With the growing social and governmental pressures for more efficient and safer cars, automotive design and test engineers face many new challenges. Today, efficiency and safety mean more technically advanced components and assemblies—dramatically increasing the time it takes to test vehicles as they are rolled off the assembly line.
More time spent testing translates into more money spent on testing. While companies are learning to develop new technologies, they are also seeking opportunities and new methods to save costs on development, production and testing.
The answer is automated testing, which can provide its own set of challenges. Essential to any automated test setup is finding the best way to power up the UUT in a way similar to how it is normally used. This can be particularly challenging for the many and varied operations that must be evaluated in automotive testing.
Power Supply Technology
Power supplies used to test automobile products must accurately simulate power levels of car batteries. For this reason, power supplies with low output impedance and fast load-generated transient response time provide the most accurate test results.
Three types of power supplies—linear, silicon-controlled rectifier (SCR)-regulated and switching—are used most often to test automobile products. Choosing the right power supply can be critical for real-life test results.
Linear Topology
Linear power supplies provide excellent regulation and tight ripple specs, typically in the microvolt range. Basically, a linear power supply consists of an AC-DC rectifier with voltage and current-control circuitry.
Linear topology is very effective for low-power testing where extremely tight regulation is critical. A good example of a linear power supply is found in a semiconductor application where testing a low-level TTL signal response requires low voltage, usually 3 VDC with a 0.01% accuracy. This means the supply fluctuates no more than +150 µV.
SCR-Regulated Topology
An SCR-regulated supply uses a phase control-type preregulator with a thyristor on the input side of a primary transformer (Figure 1). Output voltage is compared to a reference voltage. If the output voltage is too high, an error signal is generated and this signal adjusts the firing angle of the thyristor so that the input voltage of the transformer decreases. When accommodating fluctuating load requirements, the problem of continuously dissipating energy through the primary transformer is eliminated. SCR-regulated supplies are useful in applications where high-power loads, such as battery-charging systems, must be tested.
Switching Topology
In programmable switching supplies, the AC line voltage is rectified to provide bulk DC voltage (Figure 2). Switching circuits then chop the bulk high voltage into a variable-duty-cycle wave which may have a repetition rate ranging into several hundred kilohertz. The wave is transformed up or down, depending on the required output.
Because the 60-Hz AC line power is converted into a higher-frequency wave, the mechanical size of components, such as input transformers, output filters and control circuits, is drastically reduced. Due to its high efficiency, fast response time and low profile, a switching power supply is the most cost-effective solution for many automotive testing applications.
Today’s switch-mode supplies provide clean outputs with ripple in the low millivolt range, a generally acceptable parameter in the automotive test industry. In a typical car’s electrical system where many components are working off the same DC bus (the battery), the DC line can experience fluctuations reaching well above a hundred millivolts.
An Example
Ignition coils in today’s cars use pulses of high-voltage bursts to ignite fuel in combustion chambers. A simple ignition system using one transistor is shown in Figure 3. Although typical ignition systems are much more complex and use various methods to accomplish different aspects of operation, this circuit shows the basic principle. In a final-test station, a programmable power supply, instead of the battery, provides DC voltage to the circuit.
When the distributor points (DPs) are closed, current flows from the power supply through the ballast resistor (BR), the resistor R1 and to ground. This causes the transistor (Q1) to conduct from the collector (C) to the emitter (E) and through the primary (P) coil of the ignition coil. This builds up coil voltage in the secondary (S) of the ignition coil and provides a high-voltage potential to the spark plugs.
When the DPs open, the electromagnetic fields collapse, sending a high-voltage negative pulse (or a kickback of energy) through the circuit and back to the power supply. Depending on the engine design and speed, the DPs can open and close as many as 1,000 times per second, causing high EMI to be conducted back into the power supply. Load-generated EMI noise can cause the output of active power supplies (with feedback loops to monitor output voltage) and produce unreliable voltages or reduce the life span of the supply.
Using a low-output impedance power supply and incorporating a circuit such as shown in Figure 4 can reduce the EMI effects to harmless levels. A fast-recovery diode (Q1) blocks any negative pulses from the load to feed back into the power supply’s output filter stage. Another fast-recovery “flyback” diode (Q2) allows a dissipative path for these high-energy peak pulses to return to the primary of the ignition coil.
Finally, the addition of a carefully chosen R-C combination to the circuit can dampen and absorb the transients. The R-C network compensates for any added inductance of the wires from and to the power supply, the load, and finally, any other test measurement system attached to the load.
Conclusion
Needless to say, today’s cars are far more sophisticated than their ancestors of just 10 years ago. Consequently, testing also becomes more extensive and complex. As the electronics that go into automobiles become more advanced, engineers must develop new test methods to adequately verify the electronic systems.
References
1. Shacket, S.R., The Complete Book of Electric Vehicles, Domus Books.
2. Cantonwine, C.R., Automotive Tune-Up and Test Equipment, Chilton Book Co.
3. O’Shea, P., “Get on the Fast Track of Inspection,” Evaluation Engineering, November 1994, pp. 28-32.
About the Author
Nick Vaghela is an Applications Engineer at the Sorensen
Division of Elgar Corp. Before joining Elgar, he was a design engineer at Teal Electronics. Mr. Vaghela graduated from Northeastern University in Boston with a B.S.E.E. degree Sorensen, Division of Elgar Corp., 9250 Brown Deer Rd., San Diego, CA 92121, (619) 450-0085.
Copyright 1995 Nelson Publishing Inc.
June 1995