1. Inverters & DC/DC Converters

2. High Voltage Safety
Contact with voltage of less than 50 volts is unlikely to cause injury.
Voltages above 50 volts are potentially deadly.
Anytime you work on and around electrical systems that have voltages above 50 volts proper safety procedures must be observed to avoid injury.
Danger
High Voltage

3. PPE – Personal Protective Equipment
High Voltage gloves with leather covers
Test gloves for tears and punctures before use
Gloves need to be tested annually by an approved glove testing facility
Safety Glasses
Leather work shoes
Test Date

4. CAT III DVOM
A CAT III DVOM with CAT III test leads is required when testing components of the high voltage system
CAT III
rating

5. Remove the service plug
The service plug disconnects the HV battery from the rest of the electrical system
The service plug is mounted on the HV battery assembly cover
Make sure that the vehicle is turned off and the ‘ready’ light is off before removing the service plug
HV rubber gloves must be worn when removing the service plug..!

6. Check voltage after removing service plug
Before doing any work on the HV system check the voltage at the HV battery cable connections
It should be no more than a few 10ths of a volt

7. HV Safety
High voltage cables connect the inverter to the battery and motor generator.
High voltage electrical cables are identified by the orange harness cover.
Never touch these cables without first removing the battery service plug…!

8. Inverter function
The inverter converts high voltage DC current from the high voltage battery array into AC current that is synchronized to the operating speed of the traction motor[s]
During regenerative braking the inverter converts the AC current from the stator coils into DC current that is stored in the high voltage batteries
Image courtesy of General Motors Corp.

9. Inverter Voltage regulation
The inverter can step down voltage level applied to the HV batteries without using a transformer.
The inverter can also adjust the HV [high voltage] charging voltage so that the battery array is not overcharged or undercharged

10. Connector for high voltage battery cables
The DC-DC converter converts high voltage DC current from the HV battery array into 14 Volt DC current that is used by the vehicles conventional electrical system
DC-DC converter
Connector for high voltage battery cables
The DC-DC Converter is normally located inside the inverter case or underneath it
14 Volt DC output

11. Electrical symbol for an IGBT
IGBTs
Integrated Gate Bi-Polar Transistor
Function as high speed, solid state switches to convert DC voltage into AC to power the traction motor
Since power can flow through the IGBT in either direction the IGBT also can converted AC voltage generated in the stator coils into DC current to recharge the battery
Electrical symbol for an IGBT

12. How IGBTs convert DC current to AC
The inverter electronic controls turn the IGBTs on and off to vary the DC voltage pulse width
The effective current is similar to an AC sine wave
The IGBT also limits voltage by reducing the pulse width
- 300 V
+ 150 V
- 150 V

13. Inverter construction
An inverter is made up of 6 IGBTs and is similar in operation to the rectifier in an alternator
6 high current diodes are connected in parallel with the IGBT +B -B
Stator
3 of the IGBTs are connected to the HV battery positive terminal and 3 are connected to the negative terminal

14. Inverter operation – 1st phase – drive forward
Motor / Generator control module
Battery current flows through IGBT Q1 to stator terminal U, through the stator windings, then back to the battery through terminal W and IGBTQ4
High voltage battery array +B -B Q1
Stator Windings U Q4 W V

15. Inverter operation – 2nd phase – drive forward
Motor / Generator control module
Battery current flows through IGBT Q3 to stator terminal W, through the stator windings, then back to the battery through terminal V and IGBT Q6
High voltage battery array +B -B Q3
Stator Windings U Q6 W V

16. Inverter operation – 3rd phase – drive forward
Motor / Generator control module
Battery current flows through IGBT Q5 to stator terminal V, through the stator windings, then back to the battery through terminal U and IGBT Q2
High voltage battery array +B -B Q5
Stator Windings U Q2 W V

17. Inverter operation – Regenerative braking – 1st Phase
During regenerative braking current generated in the stator winding U-V passes through Q5 to the HV battery +B terminal
High voltage battery array
Current from the battery negative terminal passes through Q2 to re-enter the stator windings at terminal U +B -B
Motor / Generator control module Q5
Stator Windings U Q2 W V

18. Inverter operation – Regenerative braking – 2nd Phase
As the rotor turns the next winding that has current induced is W-U. Current flows from terminal U through Q1 back to +B
High voltage battery array
Current from the battery negative terminal passes through Q4 to re-enter the stator windings at terminal W +B -B
Motor / Generator control module Q1
Stator Windings U Q4 W V

