AN2785 datasheet(27/51 Pages) STMICROELECTRONICS

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AN2785

Application examples

Doc ID 14785 Rev 1

27/51

Due to the Cgc’s non-linearity the IGBT (or MOSFET) datasheet also reports the equivalent

total amount of charge required during the different gate charge phases: the total gate

charge Qg required for turning ON the IGBT (or MOSFET) completely, the Qge required for

increasing the Vge up to the plateau voltage and the Qgc required during the whole plateau

phase. This last amount of charge is also called plateau charge. The time required to

provide the complete plateau charge is the Tfall time and this is the same time necessary for

completing the Vce transition. For Ton time is intended the time delay between the

beginning of the gate charge and the full conduction of the IGBT (or MOSFET), when the

power switch current equals the full load current. So:

It is now possible to merge the equivalent gate driver output circuit reported in Figure 16 and

the considerations about the different gate charge phases in the total equivalent circuit

reported in Figure 19.

Figure 19.

Total equivalent circuit for turn-ON

From the above circuit the value of the transition times could be calculated (the following

formulae are related to the low-side transition, but the same are also suitable for the high-

side by changing “VCC” with “VBOOT-VOUT”).

CISS_min is used because when the Vce (or Vds) of the power IGBT (or MOSFET) is

maximum (equal to HVbus), the CISS (which depends on the Vce) shows its minimum

value.

During the turn-OFF of the power IGBT (or MOSFET), the behavior is the same as for the

turn-ON but in reverse time order. Figure 20 provides the gate charge characteristics and

Vce vs. Ic curves.

SOURCE

FALL

RGATE

RDSon_source

Vboot/VCC

Cge =

Qge

Vge_p

Vge

Isource

Ton

RGATE

RDSon_source

Vboot/VCC

Vge_p

Isource

Tfall

= Ciss_min

()

⎟⎟

⎠

⎞

⎜⎜

⎝

⎛

−

⋅

Vge

VCC

min

ISS

GATE

source

DSon

(

)

Vge

VCC

GATE

source

DSon

SOURCE

FALL

−

⋅

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