227

very small gate resistor R

Gon

is chosen, which will be increased in value once U

GE(TO)

is

reached. This procedure is also called two-stage switching.

Another method is illustrated in

The gate is switched on with an increased

gate-emitter voltage and the gate current increases accordingly. After reaching the level

of U

GE(TO)

, the boost voltage (24V) is switched off and the IGBT capacitances are

charged by the nominal gate voltage (15V) andan reducedgate current.

The design of the gate drive focuses on the static and dynamic behaviour of the IGBT

but also the corresponding freewheeling diode. The fundamental aspects of the design

are considered below. Measures to influence the switching behaviour and integrated

protective functions are discussed in later chapters.

When selecting a driver stage, one important parameter is the maximum peak current

I

peak

, which is required to drive the IGBT. For this, the turn-on and turn-off currents are

considered separately. Even if inmany applications, turn-on and turn-off are treated the

same, both should be investigated separately and calculated with the smallest gate

resistor.

An estimation of themaximumpeak current canbedonebyusin

Gext

intG

min ,GE

max ,GE

minG

GE

peak

R R

U U

7.0

R

U 7.0 I

+

−

⋅

=

∆

⋅

≈

I

peak

:

Peak current, which thediver stage has toprovide [A]

U

GE,max

: Positive gate voltage to switch the IGBTon [V]

U

GE,min

: Negative gate voltageor 0V to switch the IGBToff [V]

R

Gint

:

Internal gate resistor of the IGBT (if present) [

Ω

]

R

Gext

: External gate resistor [

Ω

]

If an additional external gate-emitter capacitor C

G

is used as part of the driver stage, in

good approximation this capacitance equals a short circuit of the internal gate resistor.

Hence, R

Gint

i

has tobe set to0

Ω

.

In practice, the correction factor of 0.7 is required for the peak current. The reason for

this is the internal driver impedance always present and parasitic effects of lead

resistances and inductances. The correction factor is derived using the following

considerations:

Assuming a constant internal capacitance C

GE

of the IGBT during the turn-on and turn-

off events, a parasitic inductance L

G

and separate lead inductances L

Gon

and L

Goff

, the

followingdifferential equation for a second order RLC circuit derives:

0 )t( i

C

1

dt

)t(di

R

dt

)t( idL

G

GE

G

G 2

G

2

= ⋅

+

⋅

+

⋅

L: Sum of the inductances in thegate path [H]

R

G

: Sum of external and internal gate resistors [

Ω

]

i

G

(t): Time-dependent gate current [A]

Theminimumgate resistanceR

G,min

in the gate path, which does not lead to oscillations,

is:

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