AN2644 datasheet(55/64 Pages) STMICROELECTRONICS

55 / 64 page

AN2644

Power MOSFET effective Coss and half-bridge midpoint's transition times

55/64

Then, assuming that the two MOSFETs are identical to one another (so that Coss25-1 =

Coss25-2 = Coss25), and referring to the voltage of the half-bridge midpoint VHB (= VDS2), their

individual Coss will be:

Equation 42

Equation 43

and their instantaneous cumulative value will be:

Equation 44

The diagram of equations (Equation 43) and (Equation 44) are shown in Figure 34. The

branch-constitutive equation of the capacitor Coss:

Equation 45

can be integrated by variable separation.

Figure 34.

Coss capacitances vs. half-bridge midpoint's voltage

oss1

DS1

------------------C

oss25

–

-----------------------------C

oss25

oss2

DS2

------------------C

oss25

---------------C

oss25

Coss V

()

oss1

oss2

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

–

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

⎝⎠

⎜⎟

⎛⎞

oss25

oss

()

oss VHB

()

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

oss

()dt C

oss VHB

()dV

⇒

100

200

300

400

7.5 .10

1.5 .10

2.25 .10

3 .10

Coss1

Coss2

Coss

Cstray

VHB (= VDS2)

100

200

300

400

7.5 .10

1.5 .10

2.25 .10

3 .10

Coss1

Coss2

Coss

Cstray

VHB (= VDS2)

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