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SAE TECHNICALPAPER SERIES 2002-01-0519

42V Power Control System for Mild HybridVehicle (MHV)

Hidetoshi Kusumi, Katsunori Yagi, Yoshihide Ny and Shouji AboToyota Motor Corporation

Hirohide Sato, Shigeki Furuta and Masami MorikawaDenso Corporation

Reprinted From: 42 Volt Technology 2002(SP–1661)

SAE 2002 World CongressDetroit, Michigan

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42V Power Control System for Mild Hybrid Vehicle (MHV)

Hidetoshi Kusumi, Katsunori Yagi, Yoshihide Ny and Shouji AboToyota Motor Corporation

Hirohide Sato, Shigeki Furuta and Masami MorikawaDenso Corporation

Copyright © 2002 Society of Automotive Engineers, Inc.

ABSTRACT

In the 42V Mild Hybrid System introduced into market byToyota for the first time in the world, the crankshaft usingbelt(s) drives the motor/generator (MG). The set-upemploys an inverter unit to control the MG electronically.This paper describes the system configuration,operations, characteristic features and developmentresults of the new power control system. The focus is onthe MG, the inverter-for-MG-control and energyregeneration, as well as DC/DC converter for the powersupply to the 14V devices.

INTRODUCTION

With the reduction of CO2 emission becoming a crucialinternational issue in recent years, "Prius" - a massproduction passenger car equipped with Toyota HybridSystem (THS)- was introduced into market by Toyota in1997 for the first time in the world. A combination of agasoline engine and an electric motor is incorporatedinto the Prius, which has sold 60,000 units so far.However, In order to facilitate more extensive use ofsuch low pollution cars in years to come, it is vital todevelop systems that are less expensive than the THS.These systems must be simple and easy to install, yetapplicable to various types of vehicles with general-purpose type functions.

On the other hand, international standardization activitiesfor the so-called 42V PowerNet are conducted mainly inthe US and Europe in order to increase the batteryvoltage to 36V. This is in line with the tendency ofincreasing electric loads in vehicles. It is promising as anext generation power supply system for vehicles. Notonly in terms of ensuring an adequate power supplycapacity, but also in weight reduction with a thinner wireharness, and the conservation of resources.

Under such constraints, Toyota developed anotherhybrid system named Toyota Mild Hybrid System, whichis intended for wider use [than THS]. This system can be

added to conventional vehicles with minimummodifications of existing power trains. A 36V-drivebattery is added according to the overall criteria of safetyagainst electric shock, operating performance, cost andtechnological advances. This is the first 42V-14V dualbattery system developed in the world.

This paper describes power control technologiesincorporated in Toyota Mild Hybrid System, together withfeatures and development results of the individual unitsconstituting this system.

OUTLINE OF SYSTEM

TOYOTA MILD HYBRID SYSTEM - This system isaimed at reducing exhaust gas emissions, fuelconsumption and idling noise, by adding new functions -idle-stop in particular - to conventional internalcombustion engines. The following functions areprovided according to vehicle running conditions:

When stopping vehicle - The engine stops promptly andsmoothly while being controlled by the motor/generator(MG). Upon request for activation of the air conditioneror power steering (PS) [by the driver], the air conditioningcompressor or PS pump can be activated with theengine stopped (auxiliary equipment driving mode).

When starting vehicle - The moment the brake pedal isreleased, MG starts the vehicle at the same time (EV-driving mode). When the driver steps on the acceleratorpedal, or after a given period of time has passed, enginefiring occurs and the engine-driving mode starts.

When running in ordinary condition - The vehicle runspowered by the engine, same as ordinary vehicles.

When decelerating - Upon braking, the wheels activatethe MG for power regeneration, and some of the brake-deceleration energy is recovered and stored in thebattery.

2002-01-0519

Figure 4-a. Photograph of a Motor/Generator.

Figure 4-b. Sectional structure of MG.

Figure 4 shows the appearance and the sectionalstructure of MG. The magnetic field winding method isemployed for the rotor magnetic field so that the intensitycan be varied easily by controlling the current. Hencethe 42V system is materialized by maximizing theintensity of magnetic field in the low speed range wherehigh torque are required to start the engine or EV mode.In the high-speed range, the intensity of the magneticfield is reduced to suppress the maximum voltageinduced by MG to 44V. Moreover, the induced voltagecan be suppressed to 58V or lower by means of three-phase short circuit control, maximum revolution speed of15000 rpm at the MG shaft.

The segment conductor (SC) method is used for thestator coil. That is, a flat and square conductor segmentis inserted from the shaft side into the core slot, and theother end of conductor is welded for connection. Owingto the above, the space for the conductor is reducedmarkedly, and the total size of the MG is reduced by 20%in comparison to the conventional round type conductor.In addition, by improving the connection method of theconductor, the magnetic noise of the MG is reduced byapproximately 7 dB in comparison to the magnetic noiseof the MG using conventional connection method.

