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AN2123 Datasheet(PDF) 8 Page - STMicroelectronics |
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AN2123 Datasheet(HTML) 8 Page - STMicroelectronics |
8 / 15 page 2-Level turn-off AN2123 8/15 6 2-Level turn-off In the event of a short-circuit or overcurrent in the load, a large voltage overshoot can occur across the IGBT at turn-off and can exceed the IGBT breakdown voltage. By reducing the gate voltage before turn- off, the IGBT current is limited and the potential over-voltage is reduced. This technique is called 2-level turn-off. Both the level and duration of the intermediate off level are adjustable. The duration is set by an external resistor/capacitor in conjunction with the integrated voltage reference for accurate timing. The level can be easily set by an external Zener diode, and its value is chosen depending upon the IGBT’s characteristics. This 2-level turn-off sequence takes place at each cycle; it has no effect if the current doesn’t exceed the normal maximum rated value, but protects the IGBT in case of overcurrent (with a slight increase to conduction losses). The principle is shown on Figure 12. During the 2-level turn-off time, the OUT pin is controlled by a comparator between the actual OUT pin and an external reference voltage. When the voltage on OUT goes down as a result of the turn-off and reaches the reference threshold, then the OUT output is disabled and the IGBT gate is discharged no further. After the 2-level turn-off delay, the OUT output is enabled again to end the turn-off sequence. To keep the output signal width unchanged relative to the input signal, the turn-on is delayed by the same value than the 2-level turn-off delay (see Figure 13). The duration of the 2-level turn-off is set by the external Rd-Cd components, and is approximately given by the formula: Ta (in µs) = 0.7 * Rd (in kΩ) * Cd (in nF) Recommended values are Rd from 10kΩ to 20kΩ, and Cd from 100 pF to 470 pF, providing a range of delay from about 0.7 to 6.6 microseconds. Figure 12. Principle schematic for 2-level turn-off feature Control Block 120µA VREF VH 3 2,5V 4 CD LVOFF 7 6 OUT VL Lvoff off |
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