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Description
Radiation- and magnetic field tolerant DCDC converters that step down the voltage from a 2.5V bus are needed for the High-Luminosity detectors. This work presents the developed prototypes, based on ASICs designed in a 130nm CMOS technology. A buck converter (bPOL2V5) is close to production readiness, showing an 89% peak efficiency and tolerance to more than 250Mrad of Total Ionizing Dose and to a fluence of 10^16n/cm^2. The ASIC and the PCB have been co-designed to guarantee high reliability. A lower-volume alternative to bPOL2V5 uses a resonant switched-capacitor architecture and shows comparable efficiency, while employing an eight times smaller inductor.
Summary
In the High-Luminosity Large Hadron Collider detectors, some modules require multiple power domains. A compact and efficient power distribution strategy for such systems uses two cascaded Point-of-Load DCDC converters: a first-stage converter having as input a 12V line creates a 2.5V domain, which powers the opto-electronic components. Second-stage converters further step down the voltage from 2.5V to supply the front-end analog and digital circuits. This work presents the status of radiation- and magnetic field tolerant second-stage converters, which provide an output voltage ranging from 0.6V to 1.5V and an output current up to 3A.
Two converters using different architectures and based on ASICs designed in a 130nm CMOS technology have been developed. A buck converter (named bPOL2V5) is close to production readiness, and the characterization results of the final prototype will be presented at the workshop. Furthermore, a functional prototype of a resonant switched-capacitor converter (named rPOL2V5) has been designed, demonstrating that it is possible to reduce the converter volume by adopting a significantly smaller inductor compared to the buck architecture, while keeping a comparable efficiency.
Early prototypes of bPOL2V5 employed devices rated 2.5V. Nevertheless, the current transients experienced by the input parasitic inductor cause voltage spikes that significantly exceed the input voltage. In order to guarantee the converter reliability, a prototype buck converter using devices rated 3.3V devices has been developed. The adoption of a flip-chip assembly to reduce the bonding inductance, the minimization of the PCB parasitic inductance and the proper sizing of the gate driver current of the power MOSFETs guarantee that the voltage spikes never exceed the device rating.
The developed prototype uses a 100nH inductor and shows peak efficiencies of 89% for the 2.5V-to-1.2V conversion, and 87% for the 2.5V-to-1V conversion. Irradiation with x-rays has demonstrated that the circuit is tolerant to more than 250Mrad of Total Ionizing Dose, while neutron irradiation up to a fluence of 10^16n/cm^2 has caused no significant degradation in the converter performances. Furthermore, bPOL2V5 will be tested for Single Event Effects using heavy ions, and the results will be reported at the workshop. The ASIC features a trimmable on-chip voltage reference circuit to guarantee an accurate and uniform output voltage. In addition, a test procedure which will be performed at wafer level together with the trimming procedure has been devised.
The resonant switched-capacitor architecture mainly uses a capacitor as the energy storage device, together with a small inductor. The developed prototype of rPOL2V5 adopts a novel control scheme that uses different modes to optimize the efficiency over the whole load range. It shows a peak efficiency of 92% for 2.5V-to-1.2V conversion and 84% for the 2.5V-to-1V conversion. The efficiency is comparable to that of bPOL2V5, despite the used inductor is approximately eight times smaller (12nH). The radiation performances of rPOL2V5 will be analyzed and reported at the workshop. In order to bring rPOL2V5 to production readiness, additional work is needed to guarantee its stability and the reliable transition between the different operation modes.
