9020 Datasheet, PDF (8/25 Pages) Fairchild Semiconductor – IGBT Basic II

Since recovery characteristics of the freewheeling diode (FWD) on the opposite side are a
function of di_C/dt, peak recovery current (I_rr) of FWD (refer to Fig. 2) (which is IGBT’s over-cur-
rent), over-voltage of FWD and dv_CE/dt are affected by changes in di_C/dt. When di_C/dt
increases, over-current of IGBT and over-voltage stress of FWD increases, while it also
causes increases in falling dv_CE/dt of IGBT and rising dv_CE/dt of diode voltage on the opposite
side. Large values of di_C/dt and dv_CE/dt indicate that switching speed, or switching loss, is
small, which could be an advantage, but from the perspective of limiting EMI noise, it is neces-
sary to set an upper limit. Fundamental method to limit over-current in I_C is to select a device
whose built-in diode has better recovery characteristics. However, once the device has been
decided and setup is completed, I_C peak current can be kept below the rated amount by
reducing V_GG+ or by increasing R_G. IGBT’s over-current and the FWD’s over-voltage on the
opposite side can also reduce di_C/dt by limiting V_GG+ or by increasing R_G. In selecting a
device with built-in diode (co-pak IGBT), the peak value of the over-voltage must not exceed
the rated voltage of the diode on the specification. The snappiness factor (refer to Fig. 2: S=tb/
ta) of the built-in diode must not be too small.
d. Effect on the short circuit capability
IGBT’s short-circuit endurance capability can be controlled with the value of V_GG+. The smaller
the V_GG+, the smaller clamping voltage during short circuit and power dissipation as shown in
Fig. 3. In another word, short circuit endurance time increases. With these characteristics,
short circuit protection can be devised with V_GE. However, with increases in V_GG+, one must
assume increases in on-state losses. RUF Series by Fairchild can withstand short circuit con-
dition for about 10µS.
e. Effect on turn-off
As the turn-on characteristics of IGBT are largely affected by V_GG+, turn-off characteristics of
IGBT are affected by V_GG- (negative gate bias voltage). However, the tail section of the I_C
comes from the BJT characteristics of IGBT, which is an integral nature of the device and can-
not be controlled from outside with V_GG-. As the value of V_GG- becomes greater, the turn-off
switching loss of I_C decreases. As the value of V_GG- increases, di/dt of I_C increases, and di/dt
of I_D increases by the same amount. di/dt of I_D and stray inductance L_S is added to V_DC
according to the equation, V = L_S * di/dt to form over-voltage. As the absolute value of V_GG-
increases, di_C/dt and dv_CE/dt increases, and as di_C/dt increases, the over-voltage of V_CE
increases. The peak value of this over-voltage must not exceed IGBT’s maximum rating, so
the value of V_GG- can be reduced to control it. On the other hand, as the value of V_GG- is
increased, the possibility of dv/dt shoot through (R_G: refer to Effect on turn-on) is reduced.
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Rev. A, April 2002