INVERTER UNIT

The inverter unit is a control unit built into a singlemodule incorporating three parts - the inverter itself,DC/DC converter and MHV-ECU, with a coolant conduitsituated among them. The size of the unit as a whole, isreduced by incorporating the MHV-ECU, resulting in, notonly cost reduction of the entire system, but alsoenhancement of reliability. Moreover, both theinstallation ease and high production efficiency areestablished, while improving wire harness design. INVERTER - Figure 5 shows the appearance of theinverter developed under this study, and Figure 6 showsthe inner structure. The size reduction is accomplishedby using the newly developed compact 6 (arms) in 1(module) Intelligent Power MOS-FET Module (IPM), andan Ellipse-type aluminum capacitor. This capacitor wasnewly developed, and has a high heat discharge rate.Furthermore, the cost reduction of the entire system isachieved by incorporating the Mild Hybrid System andthe DC/DC converter on opposite sides, and sharing thewater cooling system between them. In order to facilitatethe installation on vehicle, the three-phase output unitand the harness connector are set on the top of inverterto enable easy connection.

Figure 5. Appearance of an inverter unit.

Output cable

ECU

Capacitor

IPM

(MOS-FETs)

DC/DC

converter

Figure 6. Inner structure of the inverter unit.

Ellipse-type Aluminum Capacitor - One of requirementsfor capacitor size reduction is the reduction of elementtemperature. There are two approaches for that purpose- one is to reduce the electrical loss by lowering theimpedance of the elements, and another is to increasethe heat discharge performance. The authors et al. havetaken the both approaches in a joint study with acapacitor manufacturer, and succeeded in achieving sizereduction by 40 % compared with the conventionalcapacitor.

Conventional Newly developedcapacitor capacitor

Figure 7. The shape of the capacitor.

Figure 7 shows the shapes of the conventional capacitorand the newly developed capacitor. In case of theconventional one, a space is provided over the entirecircumference between the element and the case tosuppress the rise of inner pressure. This accounts for thedeterioration of heat discharge rate. The volume is alsolarge due to its circular shape. In case of the newlydeveloped capacitor, the case is flattened with both sidesof the element adhered to the case. Also, the cathodebox is touching the bottom of the case, in order toincrease the heat discharge rate. Figure 8 shows thecomparison of the temperatures data. It is found that thetemperature is reduced by 20% at the 20Arms ripplecurrent in the newly developed capacitor.

[Arms/1kHz]

delta T [0C]

0

10

20

30

40

0 10 15 20

Ellipse

Circula

Figure 8. Comparison of the Temperature data. 20%reduced at the 20Arms ripple current.

MG Control – The rotor field winding in the MG enablesto control both the stator current and the rotor currentindependently by the external control circuit in theinverter, so that the following operations can be done inindividual modes.

The maximum torque is required upon starting. Hencethe magnetic field current is maximized while the statorcurrent is suppressed in order to minimize the capacity ofcurrent required for IPM. When driving the auxiliaryequipment and generating power, it is in a continuousoperation mode without requiring much stator current forstarting. Therefore, the magnetic field current and thestator current are so controlled to maximize the systemefficiency. In the low speed range, the MG output powerdrops and adequate power generation cannot beprovided. Hence the inverter increases the voltage toproduce enough power. Protection against Overvoltage - Should the batteryterminal come off while the system is working in thepower generation mode, over-voltages will happen at theinverter and the DC/DC converter. This results inovervoltage failure, since the system itself is incapable ofabsorbing the power generation energy. As a protectionagainst such failure mode, over-voltages are detected atthe input voltage detection circuit and the magnetic fieldis shut off accordingly. However, the magnetic fieldcurrent keeps on flowing despite the damping of thecurrent-reversing effect of the diode, and the terminalvoltage keeps on rising. In this regard, it is so designedthat the magnetic field current is cut off. The lower arm ofthe power element constituting the three-phase bridge isturned on forcibly to cause three-phase short-circuit, andlet the MG consume the energy to prevent the rise involtage. Figure 9 shows a result of three-phase short-circuit function has caused.

Overvoltage signal

Input voltage

Stator current

Rotor current

Action of protection

Figure 9 Voltage trace of overvoltage protection bythree-phase short-circuit.

IPM (POWER MOS-FET MODULE) - This inverter usesa 6 in 1 MOS-FET power module as mentioned earlier.The authors et al. have succeeded in the cost/sizereduction with the most advanced trench process in theelement unit, minimizing the cell unit size, and using thepower MOS-FET developed anew. Figure 10 shows theappearance of the module.

Figure 10 . Appearance of the MOS-FET power module.

