AN4544 datasheet(23/35 Pages) STMICROELECTRONICS

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AN4544

Datasheet explanation

2.6.4

Reverse transfer capacitance (Cres)

This is the reverse transfer capacitance measured between the collector and gate terminals

with the emitter connected to ground. The reverse transfer capacitance is equal to the gate

to collector capacitance:

Equation 11

The reverse transfer capacitance, often referred to as the Miller capacitance, is one of the

major parameters affecting voltage rise and fall times during switching.

Figure 19. STGW40V60DF capacitance variation

Figure 19 shows a graph of typical capacitance values versus collector-to-emitter voltage.

These capacitances decrease over a range of increasing collector-to-emitter voltage,

especially the output and reverse transfer capacitances. This variation is linked to the gate

charge data. These parameters are not tested in production.

2.6.5

Gate charge (Qge), (Qgc) and (Qg)

IGBT gate charge values are useful to design the gate drive circuit, since it takes into

account the changes of capacitance and voltage during a switching transient, by estimating

gate drive losses.

They cannot be used to predict switching times, because of the minority carrier due to the

NPN and PNP structure inside the IGBT.

Qge is the charge from the origin to the first inflection in the curve, Qgc is the charge from the

first to second inflection in the curve (also known as the “Miller” charge), and Qg is the

charge from the origin to the point on the curve at which VGE equals the peak drive voltage.

Gate charge values vary with collector current and collector-emitter voltage but not with

temperature. The graph of gate charge is typically included in the datasheet showing gate

charge curves for a fixed collector current and different collector-emitter voltages. The gate

charge values reflect charge stored on capacitances.

res

C(pF)

0.1

VCE(V)

1000

100

10000

Cies

Coes

Cres

AM17397v1

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