ricardo dc-dc converter presentation for nmi

www.ricardo.com© Ricardo plc 2010RD.10/######.#

Compact, high reliability DC-DC converters for Automotive applicationsCase Study: Chery ISGPresenter: Frank Warnes

2© Ricardo plc 2010RD.10/######.#JUNE 2010NMI Presentation

Agenda

� Project background

� Outline Specification

� Key decisions

� DFMEA

� Final Design

� Test Results

� Future Development

� Conclusions


Project Background� Ricardo

– strong track record in advanced engineering and technology solutions, including EV,HEV and PHEV

– proven ability for complex technical transfer of knowledge

� Case Study for Chery Automobile

– Largest domestic car manufacturer in China (400,000/year in 2007)

– In 2005 To support local & national government aims of increasedautomotive technology and improved fuel efficiency, Chery undertook the development of a hybrid electric vehicle (HEV)

– Ricardo were tasked with the design and development of the Hybrid system, control logic and electronics

� For the DC-DC Converter there were several key challenges to overcome:

– Design and develop the DC-DC converter to Production for low cost, high volume manufacture

– Timescale – Had to be production ready with all approvals within 2 years

– Reliability – Had to work reliably in engine bay environment for life of vehicle

– Safety – Had to connect to the High voltage supply


Outline specification

� Input Voltage 100V to 210V DC

� Output Voltage 14V nominal

� External Control of output Voltage ( settable between 12 and 16V DC )

� Output Current 85A nominal (100A max)

� Load and Line Regulation <1%

� Environment - Under Bonnet -40 to +105°C

� Efficiency >80% at nominal power

� External control of DC-DC Enable and Output current monitor

� Fault limiting for Over current, Over Temperature and Output Over voltage

� Sleep mode current <1mA LV and HV

� Safety Isolation barrier between HV battery system and operator-accessible components, in accordance with accepted practice.

� Dimensions L 250 x W 200 x H 50

� Weight <1kg


� Manufacturing

– Use standard 4 layer FR4 pcb for complex control electronics and Integrated Metal Substrate (IMS) for power

• IMS has good thermal transfer for power components ( 3.1W/mK) and uses standard sm reflow processes and costs about the same as conventional 4 layer pcb

– Use Volume manufacturer Delta Electronics based in Thailand to keep manufacturing costs down

Key Decisions

� Location of DCDC

– In same enclosure as inverter – can use same cooling – can use analogue/digital control from inverter then CAN – Close to 12V battery keeps high current cables short - stable environment

� Safety/Reliability features

– Isolation and creepage and clearance to EN60950

– No opto isolators or electrolytic capacitors due to life and reliability issues

– Constant Current limit – ideal for lead acid charging

– Internal overcurrent limit – fast pulse by pulse switch protection

– Overtemperature limit – Temperature sensor on power components

– Output over voltage limit – protect load in case of control failure


Key Decisions

– Use secondary side control with isolated gate drive – simplifies gate drive – ideal for 50% d/c – accurate voltage sense – no isolation of feedback.

– Use small SEPIC converter to derive constant 12V aux from 6 to 32V supply to control (Lineartech LTC1871) – synch with main controller to avoid unpredictable emc

– Set Transformer frequency to 200kHz. Therefore Output inductor to 400kHz –minimize magnetics and capacitor size

– Use Film capacitors – very low esr high Irms – self healing so safe – sm versions won’t crack during thermal expansion

– Use Planar Transformers – low profile – low loss – repeatable. ideal heat transfer to IMS

– Use schottky diodes rather than synchronous mosfets for output – less complex

� What circuit topology and components

– Phase shifted full bridge with zero voltage switching on primary Mosfets – Industry standard for 1 to 10kW

– Use Ti UCC2895 as main controller. Arrange automotive selection of SO20 with Ti

– Use accurate reference independent of main controller to avoid need for AOT


DFMEA

� Two main outputs from DFMEA that affected design and test procedures

1. Not enough output voltage

• Few minor issues that ultimately result in lack of output but only rate as customer dissatisfaction

2. Too much output voltage.

• This could ultimately result in the Auxiliary battery at best out gassing or at worst overheating and causing a fire.

• However despite the severity of this condition it was felt that the likelihood of occurrence was low and the probability of detection was very high with the intended process controls; - i.e. Design calculations & simulation to verify tolerance spread, design reviews to check and double check circuits, bed of nails testing and module level validation testing of each board.

