STD95NH02L

Appendix A

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4

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 working

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:

●

●

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 Ls 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

●

Figure 19.

Buck converter: power losses estimation

Table 6.

Power losses calculation

High side switching (SW1)

Low side switch (SW2)

Pconduction

Pswitching

Zero Voltage Switching

δ

*

I

*

R

2

L

DS(on)SW1

)

1

(

*

I

*

R

2

L

DS(on)SW2

δ

−

g

L

I

I

*

f

*

)

Q

(Q

*

V

gd(SW1)

gsth(SW1)

in

+