User`s guide

SMPS AC/DC Reference Design User’s Guide

The inductor current is continuous and never reaches zero during one switching period

S); therefore, this mode of operation is known as Continuous Conduction mode. In

Continuous Conduction mode, the relation between the output and input voltage is

given by Equation 1-1. The duty cycle is given by Equation 1-2.

EQUATION 1-1:

EQUATION 1-2:

When the output current requirement is high, the excessive power loss inside

freewheeling diode D1 limits the minimum output voltage that can be achieved. To

reduce the loss at high current, and to achieve lower output voltage, the freewheeling

diode is replaced by a MOSFET with a very low ON state resistance (R

DS(ON)). This

MOSFET is turned on and off synchronously with the Buck MOSFET. Therefore, this

topology is known as a Synchronous Buck converter. A gate drive signal, which is the

complement of the Buck switch gate drive signal, is required for this synchronous

MOSFET.

A MOSFET can conduct in either direction; which means the synchronous MOSFET

should be turned off immediately if the current in the inductor reaches zero because of

a light load. Otherwise, the direction of the inductor current will reverse (after reaching

zero) because of the output LC resonance. In such a scenario, the synchronous

MOSFET acts as a load to the output capacitor, and dissipates energy in the R

DS(ON)

of the MOSFET, resulting in an increase in power loss during the discontinuous mode

of operation (inductor current reaches zero in one switching cycle). This may happen if

the Buck converter inductor is designed for a medium load, but needs to operate at no

load and/or a light load. In this case, the output voltage may fall below the regulation

limit if the synchronous MOSFET is not switched off immediately after the inductor

reaches zero.

OUT

⋅=

where

D is the duty cycle

------

where t

is the ON time and T

is the switching time period