AN2785 datasheet(30/51 Pages) STMICROELECTRONICS

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Application examples

AN2785

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Doc ID 14785 Rev 1

Figure 22.

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

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

Trise time, and as height the product between the maximum Vce voltage (equal to HVbus)

and the Iload current value, as follows:

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

In summary, using the results obtained by these approximated calculations, it is clear that

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

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

switching, as shown in Figure 23.

Ton

Tfall

HVbus

Iload

Vce, Ic

approx.

real

POWER DISSIPATION FOR SWITCHING

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

(

) (

)

RISE

FALL

LOAD

BUS

DISS

⋅

≅

(

) (

)

RISE

FALL

LOAD

BUS

DISS

⋅

≅

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