The challenge of designing parts for today’s electronic marketplace has grown exponentially. While components can be purchased from distributors’ warehouses, the need to minimize size while maximizing reliability forces many engineers to seek custom sizes and specifications for their various components. To do this effectively requires direct communication between the system design engineer and the component design engineer. The old method, whereby engineers at each level of design derate components to ensure reliability, not only results in an increase in the size and weight of the component, but wastes money as well. Nevertheless, it is still common practice for some companies to purchase capacitors rated for 140 percent of the actual operating voltage; that is, a company would specify a capacitor rating as 7 kV even though its intended use is at 5 kV. The 140 percent increase in voltage rating means a 183 percent increase in size, and weight and cost will increase accordingly.
When designing components like capacitors, it is critical for the design engineer to be aware of the operating environment, including temperature, shock, vibration and atmospheric pressure, as well as the electrical stresses. A good example is an aircraft ignition system, in which many of these stress factors come into play in the same package. An airplane waiting on the tarmac in Texas, for example, can subject its electronics to temperatures well above 125°C, the maximum rating for many electrical components. Shortly after takeoff, the temperature and atmospheric pressure will drop radically and, if proper encapsulation procedures are not followed, capacitors can lose up to 65 percent of their voltage breakdown strength at eight inches of mercury pressure (about 30,000 feet elevation).
In addition, typical aircraft ignition capacitors operate with pulsed DC voltage, which is not damped, and voltage reversal can be up to 80 percent, which reduces lifespan dramatically. For example, when comparing the expected life of two capacitors of the same construction, the capacitor with steady DC voltage applied at the rate of 1,250 V per mil of mica paper can be expected to last more than 100 years. The capacitor with the same voltage stress but with pulsed voltage and large current reversal can be expected to fail in fewer than two years. It should be noted that AC voltage stress has little to no correlation to DC voltage stresses or lifespan. To obtain long life with AC voltage, the design engineer should strive for corona-free operation without regards to the volts per mil stress on the mica paper, provided that the AC current does not cause internal heating in the capacitor to exceed the rated operating temperature.
Some problems are difficult to foresee, and a good relationship between the design engineers is most beneficial. For instance, if a customer was dissatisfied with its capacitor supplier because of failures in the application, the capacitor manufacturer might be frustrated because the capacitor was designed according to standard specifications, and passed all required testing. Failure analysis might even show that the returned capacitor still passed standard testing. But the problem was this: the capacitor was mounted too close to the printed circuit board and was creating stray capacitance between the capacitor body and the board.
The capacitor thus formed in parallel with the mica paper capacitor was much lower in capacitance than the mica one, the voltage stress was much higher and would cause the circuit to short between internal connections in the mica paper capacitor and the printed circuit board. Had the design engineer known that guideline from the beginning of the design process, an extra wrap of mylar tape around the mica paper capacitor would have provided the additional dielectric strength to prevent the arcing and would solve the problem. This kind of problem is difficult to correct, can cost both companies time and money and can be easily resolved if both parties cooperate from the start.
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A close working relationship between engineers can lead to other benefits. Suppose that during discussions about how certain mica paper capacitors were to be molded, it was found that resistors, diodes and other components were needed in the assembly and were part of the calculations of what space was available for the capacitor. The components could be assembled with the mica paper capacitor and molded assemblies as single units, saving space and offering convenience for the customer. Custom shapes to molded units can also provide mounting options for other devices as well. In addition to custom shapes, custom electrical terminations and mounting hardware can usually be integrated into the molded unit to offer more space and weight savings.
Additionally, the use of accelerated life testing is advantageous when trying to maximize design efficiency. By testing samples of the desired capacitor at multiple elevated stress levels that are similar to the actual operating stresses, the designer will get an accurate picture of when and how the capacitor will fail. This can be done in a relatively short time. For an aircraft ignition systems capacitor, the process should begin by making samples of the desired capacitor with less mica paper dielectric than usual, that is, 10 units with 2.6 mils paper thickness, 10 units with 2.8 mils paper thickness and 10 units with 3.0 mils paper thickness where normal design criteria would call for 3.6 mils of dielectric.
Using a spark gap of the same breakdown voltage and using circuitry as close as possible to the finished unit will provide three levels of voltage stress. By using different oven temperatures of 85°C and 125°C, there are two levels for temperature stress. By increasing the pulse rate of the spark gap, the process can be sped up even more. Working at 20 Hz, designers can accumulate 1.0 x 108 pulses in less than two months time. With data from test procedures, the probability density function of each of the various stress factors can be graphed and used to calculate the acceleration factors for volts per mil stress and temperature. Using that information, it is possible to predict life at any given voltage or temperature with a certain degree of confidence and reliability.
Lastly, as expected, good quality assurance programs such as Aerospace Standard 9100 and ISO 9000/2008 provide excellent business models for effective and efficient control of manufacturing systems to enable production of these new and challenging designs.