The speed control of a separately excited DC motor driven by DCDC converter is realized by using Neuro-PID controller. Firstly, a general and unified large-signal averaged circuit model for DC-DC converters is given. This method converts power electronic systems, which are periodic time-variant because of their switching operation, to unified and time independent systems. Therefore, it can be obtained conveniently and straightforwardly various analysis and control processes related to DC motor drive system. Some large-signal variations such as speed, voltage and current relating to DC motor, speed control are easily obtained by using the averaged circuit model. A self-tuning PID neuro-controller is developed for speed control on this model. The PID gains are tuned automatically by the neural network in an on-line way. The controller developed in this work, based on neural network (NN), offers inherent advantages over conventional PID controller for DC motor drive systems.