Timeline for Determining the correct amount of bulk decoupling capacitance for a PWM LED driver
Current License: CC BY-SA 3.0
17 events
| when toggle format | what | by | license | comment | |
|---|---|---|---|---|---|
| Jan 24 at 19:29 | comment | added | MicroservicesOnDDD | Nobody else mentioned, so I will, that twisting the power lines between the LED driver and the strip not only minimizes the inductive loop,.but also cancels out much of the inductance, which may reduce whining sounds, and how much capacitance is needed. | |
| Apr 23, 2022 at 18:20 | comment | added | Sixtyfive | As for Ignacio's question whether OP has determined if a cap is required at all: a cap on the output of an SMPS driving a PWM'd strip of LEDs can help with PSU whining sounds even if it's not required to make the light output smooth. Perhaps that could be worked into the question... | |
| Sep 24, 2016 at 0:10 | vote | accept | Andrew Spott | ||
| Sep 23, 2016 at 17:25 | comment | added | Andrew Spott | @Daniel I understand the sentiment. Unfortunately a combination of being a poor graduate student (can't do too many prototypes) and being a theoretical physicist gives me a predilection for theory when possible... or at least trying to understand where things might go wrong. | |
| Sep 23, 2016 at 17:16 | comment | added | Daniel | You've already spent too much time on that. Get out of your head and get on the bench! (You're asking people to estimate stuff you need to measure. Theory can explain stuff, but it can't predict things when you have no idea what the parameters are) | |
| Sep 23, 2016 at 17:14 | comment | added | Andrew Spott | @Daniel: that is the plan. However I would love to get as much info as possible and try and get it right the first time before I guess... :) | |
| Sep 23, 2016 at 17:13 | comment | added | Daniel | If things don't work the way you want, you're going to have to experimentally figure out what the ripple voltage is at each point, then figure out of that is ok for you. If it's not, you can estimate the current pulse magnitude and duration with an oscilloscope and a small resistor, then place the appropriate amount of capacitance there to fulfill the current pulse. Or you can do what most people do: guess, and if it works leave it alone! | |
| Sep 23, 2016 at 16:31 | comment | added | Andrew Spott | @Daniel: "put capacitance where you need it". That sounds very simple, but I'm not sure "where I need it", which is part of what this question is about. | |
| Sep 23, 2016 at 16:20 | comment | added | Daniel | @AndrewSpott Just put capacitance where you need it, and if you have a higher-impedance pinch point (like feed wires or a wire harness) make sure that you're locally bypassed. Sense wires probably will not improve this problem since it needs to respond very quickly. | |
| Sep 23, 2016 at 16:13 | comment | added | Andrew Spott | @Daniel: thanks. I'll have to keep that in mind. Maybe get a power supply with sense pins to try and mitigate that. | |
| Sep 23, 2016 at 6:39 | comment | added | Daniel | I have experienced this many times, debugging hardware, getting strange results, then cutting the supply wires down which makes the problem disappear. You have the inductance, and also the wire resistance is a problem when a partially bypassed device tries to take a large gulp of current, which can be 10s of amps. | |
| Sep 23, 2016 at 5:46 | comment | added | Andrew Spott | @Daniel: care to elaborate? I'm aware of the inductance of the wires, but didn't think it would be a huge deal at 1kHz (even taking into account harmonics) | |
| Sep 23, 2016 at 5:40 | answer | added | D.A.S. | timeline score: 2 | |
| Sep 23, 2016 at 5:13 | comment | added | Daniel | The length of the wires between the power supply and LED strip will make a big difference on how much C you need. | |
| Sep 23, 2016 at 5:10 | comment | added | Andrew Spott | @IgnacioVazquez-Abrams: I haven't. At the moment, I haven't chosen a power supply, so I can't tell if the capacitance is needed. I'm designing the PCB for it at the moment, and am trying to figure out if I should add room for the rather large caps I believe I will need. Another way of phrasing this might be "when is bulk capacitance needed?" | |
| Sep 23, 2016 at 4:59 | comment | added | Ignacio Vazquez-Abrams | Have you determined that a capacitance is required at all? | |
| Sep 23, 2016 at 4:58 | history | asked | Andrew Spott | CC BY-SA 3.0 |