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How to ensure IGBT parallel current sharing by design


Due to the inconsistency of the parameters of the parallel IGBTs and the asymmetry of the circuit layout, it will inevitably lead to uneven current distribution of the device, and in severe cases, the device will fail or even damage the main circuit. Therefore, the focus of IGBT parallel connection is to consider how to ensure current sharing through design. At present, some existing IGBT parallel current sharing measures include: derating method, gate resistance matching method, emitter resistance feedback method, and external inductance balancing method.


1. Derating method


The derating method is to use the same batch of IGBTs with a positive temperature coefficient to be connected in parallel, and at the same time, apply a large derating application to the IGBT. This method is simple and easy to implement, but the power capacity of the device is wasted seriously. It is also difficult to guarantee good reliability in the application, because the relevant parameters of the device will change with the operating conditions.


2. Gate resistance matching method


There are two forms of grid resistance matching method: the same grid resistance method and the independent grid resistance method. Although this method is simple and easy to implement, it is not universal, and the matching effect will deteriorate with the change of switch parameters, and the di/dt The inconsistency of the stray inductance of the emitter will lead to a large circulating current between the emitters.


3. Emitter resistance feedback method


The emitter resistance feedback method is to connect a feedback resistor in series with the emitter of the IGBT. This method is simple and easy to implement, but the degree of feedback is contradictory with the current sharing effect. The feedback voltage of the IGBT is weak and cannot improve the current sharing of the parallel IGBTs.


4. External inductance balance method


The external inductance balance method is to add a small inductance of μH level to the midpoint of the bridge arm of the parallel IGBT to reduce the uneven distribution of the dynamic current of the IGBT. The design is simple and the engineering can be realized. However, due to the difference in the parameters of the two parallel IGBT modules, the reverse recovery current of the freewheeling diodes during the reverse recovery process is inconsistent, which makes the IGBT turn on. The turn-on current during the reverse recovery phase is uneven.


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