Download On board diagnosis of pre-supply fuel pump control Module with

Renny Mathew, Sr Hardware Engineer
Robert Bosch Engineering and Business Solution
Controls, Measurement & Calibration Congress
On board diagnosis of pre-supply fuel pump control Module with
MOSFET’s “𝑹𝒅𝒔_𝒐𝒏” 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 “𝑅𝑑𝑠_𝑜𝑛” 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” 𝑅𝑑𝑠_𝑜𝑛” 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” 𝑅𝑑𝑠_𝑜𝑛 “used as diagnosis element.








Fault detection threshold (𝑉𝑡ℎ),
“𝑅𝑑𝑠_𝑜𝑛” 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 “𝑅𝑑𝑠_𝑜𝑛” 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” 𝑅𝑑𝑠𝑜𝑛 ” 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 the 𝑉𝑑𝑠 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
𝑅𝑑𝑠_𝑜𝑛 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 𝑉𝑑𝑠 comparator 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 ” 𝑅𝑑𝑠_𝑜𝑛” 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 𝑉𝑑𝑠 equal to the fault threshold in 𝑉𝑑𝑠 monitoring comparator. The 𝑉𝑑𝑠 magnitude
during the fault depends on the following parameters.
1.
2.
3.
4.
𝑅𝑑𝑠_𝑜𝑛 of the high side transistor
Duty ratio and frequency at which power stage operates
Wiring harness impedance
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:
𝐷
𝑇𝑜𝑛 =
(2)
𝑓
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 =
𝑉𝑏𝑎𝑡
𝑅
(1 − 𝑒𝐿𝑅 𝑡 )
(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 𝑅𝑑𝑠_𝑜𝑛 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 𝑅𝑑𝑠_𝑜𝑛 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” 𝑅𝑑𝑠_𝑜𝑛 “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 𝑉𝑑𝑠 equal 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”
𝑅𝑑𝑠_𝑜𝑛” 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” 𝑅𝑑𝑠_𝑜𝑛
“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” 𝑅𝑑𝑠_𝑜𝑛” 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.
2.
3.
4.
5.
6.
7.
8.
Robert bosch DGDI-S design specification document
Robert bosch electronic product specification Manuel 2015
THOSHIBA Power MOSFET in details description note
A Transient Temperature Prediction Model for Power MOSFET in ECU
International Journal of Advancements in Computing Technology(IJACT)
Volume5, Number15, November 2013
Infineon+-+Application+Note+-+PowerMOSFETs+-+CoolMOS™+-+C3+-+Power+Handling+Capability
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).
[email protected]
[email protected]
+49 7141 5078265
+49 1514 6860058
DEFINITIONS, ACRONYMS, ABBREVIATIONS
ECU: Electronic Control Unit
TCU: Transmission Control Unit
SCG: Short Circuit to Ground
SCB: Short Circuit to battery
OL: Open Load
OBD: On-board diagnosis
FET: Field Effect Transistor
AFW: Active freewheeling
𝑅𝑑𝑠_𝑜𝑛: Resistance of the MOSFET when it is completely on
𝑉𝑑𝑠: Voltage across the drain and source pins of the MOSFET
𝐼𝑠𝑐𝑔 : Short circuit current when SCG occurs
𝐼𝑠𝑐𝑏: Short circuit current when SCB occurs
𝑓: Frequency
𝐷: Duty ratio
DGDI-S: Diesel Gasoline Direct Injection – Solenoid
OEM: Original equipment manufactures
HS: High side transistor switch
LS: Low side transistor switch
𝑇𝑗: Junction temperature of the MOSFET
𝑇𝑐: Temperature of the MOSFET casing
DFA: Dependent failure analysis
FMEDA: Failure mode effect and diagnosis analysis
FME: Failure mode effect analysis
SPI: Serial peripheral interface
PWM: Pulse width modulation
APPENDIX
𝑹𝒅𝒔_𝒐𝒏 And MOSFET junction temperature:
The” 𝑅𝑑𝑠_𝑜𝑛” 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” 𝑅𝑑𝑠_𝑜𝑛” 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: 𝑅𝑑𝑠_𝑜𝑛 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.
Junction Temperature 𝛥𝑇𝐽𝐶 = 𝑃𝑑 × 𝑅𝑡ℎ𝐽𝐶
Figure A3: Junction temperature and Case temperature vs time
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