Selectively Sampled Subharmonic-Free Digital Current Mode Control Using Direct Duty Control

Benefits of digital current mode control are often limited by the choice of a current-loop sampling rate. A higher rate requires a fast analog-to-digital converter that consumes substantial power and increases cost. A lower rate often results in subharmonic oscillations, even using a programmable ramp compensation. This brief proposes a simple technique to compute the steady-state duty ratio in real time, when the closed-loop controller is in action. A time-to-digital converter translates cycle-by-cycle duty ratio information into digital code, and a “duty ratio computation” block generates the computed duty ratio using a moving average filter. At steady state, this enforces a virtual open-loop configuration and completely disables current-loop sampling and controller computation, thereby saving substantial power and eliminating subharmonic oscillations. Considering a dc-dc buck converter as the test case, it is found that even in the presence of high periodic behavior under the closed-loop control, a near-ideal steady-state duty ratio can be reconstructed. Design-related issues along with duty ratio saturation are discussed with test cases.