Susceptibility to EMI of a Battery Management System IC for Electric Vehicles

Transcription

1 Susceptibility to EMI of a Battery Management System IC for Electric Vehicles Orazio Aiello Automotive Business Unit, NXP Semiconductors Gerstweg 2, 6534 AE Nijmegen, Netherlands orazioaiello@nxpcom Paolo S Crovetti, Franco Fiori Electronics Dept, Politecnico di Torino Corso Duca degli Abruzzi, 24, I Turin, Italy paolocrovetti@politoit, francofiori@politoit Abstract The susceptibility to Electromagnetic Interference (EMI) of Battery Management Systems (s) for Li-ion and LiPo battery packs employed in emerging electric and hybrid electric vehicles is investigated in this paper To this purpose, a specific test board is developed to experimentally assess the EMI susceptibility of a front-end integrated circuit by direct power injection (DPI) and radiated susceptibility measurements Experimental results are discussed highlighting different EMIinduced failure mechanisms observed during the tests Index Terms Battery Management System (); Li-ion Battery Pack; Electric Vehicles (EVs); Hybrid Electric Vehicles (HEVs); IC-level EMC; Susceptibility to Electromagnetic Interference (EMI); Direct Injection (DPI); Anechoic Chamber I INTRODUCTION Battery packs based on the most advanced Lithium-ion (Liion) and Lithium-Polymer (LiPo) electrochemical technologies are nowadays the only viable options to address the challenging demands in terms of electric energy storage and deliverable power per unit mass of electric vehicles (EVs) and hybrid electric vehicles (HEVs) Unfortunately, unlike leadacid batteries and other more conventional electrochemical accumulators, Li-ion and LiPo cells can be permanently damaged and can also originate life-threatening hazards like fires and explosions in the event of overdischarging, overcharging and/or overtemperature operation [1] An electronic battery management system (), which timely detects the onset of dangerous conditions and takes the appropriate countermeasures to avoid hazards, is therefore necessary to safely operate Li-ion and LiPo cells in vehicles [1] A, which is schematically depicted in Fig1, typically includes several front-end modules, that acquire critical cell information like terminal voltages and temperatures, and a digital control unit that runs specific control and management algorithms Smart power integrated circuits (ICs) employed in the frontend ICs of a for EV/HEV applications [2] have stringent requirements in terms of accuracy and operate in a very harsh electromagnetic environment [3], where relevant radiated and conducted interference are generated by the electric powertrain, by on-vehicle electronics and by mobile phones and/or other information and communication equipment carried by the driver and by the passengers in the cockpit Such disturbances can be easily picked up by the long wires that connect front-end ICs to each other, to the control unit, to Battery Pack Li-Ion Cells Li-Ion Cells Control Unit Li-Ion Cells Li-Ion Cells Li-Ion Cells CAN Bus Data/CTRL Bus Safety switch Fig 1 Architecture of a Battery Management System () for EV/HEV applications the terminals of the electrochemical cells and to the temperature sensors, which are spatially distributed over the whole battery pack module [4], and can easily impair the operation of data acquisition circuits [5]-[6] and digital transceivers for binary communications between the front-end ICs and the control unit [7] As a consequence, the susceptibility to electromagnetic interference (EMI) of the front-end ICs can be regarded as the major potential cause of critical EMI-induced failures in the of an EV/HEV and could be a major threath to the safety of emerging electric vehicles Taking into account of the typical application scenario in Fig1, the EMI susceptibility issues of systems for electric vehicles are addressed in this paper on the basis of the results of direct power injection (DPI) tests (IEC standard [8]-[9]) and radiated susceptibility tests (ISO standard [10]) on a commercial smart power IC, widely employed as a front-end in EV/HEV applications In particular, the specific susceptibility level of the main IC pins is highlighted and different EMI-induced failure mechanisms are observed The effectiveness of filtering techniques, that can be adopted to enhance the immunity to EMI of a, is also discussed The paper is organized as follows: in Section II, the V+ V-

