Real-Time Reconfiguration in Digital Current Mode Control for Fast Transient with Robust Stability

The feasibility of real-time tuning of the digital voltage controller in mixed-signal current mode control (CMC) can achieve fast transient recovery. However, it has been reported in [1] that the sampling delay can substantially limit the stability boundary and may lead to structurally different instability in fixed frequency digital CMC with duty ratio saturation. Also, the effects due to finite sampling and quantization may lead to onset of instability, even in presence of a ramp compensation. In this paper, a robust real-time duty ratio computation technique is proposed in mixed-signal peak CMC (MCMC). This employs an extra constant off-time logic that achieves anti-windup transition because of sharing the same digital voltage controller and analog current comparator. This requires an extra timer along with existing resources under MCMC. This ensure fast-scale stability, and a time-to-digital converter accurately computes the duty ratio in real-time. At steady-state this enforces an open-loop cruise operation and completely disables the blocks associated with the closed-loop operation, which completely eliminates the sub-harmonic instability. However, the sampling process of the voltage-loop ADC remains active throughout, and the closed-loop MCMC will be activated after detecting a transient using the ADC quantization level. The impact on transient performance using the proposed control is discussed. A buck converter prototype is tested, and the proposed control is implemented using an FPGA device.