Q: Secure working area for IGBTs
In the datasheet of , you may see a Safe Operating Area (SOA), such as as in the figure below. What does this safe workspace mean?
Figure 1. Rohm's RGS30TSX2DHR secure workspace (Image source: ROHM)
The safe operating area (SOA) of an IGBT is the current and voltage conditions that enable the IGBT to operate without self-damage or degradation in performance. In fact, it is not only necessary to use the IGBT in a safe working area, but also to derate the area in which it is located. The safety working area is divided into Forward Bias Safe Operating Area (FBSOA) and Reverse Bias Safe Operating Area (RBSOA).
1 Forward offset safe work area
The forward bias safety operating area defines the available current and voltage conditions during IGBT turn-on.
Figure 2. RGS30TSX2DHR's positive bias safe workspace (Image source: ROHM)
The diagram above shows the positive bias safety workspace of RGS30TSX2DHR, which can be divided into 4 areas according to the specific situation, as described below:
(1) An area that is limited by the maximum rated current of the collector
(2) Areas limited by collector dissipation
(3) Areas limited by secondary breakdown (this area will vary depending on the device design)
(4) Areas limited by the maximum rated voltage of collector-emitter
2 Reverse bias safe work area
The reverse bias safety operating area defines the available current and voltage conditions during IGBT shutdown.
Figure 3. RGS30TSX2DHR Reverse bias safe workspace (Image source: ROHM)
The diagram above shows that the RGS30TSX2DHR reverse bias safe working area can be simply divided into 2 finite zones, as described below:
- The area that is limited by the maximum rated current value of the collector
- An area limited by the collector-emitter maximum voltage rating.
Please note that unexpected failures may occur when the VCE-IC is designed to operate outside of the safe operating area of the product itself. Therefore, when designing circuits, close attention must be paid to dissipation and other performance issues when determining the specific characteristics and circuit constants associated with breakdown tolerances. For example, the back-bias safe operating area has temperature characteristics (deterioration at high temperatures), and the operating trajectory of the VCE-IC varies depending on the gate resistance Rg and the gate voltage VGE.
Therefore, it is necessary to understand the operating environment and the minimum gate resistance value at turn-off before designing Rg and VGE.
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