What is a Synchronous Buck Converter?

Fig. 1 shows a simplified synchronous buck converter circuit. U1 is what is called a “synchronous buck controller” because of its capability to control the power switches Q1 and Q2 in a synchronous fashion using its gate drive output labeled HG and LG in Fig. 1. The controller may have additional functions not shown in Fig. 1, such as a “boot” voltage input to be able to drive Q1’s gate with a drive voltage signal above the input power rail to insure that the switch will turn on. The boot voltage may be derived from the switching node S1 or from a VCC supply voltage that is sufficiently higher than the power rail voltage.

U1 may also have a way of sensing current into S1 or through the high side mosfet Q1 or through the inductor L. Sometimes the voltage drop across Q1 is used to monitor current. The circuit may then be operated in current mode control (see my previously posted article for a description of current mode control.)

The circuit is called synchronous because the top switch is turned on when the bottom switch is turned off and vice versa. This eliminates the power loss in a diode that would otherwise need to be used in place of Q1. Of course, there cannot be any significant overlap of the on-times of Q1 and Q2 or there will be what is called “cross-conduction” which will cause heating, power loss, and possible failure of the switches due to over-current. On the other hand, there should not be too much dead time or a large Q1 turn-off voltage spike will occur at the open end of the inductor possibly causing switch failure or damage to U1.

The LC filter removes noise and ripple voltage from the output. R1 and R2 adjust the feedback voltage level to be compatible with the internal voltage reference included in U1. R3, R4, and C2 form a typical feedback compensation network. Note that the equivalent series resistance (ESR) of the filter capacitor C is important in determining the compensation network, and of course also affects the amount of noise that is seen at the output. ( See also my first article posted for the main equations governing the buck circuit.)