Automotive Application - Load Dump Analysis
Computer Aided Load Dump Analysis in 42V Automotive Electrical Systems
Bill Burns
Scott Stanton
Uwe Knorr
Ansoft Corporation, Pittsburgh PA
Abstract
Interest in the mitigation of failure effects in automotive power generation systems has been renewed by the prospective use of 42V systems and the fundamental challenges brought on by those architectures. Of particular concern are prospective secondary over-stresses of electronic parts resulting from sud-den battery cable disconnect, or load dump. Laboratory studies of load dump and the sec-ondary failures are costly, and difficult to carry out and repeat. Cost effective computer simu-lation tools are available to study these failures by executing numerous, exactly repeatable “What-If” analyses. To perform these analyses several physical domains have to be com-bined. The system used in this paper com-bines a precise motor model, the electrical load systems and several control mechanisms. In the interest of short simulation times model-ing has to be performed at different abstraction levels. A major problem is the generation of accurate motor models without introducing a detailed finite element analysis. The paper shows the combination of an electrical ma-chine design tool – RMxprt with a multi-domain system simulator - SIMPLORER. The analysis shows the results of a number of simulations produced from models mapped from the ma-chine design tool into the system simulation tool. RMxprt incorporates technology data and physical dimensions, winding characteristics, and material properties of a specific machine to develop a state space model which can be imported into SIMPLORER as a single compo-nent. Using the schematic feature, the user constructs or retrieves controlling electronic topologies, loads, and failure mechanisms to connect with the imported machine. The effec-tiveness of alternative protection schemes are evaluated until a suitable design is found. The unique combination of RMxprt with SIM-PLORER enhances the ease-of-use and accu-racy of the computer analysis, preserves ex-pensive prototypes, and speeds the engineer toward the best design or minimizes the alter-natives for laboratory test. Load Dump Worst Case
Exposure to automotive load dump warranty expenses and customer dissatisfaction from this “walk home” failure drive automobile manufacturers and suppliers to design and test methods to protect against the secondary fail-ures caused by this event. Load dumps are battery disconnects from the main voltage bus that occur during vehicle operation. Discon-nects can occur due to broken wires, loose connections and lugs, or unwitting mainte-nance. With the battery and loads discon-nected the resulting inductive spikes and alter-nator output must then be absorbed by the generation stage. Damage from the arc at the point of separation will also occur. Energy dis-sipated in the arc helps to reduce damage elsewhere confining the voltage level to some 20 volts. Arcing in 14 volt systems quickly ex-tinguish but 42 volt systems can produce arcs that exist indefinitely creating a fire hazard. Sometimes these energy dumps are accom-panied by disconnects of the electrical loads themselves. While the mutual disconnect of load and battery will save the loads from the energy surge, the rectifier bridge of the vehicle alternator remains jeopardized. The worst case condition for the alternator output stage is a disconnection of the battery and the load with-out an arc. This scenario is analyzed under different control regimes.
Approach
Testing of protection methods typically results in destruction of parts. When a limited supply of expensive alternator prototypes are at hand, computer simulation trumps physical test. Many automotive manufacturers and suppliers are currently testing new 42V alternator and integrated starter alternator combination proto-types. An efficient application of the proposed design flow can help to analyze system behav-ior before expensive prototypes are tested. SIMPLORER is a multi-domain simulation package combining circuit, state machine and block diagram simulation. It is based on a unique simulator coupling technology combin-ing the advantages of different modeling ab-straction levels with tailor made algorithms for different technical domains. SIMPLORER is directly linked to RMxprt – the analytical elec-trical machine design tool. RMxprt analyzes technology data, slot shapes, winding charac-teristics, and material properties to calculate
machine performance and parameters. Due to the analytic nature of the design algorithms results are available virtually instantaneous. The user derives a variety of important design data, such as lamination, winding schemes, performance curves and a complete design result report. The report data can be used to define specifications for suppliers or to provide customers with detailed technical information about the electrical machine. Built in interfaces allow data transfer into multiple design stages and enable concurrent engineering. While the analysis is running automatically a SIM-PLORER simulation model is generated and ready to use within a system level or circuit model. Additionally a design project for An-soft’s FEA (Finite Element Analysis) software Maxwell 2D/3D can be generated. RMxprt can solve 9 different motor types ranging from in-duction motors to permanent magnet, syn-chronous and switched reluctance motors. It can also solve for two types of generators: field wound synchronous generator and per-manent magnet generator. In calculating the machine parameters, RMxprt uses the Schwarz-Christoffel Transformation and allows for non-uniform air gaps. The solution also takes into account the non-linear material characteristics of the stator and rotor.
