Inverters & DC/DC Converters

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. DC-DC converter • 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 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. 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 + 300 V • 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 Inverter for MG2 • The IGBTs are bonded to a flat steel heat sink plate that transfers heat from the IGBT to the inverter case Inverter for MG1

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

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

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 +B -B Stator for MG1 Control module Stator for MG2

27. 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 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. 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 computers to ignition switch 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 • The DC-DC converter is made up of three components: • Oscillator • Step down transformer • Rectifier Rectifier High voltage battery current Oscillator 14 Volts DC Chassis ground

46. 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. Transformers • 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. Transformers • The input of a transformer is called the primary coil • The output of a transformer is called the secondary coil • 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 Primary coil winding Secondary coil winding

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 + _