These parts sit in a different world from the 50 V ceramic capacitors used for decoupling. Where a general purpose MLCC is judged on capacitance per unit volume, a high voltage disc is judged on how much voltage it can stand, how stable it stays over temperature, and how it behaves when it finally fails.

How the Disc Is Built

The construction is deliberately simple. A ceramic disc is pressed from dielectric powder, fired, then screen printed with silver on both faces to form the electrodes. Leads are soldered or clamped to the silvered faces and the whole body is coated in epoxy, which provides the dielectric strength along the surface and keeps moisture out. Tubular versions follow the same idea wrapped into a cylinder for higher voltage ratings.

Almost all high voltage ceramic capacitors are single layer. That is not an oversight. Holding off 15 kV requires a dielectric layer on the order of several millimetres, and stacking that into a multilayer body would multiply both the thickness and the cost without adding any useful capacitance. Single layer construction also removes the mechanical stress that builds up between layers in a stacked part, which matters when the capacitor is cycled through large temperature swings.

Class I and Class II Dielectrics

Older reference material often splits these ceramics into Class I, II and III. Current practice follows the EIA scheme, which really has two working groups, and the older “Class III” high permittivity materials are now folded into Class II.

Class I is the temperature compensating family. Its capacitance barely moves with temperature or applied voltage, so it is used where the value has to stay put, such as in resonant circuits, snubbers and timing paths. The price is dielectric constant, and therefore capacitance: a C0G disc at 15 kV is usually measured in tens or hundreds of picofarads.

Class II trades stability for capacitance. A Y5V disc can reach ten times the capacitance of a C0G part in the same case size, but its temperature coefficient runs to roughly +22% and −82% across its range, and the capacitance falls further as the rated DC voltage is applied. When a datasheet gives a Y5V capacitance, read it as a value at 25 °C and low voltage rather than as the value you will see in service.

Why Capacitance Falls as Voltage Rises

The relationship is physical, not a design choice. Capacitance follows C = εA/d, where d is the dielectric thickness. Raising the working voltage means raising d, and raising d cuts capacitance in direct proportion. The dielectric constant can be increased to claw some of it back, which is exactly what Class II materials do, but that same high permittivity is what brings the temperature and voltage drift.

The practical result is that a 15 kV capacitor and a 50 V capacitor of the same physical size will differ in capacitance by orders of magnitude. Designing around a high voltage disc means accepting that the capacitance you can get is limited, and shaping the circuit around a few hundred picofarads rather than a few microfarads.

The Failure Mode to Design Around

Ceramic capacitors fail short. When the dielectric breaks down, the silvered faces are bridged and the part turns into a low resistance connection. Film capacitors behave the other way and generally fail open. The difference matters because a shorted disc across a high voltage rail will keep drawing fault current until something upstream clears it.

Two consequences follow. First, high voltage ceramic capacitors are normally placed where a short is survivable, either downstream of a fuse or in a position where the fault current is inherently limited. Second, a disc that has been overstressed should be treated as failed even if it still reads a plausible capacitance, because the breakdown path is often a partial one that widens with each voltage excursion.

Where They Are Used

The common thread in every application is a circuit that switches or stores energy at high voltage and cannot tolerate the inductance of a wound part.

• Snubbers across switching devices in high voltage supplies and inverters
• Coupling and DC blocking in high voltage signal paths
• X-ray generators and medical imaging power supplies
• Plasma generators, electrostatic spraying and electrostatic precipitators
• Pulsed laser and rapid pulse discharge circuits
• Non-destructive testing equipment and voltage multipliers

Note that the roles above lean on two different properties. In snubbers and pulse circuits the part is chosen for low inductance and high current capability, which is why a disc with short leads is preferred over a wound capacitor. In coupling and blocking duties the part is chosen for voltage hold-off and stability instead.

Choosing One

The order that keeps a design out of trouble is simple.

1. Set the working voltage with margin for transients, then pick a rated voltage above it.
2. Decide whether capacitance stability matters. If it does, go Class I and accept a small capacitance. If it does not, Class II gives more capacitance for the same case.
3. Read the tolerance and the temperature coefficient together, not separately, because a Class II part can move far more than its tolerance band suggests.
4. Check the voltage coefficient at the working voltage rather than at the test voltage.
5. Choose the body and lead style last, based on the creepage distance the board needs.

Xuansn manufactures high voltage disc ceramic capacitors from 1 kV up to 15 kV in Class I and Class II dielectrics, with the epoxy coating and lead styles shown on our high voltage ceramic capacitor range. A typical example from that range is the 102K 1 kV disc, a Class II part used in bypass and coupling positions.

FAQ

Can I use a normal 50 V ceramic capacitor at 1 kV?

No. Rated voltage is a limit, not a suggestion, and a part rated at 50 V will break down well below 1 kV. Beyond the voltage rating, the dielectric thickness and the creepage distance of a low voltage part are both far too small for high voltage service.

Why is the capacitance of a 15 kV capacitor so small?

Because the dielectric has to be thick enough to withstand the voltage. Capacitance is inversely proportional to dielectric thickness, so a part built for 15 kV ends up with a capacitance in the picofarad range even in a large case size.

What happens when a high voltage ceramic capacitor fails?

It usually goes short circuit rather than open. Plan the circuit so that a short is cleared by a fuse or inherently limited by the source, and replace any part that has been subjected to an overvoltage event.

Do high voltage disc capacitors come in tight tolerances?

Class I parts do, commonly to ±5% or better. Class II parts are looser, and their capacitance moves with temperature and applied voltage, so the marked tolerance describes the value at the reference conditions rather than the value you will measure in circuit.

If you are matching a disc to a specific voltage, capacitance and stability requirement, send the working voltage, the capacitance you need and the temperature range to coco@xuanxcapacitors.com, or contact us here with the specification, and we will confirm a Class I or Class II part that fits.

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