450 kV Pulsed Modulated Power Supply for Industrial Irradiation Accelerators

Industrial electron-beam irradiation for sterilization, polymer cross-linking, and gemstone color enhancement demands dose rates up to 2000 kGy/s delivered in precisely controlled pulses from 450 kV accelerators, requiring high-voltage modulators capable of 10–60 kHz repetition rate, pulse widths of 2–20 µs, and flat-top stability better than ±0.8 % while surviving continuous operation in high-ozone environments.

The modulator topology employs a solid-state Marx-type configuration using 50 series-connected 10 kV IGBT half-bridges charged in parallel from a resonant supply and erected in series during the pulse. Each stage incorporates a freewheeling diode stack and active clamping to limit overvoltage transients to 7 % during the 300 ns rise time. Total erected voltage reaches 460 kV with droop below 0.6 % across a 15 µs pulse into a 2.2 A beam load.

Pulse-to-pulse repeatability is maintained by a digital pre-charge regulator that adjusts charging voltage on every cycle to compensate for facility line variation and capacitor dielectric absorption. A fiber-optically isolated gate-trigger distribution system with 3 ns channel-to-channel matching ensures simultaneous turn-on across all 50 stages, preventing localized overstress that previously caused premature IGBT latch-up.

Flat-top regulation uses a fast feedback loop sampling beam current via a non-inductive shunt at the accelerator exit and correcting stage conduction angle in real time at 250 kHz update rate. An adaptive predictive term derived from the previous 200 pulses compensates for beam loading droop before it occurs, holding dose variation below ±0.4 % across the full dynamic range.

Ozone and X-ray resistance is achieved through full encapsulation of all power semiconductors in high-thermal-conductivity epoxy and the use of ceramic substrates with gold metallization. Cooling is performed by forced fluorinert flow through cold plates bolted directly to each IGBT module, maintaining junction temperatures below 85 °C even at 60 kHz repetition and 50 % duty cycle.

Fault management incorporates staged protection: sub-microsecond desaturation detection per IGBT, coordinated stage shutdown within 1.2 µs of overcurrent, and a mechanical crowbar across the output that fires only if solid-state protection fails. Recovery to full repetition rate occurs within 80 ms after minor faults, preventing thermal runaway in continuous sterilization tunnels.

Dose control precision supports closed-loop operation with an in-line transmission ionization chamber. Real-time dose integration is compared against setpoint every 1000 pulses, with automatic adjustment of pulse width in 50 ns increments to maintain cumulative dose within ±1.2 % over multi-hour runs despite beam current drift from cathode emission variation.

These pulsed modulators routinely deliver greater than 98.5 % energy transfer efficiency from the wall plug to the beam while achieving dose uniformity better than ±2.5 % across 1.2 m wide product webs at conveyor speeds exceeding 80 m/min, enabling single-pass sterilization of medical devices and food packaging previously requiring multiple slower systems.