Film Capacitor Selection: DC Link, Snubber and Class X/Y Safety Caps

Film capacitors get chosen last and specified least. They are the part you reach for when a ceramic loses most of its capacitance under bias and an electrolytic will not survive the ripple, and the three jobs they usually do, mains suppression, DC link and snubber, have almost nothing in common beyond the dielectric. Picking one means answering a different question in each case.

Polypropylene or polyester

Two dielectrics cover nearly the whole range. Polyester, marked PET or sometimes mylar, is dense and cheap. A KEMET F161WP105K063V gives 1 uF at 63 V in a small radial box, and its dissipation factor of roughly 0.005 is fine at audio frequencies and low ripple.

Polypropylene, marked PP, has a dissipation factor around ten times lower, near 0.0005, and holds its capacitance much more steadily over temperature. That low loss is the whole reason it dominates power electronics: dissipation in a film capacitor is ripple current squared times ESR, and ESR follows the dielectric loss. A TDK B32654A6105J000 gives 1 uF at 630 V in PP and will run cool where a PET part of the same value would not.

Use PET where the part is small, the voltage is modest and the current is low. Use PP anywhere current flows continuously through the capacitor.

Metallized or film/foil

In a metallized part the electrode is a few tens of nanometres of aluminium evaporated onto the film. When a flaw punctures the dielectric, the fault current vaporises the metal around the pinhole and isolates it. The capacitor loses a sliver of capacitance and keeps working. This is self-healing, and it is why metallized film survives in mains applications where a fault would otherwise be terminal.

The trade is peak current. That thin electrode cannot carry much, so a metallized part has a limited dV/dt rating. Film/foil construction uses a discrete foil electrode, does not self-heal, and handles far higher pulse currents. A GE A28F5502S, 0.5 uF in film/foil PP, is built for snubber duty for exactly this reason.

X and Y classes: the safety part

Capacitors connected to the mains are classified by what happens when they fail, not by how well they work. This is the part that fails a safety review.

Class X sits line to line, across the mains. A failure there does not expose anyone to electric shock, but an uncleared fault would be a severe fire hazard. Class X capacitors use self-healing metallized film that vaporizes the electrode around a dielectric puncture, isolating the fault so the part loses a tiny fraction of capacitance and degrades gracefully toward an open circuit rather than welding into a dead short. X2, rated for a 2.5 kV peak pulse, covers ordinary mains equipment. X1, at 4 kV, is for higher surge environments. A KEMET R46KI315050M2K is a 0.15 uF X2 at 275 VAC.

Class Y sits line to earth, or across reinforced insulation. A short there puts mains on the chassis, so Y parts must fail open and are built with far more margin. Y2 withstands 5 kV, Y1 8 kV. A TDK B32022A3473M000 is a 47 nF Y2. Y capacitance is capped in practice by the earth leakage current the equipment is allowed, so these are nanofarads, never microfarads.

Never substitute a general purpose capacitor of the same value and voltage rating into an X or Y position. The rating that matters is not the working voltage, it is the pulse withstand and the failure mode, and only a safety-approved part carries it.

DC link: sized by ripple, not by capacitance

In a DC link the capacitor absorbs the current the converter switches. Capacitance sets the voltage ripple, but what actually limits the choice is RMS ripple current against the part's own rating, which exists because ESR turns that current into heat inside a plastic body with poor thermal conductivity.

Work in that order: compute the RMS ripple current for the topology, find a part rated above it at your ambient, then check that the resulting capacitance holds the ripple voltage inside the converter's tolerance. Voltage headroom matters too. A 400 V bus with a soft start overshoot and a braking event does not belong on a 450 V part, which is why parts like the Vishay MKP1848S55010JP2C are rated at 1 kV.

Snubber: sized by dV/dt and pulse current

A snubber capacitor sees a fast edge and must pass the current that edge demands, and the current is C times dV/dt. A 0.1 uF part across a switch node moving at 5 V/ns carries 500 A for the length of that edge. Nothing about the capacitance value tells you whether the part survives it. Look for an explicit dV/dt or pulse rating, and expect a film/foil or a heavy-duty PP construction rather than a general purpose metallized part.

Comparison table

Selection checklist

• Decide the job first. DC link, snubber and mains suppression size on different parameters.
• On the mains, specify a safety class, X or Y, and never a general purpose equivalent.
• For any continuous current, compute RMS ripple and check it against the rating at your ambient, not at 25 °C.
• For any fast edge, check the dV/dt or pulse rating explicitly. Capacitance does not imply it.
• Give voltage headroom for start-up overshoot and regenerative events, not just the nominal bus.
• Check the capacitance tolerance where the value sets a filter corner or a timing constant. Film parts are commonly 5% or 10%, not 1%.

FAQ

Q: Can I replace a film capacitor with a ceramic of the same value?

Rarely, in the positions where film is normally chosen. Class 2 ceramics lose a large fraction of their capacitance under DC bias and are strongly temperature dependent, so a 1 uF X7R at half its rated voltage may deliver well under half its marked value, while a film part holds its value. Ceramics are also piezoelectric and will sing audibly under mains ripple. C0G ceramics are stable but are not available in the capacitance range film covers.

Q: What actually happens when a metallized film capacitor self-heals?

The fault current through the pinhole vaporises the thin aluminium electrode in a small ring around it, disconnecting that patch from the rest of the plate. The clearing takes microseconds, and the capacitor loses the capacitance that patch represented, which is a tiny fraction of the total. A part late in life has accumulated many such clearings, and the usual end-of-life signature is capacitance that has drifted below tolerance rather than a sudden failure.

Q: Why is my X2 capacitor rated 275 VAC when the mains is 230 VAC?

The AC rating covers the continuous working voltage with margin for supply tolerance and long term ageing of the dielectric, and 275 VAC or 305 VAC are the standard ratings for a 230 VAC mains. The number that handles surges is separate: the class defines the pulse withstand, 2.5 kV for X2, and that is what survives a lightning-induced transient on the line.

Q: Do film capacitors need derating with temperature?

Yes, and it is usually a voltage derating above a stated corner. Most film parts hold full rated voltage to about 85 °C and then derate linearly to the maximum temperature, often 105 °C for PP. Ripple current capability falls at the same time, because the rating exists to keep the hot spot inside the winding below a limit and a hotter ambient leaves less room for self-heating.

Filter by capacitance, voltage, dielectric and safety class in Novapart's film capacitors range, or go to a brand you already qualify such as KEMET film capacitors. If the decision is really film against something else, the ceramic vs electrolytic comparison covers the other two candidates, and the AC/DC power supply guide puts the DC link in context. Building a mains front end? Upload the BOM for consolidated sourcing with cross-references and lead times.