Calculating Bulk Capacitance for Motor Driver - Page 1
Forum section: electronics / beginners

author: Rijsik
author_rank: Contributor
timestamp: January 05, 2024, 06:36:45 pm
content: I am working on a stepper motor driver board, based on the TI DRV8424 (datasheet can be found here:https://www.ti.com/product/DRV8424). I am struggling to determine the correct amount of bulk capacitance needed. The datasheet doesn't tell us precisely how much bulk capacitance should be added. Instead, it tells us to look at the datasheet of the stepper motor we are planning to use:"The data sheet generally provides a recommended value, but system-level testing is required to determine the appropriate sized bulk capacitor."I am using stepper motors bought at Stepper Online. This is an example of one that I am using:https://www.omc-stepperonline.com/nema-17-bipolar-45ncm-63-74oz-in-1-5a-42x42x39mm-4-wires-w-1m-pin-connector-17hs15-1504s-x1.The datasheets don't really mention anything about capacitance. So the other option would be to just do some "system level testing" and trial and error my way to the right value. But this doesn't sound like a very good approach. I would like to have a formula with which I can ball park the needed capacitance so that I at least have a rough idea of how much capacitance is needed.1) So my first question: Does anyone know how this should be calculated?I found a presentation of the website of TI which mentioned bulk capacitance when using motors:https://www.ti.com/video/6265440667001.At timestamp 6:01 they show the formula ½*Cbulk*ΔV^2 > ½*L*I^2. I am terrible at math so this could be wrong, but I think I can rewrite the formula as Cbulk > L ⋅ I^2 / ΔV^2. The variables in the formula can be substituted with the following values:- I= 2.1A (driver will support max current of 2.1A)- ΔV = 1.2V (5% of expected operating voltage, 24V)- L = 10mH (datasheet of the stepper motor I linked mentions a per phase inductance of 4.4mH at 20%, so I guess 10mH is a safe value to model with)When I plug these values in the formula I get the following bulk capacitance: 0.01 * 2.12^2 / 1.22^2 = 0.030. 30mF seems a bit excessive, doesn't it2) So second question, is my math correct and is this a reasonable value? Hope any of you can shed some light on this. Thank you in advance for your help.

author: Jwillis
author_rank: Super Contributor
timestamp: January 05, 2024, 08:31:13 pm
content: If you intend on stalling the motor consistently . For regular free running operation at 1.5A at 24V the capacitor is 39uF. So depending on motor load anything above 39uF would be adequate.

author: ejeffrey
author_rank: Super Contributor
timestamp: January 05, 2024, 08:53:10 pm
content: I've never done this and don't know the answer but that formula looks wrong. It seems they are trying to do an energy balance and trying to limit the ripple voltage on the bulk capacitor? If so the formula for energy change in a capacitor should be C*V*dV, not .5*C*(dV)^2. That gives the sensible result that the ripple voltage drops linearly with 1/C if everything else stays the same? Using that will reduce the calculated capacitance requirement by 10x.

author: Rijsik
author_rank: Contributor
timestamp: January 05, 2024, 09:47:53 pm
content: Quote from: Jwillis on January 05, 2024, 08:31:13 pmCould you explain to me how you came to this amount?

author: Rijsik
author_rank: Contributor
timestamp: January 05, 2024, 09:52:06 pm
content: Quote from: ejeffrey on January 05, 2024, 08:53:10 pmThat definitely sounds better. Guess they were not just calculating the bulk capacitance. But that would also mean that the formula might not be useful in this situation.

author: Rijsik
author_rank: Contributor
timestamp: January 05, 2024, 10:48:46 pm
content: For people that are interested, I found a post on the TI motor driver forums that says the following:"Generally 100uF and more is a good starting point for low power applications, and 330uF or more is good for higher power applications."Source:https://e2e.ti.com/support/motor-drivers-group/motor-drivers/f/motor-drivers-forum/1073994/drv8323-bulk-capacitance/3974734#3974734Alas, still no formula. But at least a clear guideline.

author: Jwillis
author_rank: Super Contributor
timestamp: January 06, 2024, 02:19:26 am
content: That formula is Ok but more for driving continuous running motors under load.. That step motor draws 1.5A for each phase. Or you can think of it as 1.5A per step under no load conditions at 24V. So you work out your calculation at 24V at 1.5A to give a base capacitance of the bulk capacitor. Under load that capacitance will need to be a bit higher depending on the average total load on the motor during a step.