Renny Mathew, Sr Hardware EngineerRobert Bosch Engineering and Business Solution

Controls, Measurement & Calibration Congress

On board diagnosis of pre-supply fuel pump control Module with MOSFET’s “Rdson” as diagnosis element

Renny Mathew Senior Hardware Engineer

Robert Bosch Engineering and Business Solution

ABSTRACT

In automotive, on-board diagnosis does the self-diagnosis and the fault reporting. It provides the required level of robustness to the control electronics. The amount of diagnostic information available via OBD as always varied widely depends on the category of vehicle and its usage. Pre supply fuel pump comes with the common rail system it pumps the fuel from fuel tank to the high pressure pump inlet. This paper discuss about the on board diagnosis provided in a pump driver module which has two power transistors used as switching elements for motor control and “Rdson” of these transistors as the fault diagnosis element. It operates in dual frequency and variable duty cycles, and provide maximum possible diagnostic coverage in all operating modes. The OBD that diagnose the following faults associated with the power stage, Motor in-rush current, Rotor Lock, Short circuit to ground, Short circuit to battery, Line to line short circuit between HS and LS pins and Open load. These are the normal fault conditions which are having higher probability to occur during the life span of the control electronics and can cause the undesirable operating conditions or destruction of the electronic components in the ECU. OBD against these faults with” Rdson” as the fault detection element discussed.

The wiring impedance plays significant role in the fault diagnosis discussed here, since the long wire normally used to connect the electronic control units with the pump which is immersed in fuel in the fuel tank. The resistance and inductance of the wiring harness are the function of wire diameter and length. The below listed parameters decides the fault diagnosis range of the module with respect to a particular operating condition when” Rdson “used as diagnosis element.

Fault detection threshold (Vt h), “Rdson” of the MOSFET, Fault current magnitude, Frequency at which power stage operates, Duty cycle, Thermal response of the MOSFET, Safe operating conditions, blind bands and MOSFET turn on/off delays

These parameters and its typical relationships between each other in different operating conditions are the challenges to provide maximum diagnosis coverage without compromising the hydraulic response of the pump. This paper focus in detail about the different fault conditions and the challenges to provide maximum diagnosis coverage when power stage operates in different duty cycles and operating frequencies with “Rdson” of the MOSFET as the diagnosis element.

INTRODUCTION

The pre supply fuel pump comes as an essential component in common rail system when the rail used in high pressure applications. Pre supply fuel pumps coupled with brushed DC motors are normally used in diesel applications. The operation of the pre supply fuel pump and high pressure fuel pump together makes the common rail in to the enquired pressure level. Both are controlled by the electronic control unit with respect to the pressure demand on the rail. There are multiple ways to control the dc motor to achieve required hydraulic response however this paper discuss only about the electronic power stage consists of two transistors, one high side transistor used as controlling element where other one used as freewheeling transistor. Both the transistors are driven by specially designed ASIC. The ASIC drives the power stage in dual frequency modes and variable duty cycles. The” Rd son” of both transistors are used for the fault diagnosis purpose. The current though the motor coil is monitored and regulated by the precession low side current shunt during normal operating condition. The circuit topology has been opted here by considering product cost as the important factor, must be affordable to the markets of developing countries. However it ensures the effective drivability of motor and provides the maximum possible diagnosis coverage.

FUNCTIONAL DESCRIPTION

Figure 1: Functional Block diagram

The power stage consists of two transistors one high side transistor as control element and another for freewheeling. Both the transistors are controlled by the PWM signals generated by the transistor driver ASIC. The driver ASIC has been specially designed with internal monitoring modules to monitor the voltage across the drain and source pins of the transistors, those monitoring modules are register programmable by the micro controller. The high side transistor will be switched in a particular frequency and variable duty cycle. The operating frequency of the power stage can be controlled by the main micro controller through the SPI command depends on the operating conditions. Motor operates always in continuous current mode and two operating point current control used to regulate the current. The motor current will be sensed through a low side current shunt however the current shunt has not been utilized for fault diagnosis purpose due to the limited gain of measurement amplifiers associated with its internals, except open load detection. The gain of the internal current measurement amplifiers are accurate only in the low magnitude current measurement maximum up to 10A. But during the fault conditions current will rise drastically to 30A above.

