Abstract: Introduction: Worst case circuit analysis (WCCA) is a method which is used to verify the hardware design of the circuit. It investigates the circuit behavior under worst cases, which are environmental and operating conditions. Environmental conditions include temperature, humidity and radiation, etc. Operating conditions include external electrical inputs, component tolerances and component aging, etc. The output of WCCA enables an evaluation of whether any risk exists in terms of circuit operation and component malfunction. Among the methods of WCCA, Monte Carlo Analysis (MCA) is the most realistic one which is based on the random distribution of component values regarding their tolerances. Aim: This study investigates MCA of a running board module (RBM), an electronics controller used to open and close the step module which helps passengers get on and off transportation vehicles. Scope: Among all circuits located in RBM, the most important of all, i.e. motor driver circuit is selected to perform WCCA. Restrictions: Since the available tool to perform WCCA is of limited capability, the study is restricted to only the investigation of input voltage variation, temperature variation and initial component tolerances. In this study, the only tool used to simulate the circuit behavior under specified conditions is Altium Designer. Method: The motor driver circuit is energized by a battery whose nominal voltage is 12 V. Power flow through the motor windings, which draw 4 A nominally, is controlled by a relay driver sub-circuit used for bidirectional switching. Thus, the motor can rotate in either forward or reverse direction regarding the relay’s contact position. The relay driver sub-circuit is composed of a relay coil K1 whose resistance is 160 Ohms, a 33 mOhm resistor R1, 1SR154-400 diode D1 for reverse polarity protection, a IMH9A Bipolar Junction Transistor (BJT) Q1 operated by 5 V to trigger the relay driver sub-circuit and a BAV70 diode D2 to freewheel the coil current when the trigger ceases. Power flow through the windings is also provided by a BUK7C10-75AITE NMOS transistor Q2. The electrical load i.e. the motor windings is included in the analysis and denoted by R2. As for WCCA, the circuit is simulated using 5 secs of simulation time and 2 msecs of step time. In order to obtain the response of input voltage variation, DC sweep analysis is carried out ranging from 0 to 16 V with 1 V incremental steps. Next, to perform the second analysis, which accounts for temperature variation, temperature sweep is carried out ranging from -40 to 85 °C with 65 °C incremental steps yielding three points at -40, 25 and 85 °C. And finally, the last analysis is based on component tolerances. Tolerance values of all components such as capacitors, resistors, etc. are taken into account and MCA with “Worst Case” distribution is selected. Thus, component values are selected at their own lower and upper boundaries. The simulation is carried out 5 times with the minimum and maximum values of components considering tolerance values. For each execution, the component’s value is at its either maximum or minimum. Results: The simulation results show that the output voltage applied through the terminals of the motor is nearly the same as the input voltage for both its minimum (i.e. 9V) and its maximum (i.e. 16 V). The maximum current supplied through the relay driver sub-circuit is 80 mA. All components in the circuit withstand this current. The collector-emitter voltage is at its saturation level up to 10 V input voltage. When the input goes up to 16 V, it rises up to 4 V, which is much lower than its off-state voltage (i.e. 50 V). Relay’s coil also withstands its maximum terminal voltage, i.e. 12.78 V, lower than its maximum (i.e. 16 V). The collector-emitter voltage decreases with temperature since Q1 becomes more conductive at higher temperatures. When the relay contact is closed, the load (i.e. motor windings) is energized and draws 5.27 A at maximum and 5.24 A at minimum taking the temperature variation into consideration. Its current decreases with increasing temperature since drain-source voltage on Q2 increases with temperature while operating at linear region. When analysis considering component tolerances is evaluated, it is found out that when ±%10 tolerance of motor windings are taken into account, approximately 6 A flows through it at maximum. This is safe enough to operate since a 9 A overcurrent condition causes to stop the operation. Conclusion: WCCA results clearly indicate that all components are assured to operate safely.
Anahtar Kelimeler: Worst Case Circuit Analysis, Monte Carlo Analysis, Transportation Vehicles
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