What is “dead time”?How to reduce the dead time of IGBT

In modern industry, there are more and more applications of voltage source inverters using IGBT devices. In order to ensure reliable operation, the bridge arms should be prevented from passing through. The bridge arm pass-through will generate unnecessary additional losses, and even cause heat to run out of control, which may result in damage to the device and the entire inverter.

In modern industry, there are more and more applications of voltage source inverters using IGBT devices. In order to ensure reliable operation, the bridge arms should be prevented from passing through. The bridge arm pass-through will generate unnecessary additional losses, and even cause heat to run out of control, which may result in damage to the device and the entire inverter.

The following picture shows a typical structure of an IGBT bridge arm. During normal operation, the two IGBTs will be turned on and off in sequence. If the two devices are turned on at the same time, the current rises sharply, and the current at this time will only be determined by the stray inductance of the DC loop.

What is “dead time”?How to reduce the dead time of IGBT

Figure 1 Typical structure of voltage source inverter

Of course, no one intentionally turns on two IGBTs at the same time, but since IGBTs are not ideal switching devices, their turn-on time and turn-off time are not strictly the same. In order to prevent the IGBT bridge arm from passing through, it is usually recommended to add the so-called “interlock delay time” or “dead time” to the control strategy. This means that one of the IGBTs must be turned off first, and then the other IGBT will be turned on at the end of the dead time. In this way, the shoot-through phenomenon caused by the asymmetry of the turn-on time and the turn-off time can be avoided.

1. The influence of dead time on inverter operation

On the one hand, the dead time can prevent bridge arms from passing through, and on the other hand, it will also bring adverse effects. Taking Figure 2 as an example, first assume that the output current flows in the direction shown in the figure, and the IGBT T1 changes from on to off. After a short period of dead time, the IGBT T2 changes from off to on. During the effective dead time, both switch tubes are turned off, and the freewheeling diode D2 flows through the output current. At this time, a negative DC voltage is applied to the output side, and the voltage polarity at this time meets the design requirements. Consider another situation, T1 is turned off to on, and T2 is turned on to off. At this time, since the current is still in the same direction, this current still flows during the dead time, so the output voltage is still negative Value, the voltage polarity at this time is not what the design hopes to get. The conclusion can be summarized as follows: During the effective dead time, the output voltage is determined by the output current, not the control signal.

What is “dead time”?How to reduce the dead time of IGBT

Figure 2 A bridge arm of a voltage source inverter

If we assume that the direction of the output current is opposite to that shown in Figure 2, then when T1 is switched from on to off, and T2 is switched from off to on, the situation similar to the above will also occur. Therefore, under normal circumstances, the output voltage and output current will be distorted with the addition of the dead time. If we choose a too large dead time, the system will become unstable in the case of induction motors. Therefore, carefully calculate the dead time.

What is “dead time”?How to reduce the dead time of IGBT

This article mainly describes how to measure the IGBT delay time in practice, and how to correctly calculate the control dead time based on the measured value.

2. Calculate the appropriate dead time

As mentioned above, when choosing the dead time, on the one hand, it should meet the requirement of avoiding bridge arms through-through, and on the other hand, it should be as small as possible to ensure that the voltage source inverter can work normally.

2.1 Method of calculating dead time

We use the following formula to calculate the control dead time:
in,
td_off_max: Maximum turn-off delay time.
td_on_min: minimum turn-on delay time.
tpdd_max: the maximum transmission delay time of the drive.
tpdd_min: The minimum transmission delay time of the drive.

1.2: Safety margin.

In this formula, the first term td_off_max-td_on_min is the difference between the maximum turn-off delay time and the minimum turn-on delay time. This item mainly describes the characteristics of the gate resistance used in the combination of IGBT devices. Since the rise and fall times are usually much shorter than the delay time, they are not considered here. The other item, tpdd_max-tpdd_min, is the difference between the transmission delay time determined by the driver (the delay time does not match). This parameter can usually be found in the drive data sheet provided by the drive manufacturer. For drivers based on optocouplers, the value of this parameter is usually very large.

Sometimes it is possible to calculate the dead time by multiplying a typical data sheet value by a safety factor from field experience, but it is usually not accurate enough. Because the IGBT data sheet only provides typical values ​​corresponding to standard operating conditions, it is necessary for us to obtain the maximum value corresponding to special driving conditions. To this end, a series of measurements must be performed to obtain a suitable delay time value, and then the dead time is calculated.

2.2 Switch and delay time definition

Infineon defines the switching time of the IGBT in the following way:
td_on: The time from Vge rising 10% to Ic rising 10%.
tr: the time from 10% Ic to 90% Ic.
td_off: the time from 90% Vge to 90% Ic.
tf: the time from 90% Ic to 10% Ic.

