Deadtime calculation for IGBT Modules

Application Note, V1.0, May.2007
AN2007-04
How to calculate and minimize the dead
time requirement for IGBTs properly
Power Management and Drives
Edition 2008-05-07
Published by
Infineon Technologies AG
81726 München, Germany
© Infineon Technologies AG 2008.
All Rights Reserved.
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V1.0
AP99007
Dead time estimation
Table of Contents
Page
1
1.1
1.2
Introduction ...................................................................................................................................5
Reason of IGBT bridge shoot through ............................................................................................5
Impact of dead time on inverter operation ......................................................................................5
2
2.1
2.2
2.3
2.4
2.4.1
2.4.2
2.4.3
Calculate proper dead time ..........................................................................................................6
Basics for calculating the dead time ...............................................................................................6
Definition of switching and delay times ...........................................................................................7
Influence of gate resistor / driver output impedance.......................................................................8
Impact of other parameters on delay time ......................................................................................9
Turn on delay time ..........................................................................................................................9
Turn off delay time ........................................................................................................................10
Verification of calculated dead time ..............................................................................................12
3
How to reduce dead time............................................................................................................13
4
Conclusion...................................................................................................................................14
Bibliography....................................................................................................................................................15
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Dead time calculation
Introduction
1
Introduction
In modern industry the voltage source inverter with IGBT devices is used more and more. To ensure proper
operation, the bridge shoot through always should be avoided. Bridge shoot through will generate unwanted
additional losses or even cause thermal runaway. As a result failure of IGBT devices and whole inverter is
possible.
1.1
Reason of IGBT bridge shoot through
The typical configuration of a phase-leg with IGBTs is shown in the following figure. In normal operation two
IGBTs will be switched on and off one after the other. Having both devices conducting at the same time will
result in a rise of current only limited by DC-link stray inductance.
+
Driver
Driver
Figure 1
.
.
.
.
.
.
.
.
.
.
.
.
Typical configuration of a voltage source inverter
Of course no one will turn on the two IGBTs at the same time on purpose, but since the IGBT is not an ideal
switch, turn on time and turn off time are not strictly identical. In order to avoid bridge shoot through it is
always recommended to add a so called “interlock delay time” or more popular “dead time” into the control
scheme. With this additional time one IGBT will be always turned off first and the other will be turned on after
dead time is expired, hence bridge shoot through caused by the unsymmetrical turn on and turn off time of
the IGBT devices can be avoided.
1.2
Impact of dead time on inverter operation
Generally there are two types of dead time, the first one is control dead time and the second is effective dead
time. The control dead time is the dead time, which will be implemented into control algorithms in order to get
proper effective dead time at the devices. Target for setting control dead time is to ensure that effective dead
time is always positive. Due to the fact that calculation of control dead time is always based on a worst case
consideration, an effective dead time being a significant portion of the control dead time will result.
Providing dead time can on one side avoid bridge shoot through but on the other side it has also adverse
effect. To clarify the effect of dead time, let’s consider one leg of the voltage source inverter as shown in
Figure. 2. Assuming first that output current flows in direction shown on the illustration IGBT T1 switches
from ON to OFF and IGBT T2 switches from OFF to ON after slight dead time. During effective dead time
both devices are off and freewheeling diode D2 is conducting output current. So negative DC link voltage is
applied to the output, which is desired here. Consider the other case that T1 switches from OFF to ON and
T2 from ON to OFF, then, with current in the same direction D2 still conducts the current during dead time,
so that output voltage will be also negative DC link voltage, which is undesired here. The conclusion can be
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Calculate proper dead time
summarized as follows: during effective dead time output voltage is determined by the direction of output
current but not the control signal.
If we consider output current in the opposite direction than illustrated in figure 2, then we will gain a voltage if
T1 switches from ON to OFF and T2 switched from OFF to ON. So in general output voltage and as a result
also output current will be distorted with application of a dead time. If we choose a dead time unnecessary
large, then in case of an induction motor the system will become instable and may cause some destructive
effects [1]. So the process of choosing dead time is very important and should be performed very carefully.
+
Current
Figure 2
One leg of voltage source inverter
This application note will explain how to measure delay time of IGBTs in practice and how to calculate the
control dead time properly based on measurements.
