Minimizing IGBT dead time
Verification of dead time calculation value
With the above equation, the required dead time can now be calculated based on the measured values. Using the calculated dead time, a worst-case measurement is needed to verify whether the calculated dead time is sufficient. From the measurements, it can be found that the turn-off delay time increases with increasing temperature. For this reason, it is better to test at low and high temperature conditions. The schematic diagram of the test example is shown below:
Figure 1. Test schematic for verifying the calculated dead time value
The lower IGBT needs to be turned on and off, and the upper IGBT is the same. The time interval between the two drive pulses needs to be adjusted to the calculated value of the dead time according to the actual situation. In this way, the negative current of the DC loop can be measured, and if the dead time is sufficient, no shoot-through current should be observed. Since there is no current flowing through both IGBTs at the same time, the test given above represents the worst case required for the dead time calculation. From the discussion of the turn-off delay time, it can be seen that the dead time increases with the decrease of the collector current. Therefore, when there is no current flowing, the turn-off delay time should be the maximum, which leads to the need for the maximum dead time. If there is no shoot-through current at zero collector current, then the selected dead time is sufficient.
2. How to reduce the dead time In order to correctly calculate the control dead time, the following drive conditions should be considered: 1. What is the gate drive voltage applied to the IGBT? 2. What is the gate drive resistor value? 3. What type of output level is the driver? Based on these conditions, tests can be performed, and then the control dead time can be calculated using the formula based on the test results. 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: 1. Use a driver large enough to provide peak source and sink current to the IGBT gate. 2. Use a negative voltage to accelerate the shutdown. 3. 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. 4. If a 0V/15V drive voltage is selected, then consider using independent Rgon/Rgoff resistors. As described below: There is a strong correlation between Td_off and gate resistance. If Rgoff is reduced, then td_off and dead time will also be reduced. Infineon recommends that Rgoff be reduced to 1/3 of Rgon when using a 0V/15V drive voltage. A circuit using independent Rgon and Rgoff is shown below:
Figure 2 Recommended circuit for 0V/15V drive voltage
The value of R1 should satisfy the following relationship:
From equation (3), Rgon must be greater than 2Rgint to obtain a positive R1. However, in some modules, this requirement cannot be met. In this case, R1 can be completely ignored. The diode should be a Schottky diode. Another very important point when using 0V/15V drive voltage is the parasitic turn-on effect. If we use the circuit we recommend, this problem can also be solved.