Module Unit - It is so constructed that the three-phasemotor can be driven with one 6 in 1 module. In order toensure maximum 400 A rating per arm, 200A senseMOS-FETs are connected in parallel. An adequateresistance is provided to MOS-FETs so that they arekept intact even if the current exceeds the rated value.Figure 11 shows the measured results of voltagecharacteristic curve in off-mode. It was established thatthe unit did not break down by overvoltage under the testconditions shown in the figure. It was also determinedthat the optimum off-resistance of 1.3 m omega per armis obtained

V-gate

Ic

Vce surge

Figure 11 The measured results of voltagecharacteristic in off-mode

The heat discharge unit requires a material with highthermal conductivity and a linear expansion coefficientclose to that of the insulation substrate. This is in order toendure thermal stresses caused by the difference intemperature. Al-SiC was used in past whereas CuMo isselected for this unit, resulting in improvement in thermalconductivity by 10% and the cost reduction by 20%compared with the conventional unit. Running the currentpath back and forth on one substrate, resulted in a 35%smaller element insulation than the conventionalsubstrate

Control Circuit Substrate - A control circuit substrate isprovided for the generation of the MOS-FET drive gatesignals. The control circuit acts as the self-protectioncircuit, and generates diagnostic information. Thesubstrate is subjected to radiation electromagnetic noise,due to the presence of a large current switching unitlocated immediately below the substrate. For the

prevention of such noise, a metallic shield is installed.The substrate consists of four layers and doublesurfaces.

Protection against Overheat - Appropriate protectionagainst overheat is ensured by installing a temperaturesensing diode at the center of MOS-FET chip. Thisenhances the temperature detection accuracy.

Protection against Overcurrent - As the subMOS-FETwith the sensing function detects the branched outcurrent, the current passing through the mainMOS-FETcan be determined accurately. This provides theprotection against overcurrent. Upon sensing ofovercurrent, the control IC detects the abnormality andcuts off the gate voltage. However, if the current risessharply by short-circuit, etc., the hard circuit shuts off thegate directly and immediately without waiting for stopcontrol action by the IC. This prevents the devices frombraking down.

Protection against Voltage Drop at Gate Power Supply -If the gate voltage drops, both the on-resistance and theloss will increase. Therefore, if the gate power supplyvoltage drops, the module will judge it as an abnormalityand the MOS-FETs will not be activated.

DC/DC CONVERTER

The DC/DC converter drops the high voltage (42V) ofMild Hybrid System power supply to the auxiliary powersupply voltage (14V). The reason is to supply power tothe auxiliary system, even if the engine is stopped.

Synchronous Rectification Method - Synchronousrectification method is adopted for the enhancement of42-14V conversion efficiency. Figure 12 shows thecomparison of efficiency between the cases with andwithout synchronous rectification. Aiming at high speedswitching, MOS-FET is also used here. In order toreduce the size of the L-C, we decided to accomplisheven higher switching speeds of the MOS-FETs. As canbe expected, this approach increases the problem ofheat dissipation. Reducing the generated losses by theMOS-FET, was accomplish by redesigning if for lowerresistance. Lower resistance was established by dividingthe activation range into smaller segments.

Power Module Unit - Heat sink-less feature is achievedby pushing the ceramics substrate (with elementsactually soldered) directly against the case, using thepushing force of a plastic part (leaf spring). A bare chipis used on each MOS-FET so as to minimize the currentloop length, which in turn minimizes the wiringinductance and the switching surge voltage. Figure 17shows the switching surge voltage data compared withthose of molded part(s).

As for manufacturing, production cost is reduced bysoldering bare chips and other elements within a singleprocess using eutectic crystal joining re-flow method, sothat the manufacturing process is simplified.

Peak of surge voltage

Figure 17-a. Switching surge voltage generatedby mold type MOS-FETs.

Figure 17-b. Switching surge voltage generatedby bare MOS-FETs.

RESULTS OF INSTALLATION ON VEHICLE

Figure 18 shows the current-voltage characteristic curvewhen MG started the engine after idle-stop with thesystem installed on the vehicle. A smooth engine start isfound without stalling.

Input voltage

Input current

Output current

Figure 18. Current and voltage characteristic curvewhen MG starts the engine.

CONCLUSION

The easy-to-install 42V power control system for MildHybrid Vehicle was developed and introduced intomarket for the first time in the world, with [uniquefeatures such as] the high applicability to various types ofvehicles [and other areas] as described below.• The MG (motor/generator) to start the engine

occupies the same location as the conventionalalternator. It is driven by belt.

• The inverter unit uses the newly developed Ellipse-type aluminum capacitor, and the size is reduced byintegrating the ECU for the whole Mild HybridSystem control. Also, the reduction in both size andpower loss is achieved by employing the newlydeveloped MOS-FET module for power modulation.

• The efficiency of DC/DC converter is enhanced byusing synchronous rectification method. The size isreduced by using a high speed switching device.

• Toyota Mild Hybrid System is world’s first 42VPowerNet system with a 36V battery and hasattained higher fuel economy by 15 % in the 10-15[driving] mode. This is owing to the structureconsisting of the above mentioned various units.

Since the system can be easily installed on other powertrains, it is highly practical. We expect that the systemwill contribute to facilitate more extensive use of lowpollution cars in years to come.

REFERENCES

1. Abe, Kubo et al., “Development of Hybrid System forMass Productive Passenger Car”, Journal of theSociety of Automobile Engineers of Japan. Collectionof Presentation Manuscripts 975, 1997.

2. Teratani et al., “Development of Vehicle with Idle StopSystem “, Toyota Technical Review Vol.50 No.1,Sep.2000.

3. J.Kassakian, “Challenges of the New 42V Architectureand Progress on Its International Acceptance”, VDIBerichte NR. 1415, 1998.