The EMC Implications of 42V Powernet

Transcription

1 The EMC Implications of 42V Powernet Martin Telematica Systems Ltd, Trafficmaster PLC, University Way, Cranfield, MK43 0TR, England Abstract: The proposed changing of the vehicle primary power system from a 14V power network (12V battery) to a 42V Powernet (36V battery) system has implications for the EMC of vehicles and their components. This paper examines where the existing EMC standards may be affected and which will need revising or updating for the new 42V Powernet system, if and when it is implemented. Introduction The proposed change of the automotive electrical power network from 12V to 42V has been "under development" for over five years and still appears to be as far away from full production implementation as it has been at any time since its conception. The reasons for the proposed change are well documented elsewhere [1,2], but what is less well documented (although considered in some papers at specific 42V conferences) is the EMC implications of the proposed change. This is the most significant change to the electrical architecture of the vehicle for over 50 years (when the 6V battery was replaced by the 12V battery). Even 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) may not have the impact that the change to 42V Powernet has on the overall EMC of a automobile and its component. Background The international 42V Powernet consortium, primarily organised by MIT in the US, have considered some of the EMC implications of 42V Powernet via the 42V Bordnetz group in Germany. The scope of the standardisation work was relatively limited and primarily targeted at the conducted susceptibility of 42V systems (figure 1). This organisation has so far produced a working group (WG14) under the ISO technical committee 22 umbrella (ISO TC22 N2182) and has produced two working documents on the implications for mainly transient immunity testing [4,5]. Figure 1: Scope of 42V Standardisation Working Group on EMC The working group have developed the standard up to an ISO release, but this is still under review (committee draft stage) and not yet published (ISO/CD , due 2003). This paper examines the EMC implications of a migration to 42V Powernet, including those that might be considered trivial, so that all the consequences on ESD, radiated and conducted emissions and immunity, can be referenced from this single paper. ESD There is no need for any new consideration for ESD as the human body model is obviously not changed. The existing standard (ISO 10605) is perfectly adequate. Radiated Emissions Again there should be no need to change the standards for radiated emission tests; the levels in EU directive 95/54/EC and CISPR-25 should still be applicable. The radiated emissions standard is intended to primarily protect off-board systems and on-board radio receivers, hence the levels already set need not be changed. There could be some challenges for the designers with the existing radiated emissions levels: a higher voltage rail and potentially higher voltage unintentional signals will make EMC compliance more difficult to Automotive EMC 2003 Page 1 6 th November 2003

2 achieve in a vehicle. On the plus side the currents distributed around the vehicle may be lower per system for similar power levels used today. Although the power requirements are expected to increase, hence the need for a 42V Powernet, these are required for newer systems and not existing ESA s that should mainly remain at today s power levels. 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 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.). 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 higher 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 logic requires, the regulator dissipating the 0.7W to 0.9W of wasted power in heat. With a 42V Powernet the waste power for such linear regulators becomes up to 4W for a similar system and linear regulation is no longer an attractive option, a consequence of which is that switching regulator use will be increase, even for a low power ESA. The increased switching regulator use 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 could be required on the low frequency harmonic content as well as the base switching noise levels. 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 prove functional 42V Powernet circuits (figure 2). The results of these tests are published in the 42V Powernet archives (although some only in presentation format, not in written papers) and at other SAE conferences [7]. Simulations with additional filtering suggest CISPR 25:class 4 can be achieved even for 1kW loads and switching frequencies near to 1MHz. Consequently existing conducted emissions standards need not require revising significantly. 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 Figure 2: Measured Conducted Noise from 500W 14V-42V bi-directional DC-DC converter. Note: poor results at higher frequency are attributed to use of incorrect RBW. Automotive EMC 2003 Page 2 6 th November 2003

3 required for a full 42V Powernet network, but more development work on the full 42V implementation is needed before any new standard should be considered. Conducted Immunity This is an area that will obviously have significant implications with a change of the power network and the area that has received 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 for electrical interruptions, and applied relative scaling factors where applicable [5]. 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). Consequently 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 level IV. Another difference is that there is not a series of test levels; the test level is a single fixed waveform for all ESA s on the 42V power network. Figure 3: 42V Dynamic Overload Waveform Figure 4: 42V Cranking Waveform 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 test levels but given a single fixed test 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 (working document still specifies 0.5s to 20s). 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. The same argument may be used to exempt 42V systems from the signal coupling tests used in ISO for signal line immunity. Figure 5: 42V Sinusoidal Ripple Waveform Table 1: 42V Powernet Ripple Test Specification Min Freq Max Freq Max Ripple Test 1 50Hz 1kHz 4V pk-pk Test 2 1kHz 20kHz 1V pk-pk The working document also covers transient events more commonly associated with ISO for 12V and 24V systems, not always considered EMC by Figure 6: 42V Fuse Failure Supply Interruption Automotive EMC 2003 Page 3 6 th November 2003

4 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 and covers ripple amplitude, immunity to sinusoidal ripple over 1kHz to 20kHz (table 1 and figure 5), fuse failure (figure 6) and microinterruptions to the supply (figure 7). 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 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 for immunity standards within OEM specifications. Figure 7: 42V Micro-Interruption Reset Waveform Other Implications The existing CISPR 25 automotive LISN/AMN (50Ù/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), this will have to at some point include requirements for a 42V Powernet. It is unlikely to be implemented at the next 95/54/EC revision and the European automotive EMC directive will probably 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 , -4 and 5 (despite the original electrical loads being rejected from inclusion in ISO ). The group is also considering short circuit and other load conditions and can be found at 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 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 units (both radiated and conducted) that have the greatest potential to fail existing 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 in existing ESA s. 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 electronics industry; trying to predict when volume production will commence and hence when to invest in the necessary test equipment to support the automotive industry. 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. Automotive EMC 2003 Page 4 6 th November 2003

5 References [1] Automotive Electrical Systems circa 2005, J.Kassakian, IEEE Spectrum, pp 22-27, August 1996 [2] 42V Powernet: The First Solutions, Villach, September [3] 42V Powernet Enabling Technology: Overview, Peter Hartnett, Martin O Hara and Peter Miller, IEE Seminar, Passenger Car Electrical Architecture, 21 June [4] Road Vehicles Conditions for electrical and electronic equipment for a 42V powernet Part 1: General, WGS/WD 03/2000-1, April [5] Road Vehicles Conditions for electrical and electronic equipment for a 42V powernet Part 2: Electrical loads, WGS/WD 03/2000-2, April [6] 42 Volt Technology and Advanced Vehicle Electrical Systems, Arthur Pfaelzer and G. Sadler Bridges, SAE Publications, August 2001, ISBN [7] Standardization of the 42V PowerNet, Wolfgang Bremer, SAE World Congress 2002, 42-Volt Standards - A Global Opportunity, 6 th March Automotive EMC 2003 Page 5 6 th November 2003