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Due to a common misunderstanding, many designers mistakenly believe that the EXB840 can handle both overcurrent and short-circuit protection for IGBTs in their circuits. However, this is incorrect. The EXB840 detects overcurrent by monitoring the voltage drop across the IGBT during its on-state. If the current exceeds Ic but the voltage remains below 7.5V, the EXB840 won’t trigger. Its actual role is to shut down immediately upon detecting a short circuit, not just any overcurrent. Thus, this protection mechanism is specifically designed for short-circuit scenarios, not general overcurrent conditions. Relying solely on the EXB840 for such protection without additional external measures could lead to IGBT damage, which needs to be addressed carefully.
When designing overcurrent protection for systems like ABFSD, several considerations should be made:
(1) **Overcurrent Protection**: Utilize a LEM Hall sensor to detect overcurrents. Once triggered, a timing circuit activates. If the overcurrent persists beyond a set delay—typically between 5 to 15 seconds—and reaches a threshold of 1 to 1.5 times the IGBT’s rated current—an alarm signal is issued, and the IGBT is softly disconnected.
(2) **Short-Circuit Protection**: This can either be handled by the EXB840 itself or through a short-circuit signal from the LEM Hall sensor. The latter method reacts more swiftly. In the event of a short circuit, the IGBT cuts off the overcurrent while simultaneously blocking the EXB840's input signal and sending out a short-circuit alert. To account for current spikes due to phenomena like reverse recovery in freewheeling diodes, a slight delay can be introduced, though it shouldn't be excessively long to maintain effective protection.
(3) **Protection Duration**: Both short-circuit and overcurrent protection mechanisms should have a duration to avoid damaging the IGBT through repeated cycles of activation, deactivation, and reactivation. When using the EXB840 for short-circuit protection, oscillations typically don’t occur. However, if direct IGBT current detection is used, oscillations might arise.
In conclusion, integrating comprehensive protection solutions with external components is crucial to safeguarding your IGBTs effectively.
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