A Six Legs Buck-boost Interleaved Converter for KERS Application

Abstract

This paper addresses the design of a bi-directional DC/DC power converter to interface a supercapacitor bank and a motor-generator unit. The design is based on an interleaved six legs topology in which the current is shared among six inductors to minimize their weight and cost, allowing, besides, a low switching frequency to lessen switching losses. The converter is conceived to be employed in an electric Kinetic Energy Recovery System for Internal Combustion Engine Vehicles. The system makes use of a supercapacitor as a storage system, and a motorgenerator unit connected to the drive shaft for vehicle acceleration and braking. The system uses available commercial devices, thus obtaining a cheap and high-efficiency conversion chain. It is shown how the design criteria differ from traditional interleaved converters. The same topology allows minimizing the input and output ripple and improving the reliability in case of fault as well. Losses are reduced both by sharing the currents and by a suitable control strategy, which allows more converters to be connected in parallel to increase the delivered power. Results, given in simulation, assess the stability and dynamic performance of the conversion circuit, showing a low current and voltage ripple in all operating conditions.

A regenerative braking system for internal combustion engine vehicles using supercapacitors as energy storage elements - Part 2: Simulation results

“…The data reported in Table 5 allow to determine the maximum output power P PC,max of the power converter, for each brushless machine considered. As already shown in in Figure 3 of paper Part 1, the efficiency curve of such a power converter [57] reveals that for power factor exceeding 10% of the maximum, the efficiency remains at its best value, i.e. 0.93.…”

“…Moreover, as already stated, the brushless motor is assumed to be current-controlled, i.e. the torque delivered (or received) is controlled by controlling the phase-currents, as described in [56]: this task is obviously accomplished by the power converter, which was properly designed for the purpose [57]. The maximum speed of revolution of all the motor selected does not allow a direct connection with the drive shaft, making hence necessary the adoption of a gear drive, whose efficiency η G (supposed 0.97 in this paper) will certainly condition the overall KERS efficiency and will be properly taken into account in the numerical simulations.…”

“…On the other hand, it would not be appropriate to size each PC on the same maximum current (and maximum power) of the supercapacitor, since the high cost increment would not be counterbalanced by an efficiency improvement of the whole KERS: the power limits imposed by each MGU, in effect, would cause the PC to work with low power factors and hence, as shown in Figure 3 of paper Part 1, with low efficiencies. The cost and the weight of each power converter was estimated on the basis of its component [57] and is also reported in Table 6.…”

A regenerative braking system for internal combustion engine vehicles using supercapacitors as energy storage elements - Part 1: System analysis and modelling

“…the torque delivered (or received) is controlled by controlling the phase-currents, as described in [36]. In the case here considered, the power converter, whose block diagram is reported in Figure 2, is a buck/boost converter that can be operated in both step-up and step-down configuration, thus adapting the voltage of the SC to the voltage of the MGU [37]; its interleaved topology offers several advantages compared to a traditional single inductor topology: the current can be shared among the inductors thus allowing to reduce conduction losses; when small currents are concerned, a single inductor can be used and the ripple on the current can be reduced by a phase displacement of the pulse width modulation signals, allowing a lower switching frequency to be adopted with advantage in terms of the minimization of switching losses.…”

“…These conversion topologies have been recently studied to achieve high conversion efficiency assessing a high power density and a reduced cost per kW; a discussion on the optimization of the power converter is beyond of the scope of this paper, but further information can be found in [37][38] [39].…”