STD70NH02L Datasheet, PDF (9/16 Pages) STMicroelectronics – N-CHANNEL 24V - 0.0062 Ohm - 60A IPAK/DPAK STripFET II Power MOSFET

STD70NH02L - STD70NH02L-1
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Appendix A
Figure 19. Buck converter: power losses estimation
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:
● Very low R_DS(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 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
● Low R_DS(on) to reduce the conduction losses.
Table 6.
Power losses calculation
High side switching (SW1)
Low side switch (SW2)
Pconduction
R _DS(on)SW1 * I^2L *δ
R _DS(on)SW2 * I^2L * (1 − δ )
Pswitching
V_in
*
(Q_gsth(SW1)
+
Q_gd(SW1)
)*f
*
I_L
I_g
Zero Voltage Switching
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