Power BJT Safe Operating Area [SOA]

BJT Safe Operating Area - SOA

What is Safe Operating Area?
The safe operating area is defined as the maximum voltage and current limits within which the power semiconductor device can be operated without destructive failure.

As shown below, Generally the manufacturer's datasheet will provide a safe operating area(SOA) curve.

  • In the SOA graph, the Collector current (IC) is plotted against Collector-Emitter Voltage(VCE).
  • The curve locus represents the maximum limits at which the transistor can be operated.
  • Load lines that fall within the pulsed Forward-Bias SOA curve during turn-on are considered safe, provided that the device thermal limitations and SOA turn-on time are not exceeded.

While reading about the SOA of the Power BJT it is good to refresh about the Safe operating area of Power MOSFET and compare with the SOA of Power BJT. Please click here to know about the SOA of Power MOSFET.

Forward Biased Safe Operating Area [FBSOA]:

The following figure shows the forward biased SOA of BJT.
As shown the Safe Operating Area(SOA) is effectively increased for pulsed operation.
In power electronics applications, the transistors are used in pulsed mode operation( ie, at high frequencies) so the effective SOA is larger.
We can observe one major difference between Forward Biased SOA of Power MOSFET and FB-SOA of Power BJT. ie, Secondary Breakdown limit.

Reason for Secondary Breakdown:

  • In BJT, under high voltage stress, unequal current conduction will happen.
  • Due to this, hot spots will be developed at random places over the working area of the power BJT.
  • This effect causes the phenomenon of forward-biased secondary breakdown limit.
  • These hot spots increases the local current flow and consequently the transistor will fail due to thermal runaway.

Reverse Biased Safe Operating Area [RBSOA]:

The following figure shows the reverse biased SOA of BJT.
The RBSOA is larger than the FBSOA because of BVCBO at low collector currents(IC).

As shown in the RBSOA graph, for voltages below VCEO the safe operating area is independent of reverse bias voltage VEB and is only limited by the device collector current IC.
Above VCEO the collector current must be derated depending upon the applied reverse bias voltage(VEB).
Observe that higher the voltage VEB, larger the reverse biased safe operation area.

Read More:

Power Electronics Viva Questions with Answers 
Comparison of MOSFET with BJT 
Base Drive Circuits: Driving Power BJT 
Power Semiconductor Devices Classification

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