The fault current monitoring is completely handled by theVds monitoring modules in the ASIC. The ASIC is flexible enough to configure the reference voltage of the monitoring comparators through the SPI communication with the micro controller. Different layer fault diagnosis strategy has been implemented in the power stage based on the failure mode and its effect analysis on the power stage. Some handled by the hardware, some are with software and some other faults are handle software and hardware together. The faults which are normally occurs with the power stages for the application this kind have been discussed here for example startup inrush current, blocked rotor, SCG, SCB, OL etc. Over load and short circuit to the ground faults are detected by the Vds monitoring module of the high side transistor while the short circuit to the battery fault detected by the Vds monitoring module of the active freewheeling transistor. Open load fault condition will be detected with the low side current shunt and internal load biasing current source provided inside the ASIC.

Figure 2: HS Vds and Motor current @ D=50%.f = 5 kHz, Vbat=16V

The power stage is designed to operate in two different frequencies and these frequencies are possible to change during the operating condition of the power stage by micro controller SPI command based on the operating condition. Power stage will operates in higher operating frequency during heavy load condition but always use low frequency during low load or normal load operation. The flexibility of the power stage to operate in two different frequencies helps the power stage to control the pump effectively without dissipating more and enhanced the fault diagnosis range of the power stage since Rdson as the fault diagnosis element. Ref: equation (2) and equation (3)

On board diagnosis and Description of faults:

The diagnosis strategy normally specific to the power stage and its load behavior. Faults can happen any time during the life span of the ECU, may be in assembly line, during testing, during the service, during the normal operation and so on. The potential of the faults to damage the power stage depends on many parameters like age of ECU, strength of the components, design of the power stage etc. The study about the possible failures and its effects (FMEA) are significant in power stage design and on board diagnosis strategy development. This paper discuss about the failures that can normally occur with the pre supply fuel pump drivers. The failures such as, Inrush current during start up, Rotor lock, Short circuit to the ground, Short circuit to the battery and Open load are discussed.

The fault may be permanent or temporary depends on the causes. No matter faults is permanent or temporary fault reaction switch off the power stage instantly and initiate periodic test pulses for specific duration to confirm the nature. It is important to identify the nature of the fault, it is not necessary to keep the power stage off for the fault events which are temporary. Temporary faults normally occurs for short duration and it disappears. ECU can continue to operate in this case. Although the faults are which are permanent makes power stage turn off forever. Power on restart necessary to make power stage functions again.

Inrush Current:

Inrush current is only significant during the motor start and the duty cycle changeover operating condition, the motor current will rise drastically due to the inertia of the fuel and rotor. During start up the rotor will be in steady state only the wiring harness impedance and motor coil impedance will restrict the current rise, the magnitude of the current shoot up will be decided by these parameters in together with the battery voltage. To avoid the damage of power stage due to this high current it is necessary to chop down the motor current in to the safe level. Furthermore, the duty cycle always vary with respect to the operating points of the pump. Any sudden demand from rail pressure can result the drastic change in duty cycle from one value to another, each of these sudden change of duty cycle cause high current shoot up due to the inertia of the fuel and motor rotor. If it is not controlled the current go beyond the safe level of the power stage for certain duration can damage the power stage. Normally this sudden duty cycle change over restricted by the slope function without affecting the engine performance. However this slope function will be specific to the motor and wiring harness. On board diagnosis provided in the power stage respond to any inrush over current events which can occur in the life span of the ECU. HS transistor Vdscomparator always monitors the inrush current by monitoring the Vds of high side transistor. If the current exceeded certain level for particular duration OBD will turn of the power stage and error will be reported to the microcontroller.

