Gate drivers two level turn off switching behavior

Gate driver

1. EVALUTION OF MERITS AND LIMITS OF IGBT
DRIVERS USING TWO LEVEL TURN OFF

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IGBT under Short circuit and Over current
During short Circuit condition a very high current will flow
through the device.
The magnitude of short circuit current is 5 to 6 times of rated
current.
The peak current and rate of rise(di/dt) was limited by
transconducatnce and common emitter inductance respectively
If IGBT collector current doest not reach to equilibrium causes
the device to latch up.

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NPT IGBT under Over current
The NPT IGBT under over current
Fig 1: NPT IGBT during turn off of over current
Turn off of IGBT under over current condition causes the device to be
latch up

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Field stop IGBT under Over current
Incase of field stop IGBT turn off under over current can rise second
electic field in front of buffer layer
Fig 2: TLTO driver gate voltage

Fig 2: Field Stop IGBT under over current

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Short Circuit 1
In short circuit 1, IGBT switched on to the short circuit case
This will happen when the lower IGBT switched on while upper IGBT still on
In short circuit 1 case, the collector emitter voltage is maintained to bus voltage through out the time except at intial
phase there will be small drop in Vce due to stray inducatnce in the circuit.
Short circuit current is 5 to 6 times than rated current.
Fig 3 : Block diagram of TLTO driver (1ED020I12-BT)

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Short Circuit 1 Setup
Upper IGBT supplied through -8V and Load inductance value is zero
Fig 4: Short circuit 1 setup for FF600R12ME4 module
Lower IGBT(DUT) is triggered using two level turn off driver as well as soft shut
down driver.
Fig 4: Short circuit 1 setup for FF600R12ME4 module

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Voltage and current waveforms during SC1
Fig 5:Test setup
Vge
Ic
Vce

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Voltage and current waveforms during SC1
Between time t0 to t1 there is a small drop in Vce because of stray inductance in the power circuit.
This small dv/dt will cause very low displacement current through miller capacitance.
From time t1 to t2 collector current will increase to high value and at the end of time t2 two level
turn off was intiated by driver
From time t2 to t3 gate voltage was reduced to 10V by driver mechanism there by corresponding
collector current starts reducing
From t3 to t4 there is negitive dv/dt, there by displacement current flow from gate to miller
capacitance(CGC) and also dischrages the gate capacitance there by small drop in gate voltage.
Fig 6: TLTO driver evalution board

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Soft shut down driver
Fig 8: Turn off of SC using Soft shutdown driver(Vdc=600) Fig 9: Using Two level turn off driver
Yellow – Vce ; Violet – Gate volatge ; skyblue - Ic
In case of soft shut down driver Vce is 1040V but in case of two level
turn off driver it is 880V

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Over current
The overcurrent condition can be achieved by connecting very
small Load inductance(nH) to the circuit in Fig3.

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Turn off of Over current using TLTO driver
dv/dt is positive

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Over current turn off
Between the time t3 and t4 the dv/dt is positive but there is drop in gate
voltage this can be explained as follows

Current through miller capacitance can be written as displace d
current density through the oxide
.
The electric field as a function of Vce and current in IGBT written as follows
[Ref: Negitive miller capacitance during
switching transients by Jürgen Böhmer]




(Eq.1)

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Over current turn off




Electric field
Electric field

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Two level turn off
The graph between DC applied voltage with respect to cable length
for different pulse widths is shown below

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Comparison
The comparison between Soft shut down driver and TLTO driver as
follows

420V

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Temperature
The short circuit current depends upon forward transconductance(G Fe) of IGBT
The GFE of IGBT is having negitive temperature coefficient means during the short circuit a
very high current will flow which increases the junction temperature
 So if the temperature of IGBT increases then the Short circuit current through IGBT reduces

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Temperature
DC link voltage for different temperatures as follows

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Simulation Model TLTO driver

voltage
time
Booster stage
Compensation of opamp
Load Inductance

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Short circuit 1 Results

20. Thank You