Design of a high current hot swap controller exploiting hysteresis control

The aim of this thesis is the analysis, the design and the implementation and the experimental validation of a new high current hot swap controller which allows designers to overcome the limitations of state-of-art systems related to linear mode operation of Trench power MOSFETs. A hot swap controller is a circuit that limits and manages the power flow during the start-up transient. This kind of circuitry is always present in rack electronic systems, especially in online servers and telecommunication equipment that provides services 24/7. In order to achieve a zero out of service time, maintenance and upgrade operations must be executed while the system is running. The hot swap control circuit constitutes an interface between the common power supply backplane and the load circuit. The main purpose of the controller is to limit the inrush current that would flow towards the components, particularly in discharged capacitors; indeed, when the board is inserted into a live system, the load circuit has a very large capacitive that causes a high current surge with dangerous voltage drop. To mitigate such an issue, the hot swap controller limits the current, protecting both the board and the surrounding system. Typically, this is achieved through an external MOSFET operated in linear mode by the controller, that senses the drain current, which is indeed the load current, and adjusts the control voltage accordingly. In order to get started, literature and documentation were first studied to have an idea of the state-of-the art existing solutions and the related arising problems: as growth of networking services and data size has been impressive in the last decades, the power levels involved in telecom applications have seen a large increment. Furthermore, another critical aspect that has to be considered in this context is the evolution of trench MOSFETs. The devices have indeed undergone a strong optimization of their switching and on-resistance performance, reducing the capability of working in linear mode due to thermal instability. Due to the previous reasons, the implementation of hot swap controllers has become increasingly difficult during the years. In this thesis a different control strategy is applied, aiming to avoid the problems related to the Spirito's Effect. The objective is to control and to limit the inrush current in a switching operation mode, avoiding the linear mode operation. The circuit implements a buck converter where current feedback from the inductor is used to limit the load current. To ensure the limitation of the current and the transparency of the circuit after the transient, an Hysteresis control strategy is implemented. This strategy ensures that the circuit has negligible losses when current is under defined value and cannot exceed this limit during the start-up and fault condition . LTSpice has been largely employed to check the functionality, achieving successful results. The simulated circuit allows to charge an equivalent load capacitance of 20 mF in under 20 ms while keeping under control both the current and the switching frequency. A FPGA has been included in the real circuit to implement the controller behaviour and the circuital protections. Lastly, a prototype board has been designed and realized to perform a real test and validation of the simulated system. This thesis work has been carried out at the Application Linear Power Systems Laboratories (ALPS) of Vishay Semiconductor Italiana S.p.A.