How does ESR of a X5R capacitor change as temperature falls?  - Page 1
Forum section: electronics / projects

author: AndersJ
author_rank: Frequent Contributor
timestamp: February 02, 2023, 09:41:36 pm
content: I have instablity problems with a step down converter when temperature falls below -5 degrees C.The output capacitor is a 0805, 10uF as recommended in the data sheet.The capacitor 0.85 mm high.If I change to a similar capacitor but 1.25 mm height, the instabiliy is gone.I suspect the ESR of the higher capacitor is slightly lowerThe data sheets, Murata and Kemet do not reveal any ESR data.Does ESR of a typical X5R capacitor decrease when temperature decreases?Would other materials, such as X7R be a better choice?

author: TimFox
author_rank: Super Contributor
timestamp: February 02, 2023, 09:50:02 pm
content: I found this page on a capacitor vendor's catalog (attached pdf).There is some difference between X5R and X7R at low temperatures.However, I could not find anything about temperature variation of ESR.

author: jmw
author_rank: Frequent Contributor
timestamp: February 02, 2023, 10:03:50 pm
content: Could DC bias derating be a factor? For class II ceramics, usually the larger (physically) the capacitor, the lower its DC bias derating.To test ESR as a factor, can you bodge two 4.7 µF caps together on the output? That should give you (roughly) same capacitance and half the ESR.

author: AndersJ
author_rank: Frequent Contributor
timestamp: February 02, 2023, 10:10:30 pm
content: I have tried ONE 4.7 uF, which also resolved the problem.That makes me think it might be a ESR issue, rather than low capacitance.I have also ADDED a small series resistor, which made it worse.Another reason I suspect ESR.

author: mikerj
author_rank: Super Contributor
timestamp: February 02, 2023, 11:09:18 pm
content: Have you worked out how much capacitance you are actually getting from your 10uF part at your working voltage? Wouldn't surprise me if it was well under half. The smaller capacitance parts with the same working voltage lose less capacitance in total with DC bias, so you can end up with a 4u7 having as much or more capacitance than a 10u part of the same package size. X5R capacitance tends to drop at low temperatures as well (though nothing like as bad as Y5V) so it might have been on a knife edge at nominal working voltage and the temperature pushes it over.BTW the excellentMurata Simsurfingweb app does allow you to see the effects of temperature on ESR. After selecting your part, choose frequency characteristics and there are two text boxes for DC bias and temperature. Enter you values, click "R" and you'll get a plot of ESR vs frequency for those conditions. You have to close and re-open the chart if you change the values.

author: AnalogTodd
author_rank: Regular Contributor
timestamp: February 04, 2023, 12:43:51 am
content: The ESR of MLCC capacitors doesn't make huge changes with temperature. Most of what you get from ESR comes from the connections to the plates which is why the overall ESR is as low as it is (typically around 5 milliohm). Electrolytics and tantalums have a much different construction (which makes them polarized) and ESR increases a huge amount at cold temperatures.The issue with the ceramics is a combination of DC bias and temperature shift. When you look at the Class 2/3 dielectric code (X5R, X7R, Z5U, etc.) it simply specifies minimum and maximum temperature and change across temperature. The X means -55C minimum, 5 is 85C maximum, and R is +/-15% over temperature. What isn't specified is change with DC bias voltage. As stated earlier, larger case size tends to give better DC bias characteristics, but one must be wary.A rule of thumb I use is that at 80% of their rated voltage, a class 2/3 MLCC will be 20% of their rated capacitance value. At 50% rated voltage, they can still be as low as 30% to 40% of their rated capacitance value. To top it all off, the change over temperature is only rated at zero DC bias, so a capacitor may be running at 50% rated capacitance and drop another 50% as temperature change is different with DC bias.Quite often, the change in ESR of a ceramic capacitor is not going to affect the loop of a step-down converter or other regulator circuit, most of those loops operate at well under 500kHz loop bandwidth (while they may be switching as high as several MHz). The ESR/capacitance product that sets that as a zero in the loop response is in the tens of MHz range.My bet is that your instability is caused by a drop in the capacitance value and the circuit is already operating close to the edge of stability.