Passive Optical Network (PON) are often managed by non-flexible, proprietary network management systems. Software Defined Networking (SDN) opens the way for a more efficient operation and management of networks. We describe a new SDN-based architecture for Ethernet Passive Optical Networks (EPON), in which some functions of the Optical Line Terminal (OLT) are virtualized and located in an external controller, while keeping the rest of the PON functionality around an Open Flow switch. This opens the way for an improved management of the resource usage, bandwidth allocation, Quality-of-Service (QoS) monitoring and enforcement, or power consumption management, among other possibilities. In order to maintain the time-sensitive nature of the EPON operations, synchronous ports are added to the switch. OpenFlow messages are extended in order to cope with the PON-related parameters. Results based on simulations demonstrate that our proposal performs similarly or better than legacy architectures, in terms of delay and throughput.