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This gate driver performs four operating modes:
- Turn-on
- Turn-off
- Over-current protection
- Active Vce clamping
transistors Q1 and Q2 were closed when photo-coupler U3 is turned on by input signal, let the voltage of Q8,Q9 base to increased to 24V. therefore, Q8 and Q11 is ON while Q9 and Q12 are OFF, consequently, IGBT turned on immediately, and collector voltage dropped to (1.8->3.2)V for SKM200GB12E4.
Q1 and Q4 turned on when U3 is OFF, which led Q8 and Q11 turned off while Q9 and Q12 turned on. The voltage of Gate pin dropped to OV rapidly, 8V lower than of the Emitter pin which is biased to 8v by D10 , and caused IGBT turned off reliably.
when IGBT was ON, the voltage of D16 cathode kept around 10.5V (Vd15 + Ve + Vce), and V c38 kept around 24V. If short-circuit or over-current occurred, IGBT would withstand high current and goes to desaturation state, then Vce increased rapidly, led D15 turned off. The voltage of D16 cathode increases to 13V until d16 breaks down, and then Q3 turned on. C38 discharged through R23 and Q3, which causes the voltage of "on" label drop gradually, and turn the IGBT off slowly.
IGBT modules and converter circuits have parasitic inductances that can’t be completely eliminated; their influence on
system behavior also can’t be ignored. Figure 1 illustrates the parasitic inductances contained in a commutation circuit.
The current change caused by turning off the IGBT produces an overshoot voltage at its collector terminal, as shown in
Figure 2.
The commutation speed (and therefore, the turn-off overvoltage) at an IGBT can, in principle, be affected by the turnoff gate resistance Rg(off). This technique is used particularly at lower power levels. However, the Rg(off) must then be
matched for overload conditions, such as turn-off of the double-rated current, short circuit, and a temporarily increased
link circuit voltage. In normal operation, this results in increased switching losses and turn-off delays, which reduces the
usability or efficiency of the modules. As a result, this simple technique is unsuitable for modern high power modules.
When IGBTs are driven with a pulse that is shorter than the response time
in the event of a short circuit, the fault is not detected and the driver turns off too quickly. The resulting overvoltage destroys the IGBT.
Moreover, coverage of limit cases (between overcurrent/non-overcurrent) presents a problem; for instance, a higher overvoltage may
well occur when the double-rated current is turned off than at a short-circuit turn-off
**
The feedback branch consists of a clamping element, which, as a rule, is made up of a series of transient voltage suppression (TVS) diodes. If the
collector-emitter voltage exceeds the approximate breakdown voltage of the clamping element, a current flows via the feedback to the gate of the IGBT,
raising its potential, so that the rate of change of the collector current is reduced, producing a stable condition. The voltage across the IGBT is then
determined by the design of the clamping element. The IGBT operates in the active range of its output characteristic and converts the energy stored
in the stray inductance into IGBT heat. The clamping process continues until the stray inductances have been demagnetized. The fundamental
relationships involved here on the basis of typical curves are illustrated in the lower part of Figure 5. A single high voltage TVS diode, or several lower voltage TVS diodes connected in series with a resultant high voltage, can be used to provide active clamping based on the DC power line voltage or IGBT Vce voltage.
for further details read Littlefuse Application note
VGE before clamping is like the picture below
at voltage of five TVSs dynamic clamp starts and current goes from capacitor C57 to gate until it charges up the Vge is like picture below
if overshoot voltage goes higher than all TVSs voltage then static protection starts and VGE is going to be like picture below
Clamping of IGBT collector voltage is shown HERE on YouTube note : Video is in Persian and oscilloscope probe is on 10x
Short pulse suppression realized by a RC filter which you can see in the picture below
- for more information about short pulse you can read Fuji REH984e section 3.4
- you can find RC circuit tutorials HERE
When voltage is less than (D19 breakdown voltage + Q5 Vbe) Q5 will be off and Q3 base will be driven by R27 and R26 and soft shutdown will be initiated
If you turn off IGBT fast after short circuit voltage over shoot due to di/dt may lead to IGBT destruction so you should turn off IGBT slowly but you have a limited time which is defined in IGBT datasheet as Tpsc if you don't turn IGBT off in this time your IGBT may fail
As you van see in the circuit when VCE saturation is more than a threshold Q3 base will be biased and it starts to decrease Q8 & Q9 voltage by R23,C38 RC filter this will bias Q8,Q9 in linear mode so gate voltage will decrease slowly if input pulse lowered at this time IGBT will turn off fast so D12 is used to hold input on
