Design of Radiation Hardened LinearRegulator for GaN based DC-DCConverters

Power distribution to the High-Luminosity Large Hadron Collider (HL-LHC)detectors at CERN is a challenging task, due to the limited amount of mate-rial that can be used, to the limited cooling capability and to the presenceof high levels of radiation. In this context, the use of step-down radiationtolerant DC-DC converters is of paramount importance to guarantee that thetargets of power efficiency and minimization of the used material are met. Anew generation of Gallium Nitride (GaN) based converters is now in devel-opment at CERN.During recent years, the employment of GaN devices has become more andmore widespread due their increased switching capabilities and power lossesreduction. Nevertheless, even though GaN devices are superior to Siliconones by many different metrics and parameters, they come with their ownset of challenges in terms of circuit design.The absence of a built-in body diode (unlike Silicon MOSFETs) eliminatesreverse recovery losses in GaN transistors, since there are no minority carri-ers involved in the conduction. Nevertheless, the absence of such body diodeleads to large negative voltages (about -2.5V, much lower than the -0.7V whenusing Silicon devices) on the switched node of a buck DC-DC converter dur-ing the dead times. When using a conventional bootstrapping technique topower the high-side gate drivers, this issue can lead to an overcharging ofthe power supply of the gate drivers, which in turn can damage irreversiblythe gate driver or even the GaN transistor itself.In this project, a linear regulator that supplies the gate drivers of a GaN de-vice has been developed using a commercial 0.35μmCMOS technology. Suchregulator guarantees that a steady 3.3 V voltage is obtained as the power sup-ply of the gate driver regardless of the large negative voltages reached by theswitched node. The development has included the choice of the topology,its small-signal analysis, the transistor sizing, the validation of the schematicthrough simulations and the layout. In order to reach the targeted ultra-highlevels of radiation tolerance (e.g. a Total Ionizing Dose of 100 Mrad), ra-diation hardening design techniques have been extensively adopted in thiswork, both in the schematic and in the layout phase.