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42 The PCB Design Magazine • December 2014 When laying out switch-mode power sup- plies or filters on PWM circuits, designers should think about high-frequency current paths—there are usually two of them—and keep the area of these paths minimized. This usually means placing the big components first: switches, inductors, caps and diodes. Only connect switch-mode supply and PWM filter circuits to the rest of the PCB ground in one place. This includes signal grounds to the control ICs. Use the ground terminal of the main input capacitor. If the connection is done in more than one location, the ground plane and hence other circuits may be pol- luted with switching currents. Be wary of nearby copper pours ac- cidentally touching the ground net somewhere else. Simulation Given the investment in time and effort to design PCBs it pays to simulate in order to learn as much as possible pri- or to the board order. Usually this does not mean simulating a whole circuit but focuses on key sub-circuits, or even key components of these sub-cir- cuits. A good candidate for simu- lation is to validate the choice of switch-mode power supply inductor. If the inductor is poorly chosen the circuit will smoke. Typi- cal equation-based methods for determining the peak current requirements often are opti- mistic. A quick simulation regularly shows that under certain conditions the current will go just a bit higher. Simulate at worst-case con- ditions: maximum current, minimum induc- tance, maximum input voltage, etc. This deter- mines the peak inductor current, which is often much higher than expected. Simulation is also very useful in developing feedback systems. In medical devices it's not unusual for a control system to have hundreds of Watts at its disposal. To rush to experimenta- tion in such a system is likely to result in dam- aged parts or even injuries Use Spice, MATLAB, FreeMat or even a spreadsheet to numerically model the behav- ior of a system, particularly an electronic or electromechanical one. This method allows de- tailed exploration of system performance over a wide range of conditions prior to building a prototype. It is especially useful to test condi- tions that occur infrequently or ones you can't easily replicate with the available test equip- ment. Note that simulation results are often incor- rect. Simulation models are frequently wrong, incomplete or inappropriate for a particular purpose, whether they are made from scratch or they are from an existing library or even a manu- facturer's website or datasheet. Models rarely take into ac- count actual physical limita- tions. One can get valid look- ing simulation results from a situation which would vapor- ize parts in a microsecond. Similarly, the system modeled might ignore a part's non-ide- ality or a parasitic signal path that occurs in reality and criti- cally affects performance. It is useful to think of sim- ulation like physical bread- boarding. The same thought processes that would be used as physical components are wired up in the lab and the criti- cal thinking that happens right be- fore turning on the bench power supply are helpful during simulation. Special attention should be paid to the operating parameters be- ing simulated and how the parts would behave in those conditions, especially with respect to their absolute maximums. Simulation comple- ments real lab experimentation: model it, make it and repeat as necessary. Prototyping Even after simulating a system, it is impor- tant that it be physically prototyped. Frequently a key part or characteristic of a system is found to be missing once it is soldered together and scoping begins. In that case, an iterative process PCBS FOR MEDICAL APPLICATIONS—A DESIGNER'S PERSPECTIVE continues article One can get valid looking simulation results from a situation which would vaporize parts in a microsecond. Similarly, the system modeled might ignore a part's non-ideality or a parasitic signal path that occurs in reality and critically affects performance. " "