2
Calculate proper dead time
As already mentioned, dead time should be chosen on one hand to satisfy the need of avoiding bridge shoot
through, on the other hand dead time should be chosen as small as possible to ensure correct operation of
voltage source inverter. So a big challenge here is to find out a proper dead time for a dedicated IGBT device
and driver.
2.1
Basics for calculating the dead time
For calculation of control dead time we use the following equation:
[
]
t dead = (t D _ OFF _ MAX − t D _ ON _ MIN ) + (t PDD _ MAX − t PDD _ MIN ) × 1.2
Where
Td_off_max
: the maximal turn off delay time.
Td_on_min
: the minimal turn on delay time.
Tpdd_max
: the maximal propagation delay of driver.
Tpdd_min
: the minimum propagation delay of driver.
1.2
: safety margin to be multiplied.
(1)
In this equation the first term td_off_max-td_on_min is the difference of the maximal turn off delay time and
the minimal turn on delay time. This term describes characteristic of IGBT device itself plus gate resistor
which is used. Since fall and rise time is normally very short in comparison with delay time, they will be not
considered here. The other term tpdd_max-tpdd_min is the propagation delay time difference (delay time
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mismatch) which is determined from driver. This parameter will be found normally in datasheet of driver from
driver manufacturers. Typically this value is quiet high with opto-coupler based drivers.
Sometimes dead time will be calculated from typical datasheet values just multiplying by a safety factor from
field experience. This method will work in some cases but is not precise enough in general. With
measurements shown here, a more precise approach will be presented.
Because IGBT datasheet only gives typical values for standardized operation condition, it is necessary to
obtain the maximal values for dedicated driving condition. For this purpose a series of measurements is done
in order to obtain proper value for delay time and then to calculate dead time.
2.2
Definition of switching and delay times
Since we will talk a lot about switching and delay times, it is necessary to give a clear definition here.
Infineon Technologies defines the switching time of IGBT as follows:
td_on
: from 10% of Vge to 10% if Ic.
tr
: from 10% of Ic to 90% of Ic.
td_off
: from 90% of Vge to 90% of Ic.
tf
: from 90% of Ic to 10% of Ic.
Figure 3
Definition of switching times.
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2.3
Influence of gate resistor / driver output impedance
The choice of gate resistor will have significant impact on switching delay time. Generally to say, the larger
the resistor is the longer the delay time will be. It is recommended to measure delay time with dedicated gate
resistor in application. A typical switching time vs. gate resistor value diagram is shown in the following
figures:
25°C
1200
1000
800
t [ns]
td,on
td,off
600
tr
tf
400
200
0
0
20
40
60
80
100
120
Rg [ohm]
Figure 4
Switching times vs. Rg @25°C
125°C
1200
1000
800
t [ns]
td,on
td,off
600
tr
tf
400
200
0
0
20
40
60
80
100
120
Rg [ohm]
Figure 5
Switching times vs. Rg @125°C
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All tests were done with FP40R12KT3 module, gate voltage is -15V/+15V, DC link voltage is 600V and
switched current is nominal current of 40A.
2.4
Impact of other parameters on delay time
Besides the gate resistor values, there are other parameters having significant impact on delay times:
•
Collector current.
•
Gate drive supply voltage.
2.4.1
Turn on delay time
To estimate this relationship, a series of measurements was done. First the dependence of turn on delay
time and current was investigated. The results are shown in the next figure:
td_on VS Ic
120
100
Time [ns]
80
td,on -15V/15V,25°C
td,on -15V/15V,125°C
td,on 0V/15V,125°C
td,on 0V/15V,25°C
60
40
20
0
10
20
30
40
50
60
70
80
IC [A]
Figure 6
The turn on delay time vs. switched current Ic
All tests were done with a FP40R12KT3 module at a DC link voltage of 600V, gate resistor is chosen
according to datasheet value.
From results above it can be seen that turn on delay time is almost constant with variation of collector current
Ic. With -15V/+15V gate voltage turn on delay time will get larger than with 0V/+15V gate voltage [2]. For
further calculation of control dead time this variation will be neglected since it is quiet small and provides
even additional margin.
