A zero current detector with low negative inductor current using forced freewheel switch operation in synchronous DC-DC converter

In DC-DC converter at light load, discontinuous conduction mode (DCM) and pulse frequency modulation (PFM) mode are usually used to improve efficiency instead of using low efficient continuous conduction mode (CCM) in a pulse-width modulation (PWM) control system. For these control modes in light load condition, the zero-current detector (ZCD) to prevent reverse current flowing back to the power source is essential. This paper presents a zero current detection method with enhanced time response of the freewheel switch operation. The proposed ZCD was simulated using a 0.18-μm CMOS process in a test system of PWM voltage mode type-III buck converter, with input voltage of 3.3V. The test system works in CCM at heavy load, PFM at light load, and DCM in a short transient of decision to change from CCM to PFM. The PWM switching frequency is 2MHz, and testing output voltage is 1.8V with load current from 20mA up to 300mA. Under the same test condition, the proposed ZCD shows maximum improvement of 6.9% efficiency in PFM compared to the conventional method, making the overall efficiency greater than 89.7%.