480 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 22, NO. 2, MARCH 2007 Three-Port Bidirectional Converter for Hybrid Fuel Cell Systems Jorge L. Duarte, Marcel Hendrix, and Marcelo Godoy Simões, Senior Member, IEEE Abstract—The implementation of a hybrid fuel cell/battery system is proposed to improve the slow transient response of a fuel cell stack. This system can be used for an autonomous device with quick load variations. A suitable three-port, galvanic isolated, bidirectional power converter is proposed to control the power flow. An energy management method for the proposed three-port circuit is analyzed and implemented. Measurements from a 500-W laboratory prototype are presented to demonstrate the validity of the approach. Index Terms—Battery, fuel cells. I. INTRODUCTION F UEL cells have very slow response due to the natural elec- trochemical reactions required for the balance of enthalpy [1]–[4]. Therefore, electrical output load power is not matched during transients, and the deficiency or surplus must be managed by an external leveling system. A fuel cell generator will shut down or collapse when more current is taken than it can supply; so, current demand should never exceed the available current. Current demand may be less than available current, but this re- sults in unused fuel and decrease of efficiency from the fuel cell. For these two reasons bidirectional energy storage is required to sink/source the power difference. Lead acid batteries provide a suitable choice for storage because they show fast response time to load changes, being therefore capable of handling the power difference between the load demand and the available fuel cell generation. Moreover, lead acid batteries are not expensive, and widely available. The subject of this paper is the design and the implemen- tation of a suitable interface circuit for a hybrid fuel cell/bat- tery system, aiming at feeding a small autonomous load. An overview of the complete system is shown in Fig. 1, where a converter controls the power flow between a 25–39 V, 500-W PEM fuel cell stack and 48-V lead acid batteries. As soon as power deficiency or excess occurs because of load variations, the converter regulates this extra power flow from or to the energy storage element. Furthermore, since the possibility to supply ac loads through a 400-Vdc inverter output should also be available, a three-port bidirectional topology has to be chosen Manuscript received August 25, 2004; revised May 23, 2006. This paper was presented at the IEEE PESC’04. Recommended for publication by Associate Editor J. D. van Wyk. J. L. Duarte and M. Hendrix are with the Group of Electromechanics and Power Electronics, Technische Universiteit Eindhoven, Eindhoven 5600 MB, The Netherlands (e-mail: j.l.duarte@tue.nl). M. G. Simões is with the Engineering Division, Colorado School of Mines, Golden, CO 80401 USA. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TPEL.2006.889928 Fig. 1. System overview: a power electronic converter regulates the energy flow between the fuel cell generator, an energy storage device, and the load. Fig. 2. (a) Proposed dc–ac–dc converter topology that matches sources and sinks of energy in Fig. 1 through a three-winding transformer and bidirectional high-frequency switching bridges. Full H-bridges are shown at each port; however, it would be also possible to implement this converter by using half bridges. (b) Fundamental system model: three square-wave voltage sources that exchange energy through a grid of inductors, as a consequence of the phase shift angle between the switching patterns. The network of inductors is derived from the transformer in (a) based on a -model representation. in view of the characteristic behavior of the fuel cells, batteries, and load. Of course, there should be no compromising in relia- bility and battery lifetime. Multiple-port, bidirectional converter topologies that may be suitable for the system requirements in Fig. 1 can be found in the literature [5], [6]. The main drawback of the existing concepts is that they cannot handle a wide variety of voltage range inputs. A resonant converter topology is presented in [7], but it is very hard to implement. Since the system under consideration com- bines a 25–39 V fuel cell stack and 48-V batteries with a 400-V inverter output, the use of magnetic transformers may facilitate 0885-8993/$25.00 © 2007 IEEE