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2.4.2
Turn off delay time
The most important factor in the calculation of dead time is the maximal turn off delay time. Since this value
determines almost entirely how long the final calculated dead time will be. So we will investigate this delay
time in detail.
In order to obtain the maximum turn off delay time following considerations have to be done:
1. What and how long is the turn on delay time caused by IGBT device itself?
To answer this question the following test based on a characterization driver board is done in laboratory. The
characterization driver board is considered as an optimal driver, which means that this particular driver will
cause no delay (which is almost true with an oversized driver), so the whole delay time is considered to be
caused by the IGBT device itself. Following block diagram shows test setup:
Ideal
driver
Figure 7
Rg
Block diagram of test with ideal driver
2. What is the maximal turn off delay time if the threshold voltage of IGBT has the minimal value in
datasheets? (this reflects the tolerance of Vth from module to module)
To answer this question an additional diode is connected to simulate the reduced Vth voltage. The diode has
a voltage drop of approximately 0.7…0.8V, which is quite similar to the Vth variation of FP40R12KT3
module. Following block diagram shows principle test setup:
Ideal
driver
Figure 8
Rg
Block diagram of the test to simulate variation of Vth in worst case.
3. What is the impact of driver output stage on switching times?
To answer this concrete question the drivers on the market were splitted into two categories, one with mosfet
transistor output stage and the other one with bipolar transistor output stage. For each category separate
measurements were made.
To simulate drivers with mosfet output stage, another additional resistor was connected and has been
considered as the on state resistor Rds(on) of Mosfet transistor. The diode for simulation of Vth variation
remained. The following block diagram shows the principle test setup:
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Real driver with
Mos output
Ideal
driver
Figure 9
Rint
Rg
Block diagram of test to simulate variation of Vth and driver with mosfet output.
4. What is the impact of the driver with bipolar transistor output stage?
To answer the question an additional diode which simulated the voltage drop on bipolar transistor within
output stage was connected. The following block diagram shows principle test setup:
Real driver with
bipolar transistor
output
Ideal
driver
Figure 10
Rint
Rg
v
Block diagram of the test to simulate the variation of Vth and driver with bipolar
transistor output
With the configurations shown above the measurement of turn off delay time was done in our laboratory with
module FP40R12KT3 and driver board which had been considered as optimal. Test conditions were
Vdc=600V, Rg=27Ω. Results are shown in the next two figures:
Time [ns]
25°C
-15V/15V_25°C with Vth
variation
2100
1900
1700
1500
1300
1100
900
700
500
300
100
-15V/15V_25°C normal
0V/15V_25°C normal
0V/15V_25°C with Vth
variation
-15V/15V_25°C with Vth
variation and Mosfet Output
0V/15V_25°C with Vth
variation and Mosfet Output
0V/15V_25°C with Vth
variation and Bipolar Output
0
20
40
60
80
100
-15V/15V_25°C with Vth
variation and Bipolar Output
Ic [A]
Figure 11
Turn off delay time vs. Ic @25°C
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Time [ns]
125°C
-15V/15V_125°C with Vth
variation
2100
1900
1700
1500
1300
1100
900
700
500
300
100
-15V/15V_125°C normal
0V/15V_125°C normal
0V/15V_125°C with Vth
variation
-15V/15V_125°C with Vth
variation and Mosfet Output
0V/15V_125°C with Vth
variation and Mosfet Output
0V/15V_125°C with Vth
variation and Bipolar Output
0
20
40
60
Ic [A]
Figure 12
80
100
-15V/15V_125°C with Vth
variation and Bipolar Output
Turn off delay time vs. Ic @125°C
From the results we can see that there is a significant increase of turn off delay time with decrease of the
switched current Ic. So just simply calculate dead time according to a chosen gate resistor is obviously not
precise enough. Measuring the delay time under the dedicated driving condition then calculating dead time
according to these values is a better and more precise way. Normally measurement until 1% of the nominal
current would be enough to give a sufficient overview for calculating required dead time.
Another point to be considered here is that the turn off delay time will increase with 0V/+15V gate drive, and
the impact of output stage on switching times will be bigger with 0V/+15V switching. This means that with
0V/+15V switching voltage special care has to be taken by choosing the driver. Additionally, the increase of
td_off with lower switched collector current Ic should be considered also.