Fault threshold set in the OBD specific to the wiring harness impedance, this will create unintended fault detection if wiring harness or motor changed. To overcome the problem of unintended fault reporting during the normal operation validation may be necessary whenever OEM demands any change in the wiring harness or pump.

Figure 3: Typical Inrush current shoot up during duty cycle change 15% to 95%, f=¿ 5 kHz, Vbat=16V

If wiring harness is short and motor impedance is less, there is a good possibility to shoot up the current in to higher values and detected as SCG.

Rotor Lock:

The accumulation of the particles in the filter net is the normal reason of rotor lock in pre supply fuel pump. This condition is harmful to the power stage. Sudden duty cycle rise in rotor lock condition will cause very high current shoot up through the power stage. And power stage will be switched in abnormal magnitude of current, it will heat up the transistor due to higher dissipation of power. Normally the current will not reach in to the magnitude it detect as SCB since this fault is also diagnosed through the HS transistor Vds monitoring comparator however by calculating the average value of current flowing though the transistor ” Rdson” for some specific time interval this fault will be diagnosed. So the average value of current beyond certain level for specific time duration will be detected as fault. Power stage will be turned off.

Figure 4: Typical Rotor Lock current shoot up during duty cycle change 15% to 95%, f = 5 kHz, Vbat=16V

Short circuit to ground fault:

Figure 5: SCG description

Short circuit to ground fault will result drastic current rise through the high side transistor. The fault will be identified by detecting the exceptional high current. The fault detection is possible only when the magnitude of current reached in a

magnitude large enough to generate the Vdsequal to the fault threshold in Vdsmonitoring comparator. The Vds magnitude during the fault depends on the following parameters.

1. Rdson of the high side transistor2. Duty ratio and frequency at which power stage operates3. Wiring harness impedance4. Fault threshold voltage programmed in the comparator

“Rdson” Is specific to the transistor used as high side switch. It is important to select the transistor has “Rdson” matching with the programmable range of the fault detection comparator of the ASIC. It is necessary to discard the invalid Vds values during switching transitions, every turn of events of the high side transistor will make the Vds up to the battery voltage. During the switching transitions the Vds measurement must be disabled in order to avoid the detection of it as fault. The relationship of fault current and Vds of the HS transistor shown in Figure 6.

Figure 6: Vds rises with SCG fault current

Relation between the “Rdson “and fault current:

Vds=Rdson× Iscg(1)

Iscg=¿ Fault current

Rdson= On resistance of the HS MOSFET

Vds= voltage across drain and source poin of the MOSFET

Relation between the Duty ratio and time window for fault current to rise:

Ton=Df

(2)

f = switching frequency

D= Duty ratio

Ton= On duration of the HS MOSFET

Relation between the fault current, battery voltage and wiring harness impedance:

Iscg=VbatR

(1−e LRt )(3)

Vbat= Battery voltage

R= total resistance of the fault current path

L=Inductance of the fault current path

Iscg=Fault current

When the power stage operates in lower duty ratios the on time window of high side transistor may be not sufficient enough for the fault current to rise to the magnitude at which it generates the voltage drop across the Vds of the MOSFET equal to the threshold value. In this condition the on board diagnosis provided in the power stage cannot detect the fault immediately. Power stage will be repeatedly switching for some long duration in the fault current, Figure 7. This will result the heating of transistor due to the higher power dissipation. The pump delivery rate will be reduced due to the fault the ECU detects the pressure drop and issue the command to operate the pump in higher duty cycle to increase the delivery rate. This new duty cycle will make the Ton window wide enough to rise the fault current to higher magnitude through the high side transistor and thus fault will be detected. However this will be delayed some milliseconds, Figure 7, during this period the high side transistor will undergo inevitable thermal stress in this topology. In high ambient temperature e.g. 105 degree Celsius this heating will increase the Rdson gradually, this increaseVds, finally fault detection takes place after certain delay. So during each fault events will heat up the transistor for certain duration due to the higher power dissipation. This is very important among other parameters to choose the high side transistor for this kind of circuit topologies.

