Constant-Current Feedback High-Voltage Power Supply for Blood Irradiation

Blood bag irradiation with X-ray doses of exactly 25 Gy center-dose requires tube current regulation better than ±0.6 % at 160–200 kV despite bag thickness variation and cooling water conductivity changes, demanding a high-voltage supply that operates in true constant-current mode with automatic kV adjustment to maintain dose rate.

The supply uses a phase-shifted full-bridge inverter operating at 82 kHz feeding a ferrite transformer and eight-stage oil-immersed multiplier. Tube current is sensed via a precision DC current transformer on the grounded return leg with 0.08 % absolute accuracy, digitized at 180 kS/s, and compared against setpoint in a digital PI loop that directly controls inverter phase shift. Response time to load changes is under 480 µs, maintaining current within ±0.4 % during bag transitions.

Automatic kV optimization uses a secondary dose-rate feedback loop from a transmission ionization chamber mounted after the irradiation canister. When bag thickness increases and dose rate falls, the controller raises high-voltage setpoint in 400 V increments until dose rate returns to target, typically increasing from 168 kV for empty canister to 196 kV for full four-bag load while holding tube current constant at 18.0 mA.

Stored energy is deliberately limited to 14 J through distributed filtering and low-capacitance cable, enabling arc recovery within 22 ms with current overshoot below 28 %. Arc rate is further reduced by active grid bias modulation that momentarily cuts emission during the first 2 µs of voltage collapse, preventing sustained cathode spots.

Filament current is pre-programmed as a function of kV to maintain constant focal spot size and dose profile across the operating range. A look-up table derived from initial commissioning is automatically adjusted every 500 hours via focal spot camera feedback.

Cooling water conductivity compensation prevents output drift: a four-electrode sensor continuously measures water resistivity, and the controller injects a corrective offset to the current feedback gain to null the 0.11 %/µS/cm shift observed in older designs.

Dose reproducibility is verified by an independent integrating dosimeter that triggers alarm if cumulative dose deviates >0.7 % from 25.00 Gy. These constant-current systems routinely deliver center-dose variation below ±0.48 % across 18-month calibration intervals while achieving irradiation cycle times under 88 seconds per four-bag canister in blood banks processing >1200 units daily.