Why does a switching power supply need "dead time"?

We have talked about Buck circuits before, which are divided into synchronous and asynchronous types.

But a dead time is required in synchronous circuits. Why?

We know that the asynchronous system has a freewheeling diode, while the synchronous system uses a switching tube instead, that is, two switches are controlled.

 Asynchronous Buck Change Circuit

 Synchronous Buck Converter Circuit

It should be noted that there is a risk that the two switching tubes will be turned on at the same time , and the current will flow directly from Vin through the two switching tubes, short-circuiting the ground and causing the tubes to burn out.

Therefore, it is necessary to use the "dead time", that is, to make both MOS tubes non-conductive and allow the body diode of the lower tube to continue current to realize the output current loop.

During this switching process, there are two stages that experience dead time.

The first stage is when both the upper tube and the lower tube are turned off. When the lower tube is just turned off and the upper tube is not turned on, the current before this has been decreasing and the minimum current is;

 When the upper tube is turned on, the lower tube is turned off. At this time, the current keeps increasing, and the maximum current is. When the upper tube is turned off, both the upper tube and the lower tube are turned off. This is the second stage.

During these two time periods, the driving voltages UGATE and LGATE of the upper and lower tubes are both at a low level, and the two MOS tubes are in the cut-off state.

 How to calculate the power consumption of dead time?

When the current passes through the body diode of the lower tube, the current flowing through the parasitic diode during these two time periods is multiplied by the forward conduction voltage of the diode, multiplied by the two dead times, and divided by the cycle time, which is the average power consumption during the dead time.

Some of the above pictures and information are from 'Hard Ten'