Powernet ESA testing should not be problematic.  Test houses may need to buy more batteries for their testing, but otherwise the basic EMC test equipment (receivers, antennae, signal generators, amplifiers etc.) is unaffected by the change to 42V.

When the European automotive EMC directive is revised to include transient immunity on its power network (re. ISO 7637, [10]), this will eventually have to include requirements for 42V Powernet.  It is not going to be implemented at the next 95/54/EC revision (due late 2004) and the European automotive EMC directive will delay inclusion of 42V Powernet specific tests until 42V systems hit production.

The 42V Bordnetz group have also set up working parties considering the mechanical climatic and chemical loads for inclusion into ISO 16750-3, -4 and –5 (despite the original electrical loads being rejected from inclusion in ISO 16750-2).  The group is also considering short circuit and other load conditions and can be found at www.sci-worx.com, follow links to “Partners” and “Forum”.

Conclusion

In general the change to 42V should not be a major issue with respect to EMC testing, the greatest challenges are going to be for the designers to meet many of the existing standards with higher voltages on their power leads.  Immunity is likely to be marginally assisted by the change, it will be the emissions from ESAs (both radiated and conducted) that have the greatest potential to fail existing EMC standards.

The most significant EMC change occurs in the conducted immunity testing. Use of a centralised load dump should assist with removing a significant amount of the transient protection requirement from existing ESA designs.  The removal of multiple severity levels will also make test design-for-test decisions easier.  There may need to be some updating of transient generating equipment to handle the higher DC level, but much is already available from the few manufacturers of automotive transient generators.

The most significant implication of the change to 42V Powernet is the same for EMC as it is for most of the automotive electronics industry; trying to predict when volume production will commence and hence when to invest in the necessary test equipment to support 42V products.  It appears that if anything progress has slowed significantly over the last 3 years or so as the lack of an economic benefit from the change continues to be a greater factor than any perceived performance improvement [9].  Systems that require 42V to be implemented such as integrated starter-alternator (ISA) and electric power valve train (EPVT) are still struggling to get production acceptance in an industry that is traditionally conservative and extremely cost conscious.

Acknowledgements 

Dr.-Ing. Hans-Dieter Hartmann at Sci-worx GmbH has been the prime advocate for pushing forward the EMC issues of 42V Powernet and deserves the credit for much of the work on standards reported here.

This article is based on the paper “The EMC Implications of 42V Powernet” [8] presented by the author at Automotive EMC 2003 Conference, Milton Keynes, UK (www.autoemc.net).

References

[1]  Automotive Electrical Systems circa 2005, J.Kassakian, IEEE Spectrum, pp 22-27, August 1996 

[2]  42V Powernet: The First Solutions, Villach, 28-29 September 1999. 

[3]  42V Powernet Enabling Technology: Overview, Peter Hartnett, Martin O’Hara and Peter Miller, IEE Seminar, Passenger Car Electrical Architecture, 21 June 2000. 

[4]  Road Vehicles – Conditions for electrical and electronic equipment for a 42V powernet – Part 1: General, WGS/WD 03/2000-1, April 2000. 

[5]  Road Vehicles – Conditions for electrical and electronic equipment for a 42V powernet – Part 2: Electrical loads, WGS/WD 03/2000-2, April 2000. 

[6]  42 Volt Technology and Advanced Vehicle Electrical Systems, Arthur Pfaelzer and G. Sadler Bridges, SAE Publications, August 2001, ISBN 0-7680-0830-1

[7]  Standardization of the 42V PowerNet, Wolfgang Bremer, SAE World Congress 2002, 42-Volt Standards - A Global Opportunity, 6^th March 2002.

[8]  The EMC Implications of 42V Powernet, Martin O’Hara, Automotive EMC 2003, p125-129, 6^th November 2003.

[9]  Vehicle 42 Volt Electrical Systems, Peter Hartnett, IMechE and IEE Seminar, Cranfield University, 20th January 2004.

[10]  Automotive EMC Directive Update, Martin O’Hara, EMC & Compliance Journal, p19-23, Issue 53, July 2004.

Martin O'Hara is with Telematica Systems Ltd, Trafficmaster PLC (Cranfield, UK). He can be reached via e-mail at martin@tmsl.com.