Typical Alternator – Regulator Topology
A typical vehicle charging system schematic is shown in Figure 2. In this case the system voltage is 42V and a 36V battery is employed. The alternator field is controlled by the low-side driver transistor BJT1 via a pulse width modulation signal from the comparator imple-mented through SIMPLORER’s state machine feature. Regulation is completed by feeding back the output voltage, calculating and ampli-fying the error signal, and comparing the error with a 20 hertz ramp signal in the state ma-
chine. In order to implement a worst case load dump the switch S1 would be used to remove the battery and loads from the alterna-tor/regulator output. The active side is then left to absorb the resulting inductive spike and generated energy: the diode trios, alternator field winding, driver transistor, and re-circulating diode.
Test Topology
The baseline circuit described in the preceding section is now modified to incorporate an RMxprt alternator model with a generic high-side IGBT as a dissipative regulator driver. This topology appears in Fig. 3. SIMPLORER’s state machine is used to compare a calculated mean value of the rectified alternator output voltage with high and low control limits of 43 and 41 volts, respectively. A re-circulating or fly-back diode has been included to satisfy the field inductance during IGBT turn off. Virtual instruments are inserted to monitor instanta-neous voltage, current and power dissipation. These measurements are also passed to tools to calculate mean or average values. A switch controller, CS, has been included to implement the disconnection at some user specified time. The instantaneous action of the switch and lack of an arc in the model tend to produce exceptionally large transients worse than actu-ally observed. In simulation instantaneously actually means the width of the time step used by the simulator during the event. With induc-tive voltage being inversely proportional to the time step, the values obtained from one simu-lation to the next can vary substantially. In SIMPLORER switches are ideal in that no voltage is dropped across the device when closed and no current flows through the device when open. This avoids the standard simulator voltage division problem for series connected opened switches modeled as extremely large resistances. The battery and load models are
Parameter Sets, Lookup Tables
Model Generation
Optimization
only important initially as they are subse-
quently decoupled from the generation side. They are important in establishing the circuit state at the time of the disconnection such as the current flowing through the switch. The battery is modeled as a large capacitor initial-ized to 42 volts. Various automotive loads are represented by a single load resistance. The alternator speed was set at 3000 rpm and is assumed to be maintained by the engine oper-ating off the battery.
Fig. 3 Simulation Model
Alternator Model
RMxprt offers an easy to use method to create a sufficiently accurate system level model for the simulation purpose without using detailed finite element based analyses. The definition of the actual alternator is completely dialog driven. In the Windows-based software the user can select from different slot forms, mate-rials and winding arrangements (Fig. 4).
Fig. 4 RMxprt Input Dialog - Rotor Pole
Since the design is based on analytical meth-ods, the analysis time can be neglected. After the analysis, users have a variety of design outputs available. A design output sheet is generated, that can be used as a specification for suppliers as well as lamination and winding information. Fig. 5 shows one of the available performance curves – armature current vs. excitation current. Fig. 5 Performance Curve Output
On the push of a button, a SIMPLORER model and a complete Maxwell2D project for design refinement and optimization based on finite element analysis are generated. This unique approach enables concurrent engineering and shortens design cycles.
The system model generated by RMxprt is first tested in a stand-alone mode as shown in Fig.
6. When the model is imported into a SIM-PLORER schematic automatically a small icon with the shape of the machine is generated including all electrical and mechanical connec-tion pins. Steady state is reached approxi-mately 100 ms into the simulation. These re-sults for steady state operation would usually be used to confirm the validity of the model. This is done using one or all comparisons available to the design engineer: engineering judgment; laboratory data; specifications. Since this alternator was created from artificial data, engineering judgment can accept a wide degree of response. After reaching steady state, the switch S1 opens the field current path under the control of a signal file, CS, stored on a hard disk and included automati-cally into the simulation model. The analysis shows the alternator continuing to produce output until about 14 ms after the switch is opened and producing a voltage spike (Fig. 7). The spike originates in the field side due to the instantaneous interruption of the field current by switch S1.
Fig. 6 Stand-Alone Alternator Test with RMxprt model
Fig. 7 42V Alternator Model Response to OPEN field at 0.10 seconds, Phases A, B, and C
Nominal Performance
The 42V alternator model is now incorporated into the test topology to execute nominal simu-lations and protection methods. During the simulation is running, digital reporting devices display instantaneous solutions as well as the mean values calculated by SIMPLORER’s built-in running average value tool. The com-plete schematic is shown in Fig. 8.
