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1 Special Considerations in Automotive Battery Systems 1 Special Considerations in Automotive Battery Systems

1 Special Considerations in Automotive Battery Systems - PowerPoint Presentation

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1 Special Considerations in Automotive Battery Systems - PPT Presentation

Bob Shoemaker Systems Engineering Manager Automotive Battery Management Overview Automotive Battery Design Challenges Measurement Accuracy High voltages from groups of 616 series cells per module up to 256pack ID: 667262

safety fit automotive communications fit safety communications automotive battery 26262 iso iec random cell plug management connection faults asil

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Slide1

1

Special Considerations in Automotive Battery Systems

Bob ShoemakerSystems Engineering ManagerAutomotive Battery ManagementSlide2

Overview - Automotive Battery Design Challenges

Measurement Accuracy“High” voltages from groups of 6-16 series cells per module, up to 256/packWide temperature range -40 to105⁰C

TI offers ±2mV standard accuracy in automotive BMS partsCommunicationsExtremely noisy environmentEMC: Bulk-Current Injection (BCI) susceptibility requirementsEMI: Radiated energy must stay out of common radio entertainment bandsHot Plug – random connection of cells to measurement circuitsWildly fluctuating voltages during connectionExposure to many hundreds of voltsInductive ringing during connection may produce >80V on a 30V IC

Safety – ISO 26262

2Slide3

Communications

Driving Through Noise…3Slide4

Communications

Most interference is inductively coupled from the bus bar which carries very high currents up to 100’s of amps – ~ forming a one turn transformer with the communications cableNoise from the inverter is typically a 1V/cell - 3µs pulse at a 30kHz rate, with rise/fall times in the 2-5V/ns range!The noise is wideband with lots of harmonic energyStray capacitance to (isolated) chassis is also a big contributor

4Slide5

BCI Testing

5

Standardized test (IEC, others) that mimics the noise environment

Injects noise current into communications cables with toroid

xfmr

1MHz – 1GHz injected, 50-200mA

Effect on communications measured – CRC errors, dropped packets, etc.

Toroid

to Inject Current into 1M

Comm

Line

Cu TableSlide6

BCI Testing

6

test IC

Effect of BCI Test on communications signals.Slide7

Communications “Cures”

Good PCB layout practices, ground planes are very importantShielding may not buy much in this environmentImproves VHF, but may worsen low end resultsFerrites, very small inductors, small capacitors may help, especially with VHF-UHF

Partial impedance matching can helpSmall capacitors (AC coupling) between ground planes will make a difference if wire runs are short - typical values 3.3nF – 10nFThe best solution against high common-mode noise is differential communications links

7Slide8

Hot Plug

Random Cell Connections8Slide9

Hot Plug – Random Cell Connection Order

Connector pins mate in random orderWild voltage swings across cell input pinsMay stress ICWire length + capacitance around IC causes large overshoots

9Slide10

Hot Plug Exposure Can Damage Devices

3 stacked bq76PL536 devices, 4.7µF cap and 36V zener across each from BAT to VSSWires are ~2m long, ~2.5µHDevice 1 is exposed to ~55V at connection

Zener dynamic impedance is too high to prevent failure10

This circuit causes dramatic voltage overshoot at connection time, a result of natural & parasitic inductance and board (device) capacitance.Slide11

Hot Plug Exposure

With 0.1uF cap, exposure is reduced to <40VIC must have good Power Supply Rejection Ratio (PSRR)11

3 bq76PL536 devices, 4.7µF cap and

no

36V zener across each from BAT to VSS

Wires are ~2m long, ~2.5µH

Device 3 is exposed to 49V at connectionSlide12

Minimize Hot Plug Problems

Keep wiring to cells as short as possible to minimize inductanceKeep capacitance near IC to a minimum – this includes caps for filters, LDO’s, communications, etc.The 36V zener does not help, and may cause harmInstead use 5.1 or 5.6 zener diodes across each cell connection

A series resistor may reduce overshoot, but must be kept to a minimumToo large and it causes a significant voltage dropCauses the VBRICK measurement error due to I·R dropMay cause ESD diode to turn on if BAT goes below TOP cell (i.e. VC6) due to effects of L-R-C

12Slide13

ISO 26262 Introduction

Stefano ZanellaSlide14

Who is working on ISO26262 ?

