Fixed and Variable Frequency Digital Current Mode Control: Structural Stability and Performance Limits

Fixed-frequency digital current mode control (CMC) offers high performance and energy efficiency by using real-time controller tuning and energy optimization algorithms. The paper shows that the effect due to finite sampling leads to structurally different sub-harmonic instability that results in duty ratio saturations, thereby significantly increasing RMS current and ripple parameters. Using mixed-signal CMC (MCMC) with fixed-frequency trailing-edge modulation in a DC-DC boost converter, it is demonstrated using experimental results that an increase in the proportional gain k
p
results in sub-harmonic instability, in which structural stability property is lost for a relatively larger value of k
p
. This results in duty ratio saturations with substantially higher RMS current. On the other hand, constant off-time modulation using event-based sampling is shown to retain structural stability in MCMC for the entire operating range, which may lead to smooth bifurcation for a substantially higher of k
p
. Possible evolutions of the inductor current within a switching cycle are demonstrated and the onset of structural instability for the former is analyzed. Discrete-time models are derived and the benefits of using constant off-time MCMC are presented over fixed-frequency MCMC using experimental and simulation results. This paper will be helpful to identify suitable high performance digital CMC methods.