Key to success of any product lies in its producibility, quality and reliability. Quality and reliability cannot be achieved by just arriving at a schematic to perform a required function and specifying component values. Many more steps are required to make a reliable product. Here we describe the design flow required to achieve a quality and reliable product, followed by other requirements.
Design methodology
Determine. Determine the product requirements such as:
1. Functions that the product has to perform. These functions are called requirements.
2. Operating conditions under which the product will perform its functions. These include temperature range, vibrational stresses and electromagnetic compatibility requirements. The operating condition is called lifecycle environment.
3. Conditions under which the product is to be stored and transported.
4. Power requirements (alternating current or direct current, voltage value, etc).
5. Electrical interface requirements (interfacing signals (analogue/digital), voltage range for analogue signals, serial input/output or parallel interface standard for digital signals).
6. Mechanical interface requirements (size, shape, material and finish).
7. Regulatory requirements, say, electromagnetic emissions.
Define. Define how best the unit should perform its function in terms of technical performance specifications—power output for a radio transmitter, sensitivity for a radio receiver and non-linearity for a measuring instrument.
Explore design concepts. Explore and evaluate alternative design concepts to meet the requirements determined.
Component selection. Select proven, standard and replaceable components from reputed manufacturers. If components with new technologies or sources are used, evaluate their performance for required functionality under required operational stresses (electrical, mechanical, thermal, etc).
Components should also be able to withstand stresses from handling, storage and assembly. Easy assembly should be possible. Care should be taken so that selected components do not become obsolete very soon. Design should use replaceable modules so that the unit can be repaired and maintained easily.
Performance analysis with electronic design automation (EDA) tools. Simulate the designed schematics with EDA tools. Initially use nominal values for all components in simulation. Confirm that the design is giving required outputs with required inputs. Correct any non-conformance by modifying the design and simulating again.
Tolerance analysis. Nominal component values are specified in the designs. It is, however, practically impossible to manufacture all the components exactly with required values. Hence allowable tolerances (upper and lower limits) are to be specified for the component values so that even when actual component values vary within designated tolerance limits, the design performs as required.
Simulate the design with component values set to extreme limits within the specified tolerance. Confirm that the design still performs well. Correct any non-conformance by modifying the design and simulating again.
Monte Carlo analysis. During production, all components in a particular unit will not be at their extreme tolerance limit values. In fact, actual values will vary from unit to unit randomly within the tolerance limit. This random variation can be modeled by Monte Carlo analysis.
Monte Carlo analysis simulates a hundred or thousand runs of the same schematic. It varies component values randomly within the set tolerance limits as per set distribution from schematic to schematic. The results can be analysed to determine the percentage of circuits that meet specification limits.
The results help determine whether the selected tolerance for components is adequate or not and whether it can be widened for some components. Wider-tolerance components cost lesser than narrow-tolerance components.
Margin (de-rating) analysis. A designed unit is required to function during its operational lifetime under expected operational stresses. The capability to withstand stresses (temperature, vibration, etc) varies from component to component. Component values and capabilities also vary/degrade with storage and time because of chemical and physical phenomena.
To accommodate these variations, units are to be designed to withstand more stress than required during operation. For example, if a unit is required to operate at 50°C, components that can withstand 70°C should be selected. Similarly, if a resistor consumes 0.25 watt during operation, a 0.5-watt resistor should be selected. This method of using components at less capability during operation is called de-rating. This is one way to design a reliable product.
One should also ascertain by simulation that the design performs satisfactorily at higher thermal (temperature) and dynamic (vibration) stresses than required during operation. This can be achieved by performing thermal and packaging analysis using EDA tools. This analysis is also called Margin analysis, as it ascertains that the design has margin compared to required operational stresses.