Technically:

ISO-TC22-SC3-WG13

TI’s Karl

Greb

,

a co-author of parts 5 and 10Slide15

IEC 61508 vs. ISO 26262

ISO 26262

(Road vehicles)

RTCA/DO

17B

(Aerospace)

IEC 60601

(Medical

)

EN

50128

(Railways)

IEC 62061

(Machinery)

IEC 60880

(Nuclear

power plants)

IEC 61511

(Process)

IEC 50156

(Furnaces)

IEC 61508

(The grand-daddy)

Slide16

SIL vs. ASIL

ASIL -

Automotive Safety Integrity LevelSlide17

ISO 26262 is a Process and a Mindset

System

Risks

Which ASIL level?

ISO26262 is all about managing risk by reducing the number of

undetected faults to an appropriate level for the applicationSlide18

Information Exchange

Collaboration

Specs

Specs

Hazard

& Risk

Analysis

Safety Manual

Safety Manual

Audits

FMEA/FTA

FMEA/FTA

Other documentation

Other documentation

OEM

Tier 1

Semiconductor Supplier

AuditsSlide19

Item Definition

Which one??

Cell, battery, and vehicle pictures from teslamotors.comSlide20

Systematic Faults

Design Errors and EMCReduced through compliant development processLots of checklistsRedundancy helpsDecompositionSlide21

Hazard &Risk Analysis (OEM -> Tier 1)

Example: safety item – battery management system

Functional safety goals:

Over-voltage (OV) detection: ASIL D

Over-temperature (OT) detection: ASIL CSlide22

Supplier Deliverables

FMEA bq76PL455 - 1388 lines

Fault Tree Analysisbq76PL455 – 100 pages

Safety Manual

bq76PL455 – 85 pagesSlide23

What’s in the Safety Manual

Practical guide to functional safety using a component/systemMetrics (example later)Safety mechanismsAssumptions of useAll you need to know to integrate a system with the higher level systemSlide24

Random Faults Classification

All FaultsSlide25

Failure Diagnosis and Prognosis for Automotive Systems”, by Tom Fuhrman, General Motors R&D, IFIP Workshop, June 25-27, 2010

SPFMLFM

SPFM

LFMSlide26

Probabilistic Metric for Random Hardware Faults (PHMF)

ASIL

Random hardware failure target values

B

< 10

-7

h

-1

(100 FIT)

C

< 10

-7

h

-1

(100 FIT)

D

< 10

-8

h

-1

(10 FIT)

Sub-system

FIT Budget

Host Controller

2.2 FIT

Supervisory board

1.3 FIT x 6

Total

10 FIT

Budgeting

0.5 FIT

0.3 FIT

0.1 FIT

0.2 FIT

0.15 FIT

0.05 FIT

This is an example using the PL455 Evaluation Board. The FIT rate budget may vary by application.

BudgetingSlide27

Failure Diagnosis and Prognosis for Automotive Systems”, by Tom Fuhrman, General Motors R&D, IFIP Workshop, June 25-27, 2010Slide28

Summary

Texas Instruments has launched the SafeTI™ Program, a unified corporate approach to safetyIncludes ISO26262, IEC61508, IEC60730 qualified productsAll new TI automotive battery management products developed according to ISO 26262

ISO 26262 includes prescriptions on the development process for both hardware and softwareISO 26262 is about mitigating risks by reducing the number of undetected faults to an appropriate level for the applicationISO 26262 defines metrics (SPFM, LFM, PHFM) that allow a quantitative assessment of the riskSlide29

29

Bob Shoemaker

Systems Engineering Manager

Automotive Battery Management

Battery Management Solutions

Texas Instruments, Inc.

bobs@ti.com

301-407-9599

Thank You!

Automotive Battery Management

Special thanks to: Karl

Greb

, Stefano Zanella, and others for their contributions of slides.