Limiting the rms value (L2 norm) of periodic signals can be a necessity in some controllers for power electronic devices. The prevailing existing solution is simply to compute, off-line, the L2 norm and then to multiply the signal by the factor that ensures the desired bound is achieved. This feedforward procedure could yield unsatisfactory results during the transients of the system. In this paper, a feedback-based algorithm that carries out the computations on-line is proposed. Besides the on-line computation of the L2 norm, the second component of the algorithm is a classical proportional-integral loop with an anti-windup mechanism. Although the overall algorithm is described by complicated nonlinear, non-autonomous, delay-differential equations, a detailed stability analysis is carried out, proving its robust performance. Simulation results, that confirm the theoretical results, are presented.