A new continuous-time model for current-mode control with constant frequency, constant on-time, and constant off-time, in CCM and DCM

A novel small-signal model for current-mode control of PWM (pulse width modulation) converters is extended to the analysis of converters with constant on-time control, constant off-time control, and discontinuous conduction mode (DCM). Constant on or off-time converters in continuous conduction mode (CCM) have two complex right-half-plane (RHP) zeros in the current loop feedback, but reduced modulator gain eliminates current-loop instability. The modulator gain exhibits a frequency-dependent phase which introduces an extra gain term in the circuit model. The new model for DCM operation does not have RHP zeros in the current feedback loop, but the buck converter exhibits an instability for higher conversion ratios.<>