2 To DC Supply + SMA To DC Connector Filters Via Supply - to GND Cinj+ 100nF Cell #6 { plus Cinj Cell #6 minus 100nF Cells Inputs Via Inj Points to GND Cells Inputs To To the Cells Fig 2 EMC Test Board Via to GND V+ V- IC Analog MUX DUT 5V Linear Regulator Voltage Reference ADC CTRL Inj Point SMA Connector control unit Simplified schematic diagram of the IC DPI Test Board IC under test is presented and the printed circuit board (PCB), developed to perform EMC tests, is introduced; in Section III the test bench and the test procedure adopted for DPI measurements are described and then, in Section IV, DPI test results are presented Moreover, the results of radiated susceptibility measurements are presented and discussed in Section V and finally, in Section VI, some concluding remarks are drawn II THE FRONT-END UNDER TEST In order to investigate the susceptibility to EMI of a for electric vehicles, the susceptibility to EMI of a smart power front-end IC, which is widely employed in EV/HEV applications, is discussed in this paper on the basis of DPI immunity tests and on radiated susceptibility tests In this section, the specific features of such a IC are introduced and the test PCB specifically designed to perform DPI tests, is described A Front-End IC Structure and Operation The simplified block diagram of the front-end IC, which is considered in what follows as the device under test (DUT), is reported in the pink box of Fig2 Such an IC is designed to monitor the terminal voltages of up to twelve series-connected electrochemical cells by a 12-bit (effective number of bits, ENOB) Sigma-Delta Analog-to-Digital Converter (ADC) To this purpose, the IC includes 12 cell input pins, internally connected to the ADC differential input by a 12-channels isolated high-voltage analog multiplexer The IC is designed to be operated by an external control unit via a serial peripheral interface (), through which acquired data can be also retrieved The same interface can be employed to connect the IC ton similar devices in a daisy chain structure as shown in Fig1, so that to monitor up to 12N cells, addressing the requirements of high voltage battery packs including 100 or more series-connected cells The frame of the specific protocol implemented in the DUT includes a Packet Error Code (PEC), by which bus errors can be detected on the basis of the received data { The IC operates from a DC supply voltage ranging from 10V up to 55V, which can be obtained either from the electrochemical cells to be monitored or from an external isolated DC/DC converter, and includes an internal voltage reference for the ADC and a 5V linear voltage regulator to supply the low voltage analog and digital circuitry B DPI Test Board A dual layer test PCB, whose simplified schematic is reported in Fig2, has been developed to test the susceptibility to EMI of the DUT by the DPI method [8]-[9] Taking into account of the structure in Fig1, the test board is designed to inject RF power into the IC pins which are connected to (possibly long) external wires that are likely to collect a relevant amount of EMI in a realistic EV/HEV application Following this guideline, two DPI injection points are included in the test PCB in correspondence of a couple of cell input pins and in correspondence of the pins of the IC, as depicted in Fig2 The cell input injection point includes an SMA connector and two RF coupling networks (each made up of a 68nF capacitor and a 6µH inductor), designed so that to inject RF power onto the cell input pins connected to the positive and to the negative terminals of the cell #6 in the stack, as depicted in Fig2 The cell input injection network is designed so that two RC filters with R = 100Ω, C = 100nF implemented using 0603 SMD components, can be connected immediately before the IC pins as depicted in Fig2 The injection point includes an SMA connector and four RF coupling networks (each made up of a 1nF capacitor and a 6µH inductor), designed to perform DPI on all the inputs at the same time or, alternatively, on a single line III DIRECT POWER INJECTION TESTS In this section, the test bench and the procedure followed to perform DPI tests on the DUT are introduced In particular, the failure criteria considered in the DPI tests, are stated and motivated A DPI Test Bench The immunity to EMI of the IC mounted in the test PCB in Fig2 has been tested by the DPI setup in Fig3 Here, the DUT is operated from an external 20V power supply and the cell inputs of the DUT are connected to six seriesconnected commercial Nickel Metal Hydride (NiMH) cells mounted on a separate board and tied to the DUT cell inputs by twisted wires The current flowing through the cells during the DPI test is monitored by an ammetter as shown in Fig3 The lines of the DUT are connected to an automotive microcontroller evaluation board, which acts as the control unit shown in Fig1 Such a control unit includes a Controller Area Network (CAN) bus interface, that is connected to a personal computer (PC) via a Vector CANCaseXL dongle The microcontroller on the control unit is programmed so that to forward the data content of the CAN packets with a specific identifier (ID) to the DUT