19. Inverter operation – Regenerative braking – 2nd Phase
As the rotor turns the next winding that has current induced is V-W. Current flows from terminal W through Q3 back to +B
High voltage battery array
Current from the battery negative terminal passes through Q6 to re-enter the stator windings at terminal V +B -B
Motor / Generator control module Q3
Stator Windings U Q6 W V

20. Inverter IGBT assembly
Inverters for electric vehicles use IGBT [Integrated Bi-Polar Transistors] as high speed, solid state switches
An inverter consists of 6 IGBTs mounted on a heat sink
Connectors for high voltage battery array
Control module connector
Control module connector
Connectors for stator leads

21. Inverter heat dissipation
The IGBTs are bonded to a flat steel heat sink plate that transfers heat from the IGBT to the inverter case
Inverter for MG2
Inverter for MG1

22. Inverter case
Silicone heat transfer grease improves the transfer of heat from the inverters to the case
The inverter assemblies are bolted to the bottom of the inverter case

23. Inverter case
Coolant Out
Coolant In
A serpentine coolant passage is cast into the bottom of the inverter case

24. Inverter cooling
The DC-DC Converter
The bottom cover plate for the serpentine coolant passage contains the DC-DC converter electronics

25. Buss Bars
Large steel or copper strips called ‘Buss Bars” carry electrical current from the IGBTs to the stator cable assemblies

26. Inverters for two motor hybrids
Each high voltage motor/generator needs it’s own inverter
A two motor system [Prius] has one inverter for MG1 and another inverter for MG2
Stator for MG1
Control module
Stator for MG2

27. Electronic control unit
Resolver circuit
Electronic control unit +B -B
Stator
Resolver

28. Resolver function The resolver is a rotor position sensor
The inverter control module needs to know to position of the rotor so that it can turn on the correct set of IGBTs

29. A/C compressor inverter
The A/C compressor for BEVs and most hybrids is driven by a High Voltage three phase electric motor
The inverter is normally on top of the compressor housing where it is cooled by refrigerant
MG1
Electronic control module
MG2
Inverter for A/C compressor
A/C compressor motor

30. A/C Compressor inverter
The electrical connection to the HV battery is made inside the inverter assembly case
The inverter for most electric A/C compressors is located on top of the compressor housing
Image courtesy of General Motors Corp.

31. Electric A/C Compressor
3 Phase AC Motor
Oscillator Assembly

32. Inverter for A/C Compressor
The inverter circuit board for this compressor has been pried away from the case to reveal it’s circuitry.
Inverter IGBTs are cooled by contact with the compressor housing. Refrigerant vapor passing through the compressor carries inverter heat to the A/C condenser.

33. A/C compressor with external inverter
Some older hybrids located the inverter for the A/C compressor inside the traction motor inverter housing
The electrical connector shown here is for 3 phase AC current
Image courtesy of Denso Corp.

34. Capacitors Capacitors store high voltage electrical charges
Unlike a battery, capacitors cannot deliver continuous electrical power over a long period of time
Capacitors act like a ‘shock absorber’ for electricity – smoothing out the surge of electrical voltage as the IGBTs turn on and off

35. Capacitor boost function
When the vehicle is suddenly accelerated there may be a slight lag in the delivery of electric power because the chemical reactions that produce electricity in the battery take a few seconds to build up full power
The electrical energy stored in the capacitors provides a momentary surge of electrical power to improve initial acceleration – but only for a couple seconds
Capacitor boost function
Capacitor
assembly

36. Capacitors
Three large capacitors are connected via buss bars between each of the stator windings and HV negative
Buss bar

37. Gen 2 Prius
Capacitor Pack
Capacitors can be packaged in metal cylinders or in brick shaped packs
Since there is very little heat produced in the capacitor they are normally located at the top of the inverter assembly

38. Drain resistor
The capacitors can hold lethal voltage potential so a drain resistor is wired in parallel with the capacitors
When the system is shut down the voltage in the capacitors leaks out through the resistor
It can take up to 5 minutes for the capacitors to drain completely

39. Current sensor
The Motor Control Module needs to know how much current [amps] each motor/generator is using or generating
A current sensor measures the strength of the magnetic field surrounding the wires connecting the HV battery to the inverter and sends this information to the MCU

