A new mixed-mode programming in PWM controllers

In this paper a new mixed (voltage and current)-mode programming PWM controller is presented. The proposed controller compares a control signal to the error amplifier signal in order to command the switch to turn off, while a constant frequency clock signal is utilized to turn the switch on. In the case of peak current-mode controller, the control signal is a combination of inductor or most often switch current and a suitable compensation ramp which in the most common applications is constant. In the proposed controller, the constant compensation ramp has been replaced by a ramp voltage which is derived, charging an appropriate RC circuit from the output voltage of the converter. The output impedance and audio susceptibility transfer functions have been plotted and presented for both controllers revealing the improved behavior of the mixed-mode controller. Furthermore, like in peak current-mode control, sub-harmonic oscillations inherent to these control schemes, are predicted and avoided using an appropriate condition and extended validity Bode diagrams taking into consideration the well known sampling effect. To verify the theoretical analysis and conclusions, the two control schemes have been simulated considering step load and reference changes.