AN2738 datasheet(33/54 Pages) STMICROELECTRONICS

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AN2738

Application example

Doc ID 14589 Rev 2

33/54

Figure 30.

Power dissipation during switching (approximation)

Since the T2 time for the turn-on and T3 time for the turn-OFF are much shorter compared

to Tfall and Trise, respectively, the approximated instantaneous amount of energy dissipated

during the commutation can be calculated as the triangular area having as its base the Tfall,

or Trise time and as its height the product of the maximum VCE voltage (equal to HVbus) and

the load current value:

Equation 13

Equation 14

The term fSW represents the switching frequency of the power IGBT (or MOSFET).

Based the results obtained by these approximate calculations, it should be clear that the

main consideration when dimensioning the gate resistor values is the trade-off between

electromagnetic emission and the power dissipated during power IGBT (or MOSFET)

switching, as shown in Figure 31.

Ton

Tfall

HVbus

Iload

Vce, Ic

approx.

real

Ton

Tfall

HVbus

Iload

conservative

approximation

real

HVbus

Iload

Vce, Ic

approx.

real

HVbus

Iload

Toff

Trise

Toff

Trise

turn ON

turn OFF

Pdiss_sw

real

T2 is typically very short

compared to T3

T3 is typically very short

compared to T2

conservative

approximation

DISS

–

bus

LOAD

⋅

() T

fall

rise

()

⋅

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

≅

DISS

–

bus

LOAD

⋅

() T

fall

rise

() f

⋅⋅

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

≅

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