3 GPIB RF generator PC Fig 3 Ammeter A Cells USB Cells Inputs Inj Points EMC Test Board DUT RF Amplifier CANcaseXL Vector Inj Points Reflected meter C Control Unit Incident DC Supply CAN CAN BUS Direct Injection Experimental Test Setup and to send back to the CAN bus, with a different ID, the data retrieved from the DUT during the same transaction In this way, the DUT can be fully operated and monitored from the PC via the CAN bus An RF source connected to a 10W RF power amplifier operating in the 1MHz-2GHz bandwidth is employed to perform continuous wave (CW) RF power injection The output of the power amplifier is fed to the injection points of the test board through a -20dB directional coupler, whose forward and reverse coupled ports are connected to an RF power meter so that to monitor the incident and the reflected power The RF source, the ammetter and the RF power meter are connected to the same PC employed to control the DUT via a General Purpose Interface Bus (GPIB) dongle Both the CAN bus and the GPIB are fully controlled by the PC in the Matlab environment B DPI Test Procedure During DPI tests, the DUT is operated from the PC in Fig3 so that to periodically acquire the voltages of all the cells in the battery pack The acquired data are then retrieved from the DUT by the control unit and forwarded to the PC via the CAN bus, together with the corresponding PEC code The same operations are first performed without injecting RF power (ie with the RF source in Fig3 off) and then injecting RF power at a given test frequency The data retrieved from the DUT, with and without EMI injection, are finally compared and an EMI-induced failure is recorded if one of the following conditions occurs: 1) an transmission error is detected (ie the received PEC is not consistent with the received data); 2) an EMI-induced offset in the cell voltages acquired with RF injection exceeding an error thresholdv T is detected Taking into account of the accuracy level that is required to safely manage Li-ion cells [1] and of the declared maximum error of the IC, an error threshold V T = 10mV is considered in this paper Considering the above failure criteria, DPI tests have been performed for each test frequency in the 1MHz-2GHz bandwidth 1 by increasing the injected RF incident power until a failure is detected The minimum RF incident power giving rise to the failure is then reported as the DPI immunity level at the test frequency If no failure is detected at the maximum test incident power P max = 37dBm, no immunity level indication is reported The results of DPI tests performed according with the above procedure are presented in the next Section IV DPI EXPERIMENTAL RESULTS The results of DPI immunity tests performed on the DUT cell inputs and injection points in Fig2, according with the procedure outlined in the previous Section, are reported and discussed in what follows In particular, the different failure mechanisms observed during DPI tests are highlighted and the effectiveness of PCB level filtering on cell inputs is verified A Cell Inputs Injection The immunity level of the DUT undergoing DPI on the cell input injection point of Fig2 is first considered To this purpose, two different DPI excitations have been compared: differential (DM) excitation, performed by connecting only the injection capacitor C + inj to the signal terminal of the SMA connector in Fig2, and common-mode (CM) excitation performed by connecting both the injection capacitorsc + inj and C inj to the SMA signal line Both the tests have been repeated without the RC filters in Fig2 (ie using 0Ω 0603 SMD resistors and not mounting the filter capacitors) and with the 100Ω - 100nF RC filters prescribed by the IC manufacturer The results of DPI measurements are reported in Fig4 It can be observed that the DPI immunity level of the DUT without filters is very similar for CM and DM injection and is rather low (from 5 to 15dBm) over the 1MHz-1GHz bandwidth As such, EMI-induced failures do not seem to be specifically