Inverters & DC/DC Converters

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

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

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

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..!

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

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…!

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.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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.

Electric A/C Compressor 3 Phase AC Motor Oscillator Assembly

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.

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.

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

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

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

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

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

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

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

DC / DC Converter

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

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

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

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

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

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

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

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

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

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

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 -

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 +

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

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

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

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

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

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

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

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

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

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

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

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

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

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

Inverter cooling

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

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

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

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

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

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

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

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

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

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.

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

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

Inverter service

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

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

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

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

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

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

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

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

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

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

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.

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