40. HV Battery Current Sensor
The leads for the HV battery connectors pass through the current sensor

41. DC / DC Converter

42. DC to DC converter
All BEV and Hybrid vehicles have two separate electrical systems
The high voltage electrical system provides current for the traction motor
The low voltage electrical system provides current for all other vehicle functions
The A/C compressor on BEV and two motor hybrids normally runs off of the high voltage system
BEV vehicles have a PTC heater that runs off of the high voltage electrical system to provide cabin heat
The DC to DC converter is the bridge between the high voltage and low voltage systems

43. Low and High Voltage Systems
Motor Generator
Low and High Voltage Systems
The DC to DC converter uses high voltage current from the HV battery array to produce low voltage current for the vehicles normal electrical system
Inverter
High Voltage
High voltage battery array
12 Volt battery
DC/DC converter
Power distribution center
Low Voltage
to ignition switch
to computers
to lighting system

44. DC-DC converter functions
The DC-DC converter does the same thing for a hybrid or BEV as an alternator does for a conventional vehicle
It provides electrical power at 14 volts DC for all of the lights and electrical accessories when the vehicle is being driven
Instead of being powered mechanically by a belt the DC-DC converter is powered electrically by the high voltage electrical system
When the system is turned on but the ICE engine is not running the DC-DC converter converts takes electrical power from the HV battery and converts it to 14 volts to run the lights, heater fan and accessories

45. DC-DC Converter Operation
Step down transformer
Rectifier
High voltage battery current
Oscillator
14 Volts DC
Chassis ground
The DC-DC converter is made up of three components:
Oscillator
Step down transformer
Rectifier

46. Square wave output current
Oscillator
The Oscillator employs a feedback loop to switch a circuit on and off several hundred or thousand times each second
The oscillator effectively turns DC current into AC
The current output of an oscillator will be a square wave
Feedback loop
Square wave output current

47. Transformer schematic symbol
Transformers are used to change the voltage level of an AC current
Transformers only work with AC current
A step up transformer increases voltage – a step down transformer decreases voltage
Transformer schematic symbol

48. Secondary coil winding
Transformers
The input of a transformer is called the primary coil
The output of a transformer is called the secondary coil
Primary coil winding
Secondary coil winding
Since the power flowing through the transformer primary coil is constantly reversing, the magnetic field surrounding the core is constantly expanding and contracting
The moving lines of magnetic force induce an electric current into the secondary coil

49. Turns ratio
If the primarily coil has 100 turns [loops] and the secondary coil has 10 turns the voltage induced into the secondary coil will be reduced by a factor of 10
100 turns
10 turns
120 volts 1 amp input
12 volts 10 amps output
When voltage is increased by a transformer the current [amps] is decreased in inverse proportion – when voltage is decreased current is increased in inverse proportion

50. Transformers
The transformer output is AC current with a sine waveform + _

51. Rectifier The rectifier uses 4 diodes to convert AC current into DC
Diodes are one way valves for electrical current
The diodes are arranged into a rectifier bridge
Current from each end of the secondary winding is connected to two diodes
Capacitor

52. Rectifier +
When the secondary coil shown here is positive at the top and negative at the bottom diodes D2 and D3 allow current to flow through them [forward biased]
Diodes D1 and D4 are blocking the flow of current [reverse biased] D2 D1 D3 D4 -

53. Rectifier -
When the polarity of the coil is reversed so positive is at the bottom and negative is at the top diodes D4 and D1 are turned on [forward biased]
Diodes D2 and D3 are turned off [reverse biased] so no current can flow through them D2 D1 D3 D4 +

54. Capacitor
14 V DC
0 V DC
After AC current has been rectified to DC there is normally a little bit of voltage fluctuation
The voltage fluctuation is called an eddy current and is minimized by placing a small capacitor between the positive and negative DC output terminals of the rectifier

55. Eddy current with capacitor
14 V DC
0 V DC
Excess eddy current can interfere with the operation of some electronic circuits and can also effect the operation of AM and FM radios
Adding a capacitor to the DC output flattens out the eddy current

56. Battery charger
Step down transformer
Rectifier bridge
120 Volt AC input
The DC-DC converter is very similar to a battery charger
Since the DC-DC converter works off of high voltage DC current it needs an oscillator circuit to convert DC to AC before the transformer steps down the voltage
Capacitor
14.6 Volt DC output

