AN2928 datasheet(13/21 Pages) STMICROELECTRONICS

13 / 21 page

AN2928

Design equations for the modified buck converter

13/21

Figure 5.

Real drain MOSFET current

The second part of the MOSFET losses is switching losses, which depend on the (on and

off) switching time, drain MOSFET current, drain-source voltage and the switching

frequency. The switching time, rise time and fall time is a function of the gate-to-drain “Miller”

charge of the MOSFET (Q_GD), the internal resistance of the driver, the threshold voltage

(VGS(TH)), and the minimum gate voltage which enables the current through the drain source

of the MOSFET. As the correct calculation of switching power losses is complex due to non

linear behavior of the switch, it is not possible to obtain an exact equation for the calculation

of switching losses. Moreover, the switching behavior is also influenced by the performance

of the driver and layout design (leakage inductance and parasitic capacitors). To arrive at an

estimation of the switching power losses, Equation 27 can be used.

Equation 27

tOFF_SW = switch off-time (s) (typically tens of ns).

For example, switched off-time measured on the STEVAL-ILL013V1 using the STP9NM50N

power MOSFET (400 V input voltage) is 120 ns.

Total power (P_TOT) is lost in the power MOSFET and its heat sink, so it is simple to

calculate:

Equation 28

TJMAX = maximum junction temperature (°C)

TA = ambient temperature (°C)

RthJC = junction-to-case thermal resistance (°C/W)

RthCH = case-to-heat sink thermal resistance (°C/W) (usually between 0.35 and 0.8 for the

insulating washer)

RthHA = heat sink-to-ambient thermal resistance (°C/W)

AM00371

IMIN

tOFF

tON

i(t)

IMAX

IPP

MAX

OFF SW

⋅⋅

⋅

--------------------------------------------------------------------

TOT

JM AX

–

thJC

thCH

thH A

------------------------------------------------------------

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