What are the features that make a MKP capacitor labelled as "Class-X2"?

[onemilimeter]

I wish to choose a metallised polypropylene (MKP) capacitor (0.1uF or 2.2uF) for C5 as shown in the attached figure. Several choices as follows:

[1] VISHAY BC COMPONENTS - BFC233620104 - CAPACITOR, CLASS X2, 0.1UF
VISHAY BC COMPONENTS|BFC233620104|CAPACITOR, CLASS X2, 0.1UF | Farnell

[2] EPCOS - B32922C3104M - CAPACITOR, CLASS X2, 0.10UF, 305VAC
EPCOS|B32922C3104M|CAPACITOR, CLASS X2, 0.10UF, 305VAC | Farnell

[3] EPCOS - B32652A6104J - CAPACITOR, 0.1UF, 630V
EPCOS|B32652A6104J|CAPACITOR, 0.1UF, 630V | Farnell

All of them have similar AC and DC voltage ratings. They are constructed from the same dielectric material i.e. polypropylene. However, items [1] and [2] are labelled as "Class-X2". What are the features that make [1] & [2] but not [3] labelled as "Class-X2"? Can items [1], [2] and [3] be used as R5 in the power inverter (VDC=150V, Q1~Q6 are power MOSFET, maximum PWM frequency 10kHz, C1~C4 are aluminium electrolytic capacitors) shown in the attached figure?

Cheers.

[amspire]

X2 capacitors are rated for use across the mains (Active to Neutral - not Active to Earth). They are impulse tested to 2500 volts, and have self-healing characteristics with AC applied.  They cannot necessarily self heal with applied DC. If there is enough DC volts and current available, once arcing starts, it will probably not stop until a big part of the capacitor has burnt away. The issues with the mains are that high voltage spikes can happen, and there is a massive amount of energy available.

X2 is nothing to do with direct human safety, but more in terms of being unlikely to cause the house to burn down. You cannot use an X2 or an unrated capacitor to isolate a human from a dangerous high voltage.

The non rated capacitors are fine within a circuit where the maximum available energy is usually limited anyway. They can handle the rated voltage, but in the event they fail, there are no promises as to how they will fail.

Richard.

[onemilimeter]

Thank you amspire.

Is it true to say that X2 rated MKP capacitor can be safely used to replace any non-rated MKP capacitor (providing both X2 rated and non-rated have similar AC/DC voltage ratings) but not the other way around?

Cheers.

[alm]

I definitely wouldn't use a non-rated capacitor on the primary side of a power supply, or between primary and secondary side. I believe X2 capacitors may be inferior in audio/signal coupling applications due to the thicker dielectric causing dielectric adsorption, which distorts the signal. Apart from that, they should be fine (although physically larger).

[amspire]

Is it true to say that X2 rated MKP capacitor can be safely used to replace any non-rated MKP capacitor (providing both X2 rated and non-rated have similar AC/DC voltage ratings) but not the other way around?.

X2 caps can replace other capacitors, as long as the specs for the X2 cap are good enough. If you need a precision capacitor, it is probably not going to be an X2 rated capacitor.  Also X2 caps are larger and considerably more expensive then non-rated capacitors so normally X2 caps are only used when needed.

If you need capacitors or any other devices like MOSFETS across a high voltage DC rail, you have to consider the consequences of a component failure as a failure can easily burn a board to a stage it is irreparable. Using an X2 or even a safer Y2 capacitor is no guaranty against failure so you have to look at what other protection measures are in place such as AC  fuses on the AC input or DC rated (ie expensive) fuses if you are coming from a battery.

[slateraptor]

@onemillimeter: May I inquire as to what program you used to produce the schematic?

[onemilimeter]

@onemillimeter: May I inquire as to what program you used to produce the schematic?

I used Proteus ISIS to sketch the diagram.