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© Ricardo plc 2010RD.10/######.#
Compact, high reliability DC-DC converters
for Automotive applications
Case Study: Chery ISG
Presenter: 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 increased
automotive 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
Snubber
Resistor
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
Semikron
IGBT Power
Module
HV 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 .

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