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New for January 2019. Here we will look at variations in capacitor performance with temperature.
Here's a companion page on capacitor variations with voltage. Don't hold your breath for a page on capacitor aging, but that is a very real thing too!
Here's a great article that Presidio published in Microwave Journal, that includes information on various dielectrics.It also introduces the idea of buried single-layer caps which can solve a number of design issues. (BSLCs). Trust us, it is worth a read, in particular, check out Table 1.
http://www.microwavejournal.com/articles/3395-technology-and-innovation-in-single-layer-capacitors
Our aim is to summarize what most microwave engineers need to know on this page. Let us know where we fall short. So far, the discussion below pertains to ceramic multi-layer ceramic caps (MLCCs), which are typically exhibit the most problematic temperature variations. There is a more thorough discussion of capacitor classes on Wikipedia, Standard capacitor classes are a nice idea to help engineers select products, but there is no single, worldwide system in place. More like worldwide confusion!
Note: discussing temperature effects generally leads to discussion about temperature coefficients... which are the slope of a phenomenon versus temperature. In the real world, slope changes versus temperature, so you can't just assume a single coefficient if you want accurate predictions. Class 1 caps are specified by temperature coefficient, as they have been engineered to have linear temperature behavior. Class 2 caps vary all over the place, so a window of variation is specified over a temperature range.
A common miscionception is that capacitors "X7R" and "NPO" refer to materials, like "GaN" refers to gallium nitride. The letter codes refer dielectric performance. The actual dielectric materials have long names, such as oxides of barium titanate. Microwave engineers usually don't need to dive into that.
Some electronics standards history...
Before we get deep into this discussion, you'll need some history of standards organizations. These organizations are formed as collaborative efforts of manufacturers. The grand-daddy of all electronics standards associations was Associated Radio Manufacturers, formed in 1924 when radio was rapidly expanding. By 1952, it morphed into Radio and Television Manufacturers Association (RETMA). You might be familiar with "RETMA" values for components, see our discussion on this Microwaves101 page. RETMA becamer Electronic Industries Association (EIA) in 1957. In 1997 EIA became Electronics Industry Alliance. EIA disbanced in 2007, but JEDEC spun back out and carries on some of the mission. Standards associations require money to operate, thus they sell standards on-line, as well as charging a minimum of $6400 to become a member. Last time we checked the capacitor spec EIA-198 that governs the information on this page sells for $83, on the IHC Markit web site. You can also buy it on Electronic Components Industry Association ECIA's web site. Maybe one of these days we'll buy it, but it seems a little steep for a document that was probably written 50 years ago. Maybe someone reading this has a copy and canm weigh in on how it changed their life? Meanwhile, info from EIA-198 has leaked out over the years, because hiow are you going to apply a standard it it is all a big secret? More info on electronics standards organizations here on Wikipedia.
Note: there is a competing standard from International Electrotechnical Commission (IEC) in Europe. There is apparently a lot of overlap, but the devil is in the details. In practice, the European standard seems to have taken over, just visit the Digikey web site. Time marches on!
Classes of ceramic caps
Class 1: Temperature stable (linear variations available for temp-comp circuits), but limited capacitance density (dielectric constants up to ~40)
Class 2: More temperature variation (very nonlinear), much higher capacitance (dielectrics in thousands)
Class 3: So-called barrier-layer capacitors. Highest capacitance density, but don't go there: this is practically an abandoned technology. Z5U and Y5V dielectrics ofrten come up in the context of "what not to do".
Class 4: Developed by an advanced species of aliens on the planet Metaluna, and used in the Interocitor
How to use an Interocitor (This Island Earth, 1952)
Seriously, if you enjoy science fiction movies from their Golden Age (1950s), you need to see This Island Earth.
Class 1 capacitors
In the microwave world, you will want to use Class 1 for RF circuits such as matching networks, filters, VCOs. Class one caps are specified by temperature coefficients and tolerances. If you plotted Class 1 cap value versus temperature, you would see close to a straight line, or at least a monotonic line.