related to EMI superimposed onto the differential cell voltage to be acquired, but rather to other mechanisms involving the RF voltage between each test pin and the IC reference (ground) voltage By comparing the results obtained with and without RC filters, it can be concluded that RC filters provide a significant immunity enhancement in the 20MHz-600MHz band (no failure is experienced at the 37dBm test level), whereas their effectiveness is lower above 600MHz This can be motivated considering that the impedance of the 0603, 100nF capacitor of the filter, dominated above 20MHz by the parasitic inductance ESL 1nH, in series with the PCB track and via inductance (L track 1nH and L via 1nH), gives rise to a parallel resonance with the input capacitance of the IC (C IN 15pF from S-parameters measurements) at a frequency: f 0 = 1 2π (ESL+L track +L via )C IN 700MHz (1) 1 The IEC frequency bandwidth (1MHz-1GHz) has been extended to 2GHz to include the 18GHz frequency, widely employed in wireless communications

4 Fig 4 Measured Immunity Level (expressed in terms of RF incident power) for DPI on the cell input pins: differential (DM) and common-mode (CM) injection, with and without RC 100Ω, 100nF RC filters in Fig2 Fig 5 EMI-induced offset in the cell voltage readings obtained in correspondence of DPI failures level of Fig4 (DM injection performed on the 6 th cell positive terminal, without RC filters) Close to this frequency, the actual impedance of the parallel element of the RC filter (capacitor C and parasitics) is very high and its filtering effectiveness is therefore impaired 1) Failure Mechanisms: Further insight on the mechanisms giving rise to EMI induced failures reported in Fig4 can be gained by considering all the information retrieved from the IC during the DPI test To this purpose, it can be observed that no communication failure (inconsistent PEC) have been reported when performing DPI injection on the cell inputs In all cases, in fact, failures events are related to an EMI-induced offset exceeding 10mV in the cell voltage readings Such an offset voltage, defined for each cell i as Wrong readings V OFF,i = V ADC,i,EMI V ADC,i (2) where V ADC,i,EMI is the i-th cell voltage acquired by the DUT while injecting an RF power corresponding to the immunity level in Fig4 and V ADC,i is the voltage of the same cell acquired by the DUT without RF power injection, is plotted for each cell in Fig5 for the unfiltered IC and in Fig6 for the IC including RC filters In both cases, DM injection is considered From Fig5 it can be noticed that failures up to about 300MHz are related to an EMI-induced offset exceeding 10mV in the acquired voltage of cell #6, ie on the cell on which DPI is performed On the contrary, for EMI frequencies above 300MHz, all the acquired cell voltages show a similar offset Such a behavior can be related to the direct propagation of EMI to the internal ADC and/or to its reference voltage source From Fig6, a different failure mechanism can be noticed for the filtered device undergoing RF DPI in the 8MHz-12MHz band (see wrong reading, the grey area) Here, injected EMI gives rise to completely wrong ADC readings, corresponding to the maximum or to the minimum values of the ADC range At lower and higher frequencies, however, the same failure mechanisms of the unfiltered device is observed Similar considerations apply to the cell voltages acquired while performing CM injection (not shown in this paper), even Fig 6 EMI-induced offset in the cell voltage readings obtained in correspondence of DPI failures level of Fig4 (DM injection performed on the 6 th cell positive terminal, with RC filters) though, in that case, an EMI-induced offset of about 10mV can be noticed in cell #5 and cell #6 voltages B Injection The immunity level of the DUT undergoing DPI on the injection point in Fig2 has also been assessed and the results of such measurements are reported in Fig7 Two different types of injection are considered: injection performed on the four lines at the same time (ie connecting all the four injection capacitors to the signal terminal of the SMA connector in Fig2), and injection performed on the SCK (serial clock) line only (ie connecting only one injection capacitor between the signal terminal of the SMA connector in Fig2 and the SCK pin of the IC) 1) Failure Mechanisms: By comparing the immunity level reported in Fig7 for the injection with the immunity level reported in Fig4 for cell inputs injection, it can be noticed that the immunity level for injection is lower than the

