SMPS PCB Design Signal Integrity

Question

This is going to start out like the other 53 pages of power supply questions "I'm a student working on a capstone project making an SMPS....". I have 12 months before the final design is due.

I'm thinking of purchasing some software to use at home for the designing of the PCB. The cheapest solution I can think of is Eagle for the eventual complexity I'll have (powering a ~150w load).

Now for the software addons. I was wondering how necessary are SI (Signal Integrity) tools for SMPS designs? I haven't decided on the topology yet, but I know the switcher and controls could generate high frequency signals so it seems pertinent. Now the package for Eagle also seems like the cheapest SI tool, FSPICE at $1k. Now I wouldn't' mind avoiding dropping 1k on this software but if it's unavoidable then its understandable, with a certain amount but not all being reimbursable.

Background: Pertinent to SMPS design: Course on Transmission Lines, Course on intro to Power Electronics, standard EE course on Intro to Power Systems, Course on Linear Control Theory and Discrete Time Controls. Otherwise standard courses an EE senior would have.

Is the investment worth it?

Edit: I should mention I only brought up the layout and EMI issues because of this: http://ecee.colorado.edu/~ecen5797/course_material/layout.pdf

Answer 1

EAGLE is a schematic capture and PCB layout tool, not a circuit simulator. If you want to be able to simulate your design, you will need some sort of simulator. I and many others on this site like LTSpice since it is free, relatively easy to use, and most importantly free.

Designing a 150W SMPS is not trivial and will require understanding of the physical layout effects of the PCB on the operation of the circuit. You will likely need to take an iterative approach; design a circuit to work in an ideal situation, find suitable components, layout your PCB and extract the physical connections back into the simulation model until you find something that works. To help push you in the right direction, there are dozens if not hundreds of application notes from the likes of TI, Linear Tech., Maxim, ST, etc. that describe good PCB layout techniques for SMPS.

SMPS design is not complex in principle; you simply need a switch and an energy storage element, with a feedback to your switch. The devil is in the details though, and while you may want or need a higher switching frequency to handle the power level you're talking about (and to decrease your inductor size), those higher switching frequencies will require more careful PCB design.

One more note about software, if you are a student and/or non-commercial you can get very good deals or free software from a lot of vendors. Sometimes a little groveling to a sales rep can help too ("I'm a poor college student and I need a license of HyperLynx to do SI analysis on my capstone"). If that fails, there is probably a suitable free software alternative that may not be a fancy but will probably work ok for you.

Answer 2

The best tools for SMPS (switch mode power supply) design, and most other EE designs, are your brain and a calculator.

Seriously, simulators are really not for design. Worse, simulators give detailed results from rough data, giving the appearance of knowing things that aren't actually known. I've designed circuits professionally for over 35 years, including many SMPSs, even including a patent on a new SMPS technique for a particular purpose. All these circuits were designed by first thinking about the general approach and drawing a schematic from that, then a few minutes with a calculator determining parameters and ultimately deciding part values and deliverable specs. I have never used a simulator in this process.

Simulators can be useful to verify certain parts of a design, and to rapidly do what if tests on a particular design. I've even written custom programs a few times to simulate aspects of some circuits, including SMPSs twice that I can recall. However, that wasn't to design the circuit, it was to run it thru test to verify some characteristics and to demonstrate characteristics to others.

So close the fancy software, start up Eagle (or whatever your school has for schematic capture), grab a calculator and start designing your circuit. You need to be able to look at the schematic and visualize the voltages pushing, the currents flowing, and each part reacting accordingly. You must have this understanding of your circuit yourself. True understanding is not something you can deligate to software. With the understanding of how the circuit works, it's really not hard at all to use the calculator to decide things like what the on-time of the inductor should be, the minimum saturation current it must have, the current and voltage requirements of the switches, how the high side switches will be controlled, etc.

To get anything useful out of a simulation, the results must fit into your understanding of the circuit. Without true understanding, simulation results are just a pointless pile of numbers.