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Discrete IGBTs: Forward bias safety operating area (FBSOA)

Discrete IGBTs: Forward bias safety operating area (FBSOA)

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The FBSOA curves of discrete IGBT devices are not provided in the devices datasheet. 

For example, in the datasheet of the Infineon IGBT device IKW40N120CS7, the output characteristic curve of this IGBT device shows in Figure 1. When the Vge voltage is less than 15 V, the current Ic is more likely to tend to a certain saturation value. Therefore, the driver voltage Vge of the IGBT is recommended to be at least 15 V to avoid current saturation. This operating mode is also called the ""linear region"" of the IGBT.
When the IGBT operates in the linear region, power losses will rise sharply, and can even damage the IGBT; therefore, the IGBT must avoid operating in the linear region except for switching transients.



Figure 1: Output characteristic curve of IKW40N120CS7

The FBSOA of an IGBT device is the safe operating region when the IGBT channel is in the conduction state when Vge>Vgeth. Figure 2 shows the FBSOA of an IGBT device, which is a collection of various operating states of the IGBT. The boundary constraints on the FBSOA are described in detail in this AN (Discrete IGBT: Explanation of discrete IGBTs' datasheets).



Figure 2: The FBSOA of an IGBT device

Figure 3 shows a simplified diagram of the FBSOA. In particular, the AB stage specifies the maximum operating current when the IGBT is in saturated conduction state. Note that the IGBT has two operating zones, the linear zone and the saturation zone; after crossing the AB stage, the IGBT is in the linear zone, which is an operating state that needs to be avoided.

 Figure 3: The simplified diagram of FBSOA 

An IGBT is mainly used in power electronic converters, often working in the fast switching and complete conduction state. For linear region applications such as battery management systems, MOSFETs are commonly used as protection switches or charging and discharging switches, which requires the MOSFET to be working in the linear region, has a strong linear region ability, and is able to withstand high voltage and high current at the same time.
OptiMOS™ linear FETs that have wider SOA and lower on-state resistance than standard MOSFETs are recommended. These are well suited for applications that require high linear region capability.