5 Fig 7 Measured Immunity Level for DPI on the input pins: simultaneous injection on the four lines and injection on the single SCLK line Fig 8 EMI-induced offset in the cell voltage readings obtained in correspondence of failure levels of Fig7 above 50MHz (simultaneous injection on the four pins) immunity level of the unfiltered cell inputs This could be a serious concern, since the EMI immunity for injection cannot be improved by filtering [7] because EMI filters would give rise to an unacceptable degradation of nominal digital waveforms An analysis of the mechanisms giving rise to EMI-induced failures when DPI is performed on lines has been carried out on the basis of the data retrieved during the DPI tests, in analogy to what discussed in Section IVa1 Based on these data, failures below 100MHz are related, as one could expect, to errors in the transmission detected by checking the PEC code At higher frequency, however, EMI failures for injection are related to an offset in the acquired cell voltages, equal for all the six cells, as shown in Fig8 Such a behavior, which is similar to what highlighted for DPI on the cell inputs in the high frequency range (Fig5), can be related to EMI propagation inside the DUT to the internal ADC and/or to its reference voltage source V RADIATED SUSCEPTIBILITY TESTS In order to gain more insight on the relationship between the IC susceptibility, highlighted in DPI test results presented so far, and possible hazardous system-level failures, the susceptibility to radiated EMI of a prototype based on the same IC and on the same test board introduced in Section II, without injection networks and including the EMI filters on the cell inputs prescribed by the manufacturer, has been tested in compliance with ISO [10] The antenna has been placed both in front of the DUT and the cable harness Since the measurements have shown that the former case is the less immune, the test setup in Fig 9 has been considered Here, the DUT is located inside an anechoic chamber over a metal plane and it is remotely controlled and monitored by means of optical links The two terminals of the battery pack, monitored by the, are connected through a 15m-long cable to two LISNs Following [10], the operation of the DUT has been tested radiating the DUT with a 200V/m incident EM field, a typical test level for safety critical automotive applications, in the 200MHz-14GHz bandwidth, considering both horizontal and vertical polarization The EMI-induced offset in acquired cell voltages measured during radiated susceptibility tests are shown in Fig10c and Fig10d for vertical and horizontal polarization, respectively, while, in the same figure, the immunity level measured by DPI tests on cell inputs (Fig4) and on the lines (Fig7) are reported in Fig10a and in Fig10b, respectively, for a direct comparison It can be observed that both for vertical and horizontal polarization, the radiated field gives rise to completely wrong ADC readings, corresponding to the maximum or to the minimum values of the ADC range for frequency lower than 400MHz (see wrong reading, the grey area both in Fig 10c and Fig 10d) The same phenomena are also observed in the range 850MHz-950MHz for horizontal polarization Moreover, the results of radiated tests in Fig10c and Fig10d highlight an EMI-induced offset in the acquired voltages in the range 650MHz-900MHz, ie in the same range where the lowest DPI immunity level is observed in Fig10a and in Fig10b To obtain an approximate relation between DPI immunity level and radiated field strength, it has been observed that the failure threshold considered in DPI tests (EMI-induced offset in acquired cell voltages equal to 10mV) has been reached irradiating the EUT in the bandwidth from 600MHz to 900MHz by a vertically polarized E field E v = 100V/m, which approximatively induces a CM voltage on the PCB lines connected to the cell inputs with a peak amplitude V cm = h 0 E z (z)dz E v h = 85V, (3) where h = 85cm is the height of the harness connecting the to the cells with respect to the ground plane The EMI amplitude estimated by (3) is consistent with the results of DPI tests in Fig 10a, which show that the immunity of the EUT for CM injection in the bandwidth around 700MHz, where the effectiveness of RC filters is impaired by the resonance highlighted in (1), is as low as 23dBm, corresponding to an

