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Effect of Gate-Emitter Capacitor C GE
Attached you find some informations, which should give explanations about the advantages of using an additional gate capacitor C G .
The main idea behind this is to control dI C /dt and dV CE /dt independently  at  turn-on . The need for this is given by the wish for lowest IGBT turn-on losses and a not unlimited di/dt-capability of the free wheeling diode.
By just raising the R G  (until the dI C /dt max  is ok) would increase the turn-on losses E on . With the additional component C G  the dI C /dt can be controlled by the time constant given by R G  and C GE //C G . For the turn-on dV CE /dt only R G  and C G //C GC  (C GC : Miller-capacity) are effective. Be-cause C GC  >> C G , finally only R G  (and not C G ) is responsible for the resulting  dV CE /dt value !By this measure the R G  and therefore the switching losses can  remain on a low value.
1        2                    3                                      4
range determined by condition influenced by
influence on
1V GE  < V GEth
C iss  = const.C‘GE t don 2V GEth  < V GE  < V GEM C iss = const.C‘GE dI C  / dt 3V GE  = V GEM V GE  = const.R G , C GC dV CE  / dt 4
V GE  > V GEM
C iss  = const.
C‘GE  // C GC dV CEsat  / dt
C’GE  = C GE  // C G
Example : FZ1200R33KF1, turn-on without C G
V CE: 500V / div,  I C: 600A / div
high dI C/dt due to low gate resistor R G
low dV CE/dt and high turn-on losses due to high gate resistor R G For further information contact:
eupec
Solution:
An additional Capacitance between Gate & Emitter allows independent control of dI C/dt and dV CE/dt
•dV CE/dt is controlled via R G and C GC
•dIc/dt is controlled via R G and C GE//C G
For further information contact:
eupec
For further information contact:eupec
const的作用
R G =8,2Ω, C G =0, I C /dt= 5kA/µs, dV CE /dt=0,6kV/µ
s, E on =6,4J
R G =3,3Ω, C G =100nF, I C /dt= 4,5kA/µs, dV CE /dt=1kV/µ
s, E on =4,1J
R G =1,0Ω, C G =330nF, I C /dt= 5,1kA/µs, dV CE /dt=2,8kV/µs, E on =2,8J
Example:FZ1200R33KF1Variation of R G  and C G

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