As you progress into developing a circuit from home, schematic capture is an important part of the process. However it requires a little forethought especially if you are planning to transfer the schematic to a board layout software.
Simulation software has gone beyond the traditional SPICE analysis. The expansion of libraries offers additional information to accompany components. This includes packaging dimensions and other useful information. For example you can choose a particular resistor size such as a 1206.
In the same way libraries assist you, they can also cause you issues by offering limited parts. Often times these parts are in different packages. In addition, the models cause additional computing that sometimes keeps the simulator from converging. This is especially true in power supplies that have abrupt switching waveforms.
The solution that I use is to create two different schematics. I will start with my basic schematic using parts from the library. As things fail to converge, I will replace them. For example, I will used a pulsed voltage source when my 555 timer runs into convergence problems.
Convergence problems have always been an issued with SPICE . The difference is you get scolded immediately rather than wait for hours while a cursor blinks as it was in the early days of PCs. In fact, reference  “Step-by-step procedures help you solve Spice convergence problems,” was written almost 20 years ago in 1996. Not much has changes since then. Models only go so far to simulate the real world. There are however ways to progress without a lot of additional effort.
So at this point, you’ve got to realize that rather than solve these problems, it’s often best to replace the troublesome components with generic models. Some examples are:
- PWMs and timers with voltage sources or a combination of gates, op amps, and comparators
- Transistors with voltage controlled switches
- Transformers with dependent voltage and current sources plus a magnetizing inductance
Of course all of this does not do the best in recreating the parasitics that occur. Those have to be factored in as well. However if you want to get up and running this method of using generic blocks is best for quick results. You can get your parts ordered and sort it out on the bread board which is always advised anyway.
Traditional schematics are laid out like the pages on a book. One starts from the upper left and finishes on the lower right. Although this helps to understand signal flow, I found that altering the traditional placement in the schematic helped when transitioning to the Ultiboard. Instead of dropping capacitors to ground, I returned them to the base of the component they supplied. This seemed to help the “rubber band” connections when dragging components in Ultiboard. The next blog will go into more detail on board layout. However it’s nice to note that transitioning to Ultiboard from the schematic can help you identify missing connections. I had components that appeared to be connected in Multisim however when dragged out onto the board in Ultiboard there was no “rubber band” stretching back to other components.
One final note on schematics, think in two directions.
Did he say, “two dimensions?”
No I said two directions. Schematic components typically run north-south or east-west on a board and on the schematic. Very few layouts have angles other than 90 degrees. I have seen some on round objects such as motor controllers, traffic lights, and utility meters with radially placed components, however it’s not common. Therefore, the ability to rotate components in 90 degree increments is your best friend when designing the schematic. Simply press Ctrl plus “R” for rotating a component. It saves you time and frustration.
Schematic layout requires forethought for two different end goals, simulation and board layout. By implementing the hints and short cuts mentioned within, you stand more of a chance to have working hardware the first time while saving time when laying out the board and working with the assembly house. Use libraries wisely as they can create problems in the same manner that they solve them. Rather than fuss with convergence problems and difficult simulations, plan on two schematics, one for board layout and one for simulation. It will save you time in the end.
- National Instruments Multisim schematic design and evaluation tool, NI Multisim
- National Instruments Ultiboard board layout tools, Ultiboard
- You Need a Power Supply Designed by When?
- “Step-by-step procedures help you solve Spice convergence problems,” EDN magazine, 1996, Charles Hymowitz, Intusoft SPICE convergence tips