My favorite electronics book is "High Speed Digital Design: A Handbook Of Black Magic". I highly recommend this book. It seems expensive, but it is totally worth the money. This book has 12 pages on choosing a bypass cap! The author, Howard Johnson, also teaches some classes with decoupling caps as one of the topics.
Some important things that I've learned over the years, and have been backed up by this book, is that the "standard practices" with decoupling caps are almost always wrong and there is more art than science when it comes to choosing and routing them.
There are lots of calculations that you can do regarding decoupling caps, but much of those are not accurate due to many things. The caps themselves are vary wildly (especially the higher dielectric caps like X7R). The PCB layout changes things greatly (and you'll need to think in 3-D for this one). Temperature and voltage will change the behavior of the caps. A single cap will behave as both a "power supply smoothing cap" and a "AC signal return bypass cap". Etc.
What Johnson did was, after a lot of experimentation, figure out that inductance is the most important factor and it swamps almost every other consideration. So the goal when selecting and placing decoupling caps is to use a lot of physically small caps, with the highest practical value, and route them so the total inductance is as low as possible.
The ideal would be to use lots of 0.1 uF caps in an 0402 package. Place them under the chip on the back side of the PCB. The cap be routed as in the image below. And the vias go directly to the power/ground planes (not to the chip's power pins, as that would usually increase the inductance). If you place the cap under the chip then sometimes you could share the same via without any issues.
The reason why a 0.1 uF cap was chosen is because it is the highest practical in an 0402 package. The reason why 0402 was chosen is because it is the smallest practical size, and you want to use a lot of them to get the effective ESL/ESR down. Of course all bets are off if you have a 2 layer PCB without power and ground planes.
I don't want to belittle the use of the math, that is important, but the complexity of power supply decoupling and AC return paths often makes the math not so practical in the real world. In the real world, a "rule of thumb" really helps. Of the many rules of thumb for this topic, it has only been Howard Johnson that has proven the other rules don't work and provided this better rule. My experimentation and experiences has shown this to be true.
That is an X-type safety capacitor).
Printed on the side:
MKP/SH: denotes polypropylene film dielectric, self-healing
B32921: denotes the case size; this case size is typically rated 305VAC.
40/105/56: Climatic Code from IEC 60068-1, interpreted as follows, obtained from here
CLIMATIC CATEGORY The climatic category indicates the climatic
conditions which the capacitor may be operated.
According to IEC 60068-1 the climatic category is expressed by a
three group coding e.g. 55/100/56.
- The first group indicates the lower category temperature (- 55 °C).
- The second group the upper category temperature (+ 100 °C).
- The third group indicates th e number of days (56) which the capacitor can withstand within specified limits if exposed to
a relative humidity of 95 % and a temperature of + 40 °C.
I suspect if you can view the rest of the component markings you will find the capacitance, tolerance, and probably the voltage.
Best Answer
The X2 rating is mainly its ability to withstand high voltage surges. It is somewhat lower than the X1 cap and quite a bit lower than the Y1 and Y2 caps. Otherwise, you select voltage rating and value in much the same way as any other place. X1, X2 caps are for across-the-line applications and Y1, Y2 are for line to earth connecting. Although these caps are intended for AC applications as RF (EMI) filters, you will find them unsuitable for applications such as snubbers, etc. They do not have the greatest dissipation factors.