57. DC-DC Converter operation – Key Off
Low voltage Battery
Lights
HV Battery
Horn, clock etc
Ignition switch
Computer KAM
Injectors
Fuel pump
Accessories
A/C compressor
12 .8 volts
Hybrid control module
Control signal for master relay is off
DC-DC converter
DC-DC converter is turned off
HV power is turned off
MG1
HV Master relay
MG2
Inverter
When the vehicle is turned off the HV master relay is open
The HV battery array is disconnected from the inverter/DC-DC converter assembly

58. DC-DC operation – Key On / ICE off
Low voltage Battery
Lights
HV Battery
Horn, clock etc
Ignition switch - on
Computer KAM
Injectors
Fuel pump
Accessories
A/C compressor
14 .4 volts
Hybrid control module
Control signal for master relay is on
DC-DC converter
DC-DC converter is turned on
HV power is turned on
MG1
HV Master relay
MG2
Inverter
When ignition switch is on, but the ICE engine is not running HV power is converted into 14.4 volts to operate the electrical system and keep the LV battery fully charged

59. DC-DC operation – Key On / ICE running
Low voltage Battery
Lights
HV Battery
Horn, clock etc
Ignition switch - on
Computer KAM
Injectors
Fuel pump
Accessories
A/C compressor
14 .4 volts
Hybrid control module
Control signal for master relay is on
DC-DC converter
DC-DC converter is turned on
HV power is turned on
MG1
HV Master relay
MG2
Inverter
When the vehicle running current generated by the traction motor/generators provides power for the DC-DC converter and the HV Batteries

60. DC-DC Converter location
The DC-DC converter is normally located at the bottom of the inverter housing
High voltage battery input
Low voltage output
The coolant flowing through the passages in the inverter housing removes excess heat from the DC-DC converter

61. DC-DC Converter Step down transformer Low Voltage +B output
High Voltage +B input
High Voltage - B input

62. DC / DC Converter
This Toyota Highlander has its own DC-DC converter that is provides low voltage electric current to the power steering system
2006 Highlander Hybrid

63. Backup power supply for power brake booster
Low Voltage Battery
Chassis ground connection
AGM Battery
The low voltage battery is usually a 12 volt AGM type battery
Conventional [flooded cell] batteries may also be used

64. Jump starting
If the headlights or interior lights are left on overnight the 12 volt battery will be depleted and the vehicle will not start
Current from the 12 volt battery is needed to turn on the master relays for the HV batteries
A jump box or jumper cables to another vehicle’s battery will provide the electric power needed to for the computer system and HV system relays to bring the HV battery system online so the vehicle can be driven

65. Jump starting another vehicle
Never use a hybrid or BEV vehicle as a donor power source when trying to jump start a conventional vehicle ..!
Thousands of dollars in damage to the DC-DC converter and inverter could occur if a hybrid or BEV system is used to power a conventional starter
The DC-DC converter is designed for relatively low amperage [less than 40 Amps]. Operating a conventional starter normally requires 150 amps or more

66. Jump Starting
If the LV battery is located in the rear hatch the hybrid electrical system may need to be energized from the under-hood fuse box to activate the hatch release solenoid

67. Terminal for activating the hatch release solenoid
Jump Starting
Once the hatch is open connect the jump box directly to the LV battery terminals
Terminal for activating the hatch release solenoid

68. Inverter cooling

69. Inverter Cooling
The IGBTs and other electronic components require cooling
Mild hybrids that have single motor/generator that has a relatively low power output can use a fan driven air cooling system
Full hybrids with 2 motor/generators will require liquid cooling
The in liquid type cooling systems the inverter and the transmission share a common cooling system that has its own radiator and electrically driven coolant pump
Coolant inlet and
outlet tubes

70. Coolant Inlet and Outlet Tubes
Inverter Cooling
The Inverter is often mounted directly above the transaxle so coolant can flow from the transaxle upward into the inverter
Coolant passages are cast into the inverter and transmission to cook the IGBTs and stator coils
Coolant Inlet and Outlet Tubes

71. Inverter + DC-DC converter
144 Volt Battery pack
Air cooled inverters
Inverter + DC-DC converter
Light hybrids [Honda IMA] don’t generate as much heat as a 2 motor hybrids so they can be air cooled
The Inverter and DC-DC converter are mounted next to the battery array – behind the rear seat

72. Inverter Cooling System
Reservoir and
Pressure Cap
Electric
Water
Pump
Dedicated Radiator
for Inverter
and Motor / Generator
Image courtesy of Toyota U.S.A.

