Graduate Research

Sliding Mode Control of Converters with an Independent Neutral Point

With the increasing footprint of renewable energy, the drive towards a cleaner environment has consistently pushed forward the development of power electronics based power converters. While the basic principles of operating the power electronics in these power converters have been very effective in providing for a very efficient system, new topologies, and advanced control strategies enable us to achieve a still higher efficiency and simplification and help us overcome some of the fundamental problems encountered in operation.

One of the fundamental requirements of power electronic converters is that they require significantly large output capacitors. it is necessary to remove ripples in the rectified AC voltage. Numerous approaches have been presented in the past to overcome these issues including the addition of a ripple compensator to a conventional H-Bridge rectifier as well as using one leg of the H-Bridge itself as a neutral leg. A new controller; based on sliding mode has been proposed here for a neutral leg topology as well as the conventional H-Bridge topology of a single-phase power converter. In the case of a rectifier, the ripple energy is separated and directed towards the lower split capacitor present at the neutral leg so that the upper split capacitor may have very small ripples while in the case of an inverter, the lower capacitor actually acts as an independently controlled DC source. all the while the capacitance is kept to be very small. The control of the two legs in the rectifier is performed independently granting the controller an extra degree of freedom and an easier extrapolation to the 3-phase implementation. The controller operates the power electronic switches to regulate the input grid current and achieve unity power factor as well as to maintain a stable DC bus voltage removing the need for any other power factor correction circuit.