Since, the fault diagnosis is not possible in lower duty cycles below a certain value. Whenever a new wiring harness or pump needed to be interfaced with the power stage it is necessary to validate and identify the fault diagnosable window for SCG in this power stage. As wiring harness impedance increases the fault diagnosable duty cycle will be reduced for a particular operation temperature and battery voltage level.

Figure 7: SCG detected with delay of 5ms in D=20%, f=¿ 5 kHz, Ambient =100°C. Thermal rise of Rdson results the fault detection.

Short circuit to battery fault:

Figure 8: SCB description

During this fault the high side transistor will be bypassed by the fault current path. So irrespective of the switching state of the high side transistor the fault current will flow through the active freewheeling transistor during its turn on window. This will produce the large voltage drop across the” Rdson “of the active freewheeling transistor. The turn on time Vds is under the surveillance of Vds monitoring comparator and whenever the Vds reaches to the programmed threshold level it will be detected as fault. Unlike short circuit to ground fault, the short circuit to battery fault will not be detected during the higher duty cycles, the active freewheeling transistor turns on when HS transistor off. During this fault the pump delivery will be drastically increased. This sudden pressure rise will be detected by the ECU and pull the power stage to lower duty cycles. This lower duty cycle will provide big turn on window to the active freewheeling transistor so that the fault current will get enough time to rise to the magnitude at which it produce the Vdsequal to the threshold programmed at which fault would be detected. The normal fault detection interval comes in milli seconds and produce an inevitable thermal stress on the active freewheeling transistor due to the higher power dissipation.

Figure 9: AFW FET during duty cycle change over SCB detected

Open load fault:

OBD detects this fault whenever the motor unplugged from the power terminals. The unplugging can happens with the corrosion or vibration those are decided by the operating environment. There is an internal load biasing current source provided in the ASIC to detect this fault before the power stage activate. During the normal operation this fault diagnosis has been implemented by monitoring the minimum current flow through the low side current shunt. If the power stage is active and load present, there is minimum quantity of current flow occurs always through the low side current shunt. This current will come below a particular value if and only if load has been disconnected from the terminals. Whenever any such current flow observed below the threshold level, power stage will shut off immediately and initiate the periodic test pulses to confirm whether it is permanent or temporary. During the test pulse intervals load has been connected back to the terminals again power stage will resume its normal operation. If not it will shut of permanently.

Figure 10: Temporary OL detected and pump operation resumed

Figure 11: Permanent OL detected and pump operation stopped

CONCLUSION

The fault diagnosis and reporting normally contains large steps of process in modern automotive electronics. It increased the cost of development and development time significantly. Multi-level on board diagnosis are used now a days in automotive industry some are with hardware level, some are with software and some OBDs are with the combination of both hardware and software. The fine tuning of diagnosis concept specific to the systemic behavior for any system is always very important and complicated. The complexity depends always on the system architecture and the technology used. Here” Rdson” Based fault diagnosis is a cost effective solution for the kind of power stage discussed here. Unlike precession current shunts the accuracy of the” Vds” measurement normally limited. Because” Vds “will widely vary with respect to the temperature at which the component operates. However the proper simulation and validation of the power stage in the various operating conditions will help to choose the appropriate fault detection thresholds against various fault. The” Rdson “of the transistor must be matching with the programmable fault thresholds range of the internal comparators which monitors” Vds “inside the driver ASIC. It is important to provide effective diagnosis against the flexibility of configuration. This makes selection of the transistors with certain” Rdson” is very important. It decides the limit of the faults current shoot up. The above discussed topology shows the difficulty to detect the faults when the power stage operate in the lower duty cycles. The difficulty will be increased further when the powers stage operates in higher operating frequencies in this design.

REFERENCES

1. Robert bosch DGDI-S design specification document2. Robert bosch electronic product specification Manuel 20153. THOSHIBA Power MOSFET in details description note4. A Transient Temperature Prediction Model for Power MOSFET in ECU5. International Journal of Advancements in Computing Technology(IJACT)6. Volume5, Number15, November 20137. Infineon+-+Application+Note+-+PowerMOSFETs+-+CoolMOS™+-+C3+-+Power+Handling+Capability8. Infineon Application note Relation between dynamic and static thermal impedance

CONTACT

This is where main author information is typed, if desired, such as background, education, e-mail address, and web address. This is an optional section.

