AN2644 datasheet(59/64 Pages) STMICROELECTRONICS

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AN2644

Power MOSFETs switching losses at turn-off

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Appendix D

Power MOSFETs switching losses at turn-off

The nonlinearity of Coss (refer to Figure 34) makes the voltage of the node HB increase

slowly at its low values and its high values, and much faster when it is half way in its swing

(see the solid VHB waveform labeled "Actual" in Figure 36). Typically, the current through

either Q1 or Q2 during their respective turn-off transients flows during the initial part of the

transition of the node HB, i.e. when its value is changing more slowly. This reduces the

voltage-current overlap and, consequently, the associated power loss with respect to a

linear capacitor. However, the mathematical expression of the loss is extremely complex,

therefore it is more useful to provide a conservative estimate using the equivalent linear

capacitance CHB defined by (Equation 53). The associated VHB curve is the dotted one

labeled "Approx." in Figure 36 (under the assumption Tf < TT).

Again with reference to the turn-off of Q2, the voltage on the node HB can be found by

integration of the branch-constitutive equation of CHB:

Equation 58

Power loss due to voltage-current overlap (switching loss) will occur only during the time

interval (0, Tf) where IQ2 is non-zero, then (Equation 54) will be considered in this interval

only; substituting (Equation 48) in (Equation 58) we find:

Equation 59

Note that in the remainder interval (Tf, TT) the voltage VHB will change linearly with time

because charged by the constant current IR0. The energy lost because of voltage-current

overlap will be calculated by integration of the product of (Equation 59) times (Equation 48):

Equation 60

Power loss will be obtained multiplying the energy loss given by (Equation 60) by the

operating frequency fsw:

Equation 61

The same loss occurs to the high-side power MOSFET Q1 as well, thereby, the total

switching loss will be:

Equation 62

()

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

HB t

()

-----------

() t

-----------

Q2 t

()

–

[] t

∫

⇒

HB t

()

-----------

--------tt

⋅⋅

----------------------------t

∫

∈ (0,Tf)

off Q2

()

Q2 t

() V

HB t

()

⋅

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

-----

–

⎝⎠

⎛⎞

⋅

()

24C

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

∫

off Q2

()

off Q2

()f

⋅

()

24C

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

off

TOT

⋅

()

12C

------------------------- f

⋅

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