Which type of capacitor should you use?
Well, it depends on the circuit. There are many different types of capacitors, bulk ones such as electrolytic, aluminum-polymer, polymer-tantalum, and tantalum. Film capacitors such as polypropylene, polyethylene, polyester, polyethylene, and acrylic metallized in SMD type. Ceramic capacitors with type two dielectric and type one dielectrics. So when choosing a capacitor, you basically have to weigh the pros and cons of the capacitor type for the intended application in circuit.
So why would you want to use electrolytic capacitors?
Well, electrolytic capacitors are available in much higher values than other types of dielectrics. For instance, you can get many Farad’s in an electrolytic capacitor. So if you have a power supply rail of a high power output device which needs a large amount of bulk charge storage, for instance, a high power amplifier, then you may have to resort to electrolytic at some point. They are also desirable in that they have low leakage currents when they aren’t operating close to their rated voltage. Choosing the highest voltage rating you can find for your given footprint size, is a great way to assure you are getting less leakage and usually lower ESR as well.
Why you may not want to use an electrolytic?
They are subject to wear out over time due to the liquid electrolyte and the typically higher values of ESR. So if they’re not used correctly in circuit, you could actually end up going over the ripple current rating quite easily, which can cause them to heat up and they can fail prematurely. So if you’re going to use them in circuits where you have to, it’s usually a good idea to parallel a couple in there so they can share the ripple current (which will also lower their affective ESR) or at least check the data sheet and make sure you’re not going over the maximum allowed ripple current rating.
Final thoughts on electrolytics
There are some higher quality type electrolytic capacitors. Nichicon, Panasonic, and Rubycon make some higher quality ones. Be sure to check and see what their lifespans are rated at for certain temperature ranges. Some electrolytics these days don’t list full specs on them such as ESR for instance. They do usually list the tan Φ though which is the dissipation factor, but only at a specific frequency. You can actually calculate the ESR value of these capacitors with the following formula. Lets assume we have a capacitor with a tan Φ or Dissipation Factor of 0.12 at 120hz and it is a 47uF. We would take:
0.12_______________________________= 0.33ohms ESR at 120hz2*pi*120*0.00047
Why you would want to use Polymer Aluminum Capacitors
These are really low ESR capacitors. Typically these will be the lowest ESR capacitors that exist. So they’re really great for decoupling of power supply rails where you really need to have low ESR values such as in switch mode power supplies. They have very high ripple current ratings due to the low ESR values. To get lower ESR in the lower frequency ranges you will want to use a higher capacitance value. They do not need as much of a voltage derating as tantalum either. Only a 10% derating for 10v or less, or 20% derating for over 10v. This means you can safely use a 6.3v polymer on a 5v rail.
Why you may not want to use an aluminum polymer
Now, there are some cases where you may need to have higher ESR values in your circuit, for instance, on the output of some older types of low dropout regulators. They actually need a certain amount of ESR to mainstay maintain stability. So you wouldn’t want to use something like that there. For instance, you wouldn’t want to use them in circuits which are sensitive to leakage current or circuits where a low dissipation factor are required such as in a coupling circuit or the input coupling for an amplifier. Also, you may not want to use them in some portable devices which are very low power devices and that need to have long battery life or very high efficiency because the large amounts of leakage current can eat up some power. It would make little sense to use them in any ultra low power design anyway, because you can usually get away with much smaller value capacitors.
Why you would want to use Polymer Tantalum Capacitors
These are often available in small SMD packages, for instance 0805, 0603, and 0402 packages and are a bit higher ESR value than a polymer aluminum. They could be fit in a much smaller space, so if size constraints are problem and you need to use a polarized capacitor, a polymer, tantalum might work really well there. They are typically low enough ESR that they could be used quite well for power supply decoupling even of switch mode power supplies. These also offer decent ripple current capabilities and do not require much voltage derating- 10% for 10v or less rated parts and 20% for over 10v rated parts.
Why you may not want to use Polymer Tantalum?