Renny Mathew works as senior hardware engineer at Robert Bosch engineering and business solutions India limited. The responsibilities are to design and develop hardware modules for solenoid fuel injector valve, pre supply fuel pump controls, and quantity control valve applications for diesel gasoline projects. Currently working at Robert Bosch GmbH in Germany on the requirement management for transmission control electronic hardware developments and functional safety ASIL-D compliances of automatic transmission control units (Dependent failure analysis and FMEDA).

renny.mathew@in.bosch.comrennymathew.r@gmail.com+49 7141 5078265+49 1514 6860058

DEFINITIONS, ACRONYMS, ABBREVIATIONS

ECU: Electronic Control UnitTCU: Transmission Control UnitSCG: Short Circuit to GroundSCB: Short Circuit to batteryOL: Open LoadOBD: On-board diagnosisFET: Field Effect TransistorAFW: Active freewheelingRdson: Resistance of the MOSFET when it is completely onVds: Voltage across the drain and source pins of the MOSFETIscg : Short circuit current when SCG occursIscb: Short circuit current when SCB occursf : FrequencyD: Duty ratioDGDI-S: Diesel Gasoline Direct Injection – SolenoidOEM: Original equipment manufacturesHS: High side transistor switchLS: Low side transistor switchTj: Junction temperature of the MOSFETTc: Temperature of the MOSFET casingDFA: Dependent failure analysisFMEDA: Failure mode effect and diagnosis analysisFME: Failure mode effect analysisSPI: Serial peripheral interfacePWM: Pulse width modulation

APPENDIX

Rdson And MOSFET junction temperature:

The” Rdson” of the MOSFET possesses wide margin thermal tolerance. Which will vary widely with operating temperature of the MOSFET. In high temperature ambience the heat exchange from the ECU to the surroundings will be slow down, it results the accumulation of heat generated by the switching events. Which will crease the” Rdson” to higher values, in response to that Vds of the MOSFET also will increases and this will influence fault threshold. The actual fault detection threshold always vary depends on operating temperature of the MOSFET varies.

Figure A1: Rdson vs Junction temperature

The below image Figure A2 show the relationship between the case temperature development with respect the junction temperature rise. Continuous switching events makes MOSFETs heat up initially however it settle in a constant value once the heat transfer reaches in an equilibrium( Heat generated = Heat dissipated). Thus the gradient at which junction temperature rises is the function of the heat conduction factor from the junction to the casing.

Figure A2: Case temperature vs time

At the beginning the junction temperature and case temperature of the MOSFET varies almost linearly together. Once the heat conduction reached in the thermal equilibrium, both the parameters will be settled in a constant level Figure A3.

Figure A3: Junction temperature and Case temperature vs time

Junction Temperature ΔT JC=Pd×R thJC

During the fault condition MOSFET will generate more heat due to the higher switching loss Figure A4 compared to the normal switching conditions. This will disturb the thermal equilibrium of the Power MOSFET and Junction temperature of the MOSFET will start to increase further. It is very important that to restrict the heat development below certain limit which will not stress the component thermally. If the component undergo frequent thermal stresses that can bring down the age of the component less than the calculated range or left behind the probability of failure. So that the PCB design and components selection must be considering this parameter as well.

Figure A4: Junction temperature during normal operation and Fault conditions

For the prolonged pulsed switching operation the steady state thermal resistance (Rth) must be considered for the junction temperature calculation or simulation. However the junction temperature calculation must be with Zth when the junction temperature is being calculated for the faults, which occurred for short duration, where the thermal resistance offered by the component to the PCB will be less that the steady state value. Typical relation between the pulses duration and the thermal conductivity factor showed in below image.

Figure A5: Transient thermal impedance of the MOSFET