If an engineer’s design demands a precision and stable capacitor, she or he will usually turn to NPO-type multi-layer ceramic (MLCC). This old standby is still the best. If you haven’t checked them out recently, you will find that manufacturers have been able to reduce their case size and extend their value range considerably. But you could look at film, silicon, and mica capacitors as well.
NPO dielectric capacitors
Ceramic NP0 capacitors from all manufacturers have an excellent thermal stability of <0.4% change within the total temperature range of −55°C to 125°C. Tolerances as low as 1% are easily available, and their aging rate is a nice 0%/decade hour. You can get NPO capacitors with voltage ratings from 10 V to 3 KV. Best of all, the voltage coefficient for NPO is nill.
As I said earlier, manufacturers have improved on the NPO dielectric in the past few years and a few of them offer NPO with values all the way to 0.1 µF. This relatively high value comes in a 1206 or 1812 SMT case size.
The X7R characteristic MLCC capacitors are an alternative, but with some caveats. They vary only ±15% over −55°C to 125°C and have a ±5% or ±10% tolerance. Aging rate is 3%/decade hour. The specs are not nearly as good as NPO, but not bad. And they have a dielectric constant of 2,000 to 4,000, maybe 20 times that of NPO, so they are much smaller for the same capacitance and voltage rating.
It’s their voltage coefficient that designers sometimes forget about. For an X7R dielectric, at 50% of rated voltage, you can lose 35% of capacitance. I say “can” lose this much because the loss varies by value, package size, and voltage rating. A designer should figure a 35% loss of capacitance at one-half rated voltage, and 80% with full rated voltage applied for X7R devices. X5R and Y5V dielectrics are much worse than X7R in this respect.
Cornell Dubilier offers NPO SMT capacitors with values from 0.5 pF to 0.012 µF and voltage rating from 10 to 200 Vdc in 0402, 0603, 0805, 1206, and 1210 case sizes. If you go bigger than a 0402, you step up to a 50 V rating. You can get a 5600-pF device in a 1206 package.
AVX offers voltage ratings from 6.3 to 500 V for NPO and has expanded its product range to include case size 0101 with values to 100 pF at 16 V. You can get values as large as 0.1 µF at 25, 50, or 100 V in an 1812 package (1812YA104FBT1A) with 0.11-inch thickness. These devices, however, are not inexpensive at $7.39 ea/1,400 with 1% tolerance. The 2225 case 50-V version is a bit less expensive at $5.00 ea/1,000.
The 5% tolerance version is much cheaper. Of course, lower values are also much cheaper. A 1,000-pF NPO 1% device can be found in an 0402 package at 16, 25, or 50 V for $0.11 ea/10,000.
NPO ceramic capacitors.
Kemet offers a 0.10-µF 25-V 1% NPO part in a 1206 package (C1206C104F3GACTU). It is $2.50 ea/2,000. At the other end of the spectrum, we have their 10-pF 50-V 0402 1% NPO device that costs just $0.026 ea/10,000.
Mica capacitors
I think of mica capacitors as being in antique radios, and they have been around for a very long time. Nowadays, they are available in 2220 SMT packages, but are still usually found in a dipped radial lead type. They come with 1% tolerance with values from 1 to 40,000 pF, −55°C to 125°C operation and up to 1-kV rating, with 20,000-V/μs dV/dt pulse capability. They are large compared to ceramic. A 0.01-µf 100-V leaded part is 0.67 x 0.51 inches and 0.31 inches thick. Cornell Dubilier is a leader in mica capacitors and Murata and TDK also offer them. A 1,000-pF, ±1%, 100-V device from Cornell Dubilier (MC22FA102F-TF) comes in a 2220 package and costs about $3.44 ea/1,000.
A mica SMT capacitor from Cornell Dubilier.
Silicon capacitors
Silicon capacitors are available in different types. AVX, Murata, and Vishay offer them in low values (100 pf or less) targeting RF and microwave applications.
IPDIA provides silicon capacitors in values from 10 pF to 3.3 µF in 1005 to 1812 package sizes. Key benefits touted by the manufacturer are low profile (400 µm), low leakage, high stability, high reliability, and extended temperature range. Temperature stability is an excellent ±0.5% (−55°C to 150°C) and their voltage coefficient is <0.1%/V. Like NPOs, they offer negligible aging (<0.001%/1,000 hours), but their tolerance is ±15%. The HSSC423xx series features an ESR of 400 mΩ and an ESL of 100 pH. A 1000-pF 11-V 0402 size unit (935131424410) costs about $0.94 ea/single quantity.
The IPDIA 935 silicon capacitor.
Film capacitors
Film capacitors are often used in DC blocking, coupling, bypass, DC line filtering, power factor correction, and AC line filtering applications. There are a number of different types. The type that fits a need for precision is polyphenylene sulfide (PPS) film. These capacitors can offer +0.5% capacitance change from −25°C to 85°C and a ±2% tolerance. They also feature a dissipation factor of 0.2% typical and very low dielectric absorption. PPS parts come in values from 100 pF to 0.22 µf. Standard tolerance is ±5%, 2% is optional.
The temperature stability graph for PPS film capacitors.
API Technologies offers film PPS capacitors in leaded epoxy cases with 50- to 600-V ratings and 0.001- to 5.0-µF values. The 0.1-µf, 50-V version (WV2A104) has as a 0.16 x 0.24 x 0.42-in. package.
Cornell Dubilier has PPS film capacitors with ±2% tolerance in 1210 to 4833 SMT packages and 16- or 50-V ratings. They have values between 100 pF and 0.22 µf with 2% or 5% tolerances. The FCP1210C104J-G3 is a 0.1-µf, 16-V version with a 5% tolerance in a 1210 package. It costs just $0.38 ea/1,000 and is in stock in distribution.
Kemet offers the LDB Metallized PPS stacked devices. The 0.1-uF, ±2% (LDBAB3100GC5N0) is rated for 16 Vdc at 125°C. They cost about $0.53 ea/1,000.
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