Driving IGBTs with unipolar gate voltage

Application Note
Page 1
Department: AIM PMD ID AE
Date: 15-12-2005
AN-Number: AN-2006-01
Driving IGBTs with unipolar gate voltage
Introduction
Infineon recommends the use of negative gate voltage to safely turn-off
and block IGBT modules. In areas with nominal currents less than
100tA the negative gate voltage is often omitted for cost reasons. The
following paper describes special considerations for a unipolar drive of
IGBT modules.
Turn-off to 0 V
The latest Infineon IGBT chip generations have several advantages.
Some highlights especially are a wider dynamic range, faster switching,
less switching losses and lower conduction losses.
When switching to 0 V two effects may come into play:
-
parasitic turn-on via the Miller capacitance
parasitic turn-on via stray inductances
Turn-on via the Miller capacitance
When turning on the lower IGBT in a half-bridge a voltage change
dvCE/dt occurs across the upper IGBT / diode. This causes an approach
dv
current iCG = CCG CE to flow which charges the parasitic capacitance
dt
CCG of the upper IGBT. The capacitances CCG and CGE form a
capacitive voltage divider. Figure 1 depicts the current path via the
Miller capacitance of the upper IGBT.
iCG = CCG ⋅
+15 V
dt
Treiber
RDriver
C
dvCE
VCE
CCG
RGon/off RGint
CGE
dvCE
dt
t
0V
vGE = ( RDriver + RGon/off + RGint ) ⋅ iCG
E
Figure 1: Current via the Miller capacitance of the upper IGBT
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Application Note
Page 2
Department: AIM PMD ID AE
Date: 15-12-2005
AN-Number: AN-2006-01
The current iCG flows via the Miller capacitance, the serial resistors, CGE
and the DC-bus.
If the voltage drop across the gate resistor exceeds the threshold
voltage of the IGBT, a parasitic turn-on occurs.
With rising chip temperature the threshold voltage drops by several
mV / K.
When the upper IGBT switches, a current flows via the Miller
capacitance of the lower IGBT and may lead to parasitic turn-on here as
well.
Turn-on via stray inductances
When turning off the load current a voltage vσE2 = LσE2
diC2
dt
is induced
across the emitter stray inductance.
C1
T1
D1
E1/C2
+15 V
iC2
T2
RDriver
D2
RGon/off RGint
LσE2
0V
vσE2
E2
Figure 2: Induced voltage across the emitter inductance
When switching the IGBT T1 on the main current will commutate from
the free wheeling diode D2 to the IGBT. The diC2/dt produced from
decay of the reverse recovery of the diode, induces a voltage on LσE2
and shifts the emitter potential of T2 to the negative.
If the induced voltage produced through a high diC/dt is higher than the
threshold voltage of the IGBT this will result in a parasitic turn-on of T2.
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Application Note
Page 3
Department: AIM PMD ID AE
Date: 15-12-2005
AN-Number: AN-2006-01
Parasitic turn-on in modules with common auxiliary emitter
In modules where the auxiliary emitter connections of several IGBTs
are joined to a common emitter connection, very fast switching may
result in an induced voltage across stray inductances of the emitter.
The equivalent circuit diagram is depicted in figure 3:
U
V
T2
T4
Lσ1
main
emitter
W
Lσ2
Lσ3
Lσ4
Lσn: stray inductance
T6
Lσ5
Lσ6
B
chopper
Lσ8
Lσ7
Lσ9
auxiliary
emitter
Figure 3: Parasitic turn-on via the common emitter inductance
The parasitic inductances in the module are here numbered Lσ1 to Lσ9.
When turning on IGBT T6 an induced voltage across Lσ2 to Lσ3 results,
which affects T2. The emitter potential of IGBT T2 is thus shifted
resulting in parasitic turn-on of IGBT T2 when the voltage change
exceeds the threshold voltage.
Proving parasitic switching
To prove parasitic turn-on, it is necessary to insert a current sensor in
the bridge arm of the module. Two measurements may lead to the
definite proof.
1. Double-pulsing the lower IGBT while blocking the upper IGBT
with a negative voltage.
2. Double-pulsing the lower IGBT while the upper IGBT is shut off
as done later in the application.
It is recommended to make two measurements with different current
between 1 ⋅ I C nom to 2 ⋅ I C nom .