How do you apply a temperature coefficient? It seems silly that an engineer would ask that, but we'll show you here so we don't have to answer the occasional email question on this topic.
C(T)=C0*(1+Tempcoefficient*(25-TempC))
Where:
C(T) is capacitance value at temperaure
C0 is room temperaure (nominal) value of capacitance
Tempcoefficient is guess what?
TempC is temperature in Celsius
Note that when temperature coefficient is specified in parts per million (ppm), you need to divide it by 1E6.
Below is the EIA-198 (United States) Class 1 standard:
| Ceramic specifier |
Temperature coefficient α 10−6 /K |
Temperature coefficient tolerance (ppm/K) |
| P100 |
100 |
±30 |
| NP0 |
0 |
±30 |
| N33 |
−33 |
±30 |
| N75 |
−75 |
±30 |
| N150 |
−150 |
±60 |
| N220 |
−220 |
±60 |
| N330 |
−330 |
±60 |
| N470 |
−470 |
±60 |
| N750 |
−750 |
±125 |
| N1000 |
−1000 |
±250 |
| N1500 |
−1500 |
±250 |
The European standard EIA-RS-198 decoder ring is summarized as:
| Temperature coefficient α 10−6/K |
Temperature coefficient multipler |
Temperature coefficient tolerance (ppm/K) |
| C: 0.0 |
0: −1 |
G: ±30 |
| B: 0.3 |
1: −10 |
H: ±60 |
| L: 0.8 |
2: −100 |
J: ±120 |
| A: 0.9 |
3: −1000 |
K: ±250 |
| M: 1.0 |
4: +1 |
L: ±500 |
| P: 1.5 |
6: +10 |
M: ±1000 |
| R: 2.2 |
7: +100 |
N: ±2500 |
| S: 3.3 |
8: +1000 |
|
| T: 4.7 |
|
|
| V: 5.6 |
|
|
| U: 7.5 |
|
|
Note that the European standard allows the temperaure coefficient and its tolerance to be specified separately.
NP0 (EIA standard) and C0G (EIS standard) both have nominally zero temperature coefficients. Note that the both NP0 and C0G have numeral zero, not letter O, in the name. Zero implies zero variation. Below, "Podotresno" in Indonesia shows the difference between a temperature stable and capacitor compared to a cheaper alternative, using a $30 multimeter and a butane lighter. Brilliant!
If you were to go to Digikey and pull up all the options for a 1 pF ceramic MLCC temperaure coefficients, you will find an alphabet soup:
A
BG
BP
C0G, NP0
C0H
C0K
CH
CL
M
P2H
P90
R2H
S2H
T2H
U2J
U2K
8XR
Of the available dielectrics, only P90 refers to an EIA code. The other three letter codes are European (EIS) standards. The single-letter and two-letter codes are manufacturers' codes, you'll have to dig through some data sheets to figure out what they mean, but that's why they pay you the Big Bucks. If you thought picking a capacitor was easy, you probably have never done it!
Class 2 capacitors
If you plotted Class 2 capacitor value versus temperature, you will see a function that cannot be fit to a straight line. The technical term for this is "squiggly mess".
It seems like the EIA-198 spec is still in common usage for Class 2 caps. Go figure... In the EIA-198 Class 2 spec, the first letter refers to the lower temperature:
| X = −55 °C |
| Y = −30 °C |
| Z = +10 °C |
The second character, a numeral, refers the upper temperature limit:
| 4 = +65 °C |
| 5 = +85 °C |
| 6 = +105 °C |
| 7 = +125 °C |
| 8 = +150 °C |
| 9 = +200 °C |
The third character, a letter, refers to tolerance.
| P = ±10% |
| R = ±15% |
| L = ±15%, +15/-40% above 125 °C[10] |
| S = ±22% |
| T = +22/−33% |
| U = +22/−56% |
| V = +22/−82% |
So, the worst performer would be Z4V.
Real-world horror story: once upon a time, someone was testing a new MMIC over temperature. At room temperature it was stable, but at 85C it oscillated. The problem was found to be Y5V bypass caps that lost 82% of their capacitance at high temperature!
More to come... we need to add some plots!