In cases where higher ESR is needed to maintain stability, again, such as in a some of the older low drop out regulators. These would sometimes need 2 to 20 ohms ESR on the output to maintain stability. A polymer tantalum is not really going to give you that because they’re typically in the millohm range of ESR. Always check the datasheet of the components you want to use. there are many new LDO’s which are stable with low ESR capacitors. Polymer tantalums have some leakage current, typically not quite as much as a polymer aluminum, but then again, they’re also not available in as high of values as a polymer aluminum. Therefore the dissipation factors are similar to a polymer aluminum, but they’re not going to have as low leakage as electrolytic or tantalum.
Why you may want to use Tantalum Capacitors
These are kind of similar to electrolytic in the in that they have generally quite high ESR values. Now these are available in small SMD packages as well, where electrolytic are not. But still you’re looking around probably five ohms to 10 ohms ESR in a tantalum capacitors. So these would work well in cases where you’d need a high enough ESR value to maintain stability. They also have low leakage current. If you need a polarized capacitor and these factors fit the bill you can use them.
Why you may not want to use Tantalum Capacitors
They probably won’t work really good in power supply or decoupling for anything that needs to respond to high transients, for instance, switch mode power supplies. The high ESR and low ripple current ratings can mean you have to be careful in using them. The high ESR can also cause a voltage drop. They’re going to be very power inefficient due to the highest ESR. They generally want to be avoided as much as possible, just like Electrolytic would probably want to be avoided as much as possible unless absolutely necessary. Tantalums can also fail easier than some other dielectrics if you dont follow the guidelines. They require a very large voltage rating, typically about 50 percent or more, so you’d generally need to use a 10v rated one on a 5v rail.
Why you may want to use Film Capacitors:
There are many types of film capacitors such as polyethylene, polyester, polystyrene, and SMD metalized acrylics which are available in really small packages. Film capacitors work great for coupling applications, for instance on the input filtering of an amplifier. This is due to their very low dissipation factor. They can also be useful in EMI suppression circuits. Since they usually aren’t available in very large capacitances, they generally don’t work well for decoupling unless you only need a small value. The best film capacitors are usually polypropylene since they have the lowest dissipation factor. Polystyrene and polyester are a bit higher dissipation factor. If you need an SMD solution then the metalized acrylics can work but keep in mind they usually have slightly higher dissipation factors.
Why you may not want to use a film capacitor
They are available in smaller values and are often physically large. Due to this they may not be desirable in decoupling applications.
Why you may want to use Ceramic Capacitors
Now, ceramics, we have type two and type one dielectrics.
Type 2 So an example of type two ceramics is Y5V, X7r, X5r, and JB. First off Y5V is far less superior and should generally be avoided in all cases because there is no benefit to using them for anything other than sometimes cost being cheaper. So I’ll focus mainly on the higher quality ones here which include X7R and X5R. These are great for decoupling of power supply rails due to their low ESR/ESL and small case sizes. They also have fairly small leakage current. To minimize this even further you can choose a higher voltage rated part which will have a higher insulation resistance and therefore less leakage. A higher voltage rating than needed as helps to prevent the drop off in capacitance with the applied voltage.
Type 1 A type one dielectric would be something like C0G, NP0, or even U2J dielectric. These kind use calcium zirconate as the dielectric and dont exhibit any of the piezoelectric effects that type 2 are known for. They also offer excellent stability, have very low voltage coefficients, meaning they retain their full capacitance even when voltage is applied. These will work just as as film caps, if not better at coupling. They have an extremely low dissipation factor, often below 0.1%.
Why you may not want to use type 2 ceramics
These use barium titanite for their dielectric material which is piezoelectric. They could have mechanical vibration in circuits. So they’re usually not suited for coupling applications. They also have higher voltage coefficients, which means that as a voltage is applied to the type two ceramic capacitor, It’s going to have a drop off in capacitance. So these generally aren’t advisable for coupling applications.
Why you may not want to use type 1 ceramics
They are only available in very small values. You can “stack” them by soldering the ends together when mounted on top of one another but due to structural stability I wouldn’t suggest stacking more than one, and that still makes their capacitance values a bit limited.
I hope this gives you a better idea of which type of capacitor you should use in your circuit. And if you still have questions and you’re not sure which one you should use for your application, feel free to contact us and we’ll help you out if we can.