6 Fig 9 LISN LISN PC USB Cells 15m long 5cm from the table Anechoic chamber CANcaseXL Vector EMC Test Board Filters nF Antenna Cells Inputs CAN BUS DC Supply DUT CAN Optical link 9V battery Optical RX-TX Optical TX-RX C Control Unit Radiated Susceptibility Test Setup Antenna in front of the EUT Wrong readings Wrong readings Wrong readings Fig 10 a - Measured Immunity Level for DPI on the cell input pins: differential (DM) and common-mode (CM) injection, with 100Ω, 100nF RC filters in Fig2 b - Measured Immunity Level for DPI on the four lines c - EMI-induced offset in the acquired cell voltages, E=200V/m Vertical Polarization d - EMI-induced offset in the acquired cell voltages, E=200V/m, Horizontal Polarization EMI peak amplitude of about 9V Further considerations on the correlation between IC-level DPI test results and systemlevel radiated immunity tests are difficult to be established and are also scarcely significant, since the results of radiated tests could be strongly influenced by the actual structure of the battery pack including the which could be rather different with respect to the prototype considered in our investigation Nonetheless, it is worth noting that the same failure mechanisms, and also the same critical EMI bandwidth highlighted by DPI tests have been found in system level tests VI CONCLUSION The susceptibility to EMI of a front-end IC for EVs and HEVs has been investigated in this paper by DPI tests performed according with IEC on a specificaly developed test PCB On the basis of the experimental results, it has been highlighted that the IC under test can be significatively susceptible to EMI injected on its cell input terminals and on its digital communication () lines For what concerns cell inputs injection, in particular, it has been observed that low pass RC filtering can be effective to improve the immunity to EMI of the DUT in the 10MHz-600MHz bandwidth, while its effectiveness is reduced above 600MHz On the other hand, the susceptibility to EMI applied on the input lines, which cannot be filtered, is likely to be a major concern for the specific application Finally, the different mechanisms giving rise to EMI induced failures of the specific DUT have been highlighted Depending on the EMI frequency and on the injection points, an EMI-induced offset in the cell readings, failures and completely wrong acquired values have been reported An abnormal current absorbtion from the cells while performing DPI has also been observed The immunity to EMI of the same system has been addressed even by radiated susceptibility measurements performed in compliance with ISO to establish a correlation between the EMC performance of the IC, previously assessed by the DPI method, and the susceptibility to EMI of a realistic system based on the same IC During the radiated tests, which have been performed for a field strength of 200V/m and 100V/m, the same failure mechanisms highlighted during DPI tests have been observed (offset in acquired cell voltages and communication failures) ACKNOWLEDGMENT The authors would like to thank Eng F Vercelli, L Parodi and P Dovano of FCA, Orbassano, Italy, for the valuable discussions and Eng G Borio, M Scaglione and A Bertino of the LACE, Torino, Italy, for setting up the test bench used for the radiated emission measurements The work has been supported by the Regione Piemonte within the Biomethair Project, Fondo Europeo POR REFERENCES [1] DDoughty, PRoth, A general discussion of Li-ion battery safety Electrochemical Society Interface, vol21, n2 pp 37-44, 2012 [2] D Andrea, Battery Management Systems for Large Lithium-Ion Battery Packs, Artech House, Boston, 2010 [3] NMutoh et al, EMI noise control methods suitable for electric vehicle drive systems, IEEE Trans on EMC, vol47, no4, pp930,937, Nov 2005 [4] GSpadacini, SAPignari, Numerical Assessment of Radiated Susceptibility of Twisted-Wire Pairs With Random Nonuniform Twisting, IEEE Trans on EMC, vol55, no5, pp956,964, Oct 2013 [5] JGago et al, EMI Susceptibility Model of Signal Conditioning Circuits Based on Operational Amplifiers, IEEE Trans on EMC, vol 49, Issue: 4, pp: Nov 2007 [6] FLFiori, PSCrovetti, Prediction of high-power EMI effects in CMOS op amplifiers, IEEE Trans on EMC, vol48, no1, pp153,160, Feb 2006 [7] PCrovetti, FFiori, IC digital input highly immune to EMI, ICEAA 2013, pp1500,1503, 9-13 Sept 2013 [8] IEC Integrated Circuits-Measurement of EM Immunity-150kHz to 1GHz-Part 4: Direct RF Injection Method, 2005 [9] Generic IC EMC Test Specification, Ed l2, 2007; available online: EMC Test Specification [10] Road vehicles - Vehicle test methods for electrical disturbances from narrowband radiated electromagnetic energy, ISO Std , 2005