Dead Time Losses in Synchronous Rectifying Step-Down Converters

In the previous article, switching losses occurring at switch nodes in a synchronous rectifying step-down converter were explained. This time, we consider dead time losses that likewise occur at switch nodes.

Dead Time Losses

The dead time loss is the loss that occurs due to the forward voltage of the body diode of the low-side switch (MOSFET) and the load current during dead time. Here, the symbol Pdead_time is used.

In synchronous rectification, a high-side switch and a low-side switch are turned on and off in alternation. Ideally, switching should be performed such that there is never any time in which switches on both sides are simultaneously either turned on or turned off. However, in actual operation this is difficult to achieve, and in order to ensure safe operation, an interval is provided in which the switches on both sides are simultaneously turned off. This interval is called the dead time. “Ensuring safe operation” is necessary because if the switches on both sides are simultaneously turned on, a current that is called a through-current, a shoot-through or the like would normally flow from VIN through the high-side switch and the low-side switch to GND. It is easy to see that this is effectively a short-circuit state from VIN to GND, so that a large current flows, and the MOSFET switches might well fail. In order to avoid this situation, a synchronous rectifying DC-DC converter IC is provided with a control circuit to prevent the switches on the two sides from turning on simultaneously, that is, both sides are turned off before turning on the appropriate switch.

Returning to the issue of dead time, both sides are switched off during the dead time, and so as a rule, no current should flow from the output of either switch. However, the actual switches are MOSFETs, and MOSFETs have a parasitic diode called a body diode. In the following diagram, the diode drawn as though it is connected between the MOSFET source and drain represents the MOSFET body diode.

When the switches on both sides are in the off state, the body diode of the low-side MOSFET is directed in the forward direction with respect to the load current, and so a current flows to the load through this body diode. The resulting loss, Pdead_time, can be calculated using the following equation.

As is clear from the equation, smaller values for each of the factors reduces the loss. In the IC dead time control, the dead time is set so as to ensure safety while minimizing losses.

Next time, we will discuss losses in the IC control circuit.