2.3 The influence of IGBT gate resistance and driver output impedance

The gate resistance setting will significantly affect the switching delay time. Generally speaking, the larger the resistance, the longer the delay time. It is recommended to measure the delay time under the conditions of the dedicated gate resistance in the actual application. The relationship between typical switching time and gate resistance is shown in the figure below:

What is “dead time”?How to reduce the dead time of IGBT

Fig. 4 Relationship between switching time and Rg at 25°CFig. 5 Relationship between switching time and Rg at 125°C

What is “dead time”?How to reduce the dead time of IGBT

All tests are carried out with the FP40R12KT3 module, the gate voltage is -15V/+15V, the DC link voltage is 600V, and the switching current is a nominal current of 40A.

2.4 The influence of other parameters on the delay time

In addition to the gate resistance value, there are other parameters that have a significant impact on the delay time:
? Collector current
? Gate drive supply voltage

2.4.1 Activation delay time

In order to estimate this effect, a series of measurements must be taken. First study the relationship between the turn-on delay time and the current. The result is shown in the figure below:

What is “dead time”?How to reduce the dead time of IGBT

Figure 6 The relationship between the turn-on delay time and the switch current Ic

All tests use the FP40R12KT3 module, the DC link voltage is 600V, and the gate resistance is selected according to the data sheet value.

It can be seen from the above results that when the collector current Ic changes, the turn-on delay time remains almost unchanged. The turn-on delay time under the gate voltage of -15V/+15V is longer than that under the gate voltage of 0V/+15V. However, this change is small, and considering the additional safety margin, it can be ignored.

2.4.2 Turn-off delay time

The maximum turn-off delay time is the most important factor that should be considered when calculating the dead time. Because this value almost completely determines how long the dead time of the final calculation is. So we will study the delay time in detail.

To obtain the maximum turn-off delay time, the following issues must be considered:

1. What is the turn-on delay time generated by the IGBT device itself?
2. If the threshold voltage of the IGBT is the minimum value in the data sheet, what is the maximum turn-off delay time? (This value reflects the allowable error of Vth between modules)
3. How does the driver output level affect the switching time?
4. What is the effect of the bipolar transistor output level driver?

Considering the above variables, we tested the turn-off delay time in the laboratory using FP40R12KT3 and an ideal driver. The test conditions are Vdc=600V, Rg=27?. The test results are shown in the figure below:

What is “dead time”?How to reduce the dead time of IGBT

The relationship between the turn-off delay time and Ic at 25°C The relationship between the turn-off delay time and Ic at 25°C

What is “dead time”?How to reduce the dead time of IGBT

It can be seen from the test results that as the switch current Ic decreases, the turn-off delay time increases significantly. Therefore, simply calculating the dead time by selecting the gate drive resistance is not accurate enough. It is a better and more accurate method to measure the delay time under specific driving conditions, and then calculate the dead time based on the measured value. Normally, by measuring the delay time under the condition of 1% normal current, it is enough to calculate the required dead time.

There is also a problem that should be considered here, that is, when the gate drive voltage of 0V/+15V is used, the turn-off delay time will increase, and when the drive voltage of 0V/+15V is used, the effect of the driver output level on the switching time will be even greater. big. This means that when using 0V/+15V drive voltage, you need to pay special attention to the selection of the drive. In addition, the problem of increasing td_off when the collector current Ic is small also needs to be considered.

3. How to reduce the dead time
In order to correctly calculate the control dead time, the following driving conditions should be considered:
? What is the gate voltage applied to the IGBT?
? What is the selected gate resistance value?
? What is the output level of the driver?
Based on these conditions, the delay time can be tested, and then through the test results, use the formula (1) to calculate the control dead time. Since the dead time has a negative impact on the performance of the inverter, the dead time needs to be reduced to a minimum. The following methods can be used:
・Use a large enough driver to provide peak sink current to the IGBT gate.
・Use a negative voltage to accelerate the shutdown.
・It is best to choose a driver that transmits signals quickly. For example, a driver based on coreless transformer technology is better than a driver using traditional optocoupler technology.
・If you choose a 0V/15V drive voltage, you should consider using an independent Rgon/Ggoff resistor.

It can be seen from the measurement results shown in section 2.3 that Td_off has a strong correlation with the gate resistance value. If Rgoff decreases, td_off and dead time will decrease. Infineon recommends that when using a 0V/15V gate voltage, the Rgoff value should be reduced to 1/3 of the Rgon value. A circuit that uses independent Rgon and Rgoff is as follows:

What is “dead time”?How to reduce the dead time of IGBT

Recommended circuit when the gate voltage is 0V/15V

The value of R1 should satisfy the following relationship:
It can be seen from the formula that if R1 is to be positive, Rgon must be greater than 2Rgint. But in some modules, this requirement may not be met. In this case, R1 can be completely ignored.

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