73. Inverter & traction motor
Dual radiator
ICE motor
Inverter & traction motor
The radiator assembly is often divided into two separate radiators
The upper radiator services the ICE engine
The lower radiator cools the inverter, DC-DC converter and traction motors

74. Inverter / Transaxle Coolant
Inverter Cooling
The Inverter / transaxle has it’s own expansion tank and pressure cap.
Inverter / Transaxle Coolant
Engine Coolant

75. Inverter cooling passages
Coolant Out
Coolant In
Passages in the base of the inverter remove heat from the IGBT modules.

76. Inverter Cooling System
Coolant Hoses
Drain for Inverter
/ traction motor coolant
Drain for Transmission Fluid
Note how the transaxle has 2 drain plugs.

77. Electric water pump
Coolant out
To circulate coolant through the radiator, inverter and transmission case an electric water pump is used
The pump for the inverter/traction motor system uses 14 volt DC current and is controlled by the inverter module

78. Inverter Cooling System Air Bleed
Bleeder nipple for Inverter
/ Transaxle coolant
2007 Prius
To bleed the inverter/transmission system connect a small vacuum hose between the bleeder nipple and reservoir.
Open the bleeder.
Run the inverter pump until no air bubbles are seen at the end of the hose.

79. The coolant type used by the inverter/traction motor is the same as used in the ICE cooling system – a 50/50 solution of 5 year anti-freeze and water
The coolant level should be between the ‘L’ and ‘F’ line on the reservoir
Inverter coolant
‘F’ line
‘L’ line

80. Topping off inverter coolant
Most manufacturers recommend that service technicians do not top off the inverter coolant during routine service checks
Topping off the coolant will mask a slow leak in the system
A fluid level sensor in the reservoir will trigger a warning light to alert the driver when the coolant level is too low
If the coolant is constantly topped off the warning light will not turn on until the slow leak becomes a big leak

81. Inverter service

82. Inverter R&R
Since the inverter often is located above the transmission you may need to removed it to service the engine, transmission, ABS controller or the steering rack
Removal of the inverter normally involves:
Disconnecting the HV battery service plug
Draining inverter coolant
Disconnecting the coolant hoses
Disconnecting HV DC and 3 phase electrical cables
Disconnecting low voltage electrical connectors
Disconnecting the inverter from its mounting brackets

83. To gain access to the rear of the inverter housing the wiper motor and wiper transmission had to be removed on this vehicle
Inverter R&R

84. With the wiper transmission removed the components at the rear of the engine compartment are easily accessible
Inverter R&R
The plastic trim panel has also been removed for access to the front of the inverter

85. Remove service plug Make sure the vehicle is turned off
Using HV rubber gloves remove the service plug
On this Prius the loop at the top of the plug is pulled up first – then arm on the service plug is rotated 90 counterclockwise

86. Safety plug A safety plug is incorporated into the inverter cover
The safety plug is in series with the HV battery master relay control circuit
If the inverter cover is removed the master relay will turn off automatically
Safety plug

87. Allow 5 minutes for the capacitors to drain and then check for voltage between the two HV battery terminals
Test for voltage
HV battery terminals
Safety plug connector

88. Three phase cables
Three phase cables for the traction motor[s] are connected to the buss bars with 6mm bolts

89. Threaded cable end terminals
Some 3 phase cable have threaded terminal ends

90. Two wire connectors
Two wire HV connectors for the A/C compressor normally have a push-on type connector
A lock tab is used to prevent the connector loosening

91. HV Battery cables
The HV battery cables may also use a push-on type connector

92. Inverter storage
After removing the inverter the cover[s] should be reinstalled and any openings for cables should be covered with tape to prevent dust from contaminating the internal components
Image courtesy of Toyota U.S.A.

93. No serviceable parts
The are no replacement parts currently available for inverters/DC-DC converters
Any damage to the inverter or DC-DC converter requires replacement of the entire inverter assembly at a cost of several thousand dollars