Fig. 8 Test Topology
The load dump event is set to occur 350ms into the simulation when the system can be considered in steady state conditions after startup. A spike is produced at the output due to the load dump switch open event as seen in Fig. 9. With the IGBT in active conduction, the spike also appears at the field. The infinite im-pedance of the switch diverts additional current
to the field causing the alternator output to in-crease. 14ms later the IGBT is turned off by the state machine comparator as the upper 43V limit is reached. At 487ms the 41V lower limit is reached turning the IGBT back on. In the interim the field current drops but does not reach zero and continues to drive the alterna-tor. Only the regulator can now stop the alter-nator from driving itself into saturation and does this by turning off as the upper limit is reached at 847ms. The increased rate of change in the rectified output voltage causes overshoot (see Fig. 7) in the average value taking longer to settle to the lower limit. That effect keeps the IGBT off longer allowing the field current to reach zero, de-energize the field electromagnet and shut down the alterna-tor output after approx. 1.08s.
Fig. 9 Nominal Problem - Rectified Alternator Output Voltage
Fig. 10 Field Voltage
Load Dump Spike Removal
By changing the fly-back diode to a zener di-ode, the load dump spike is easily suppressed by this device when the IGBT is active. The localized schematic for this connection is shown in Figure 11. Fly-back conduction oc-
curs in Quadrant I of diode operation where the regular and Zener characteristics are the same. So fly-back operation is not effected. This cures the load dump spike and the propa-gated spike as seen in Figures 12, 13 and 14. For this configuration, the alternator cannot sustain itself and becomes quiescent.
Fig. 11 Load Dump with Zener Diode for Fly-Back and Spike Suppression
Fig. 12 Load Dump Spike Removal
IGBT Spike Removal
The addition of a Zener placed at the collector of the IGBT limits the turn off spike and the load dump spike to 56 volts. The response to this approach is shown in Fig. 13. The Zener also has the effect of delaying the alternator shutdown by diverting current that had previ-ously gone through the field coil and damping the average output voltage through the clamp-ing action. The voltage output does not rise as sharply so the average value overshoot does not occur. With the control signal to the IGBT keeping it off for a shorter period of time, the alternator sustains itself through another cycle. Subsequently, the damped average increases both the IGBT turn on and turn off times until the field current (Fig. 14) falls to zero at approx. 2.9 seconds allowing the electromag-netic field to collapse, and the output voltage (Fig. 13) is no longer sustained.
Fig. 13 Rectifier Output Voltage with Zener
Fig. 14 Filed Current with Zener
PWM Implementation
A more efficient but noisy method of alternator voltage regulation is pulse width modulation (PWM). Fig. 15 shows one possible way of implementing a PWM in SIMPLORER. Here the block diagram feature and state machine are used to set a 90% duty cycle when the average output voltage was less than the desired value, otherwise, 10%. In the com-parator block the averaged rectified output voltage is compared to that threshold and generates an output level of 90% of the saw-tooth peak value when less than the threshold. The saw-tooth runs at a frequency of 17Hz and symmetric to zero to facilitate a difference
zero to facilitate a difference calculation by the summation.
Fig. 15 PWM Control
When the saw-tooth exceeds 90% of the com-parator level, the sum is a negative value. The state machine controlling the IGBT turns the IGBT off for the remaining 10% of the cycle due to the negative value. When greater than the threshold, the comparator switches to 10% and the IGBT is off for 90% of the cycle. The duty cycles do not vary but switch from 10% to 90% as required.
The controller was completed by adding a sample and hold block to fix the period for the duty cycle, as shown in Fig. 15. The alternator shuts down after about 550ms. The field cur-rent is shown in Fig. 16 and, at an expanded scale covering the time from 300ms to 900ms, the control signal for the IGBT is shown in Fig.
17.
Fig. 16 Field Current Fig. 17 IGBT Control, Zoomed
Summary
Controlling the transient voltages resulting from a vehicle load dump event in a high-side driver configuration using a Zener diode on the field side of the alternator can successfully limit these excursions. Ansoft’s tools RMxprt and SIMPLORER are effective tools for studying protection methods. The combination of these tools can predict the future behavior of the sys-tem accurately and within short simulation time. Both dissipative and non-dissipative regulation can be used to shut down the alter-nator a short period of time following the load dump.
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