The common belief among many designers is that the EXB840 can provide overcurrent protection for IGBTs by itself, but this is actually a misconception. The EXB840 detects overcurrent by monitoring the voltage drop across the IGBT when it turns on. However, it only triggers if the current exceeds the IGBT's maximum rated current (Ic) while the collector-emitter voltage (VCE) remains below 7.5V. For currents exceeding Ic but with VCE below this threshold, the EXB840 does not respond. Its primary role is short-circuit protection, which involves shutting down the IGBT almost instantaneously upon detecting a short circuit. This function does not cover general overcurrent conditions, meaning the EXB840 isn’t truly designed for comprehensive overcurrent protection. Relying solely on the EXB840 for both short-circuit and overcurrent protection without additional measures can lead to IGBT damage. Hence, it’s crucial to address this properly in the design process.
When designing overcurrent protection for systems like ABFSD, several considerations are essential:
Firstly, overcurrent protection can be achieved using a LEM Hall sensor, which monitors the current flow. Upon detecting an overcurrent, the system activates a timing circuit. If the overcurrent persists after a set delay, the IGBT is softly disconnected, and an alarm is triggered. The delay time typically ranges from 5 to 15 seconds, and the current threshold can be set between 1 to 1.5 times the IGBT's rated current.
Secondly, short-circuit protection can be implemented either through the EXB840 or via a short-circuit signal generated by the LEM Hall sensor. The latter method offers quicker response times. When a short circuit occurs, the IGBT is immediately shut off, and the EXB840's input signal is blocked at a high level, triggering a short-circuit alert. To account for current spikes caused by factors like reverse recovery in freewheeling diodes, a slight delay can be introduced, but it shouldn’t be prolonged, as this could compromise the short-circuit protection.
Thirdly, both short-circuit and overcurrent protection mechanisms should operate over a certain duration to prevent continuous cycles of activation and deactivation, which could damage the IGBT. When using the EXB840 for short-circuit protection, there is no risk of oscillation. However, directly detecting the IGBT's current can sometimes lead to oscillatory behavior. Thus, a balanced approach is necessary to ensure reliable protection while minimizing risks to the IGBT.
In summary, while the EXB840 plays a critical role in short-circuit protection, it cannot handle all types of overcurrent scenarios on its own. Designers must integrate complementary overcurrent protection strategies to safeguard the IGBT effectively.
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