STD90N02L - STD90N02L-1

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Appendix A

The power losses associated with the FETs in a Synchronous Buck converter can be

estimated using the equations shown in the table below. The formulas give a good

approximation, for the sake of performance comparison, of how different pairs of devices

affect the converter efficiency. However a very important parameter, the wotking

temperature, is not considered. The real device behavior is really dependent on how the

heat generated inside the devices is removed to allow for a safer working junction

temperature.

The low side (SW2) device requires:

Very low RDS(on) to reduce conduction losses

Small Qgls to reduce the gate charge losses

Small Coss to reduce losses due to output capacitance

Small Qrr to reduce losses on SW1 during its turn-on

The Cgd/Cgs ratio lower than Vth/Vgg ratio especially with low drain to source voltage to

avoid the cross conduction phenomenon.

The high side (SW1) device requires:

Small Rg and Lg to allow higher gate current peak and to limit the voltage feedback on the

gate

Small Qg to have a faster commutation and to reduce gate charge losses

Low RDS(on) to reduce the conduction losses

Figure 14.

Synchronous buck converter