As an example: the HCPL-3120 driver IC will be considered here. This driver IC has a Mosfet output stage
for switching off. From diagrams above we can read the value for td_off under 0V/+15V switching condition is
roughly 1500ns. The td_on in this case is about 100ns. The tpdd_max-tpdd_min of this driver IC according to
datasheet is 700ns. Applying these values to the formula (1) results in a dead time of about 2.5µs.
2.4.3
Verification of calculated dead time
With the discussion above and the formula (1) given in chapter 2.1 it is now possible to calculate the required
dead time based on the measurements above. With the calculated dead time, a worst case measurement
can then be performed to verify if the chosen dead time is enough or not.
From the measurement it can be seen that the turn off delay time increases with temperature. From this
reason it is preferable that the test should be done both at cold and hot condition.
The schematic illustration of the test looks like following:
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Dead time calculation
How to reduce dead time
Figure 13
Schematic illustration of test to check calculated dead time value
The bottom IGBT has to be switched on and off, then the same for the top one. The time between the two
pulses should be adjusted to be the value of calculated dead time for the dedicated driving condition. The
negative dc-link current can then be measured and if the dead time is sufficient, a shoot through current
should not be observed.
Since there is no current through both IGBT, the described test represents the worst case condition for dead
time calculation. From the discussion of turn off delay time it is known that dead time will be longer with
decrease of collector current, so in case there flows no current, turn off delay time should be largest, which
leads to a need of largest dead time. If there is no shoot through current at zero collector current then the
chosen dead time is for dedicated driving condition sufficient.
3
How to reduce dead time
For a proper calculation of control dead time the dedicated driving condition should be considered:
•
What is the applied gate voltage to the IGBT?
•
What is the chosen gate resistor value?
•
What type of output stage does the driver have?
Based on these conditions a test should be made, from the test results the control dead time can then be
calculated using equition (1).
Since dead time has a negative impact on the performance of inverter, it has to be minimized. Several
methods can be taken.
•
Take a driver strong enough to sink or source the peak IGBT gate current.
•
Use negative power supply to accelerate turn off.
•
Prefer drivers based on fast signal transmission technology like Coreless Transformer
Technology to drivers based on traditional opto-coupler technology.
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Conclusion
•
If 0V/15V gate drive is used then consider use of separate Rgon/Rgoff resistor as described
below.
From measurements shown in chapter 2.3 a very strong dependence of Td_off and gate resistor value can
be observed. If the Rgoff reduced then the td_off will be reduced as well as dead time. Infineon suggests
reducing the Rgoff to 1/3 of the Rgon value if 0V/15V gate voltage is used. One possible circuit for separate
Rgon and Rgoff is as follows:
Figure 14
Suggested circuit with 0V/15V gate voltage.
R1 should be chosen to satisfy the following relation:
R1 ⋅ Rgon
R1 + Rgon
=> R1 =
1
+ Rg int = ⋅ ( Rgon + Rg int )
3
(2)
R gon − 2 R g int
1
⋅ R gon ⋅
R gon + R g int
2
(3)
From equation (3) it is to be noticed that the requirement Rgon>2Rgint has to be fulfilled to get a positive
value of R1. However, with some modules this requirement can not be true. In this case, R1 can be omitted
completely.
The diode should be a schottky type diode.
Another very important issue with 0V/15V gate voltage is the parasitic turn on effect. This issue can be also
solved if suggested circuit is used. For more details on parasitic turn on please refer to AN2006-01[2].
4
Conclusion
In this application note an approach of measuring switching times of IGBT and then calculating the control
dead time is introduced. First dependence of switching time on gate resistor value was shown, and then
influence of gate driver and collector current on switching times was discussed. Finally possible methods to
reduce dead time were introduced.
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Dead time calculation
Bibliography
Bibliography
[1]
D.Grahame Holmes, Thomas A. Lipo: „Pulse width modulation for power converters: principles and
practice“, IEEE Press, 2003. ISBN 0-471-20814-0
[2]
Driving IGBTs with unipolar gate voltage.
http://www.infineon.com/dgdl/an-200601_Driving_IGBTs_with_unipolar_gate_voltage.pdf?folderId=db3a304412b407950112b408e8c9000
4&fileId=db3a304412b407950112b40ed1711291
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