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I generated a DC/DC PS circuit from Maxim Integrated, and got this output: schematic

You'll notice that both the inductors and the capacitors have their respective resistances marked (in milliohms). Are these values important enough (generally speaking) for me to include them? What is the threshold of precision where it becomes important?

CATboardBETA
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3 Answers3

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The important question is what do you want to get out the simulation?

If you want to see the basic operation, rough idea of duty cycle, peak inductor current, its probably not important to include parasitics.

If you want to estimate something like efficiency, load regulation, output ripple then parasitics will become more important. Take for example output ripple. This is dictated by the main inductor value and the impedance of the output capacitors. It is not uncommon for the reactance of the capacitors to be smaller than the ESR at the frequency of operation for electrolytic caps. Hence if you don't add a realistic ESR your output ripple voltage ill be completely unrealistic, both in magnitude and overall shape. In your case it appears to be ceramics so not so bad.

A way I like to work is develop the sim without these parasitics, once the basic operation is good then start adding them in. The parts that show ESR values are all the important one. Items like C1 don't need it

One other reason for adding parasitics is to make the simulation work properly. Circuit with inductors & caps and rapid switching can have severe (unrealistic) ringing if there is no loss mechanism.

RYR051
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Usually the parasitics and other values are not displayed as it would clutter the design. If you have a component that you know is going to be differing from application to another (e.g. external power resistor or your sample of a reference circuit to be adopted to another design) you could add requirements as displayed or as notes in the schematic.

Ralph
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Notice that only the elements where the parasitic resistance is critical have this marked. In a switching converter, there are four main places where parasitic elements are important to consider: the input capacitors, the inductor, the switching MOSFETs, and the output capacitors.

In your schematic, C1 and C2 do not have parasitics indicated; these are places where the ESR doesn't really matter to the circuit's functioning. Instead, the ESR is marked on CINB, CIN1, CIN2, L1, COUT1, and COUT2--the input capacitors, inductor, and output capacitors. This circuit doesn't have any switches on it as they're internal to the IC in this case, so you don't have to select ones that will work.

I wouldn't be surprised if EESim showed on resistance and maybe even input capacitance and/or gate charge of switches if you did use an external-switch converter, though. (Or it might bypass all that and just give you a specific part number to use.)

There are a few other places where you need to be careful with parasitics as well, depending on your converter topology. If your topology uses a sense resistor, its parasitic inductance can be significant. The coupling capacitor in a SEPIC will also require a low ESR, and the transformer used in a flyback converter needs to have a high leakage inductance. And of course, any diodes used in non-synchronous converters should be as fast as possible and as low-Vf as possible--Schottky diodes are ideal.

Hearth
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    In some cases even the parasitics *of the pcb itself* becomes important. (Though most of the time the answer is "if this is important you're doing it wrong".) – TLW May 07 '22 at 19:34
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    @TLW Well, PCB layout is always an important consideration when making switching converters! Sometimes quite tricky to get things to actually fit, especially if you're using an inductor that's twice the size of the one the recommended layout assumes you're using... – Hearth May 07 '22 at 21:40
  • Absolutely; when it starts being a concern for switching converters people tend to preemptively adjust the PCB layout, as opposed to calculating the parasitics of the PCB layout and adding those in sims to check. (With exceptions, of course.) – TLW May 07 '22 at 21:52
  • Also, re: sense resistor. Another fun case is when people reroute a 4-point connection into an 'equivalent' circuit and inadvertently include a chunk of trace in their measurement. Or put a spot for something and populate it with a "zero"-ohm resistor. ("Zero"-ohm resistors can have surprisingly high resistances. Often better to populate with a cheap current-sense resistor, as they tend to have much better tolerances.) – TLW May 07 '22 at 22:03
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    @TLW I'm constantly having to stop KiCad from rerouting kelvin sense terminals and including a bunch of trace in the measurement--since KiCad doesn't properly support net ties yet, every time I have to adjust the positioning of the sense resistor or anything it's connected to, I have to manually reroute the sense lines without being able to use the interactive router to walk around obstacles, because it just sees that there's this other big trace that's the same net and thinks it's fine to touch it. – Hearth May 07 '22 at 22:10
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    @TLW Yes, as they currently exist in KiCad they have a number of problems I can't deal with. They take up board space, for one, and they can't be used on internal layers. I've used them before, but they're just not usable for my current project. – Hearth May 08 '22 at 00:17