Automatically Shift Seemlessly Between Peak And Valley Current Modes

by Patrice Lethellier, It Can Be Done, Salt Lake City, Utah, How2Power Today, Apr 15 2018

Focus:
At and above 50% duty cycle, peak current mode control begins to show a cycle-by-cycle instability of the PWM section where the peak current stops the duty cycle. Valley current mode control is the opposite. It is unstable below 50% duty cycle and is stable above 50%. To correct either type of control, we need to introduce a slope compensation which is in fact a bit of voltage-mode control. At very large duty cycle for the peak mode or at very low duty cycle for the valley mode, the required amount of slope compensation increases and overwhelms the current mode. We are shifting to a hybrid current mode/voltage mode, which is going to show some of the 180° of dephasing coming from the output filter in the closed loop. The ideal solution to this problem is a current mode which is always stable without slope compensation. This short article describes how to achieve this goal using a very simple circuit that allows the control to shift automatically and seamlessly from peak to valley when we go above 50% duty cycle and from valley to peak below 50%. Simulation results for this circuit are presented.

What you’ll learn:

• How to implement peak and valley current mode control in the same power converter circuit
• How to eliminate the need for slope compensation when applying current mode control

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