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30 The PCB Magazine • November 2014 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 in- put capacitor. If the connec- tion is done in more than one location, the ground plane and hence other circuits may be polluted with switching currents. Be wary of nearby copper pours accidentally 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. Typical equation- based methods for determining the peak cur- rent requirements often are optimistic. A quick simulation regularly shows that under certain conditions the current will go just a bit higher. Simulate at worst-case conditions: maximum current, minimum inductance, maximum in- put voltage, etc. This determines the peak in- ductor 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 behavior of a system, particularly an electronic or elec- tromechanical 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 equipment. Note that simulation re- sults are often incorrect. Simu- lation models are frequently wrong, incomplete or inap- propriate for a particular pur- pose, whether they are made from scratch or they are from an existing library or even a manufacturer's website or datasheet. Models rarely take into account actual physical limitations. One can get val- id looking simulation results from a situation which would vaporize parts in a microsec- ond. Similarly, the system modeled might ignore a part's non-ideality or a parasitic sig- nal path that occurs in reality and critically affects perfor- mance. It is useful to think of simu- lation like physical bread-board- ing. The same thought processes that would be used as physical components are wired up in the lab and the critical thinking that hap- pens right before turning on the bench power supply are helpful during simulation. Special at- tention should be paid to the operating param- eters being simulated and how the parts would behave in those conditions, especially with re- spect to their absolute maximums. Simulation complements 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 Models rarely take into account actual physical limitations. 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. " " PCBS FOR MEDICAL APPLICATIONS—A DESIGNER'S PERSPECTIvE continues