Digital control brings advanced features and excellent efficiency levels to power supplies.The design process should be documented in sufficient detail for a thorough design review, but, more importantly, so that an engineer can pick up the design to add features or make improvements.DocumentationAny supplier with ISO9001 approval should have a documented procedure. The design process should also document the algorithms used by the digital controller and its key constraints, such as ADC conversion frequencies, PWM update rates and delays, introduced by the control-loop calculations.This level of detail should be sufficient to enable the engineer to simulate the system with, for example, a number of mathematical or system simulation tools or any circuit simulation tool that has behavioural modelling capability or that accepts C code blocks.There are four specific questions that should be answered as part of a supplier review. The first is: How did the power supply manufacturer programmed the controller? (i.e. via GUI, high-level program and/or C.)The tools used to design, simulate, build and test code should be documented, including version numbers. Changes in the toolchain can introduce changes to compiled code. Where third party libraries or code generation tools are used, one would expect to see similar levels of test detail. Ideally, test documentation would be supplied by the third party that demonstrated the quality of the code or tools.A mixture of tools is not unusual, e.g. a third-party task scheduler, C for the higher level ‘housekeeping’ functionality and low level assembler for time-critical functionality integrated with the hardware of the digital controller.Design processWhat design processes have been used? A top-down design approach should be documented showing the main functional blocks; this allows a hierarchical review of the design.Each functional block should be documented, with, for instance, state diagrams showing the processes for setting and clearing of alarms or flags and defined ranges for all inputs and outputs. This makes test case definition and verification of compliance a straightforward task. Flow charts are another useful tool for showing processes of low complexity.Review processWhat review process is used in software and hardware elements of the design?The basic project steps are identical for software and hardware elements of a design (requirements capture, design, design verification and design validation). One would expect to see design reviews, or stage gates, at the end of each of these steps, as well as regular team and peer reviews.What testing/validation processes have been used? Typically, each software function would be tested separately, followed by integration testing where the various functions are brought together. This testing would also include validation of error correction routines. This can be quite time consuming where DSC/DSP hardware is responsible for key functions or constraints.The last step is validation testing, where the functionality of the complete system is confirmed in both hardware and software.Armed with the answers to these questions, engineers can specify digitally controlled power supplies with confidence.
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