Comparison of Duty Cycle Generator Algorithms for SPICE Simulation of SMPS

The paper presents and discusses an algorithm for average modeling of the PWM modulator in switch-mode power systems by general purpose electronic circuit simulators such as PSPICE. A comparison with previous theoretical models is conducted. To test the accuracy of the average PWM models comparison to cycle-by-cycle simulation was conducted. The proposed algorithm shows better accuracy than earlier counterparts. 1. Introduction Today circuit simulation and computer-aided design are universally accepted engineering tools and have become industry standard method of product development. Two approaches are possible for simulation of switched mode systems: cycle-by-cycle simulation and average behavior simulation. Cycle-by-cycle simulation is a quite straightforward approach. Cycle-by-cycle simulation can be performed programming the complete power electronic circuit to the simulator. Cycle-by-cycle simulation allows studding the power stage at the switching frequency scale and observing the instantaneous voltages and currents at any point in the circuit. First disadvantage of cycle-by-cycle simulation is that simulating the detailed switching process is time consuming. This is particularly true for nontrivial practical cases. The second and by far more important limitation is that the cycle-by-cycle model of a switching circuit does not lend itself to frequency response analysis. This is because the switching stage has no stable operating point and, hence, does not allow the PSPICE simulator to perform linearization and calculate the small signal gains required for frequency response analysis. Therefore, a different approach is needed to attain frequency domain simulation of the control loop. State Space Averaging is a classical theoretical analysis method of switch-mode power electronics systems [1–5]. Average modeling of the power stage can also be helpful in simulation as they can be readily implemented using PSPICE behavioral sources [6–8]. Average models are continuous and, hence, can be automatically linearized by the PSPICE simulator and prepared for the frequency domain analysis. The ability to obtain the frequency response of the feedback loop allows the practicing engineer to evaluate the system’s stability and to design the compensator network to meet the design objectives. The distinct characteristic of SMPS is that a switched-mode stage is employed as power processor, whereas the control circuits are mostly analog where Pulse Width Modulator (PWM) is used as an interface. A typical structure of a PWM switch-mode power system (SMPS) is