Ozonerosion der HV-Kondensatorverkapselung in UV-Lampen

Ozonerosion der HV-Kondensatorverkapselung in UV-Lampen

Abstract

A mercury lamp makes ozone as a by-product of the 185 nm line, and the curing chamber holds 0.1–1 ppm of it all day. Ozone splits carbon-carbon double bonds, which is the backbone chemistry of every epoxy dome, silicone gel and solder mask in the high-voltage section. This article gives the ozonolysis and crack-growth arithmetic, four field failure modes, the 500 h ozone qualification, and the inorganic surfaces that remove the reaction site.

1. Einleitung

A mercury UV lamp generates ozone as an intrinsic by-product. The 185 nm line photodissociates oxygen, and even filtered lamps leave 0.1–1 ppm in the curing chamber, with far higher concentrations within a few centimetres of the lamp envelope.

Ozone is one of the strongest oxidizers available at ambient temperature. It attacks carbon-carbon double bonds, aromatic rings and sulfur-containing groups, which are exactly the structures that make up epoxy domes, silicone gels, polyimide tape and solder mask.

Those layers are the first line of defence against voltage breakdown, and they are the materials ozone attacks. An encapsulant that yellows, cracks and lifts converts a protected part into an exposed one, and the capacitor inside starts to age from the outside in.

Ozone is missing from most component specifications because it is invisible, odourless at the concentrations that matter, and hard to reproduce on a bench. A part qualified for temperature, humidity and vibration can still fail in two years through ozonolysis alone.

2. Kernmechanismus

2.1 Ozone formation from the 185 nm line

Oxygen dissociates when a photon carries more energy than the O=O bond, and the mercury lamp supplies that photon at 185 nm.

E_ph = 1240 / λ

where E_ph is the photon energy in eV and λ is the wavelength in nm. At 185 nm the photon carries 6.7 eV, which is above the 5.1 eV of the oxygen double bond, so each absorbed photon splits O₂ into two oxygen atoms and ozone follows within milliseconds.

2.2 Ozonolysis of the encapsulant

Ozone adds across a carbon-carbon double bond, the intermediate rearranges, and the polymer backbone breaks. The mass loss tracks the ozone concentration and the exposed area.

dm/dt = k_O3 · [O₃] · A_s

where dm/dt is the surface mass loss rate, k_O3 is the reaction rate constant for the material, [O₃] is the ozone concentration and A_s is the exposed surface area. At 1 ppm the surface degradation rate runs 10–100× faster than at the 0.01 ppm background of ordinary industrial air, because the two differ by a factor of 100 in concentration.

2.3 Crack growth under partial discharge

Oxidized epoxy loses elongation, and differential expansion against the ceramic body cracks the skin. Each crack is then a discharge site, and the discharge erodes the crack walls.

g = k_PD · V² · f / d_crack

where g is the crack growth rate, k_PD is a constant for the material, V is the applied voltage, f is the discharge repetition rate and d_crack is the crack width. A 1–10 µm crack holds a low PD inception voltage at 5–20 kV, and a narrower crack concentrates more field, so the growth accelerates as the crack closes on itself.

2.4 Termination oxidation and pulse headroom

Ozone also oxidizes silver and solder surfaces, and the contact resistance that follows subtracts directly from the trigger pulse.

V_drop = I_pulse · R_c

where V_drop is the pulse amplitude lost at the contact, I_pulse is the pulse current and R_c is the contact resistance at the lead or the solder joint. A trigger loop carrying 50 A pulses loses 2.5 V for every extra 50 mΩ, which is enough to drop ignition below threshold on a marginal day.

Fig. 1 — Ozone cleaves the C=C bonds of an epoxy dome and starts a 1–10 µm PD crack, while a glass glaze offers no carbon bond to attack.

3. Schwachstellen im Engineering

Pain Point 1: Epoxy dome cracking on a HV capacitor

After 12–24 months in an ozone-laden cabinet the epoxy dome of a 10 kV trigger capacitor shows a web of micro-cracks. The machine begins to flash over at the capacitor, and each flashover erodes the crack further. The part returns as a capacitor failure, and the cause was the atmosphere.

Pain Point 2: Silicone gel delamination

Gels and conformal coatings are chosen for UV tolerance, and ozone oxidizes the silicone surface over years. The gel hardens, loses adhesion and lifts from the body, leaving an air gap. The gap has a lower PD inception voltage than the gel it replaced, so discharge starts inside the part.

Pain Point 3: Termination oxidation and resistance growth

Silver and tin surfaces oxidize, and the contact resistance at the lead and the solder joint climbs from milliohms to ohms over years. In a trigger loop carrying 50 A pulses an extra 50 mΩ costs 2.5 V of pulse headroom, enough to lose ignition on a marginal day.

Pain Point 4: Board-level surface degradation

The laminate and the solder mask around the high-voltage parts ozonize exactly as the encapsulants do. Cracking near a high-voltage node is often misdiagnosed as a capacitor fault, which delays the real fix and repeats the failure on the replacement part.

4. Entwurfsschritte

Strategy 1: Remove organic surfaces from the highest-stress sites

For the trigger, coupling and filter capacitors in the lamp head, specify glass-glazed or hermetically sealed bodies whose external surface is inorganic. A Folienkondensator with a polymer surface does not meet that requirement at these sites.

