Usage of polarised capacitor in parallel with non-paralised capacitor - Page 1
Forum section: electronics / general

author: insanoff
author_rank: Contributor
timestamp: November 27, 2022, 06:17:13 pm
content: Hi there,I am studying the user manual of an evaluation board for a charging circuit. Link to the manual is below.One design consideration caught my eye. I would like to know what the purpose of polarised 2.2uF capacitor (C204) is when there is already a 4.7uF capacitor (C203).Many thanks in advance!https://www.st.com/resource/en/user_manual/um2940-getting-started-with-the-stdes7kwobc-7-kw-onboard-charger-reference-design-stmicroelectronics.pdf

author: T3sl4co1l
author_rank: Super Contributor
timestamp: November 27, 2022, 08:42:30 pm
content: Compare the ESRs, and model the equivalent parallel impedance.Also, it's not obvious where they've placed all those capacitors, but it's generally a bad idea to put multiple different values in parallel right beside each other. If they're spread around doing local bypass, that may be okay, but that might not be necessarily either.Tim

author: TimFox
author_rank: Super Contributor
timestamp: November 27, 2022, 09:41:18 pm
content: In grad school, we had undergraduate students as cheap, yet clever, technicians.I once hand drew a circuit (for a onesie) with analog and CMOS devices on the same Vectorboard, with +15 V and -15 V supplies.To make the drawing less messy, I put a rectangle for each IC into the lower left-hand corner of the drawing, showing the power and ground connections, with appropriate bypass capacitors on each socket.Actually, IIRC the drawing was explicit, showing the capacitor directly at the pins and wires from there to the power lines and ground symbol.Invoking circuit theory, the tech substituted a single 1 uF capacitor for 10 0.1 uF capacitors.With modern drafting, the careful location of the bypass capacitors is done at the PCB layout stage, moving the parts from a rats' nest to the neighborhoods of the devices.

author: David Hess
author_rank: Super Contributor
timestamp: November 28, 2022, 02:03:04 pm
content: The parts list says C204 is 2.2uF ceramic, so the schematic symbol is wrong.

author: Someone
author_rank: Super Contributor
timestamp: November 28, 2022, 09:52:12 pm
content: Quote from: insanoff on November 27, 2022, 06:17:13 pmAre they all populated? Evaluation boards often have extra footprints so the customers can try different solutions.

author: insanoff
author_rank: Contributor
timestamp: December 06, 2022, 07:15:39 pm
content: Quote from: David Hess on November 28, 2022, 02:03:04 pmYes, that's right. I did not notice it. However, I still don't think it's necessary to use multiple values more than 2. Also, there is probably no reason to suspect rocket science behind reference designs. This is probably just another subjetive design approach.

author: David Hess
author_rank: Super Contributor
timestamp: December 07, 2022, 02:06:11 am
content: QuoteThe most common case in a PDN (power distribution network) involves low value decoupling capacitors at the loads and a smaller number of larger bulk decoupling capacitors which serve to terminate the characteristic impedance of the PDN preventing reflections which would result in excessive ringing.But there are cases where multiple local decoupling capacitors of different values at the load are required to maintain a sufficiently low impedance over a wide frequency range.

author: uer166
author_rank: Super Contributor
timestamp: December 07, 2022, 03:12:34 am
content: Quote from: David Hess on December 07, 2022, 02:06:11 amIs that still the case? I recall content about how such caps with different values close together and low ESR (think 100n + 10n ceramic caps) have parallel resonance (or anti-resonance from PDN perspective), resulting in very high impedance and a crap PDN. I'm more careful now and try to only have 100nF distributed around with same length traces to a plane, to avoid such issues.LTSpice impedance sims also confirm that thought.

author: David Hess
author_rank: Super Contributor
timestamp: December 07, 2022, 07:14:13 am
content: QuoteI have seen the same thing but I have absolutely run across it as a problem under ideal conditions with coaxial transmission lines where I *had* to use multiple 0.001, 0.01, and 0.1 microfarad ceramic capacitors in parallel when operating from 100 MHz to 1 GHz.Ok, Why is the forum adding text which is invisible in the text entry box?