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Body diode stress and SOA in MOSFETs

Body diode stress and SOA in MOSFETs

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Why the body diode stress should not be judged using the SOA in MOSFETs?


Safe operating area (SOA) basically defines the maximum Ids under the given Vds for the respective period of operation, which ensures safe operation of the MOSFET in forward bias. SOA boundaries are defined by:

  • Rds_on limit
  • Id_pulse /Package limit
  • Maximum power limit
  • Thermal instability limit
  • Breakdown voltage limit

Figure 1   Safe operating area(SOA) curve for MOSFET


Note: See Linear Mode Operation and Safe Operating Diagram of Power-MOSFETs to understand more about SOA.

Body Dioides in MOSFETs


Rds_on is the on-state resistance and Id_pulse/ package limit defines the maximum current the package (or silicon chip whichever lower) can handle. Also, the breakdown voltage is the voltage at which reverse-biased body diode breaks down and the current starts to flow from drain to source during the off-state. All these limits are not related to the stress when body diode is in conduction. Therefore, SOA should not be referred to assess the stress on the body diode.

Parameters to be considered while defining the body diode operating limits under different operating modes:

  • Forward operation: When the body diode is conducting, the current limits are to be set based on current rating and the power dissipation limitations. Normally body diode current rating is identical to MOSFET drain current rating. However, the maximum steady state current through the body diode is specified based on the total power loss allowed for the device, which can be calculated with the typical diode characteristic plot on datasheet. Therefore, the diode current rating may differ from the channel rating because of the difference in the power losses in both modes of operation.
  • Reverse operation: During reverse recovery of the diode, the forward current reduces with the slope -dIf/dt which is controlled by external circuit and once the current is reduced to certain limit, the drain-source voltage (Vds) increases with the rate dV/dt. Higher the dIf/dt and dV/dt, more chances of diode damage. Therefore, both these dI/dt and dV/dt along with the chip temperature should be under check to have safe operation of the diode.


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Community Translation: MOSFET:SOAを使用してボディダイオードのストレスを判断しない理由– KBA236325