10
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Application Note
Page 4
Department: AIM PMD ID AE
Date: 15-12-2005
AN-Number: AN-2006-01
Parasitic turn-on has been proven when the two current curves differ
considerably. To be noted here especially is a higher current peak, a
wider reverse current peak and/or an additional current pulse. Means to
suppress the inadvertent turn-on are described in details in the chapter
“Suggestions for Solutions”.
C1
T1
D1
RGint
vσE1
E1/C2
T2
D2
RGint
vσE2
A
E2
Figure 4: Bridge arm with current sensor
In applications with screw terminal power connections it is often
possible to use a Rogowski coil for measurements. In most cases,
however, it is not possible to measure directly in one arm. In smaller
modules the load current is often brought to the PCB via solder pins.
Here it is recommended to measure in the DC-bus e.g. with a Rogowski
coil or a shunt resistor.
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Application Note
Page 5
Department: AIM PMD ID AE
Date: 15-12-2005
AN-Number: AN-2006-01
Suggestions for solutions
Variation of the gate resistor
The voltage change -dvCE/dt and the current change diC/dt during the
turn-on process may be influenced by varying the gate resistor RGon.
Increasing the gate resistor reduces the voltage and current changes.
The IGBT switches slower; see also table 1.
The capacitive parasitic turn-on may be obviated by reducing the RGoff
value. The inductive parasitic turn-on, however, is prevented by
increasing the RGoff value.
Separate gate resistors to achieve non-critical turn-on and turn-off
In many applications a non-critical switching characteristics may be
achieved when separate turn-on and turn-off resistors are used.
T1
D1
DGoff RGoff
RGon
T2
D2
Figure 5: Separate turn-on and turn-off resistors
Choosing RGoff < RGon prevents a capacitive turn-on via the Miller
capacitance; s. paragraph “Turn-on via the Miller capacitance”.
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Application Note
Page 6
Department: AIM PMD ID AE
Date: 15-12-2005
AN-Number: AN-2006-01
Additional gate emitter capacitor to shunt the Miller current
The switching behaviour may be influenced with an additional capacitor
CG between gate and emitter. The capacitor is to take up additional
charge originating from the Miller capacitance. Due to the fact that the
total input capacitance of the IGBT is CG||CGE, the gate charge
necessary to reach the threshold voltage is increased.
T1
D1
RGon/off
T2
D2
CG
RS
Fig. 6: Additional capacitor between gate and emitter
In applications where the IGBT module does not have an internal gate
resistor it is recommended to place an additional resistor RS in series
with the capacitor, to prevent oscillation. The recommended value for RS
1
⋅ RGon/off . These are values derived from experience.
is: RS ≈
20
The capacitance recommended for the additional capacitor is also
derived from experience and is calculated by:
CG ≈
Qge
3 ⋅ 30 V
.
Due to the additional capacitor the required driver power is increased
and the IGBT shows higher switching losses depending on how the
RGon/off where modified.
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Application Note
Page 7
Department: AIM PMD ID AE
Date: 15-12-2005
AN-Number: AN-2006-01
Transistor to shunt the Miller current (active Miller clamping)
An additional measure to prevent the unwanted turn-on is shorting the
gate to emitter path.
This can be achieved by an additional transistor between gate and
emitter.
This “switch” shorts the gate-emitter region after a time delay, as long
as the driver shows a 0V signal at its output. The Schottky diode
prevents a current flow coming from the Miller capacitance back
through the gate resistor.
T1
D1
T2
RGon/off
D
D2
RB T
RE
Fig. 7: Possible set-up with additional transistor
The occurring currents across the Miller capacitance are shunted by the
transistor in a controlled manner. This guaranties safe switching.
Conclusion
Table 1 gives an overview of the measures discussed above with their
advantages and disadvantages accordingly. RGon is used to turn the
IGBT on; RGoff is used to block and to turn off the IGBT.
RGon/off is the common resistor for turn-on and turn-off the IGBT.
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Author: Baginski
Application Note
Page 8
Department: AIM PMD ID AE
Date: 15-12-2005
AN-Number: AN-2006-01
Effect
Turn-on due to
the Miller
capacitance
Measure
Reducing RGon/off
Increasing RGon/off
Additional CG
Reducing RGoff
Reducing RGon
Increasing RGoff
Increasing RGon
Additional
transistor
Turn-on due to the
stray inductance
Switching losses
+
-
+
↓
↑
+
-
↑
+
+
o
+
+
↓
↓
↑
↑
++
-
↓
Table 1: Effectiveness of different measures
++: Very good results
+: Improvement
- : Deterioration
o: no change
↑: increase
↓: decrease
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Author: Baginski
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