Strategy 2: Select the encapsulation chemistry by ozone test

Before qualification, run 500 h at 5 ppm ozone and 40 °C with the part energized, and inspect weekly for cracks, discolouration and leakage growth. Accept a material only when its surface stays crack-free and its leakage stays inside specification.

Strategy 3: Shield the high-voltage compartment

Separate the high-voltage board from the lamp zone with an ozone-resistant barrier of PTFE or glass, and hold the compartment at slight positive pressure with filtered air. This cuts the ozone concentration at the components by 10–100×.

Strategy 4: Use fluoropolymer and glass where epoxy is not required

PTFE standoffs, glass-filled terminal blocks and ceramic-bodied capacitors remove the polymer surface that ozone attacks. Each substitution removes a reaction site, and no rate constant has to be improved.

Strategy 5: Design for ozone-accelerated leakage

Increase the creepage margin by 30–50% beyond the clean-air requirement, then coat the zone with a material that has documented ozone resistance. Fluoropolymer outperforms silicone, and silicone outperforms acrylic.

Strategy 6: Monitor the canary nodes

Fit a leakage-current monitor and a 10–100 MΩ Abtastwiderstand on the highest-voltage node. A slow upward trend is the earliest electrical sign of an ozonized surface, and it schedules inspection into planned maintenance.

Komponentenparameter	Typischer Wert	Testbedingung
Nennspannung (HVCT8G-Serie)	10 kV DC	Hi-Pot 1.5× für 60 s
Kapazität	4700 pF ±20% (472M)	1 MHz, 1 V<sub>eff</sub>
ESR bei 1 MHz	unter 50 mΩ	25 ° C
Schleifeninduktivität (E40-Sehne)	unter 8 nH	40 mm Anschlussabstand
Isolationswiderstand	über 10 GΩ bei 500 V Gleichstrom	25 °C, 60 % relative Luftfeuchtigkeit
Temperaturbereich	−40 °C bis +125 °C	IEC 60068-2-Reihe
Impuls-dv/dt-Fähigkeit	über 50 kV/µs	bei Nennspannung

5. HVC capacitors for ozone-laden lamp heads

Three properties decide whether a capacitor survives an ozone atmosphere: the chemistry of its outer surface, the stability of its terminations and the margin of its partial discharge screen. HVC builds N4700 parts for all three.

Sechs Parameter, die hier von Bedeutung sind

• N4700 (T3M) strontium titanate under a glass glaze has no carbon-carbon bond in its outer surface, so there is no backbone for ozone to cleave.
• Corrosion-resistant, ozone-tested terminations hold contact resistance over years, which protects the 50 A pulse amplitude in the trigger loop.
• tanδ at or below 0.1% at 1 kHz and ±30 ppm/°C keep the body cool, so the encapsulation is not stressed from the inside.
• PD below 5 pC at 1.2× rated voltage stops the autocatalytic erosion that normally follows an ozone crack.
• Ratings from 1 kV to 100 kV come in sealed screw-terminal and glass-glazed packages that cover every stage of the lamp supply, including HVCT8G and 441-type bodies.
• Feldvalidierung: Ein HVC 40 kV / 441 Schraubkondensator ersetzte ein Bauteil einer erstklassigen Marke in Hochspannungs-Serviceprogrammen, und die gleiche Konstruktion entspricht den elektrischen Parametern von Vishay Radialanschluss-Baureihen von HVCC und KEMET.

Lieferung und Support

Standard parts ship in 4–8 weeks and matched stacks in 6–10 weeks. HVC applications engineering reviews the ozone concentration, temperature, humidity and voltage stress of the site.

Send the ozone concentration at the components, the cabinet temperature, the humidity profile and the node voltages. HVC returns an encapsulation recommendation with ozone-test data and a glass-glazed N4700 sample for your own 500 h qualification.

6. Häufig gestellte Fragen

Q1: Where does the ozone in a UV curing chamber come from?

From the 185 nm mercury line. A photon at that wavelength carries 6.7 eV, above the 5.1 eV of the oxygen double bond, so it splits O₂ and ozone follows. Even filtered lamps leave 0.1–1 ppm in the chamber air.

Q2: Which encapsulation materials does ozone attack?

Any polymer with carbon-carbon double bonds, aromatic rings or sulfur groups. Epoxy domes, silicone gels, polyimide tape and solder mask all qualify, and the reaction proceeds at every hour of lamp operation for the whole life of the machine.

Q3: How fast does a crack grow once PD starts?

Crack growth scales with V² and with the discharge repetition rate, and it rises as the crack narrows. That combination makes the path autocatalytic: a wider crack holds lower field, so growth slows until the discharge finds a new site.

Q4: What does oxidized termination resistance cost?

At 50 A pulse current, every 50 mΩ of added contact resistance removes 2.5 V from the pulse. On a marginal ignition day that is the difference between a lamp that strikes and one that misses.

Q5: How do I qualify a material for ozone service?

Run 500 h at 5 ppm ozone and 40 °C with the part energized, and inspect weekly for cracks, discolouration and leakage growth. A datasheet claim is not a substitute for that test.

Q6: Does conformal coating solve the ozone problem?

It helps and does not solve it. Coating raises the surface barrier, and the creepage margin underneath still needs the 30–50% increase, and the coating chemistry still has to be ozone-resistant. Fluoropolymer beats silicone, and silicone beats acrylic.

7. Referenzen

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