Due to the overcurrent protection function of the EXB840, many designers use the EXB840 to over-current protect the IGBT when designing the circuit. This is a misunderstanding. The EXB840 determines whether it is overcurrent by detecting the tube voltage drop when the IGBT is turned on. For overcurrents where the current exceeds Ic and VCE is less than the threshold of 7.5V, EXB840 does not react. The function of the overcurrent protection circuit is to turn off immediately after detecting a short circuit instead of immediately after detecting an overcurrent. It can be seen that this protection function only refers to short circuit protection, rather than the so-called overcurrent protection. Therefore, it is a misunderstanding that the integrated circuit such as EXB840 can perform short-circuit protection and over-current function without external overcurrent protection, which may cause damage to the IGBT and must be fully taken care of. When designing overcurrent protection for ABFSD, the following issues should be considered: (1) Overcurrent protection: Overcurrent detection is performed by the LEM Hall sensor. Once an overcurrent occurs, the timing circuit is turned on. If the overcurrent still exists after the delay, the IGBT is soft-off and an alarm signal is given. The delay time can be 5-15s, and the current threshold can be 1-1.5 times of the rated current of the IGBT. (2) Short circuit protection: Short circuit protection can be completed by EXB840, or it can be completed by the short circuit signal output by LEM Hall sensor, and the latter has faster reaction speed. While the IGBT is short-circuited to cut off the overcurrent, the EXB840 input signal is blocked at a high level and a short-circuit signal is given. If you want to consider the current spike caused by the reverse recovery of the freewheeling diode, etc., you can introduce a delay, but not too long, otherwise it will not be able to protect the short circuit. (3) The action of short circuit protection or overcurrent protection should last for a period of time, otherwise the IGBT may be damaged due to the continuous cycle of protection action - current drop - protection release - current rise. When the EXB840 is used for short-circuit protection, no oscillation occurs; however, when the IGBT current protection is directly detected, oscillation may occur.
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