IGBT Reverse Recovery Time
The reverse recovery phenomenon of the IGBT module refers to some specific physical phenomena and changes in electrical characteristics that occur when the integrated freewheeling diode (FWD) inside the IGBT changes from the forward conduction state to the reverse cutoff state when the IGBT is turned off. 1. Phenomenon manifestation 1. Reverse current peak When the IGBT is turned off, the freewheeling diode is subjected to reverse voltage. The large amount of minority charges originally stored in the diode during forward conduction cannot disappear immediately. These charges form a reverse current under the action of the reverse electric field, causing the reverse current to rise rapidly to a peak value. 2. Reverse recovery time The time from the IGBT turning off to the reverse current dropping to a specified small value (such as 10% of the peak value) is called the reverse recovery time, which includes the time of the current rising stage and the time of the current falling stage. 3. Voltage overshoot During the reverse recovery process, due to the rapid change of the reverse current, an inductive effect will be generated in the circuit, resulting in a voltage overshoot phenomenon at both ends of the freewheeling diode, that is, the reverse voltage will exceed the power supply voltage, which may cause damage to the IGBT module and other circuit components. 4. Here are the reverse recovery energy consumption characteristics under two test conditions:
(1) Tj=25 degrees; (2) Tj=125 degrees. The current is IF,NOM (nominal current of the module).
2. Causes 1. Minority carrier storage effect When the freewheeling diode is forward-conducted, holes in the P region are injected into the N region, and electrons in the N region are injected into the P region. These injected minority carriers are stored in their respective regions. When the diode is subjected to reverse voltage, these stored minority carriers need to disappear by recombination or drift, thus forming a reverse recovery current. 2. PN junction capacitance effect The PN junction of the freewheeling diode has a junction capacitance. During the reverse recovery process, the junction capacitance needs to be charged and discharged, which also affects the change of reverse current and reverse recovery time. 3. Effect of reverse recovery phenomenon 1. Increase switching loss During the reverse recovery process, the change of current and voltage will cause additional energy loss, increase the switching loss of the IGBT module, reduce the efficiency of the circuit, and also increase the heating of the module. 2. Limiting the switching frequency The existence of reverse recovery time limits the switching frequency of the IGBT module. If the switching frequency is too high, the next turn-on will be performed before the reverse recovery is completed, which will cause the reverse recovery current to be superimposed on the forward current, further increasing the loss and even damaging the IGBT module. 3. Generate electromagnetic interference
The voltage overshoot and current mutation generated during the reverse recovery process will generate high-frequency electromagnetic noise, generate electromagnetic interference to surrounding electronic equipment, and affect the normal operation of other circuits. IV. Improvement measures 1. Optimize the diode structure. Use advanced semiconductor processes, such as using a buffer layer structure, optimizing the doping concentration of the P region and the N region, etc., to reduce minority carrier storage, thereby shortening the reverse recovery time and reducing the reverse recovery current. 2. Select suitable materials. Use new semiconductor materials, such as silicon carbide (SiC). SiC diodes have shorter reverse recovery time and lower reverse recovery current than traditional silicon diodes, which can effectively improve the reverse recovery phenomenon. 3. Circuit design optimization. Add appropriate absorption circuits to the circuit, such as RC absorption circuits, RCD absorption circuits, etc., to absorb the energy generated during the reverse recovery process, suppress voltage overshoots and current mutations, and reduce the impact of reverse recovery on the circuit.
