Bulk capacitors: ripple vs inrush currents - Page 1
Forum section: electronics / projects

author: IU8BBM
author_rank: Newbie
timestamp: November 29, 2020, 10:58:12 am
content: Hello folks,here follows my spaghetti-design issue which is as simple as it's driving me made. Since useless simulations are driving me nowhere, I hope some thumb-rules of yours could lead me to a non-pyrotecnic design.ProemCustomer asked me for a submersible acquarium pump driver (marked 12V 10W), which consist no more of a 12V DC-AC converter. Bench tests showed 2.2A startup and 1.2A running currents @12VAC.Since a tremendous low-budget design is required, a single TI DRV8870 was chosen to do the job, a H bridge integrated driver, which is controlled by 2 signals generated from a microcontroller which consist in a 25kHz PWM signals modulated by a 50Hz sinewave phase in order to produce a pure sine output.Take a look to the attachment and please ignore the missing driver current limiting resistor on Isen pin, ignore also that such design won't output a 12Vrms but a 12Vmax sine, and focus on the 22uF capacitor (C9+C10+...C11) parallel. The DRV8870 datasheet suggest for a 47uF bulk capacitance to avoid voltage drops because of cable inductances. So i've put 2x22uF (+1 left as DNP as safe zone while prototyping) Samsung Electro-Mechanics CL21A226MAYNNNE (MLCC 25V 22uF X5R 0805 20% : <0.1€@1k each!) for the job.Ripple current...Since I'm scared by high ripple currents which I can't calculate because this inverter is exactly in the middle of the machine cabling (2m from the supply, 1m from the motor), i already removed filtering for motor noise going back into the supply (the class-2 wall adapter will already do it for me, and loads shared on the 12V won't suffer for it) but, since we don't live in a simulator, cable L and R should be taken into account so I'm forced to install bulk capacitor (as datasheet says).The first question is, without making unreasonable calculations which should take into account parameters i can't extimate now (cables, connectors and boards distributed parameters which carry the power signals)will the 44uF caps be enough? Would the MLCCs survive to the ripple currents?(MLCC were preferred over electrolytics for low ESR>low dissipated power>lower temps).VS ...inrush current...What is cranking my mind is: the bulk capacitors are placed straightforwardly to the 12V input, so when circuit is powered, inrush current will kick in the capacitors so I do expect a voltage drop on the 12V rail (which is not good since it's shared on the system with other loads) and a large peak current flowing into the caps. In order to avoid these I've explored all kind of inrush current limiters and filtering ideas: ICL NTCs, LC filters, R+MOS circuits..., which not only drove me to unreasoneable prices (whole BOM is <3€) but also drove me to the other side of the coin: increasing bulk capacitors ripple currents. So the simplest idea was going back to the 15yr me, when input filtering filtering was a still unexplored land.So the second question is: will the caps survive over time to the inrush current pulses?Colleagues lead me only to additional confusions...so here I am, I'm close to delivery the design to the prototyping, but I'm really worried that those capacitors would blow up someday after the product release.Help me eevblog forum, you're my LAST hope.Luca

author: Siwastaja
author_rank: Super Contributor
timestamp: November 29, 2020, 12:45:11 pm
content: The capacitors are fine with the hot-plug current. It won't happen millions of times.The problem with large MLCC bank is the inrushvoltagefrom LC ringing when hot-plugged to the 12V source, this easily goes somewhere around 24V.The simplest way to deal with this is to dampen the LC circuit by adding significantly larger value of C with considerable ESR in series. With your 44uF of MLCC, a bare minimum of 100uF lowest cost normal-ESR electrolytic capacitor would do the job. The value and ESR is not critical. It can be larger as well. The only thing left is to try to analyse if this high-ESR capacitor carries significant part of the ripple current, if yes, then it becomes a heating/lifetime issue. The obvious answer is then to increase the amount of low-ESR MLCC to take that ripple (which then forces you to increase the value of the elcap as well, making it handle ripple better, as well). You can simulate the ripple current sharing of parallel capacitors modeling the layout+capacitor ESL/ESR until the cows come home, but really the easiest way to assess this is to solder in some 220uF elcap, run the thing for 15 minutes and try the elcap with your hand. It shouldn't heat up more than by a few degrees C.For such small driver with such little dc link capacitance, don't bother with precharging circuits / inrush current limiters, IMHO. The resistance of the power supply, crappy connectors and wiring is enough. MLCCs have low ESR, but as a result, they also deal with high pulse currents just fine.My gut feeling says 44uF is definitely enough for said specs. I have used such capacitance values for motor drivers ten times larger.About reliability, the thing you need to worry most is the possibility of cracking those ceramics. With 0805, the risk is small; still remember to avoid board flex (mind the location of mounting holes). To mitigate it further, change to a part with soft terminations.You probably realize this already but a 25V 22uF in 0805 is almost "too good to be true". This means the actual capacitance at 12V will be significantly less, I don't think Samsung publishes the data (at least I have wasn't able to find it some years ago) but consider them 5uF capacitors, maybe 10uF if they are really good. Which could be fine because the TI appnote writer also considers 47uF of MLCC as something much less than that. If you can get the data, you might find out the 10uF part would be really close in actual capacitance, and if it's cheaper, it may have better (or at least similar) $/uF.Seeing your "bulk" MLCCs are in such small packages (so low-ESL), you can remove the 100nF capacitor to save cost, and free an optimal layout spot for one of the "bulk" MLCCs. So after this you might be able to fit three of them again.

author: IU8BBM
author_rank: Newbie
timestamp: January 14, 2021, 10:08:12 am
content: A quick followup after the product release.Images attached will speak even more than words.Quote from: Siwastaja on November 29, 2020, 12:45:11 pmTested a lot of @1kHz: 15.268, 15.041, 15.339, 15.621, 15.437, 15.345, 14.957, 15.265, 15.265 ,15.593, 15.259 . 0.284R@10kHz 0.581R@1kHz.Mounting 2x "22uF" caps Yellow is current, cyan is the supply bounce.Inrush, it looks cabling parasitics would totally confirm that my doubts are just an over-engineering practice as usual.Thanks for your kind support, covid pandemic lockdown put me in an unreasoneable panic state upon totally useless problems. A friendly word helped me exiting from this, i thank you warmly.LG

author: Siwastaja
author_rank: Super Contributor
timestamp: January 14, 2021, 12:48:46 pm
content: Test the capacitance with DC bias. That's the whole point.If you are interested about the amount of capacitance, that's it. I think you have enough in any case.

author: NiHaoMike
author_rank: Super Contributor
timestamp: January 14, 2021, 02:15:30 pm
content: The 15V TVS diodes are a bit low in voltage, especially if the circuit might be powered from a lead acid battery. The DRV8870 is rated to 45V and I assume you'll also have a LDO for the microcontroller, although I don't see which one you're using. You could consider going to 5V (if your microcontroller is rated for it) and then use a 78L05 which is rated for up to 35V. That leaves the 25V caps as the weakest link which means a 20V TVS would be about right. Or you could use a regular zener if you just need to suppress the initial overshoot. The TVS on the output should not be necessary since the motor driver already has provisions to suppress inductive spikes.Since the device is used near water, moisture is a significant concern. Best option is to conformal coat the board and use dielectric grease in connectors.