• Despite this an additional independent 18V overvoltage shutdown circuit was added for real “belt and braces”


Final Design – Power Board

LV to HV Isolation Barrier

Input Capacitance

Input Mosfets

Output Diodes

Output Capacitance

Switched Current sense

DC Current sense

Planar Transformer

Planar Output

Inductor

Temperature sense

Input Connections

Gate Drive connections from Control

PCB

12V output connection

Output Ground Connection

Control PCB connections

SnubberResistor

Bridging Link


Final Design – Control Board

LV to HV Isolation Barrier

Gate Drive Transformers

Gate Drive Connections to

IMS

Main PWM Controller Texas Instruments

UCC2895

DC Current Monitor circuit

I/O Control Connector to Inverter

SEPIC Converter

Fault Monitor circuits

IMS control connections


Control PCB

Power IMS Board

Cooling inlet/outlet

Inverter DC Link Capacitance

Auxiliary LV Battery Connections

Inverter Gate Drive Board

Inverter Control Board

SemikronIGBT Power

ModuleHV Battery input

Three Phase HV Output

Final Design – Mounted in Inverter enclosure

ISG machine control unit (MCU)15kW peak power / 10kW continuous control to ISG machine

DC-DC Converter


Test Results – Key Parameters

TARGET

� Load and Line Regulation <1%

� Efficiency >80%

� Sleep mode current <1mA LV and HV


� Weight <1kg

ACHIEVED

� Load and Line Regulation <0.1%

� Efficiency 85%

� Sleep mode current <900uA for LV and < 100uA for HV


� Weight = 600g


Test Results – Electrical Stress

� Sample units were subjected to and passed several electrical stress tests including:

– Radiated Emissions and Radiated Immunity to EC2004/104

– Conducted transient immunity to ISO7637-2

– Adverse LV Battery supply – double battery, reverse battery, under voltage, drop out and supply dip, ramp up/down and short circuit

– Isolation voltage test – 2.1kV DC for 1 minute

– ESD to EN61000-4-2 – 8kV

EMC Testing in TEM Cell Radiated emissions


Test Results - Environmental

� Sample units were subjected to and passed several environmental tests including:

– Temperature cycling -40 to 105°C operational (240Hr s)

– Humidity/temperature cycling -25 to 85°C 95% RH (24 0Hrs) non operational

– Sinusoidal vibration at 10g for 20Hrs in each axis operational

– Half sine shock at 40g for 60 cycles non operational

– Drop/Topple test at 0.5m

Temperature cycling Thermal image of IMS at Full output power and 70°C coolant


Test Results

� During our design validation phase our customer was invited to supply a taxi fleet of 50 hybrid cars to the Olympic games.

� Driven by experienced Beijing taxi drivers, the cars were working seven days a week, 24 hours a day, transferring dignitaries and athletes. This continued for the 17 day duration of the main Olympics and the 12 days of the Paralympics. During this time not a single fault was reported.

� This was quite an achievement with a fleet of vehicles which – despite significant fleet-specific validation testing – was in effect midway between prototype and preproduction status.


Future Development

� A second design of the DC-DC converter was created and tested with a higher input voltage of between 200 and 460V and is now being fitted to a production EV from a European customer

� Additional testing has been successfully carried out to assess the viability of connecting the DC-DC converter in parallel to achieve higher output power capability.

This is easily achieved by adopting a Master Slave approach where an external processor monitors the output current of a Master unit and drives the output voltage of a Slave unit to achieve exact current sharing

� We are also currently looking into a 4kW design for a high performance PHEV


Conclusions

� In conclusion the DC-DC converter for the Chery HEV project:

– Was successfully developed for low cost, high volume manufacturing.

– Was available for production within a year. Subsequent variants have been delivered to advance prototype level within 6 months.

– Exceeds the customers performance standards

– Has proven it‘s reliability by completing substantial automotive testing and Olympic drive trials

– Meets and exceeds all of the isolation and safety standards expected for consumer use.

� The DC-DC converter is an essential part of any EV/HEV system and it’s reliability is paramount. Failure of a DC-DC converter results ultimately in a stopped vehicle in the same way that the failure of an alternator would in a conventional internal combustion engine vehicle.

� In time as the EV/HEV becomes common the DC-DC converter will become a commodity item like the alternator is now.

� We believe we have a solid and reliable design. The drive now is to continually improve the performance and ultimately to push down the cost to meet this market .

ricardo dc-dc converter presentation for nmi

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