Previously unobserved effects of delay on current-mode control

Abstract

A new sampled-data model for the current-mode controlled buck converter includes for the first time the effects of delay in the current loop. Modified z-transforms are used in this new model for constant frequency trailing-edge modulation. Realistic amounts of delay are found to be particularly significant when the buck converter is operating in the continuous conduction mode near the discontinuous conduction mode boundary. The new model is used to predict the loop gain measurements obtained with the digital modulator and with conventional measurement techniques. It is shown that conventional loop gain measurement techniques are insufficient to measure the loop gain in this region of operation. It is also shown that the digital modulator can add a significant amount of delay, thereby altering the loop gain of the circuit being measured. However, if care is taken to minimize this delay, a good loop gain measurement can be achieved. The new model accurately predicts the boundary condition for subharmonic instability. In addition, it reduces to Ridley's current-mode control model for the case of zero delay.