Speaker
Description
The Low Voltage Power Supply is a modular power converter stepping down 380VDC to 12VDC to power the widely used bPOL12V point-of-load converters. It is designed to operate in the radiation and magnetic-field environment of the CMS towers of experimental cavern. Three prototype iterations underwent radiation and magnetic-field testing, followed by progressive technical qualification. We present the environmental test methodology and summarize results from the latest irradiation and magnetic-field test campaigns. Key performance metrics, like conversion efficiency, output voltage stability, over-current and over-voltage protection, interlock behaviour - are evaluated against the requirements, aiming at proofing production readiness.
Summary (500 words)
Low Voltage Power Supply (LVPS) is a second conversion step in a power cascade, designed to step down 380 VDC to 12 VDC, which will serve as an input voltage for bPOL12V converters used in the Electromagnetic Calorimeter Barrel, the High-Granularity Calorimeter, and the Minimum Ionizing Particle Timing Detector. The unit must tolerate a total ionizing dose of 32 Gy and a hadron fluence of 2×10¹¹ p/cm² over its operational lifetime, corresponding to 4000 fb⁻¹ integrated luminosity, as well as stray magnetic fields up to 120mT. These environmental requirements necessitated four irradiation campaigns and three magnetic field test campaigns to date.
Three dedicated boards and two full-scale prototypes were tested across all configurations: 12x60W, 6x120W and 3x240W. A dedicated, automated test system was developed to extensively evaluate all relevant features to integrate the units with existing CMS infrastructure: reboot test, turn on/off test with variable ramp up/down settings and loads, interlock test with fast voltages and currents acquisition, over-current protection threshold, over-voltage protection threshold, efficiency, output voltage stability and communication robustness. During all tests, input and output voltages and currents were measured using laboratory-grade DMMs with customized acquisition rates to preserve transient waveforms, aiding root cause analysis in case of failure. For example, sampling the input current at 200 kHz previously helped identify the source of a critical failure.
The most recent (second) fully featured prototype marked a significant step towards production readiness, being able to operate reliably in a radiation environment. The majority of tests mentioned earlier were passed. However, it exhibited a substantial current-readout drift of up to 30%, adversely affecting the over-current protection threshold, which deviated far beyond the desired ±3% tolerance. Another discovered issue was single-event related damage of digital isolators. Although these failures occurred with a very low cross-section across three tested modules, they are projected to cause several failures per year across the full system.
External magnetic field, on the other hand, caused output voltage instabilities, manifested as sinusoidal voltage distortions with magnitude exceeding 200 mV peak-to-peak. The issue was most evident in the 6x120W configuration module under full-load at 380 VDC input.
Additional non-radiation-related vulnerabilities were uncovered, such as internal EMI causing random corruption of CAN address assignments. This could result in duplicate addresses and loss of communication with affected modules.
Next-generation, fully-featured prototypes addressing all of the above issues have already been produced and will be evaluated starting May 25, 2025 at CERN’s CHARM mixed field irradiation facility and starting June 23, 2025 in CERN’s Goliath magnet for their radiation tolerance and magnetic field compatibility, respectively. The test results and readiness of LVPS for mass production will be reported.