Confirmation that Operation is within the SOA (Safe Operating Area)

In this chapter, we explain methods and procedures for determining whether or not a selected transistor is appropriate in actual operation.

This time, we explain ③“ Confirmation that operation is within the SOA (safe operating area)” in the flowchart on the right.

• ① Measurement of the actual current and voltage waveforms
• ② Confirmation that absolute maximum ratings are satisfied

• ③ Confirmation that operation is within the SOA (safe operating area)

• ④ Confirmation that operation is within the SOA derated at the ambient operating temperature
• ⑤ Continuous pulses (switching operation)
• ⑥ Confirmation that average power consumption is within the rated power
• ⑦ Confirmation of the chip temperature

③Confirmation that Operation is within the SOA (Safe Operating Area)

The SOA (Safe Operating Area) is information used to check whether a transistor is operating under safe conditions. In essence, in a graph of the relationship between the drain current ID and the drain-source voltage VDSS, it is the safe area with respect to the rated voltage and current and the allowable power dissipation (heat generation), that is, the area in which reliability is not impaired and breakdown will not occur.

The following is an SOA graph of the MOSFET R6020ENZ used in this example, and is excerpted from the datasheet.

Using this graph, whether the operating state of a circuit is within the SOA can be judged. However, there are a number of conditions to be considered when using this data.

With these limitations, it may seem that transistors cannot be used under actual conditions. However, for example package thermal resistance is a known value, but thermal resistance in the mounted state depends on the mounting conditions. Further, the single-pulse condition is reasonable, because if the number of pulses is five or 100, the SOA will be imprecise. Hence it should be clear that the SOA must be indicated for conditions that have been clarified to some degree. In actuality, judgments are made using averaging and conversions, as well as the concept of derating, explained later.

About SOA Failure

When a transistor operates out of the SOA range, the possibility of failure occurs. Using normal operating conditions that are outside of the SOA is out of the question, but overvoltages and short-circuits, which do not occur during normal operation, can cause overcurrent to flow when the applied voltage is maintained, and this can cause momentary local heat generation that results in failure.

Moreover, inappropriate thermal design or a higher repetition frequency can lead to inadequate heat dissipation of a transistor chip, so that continuous heat generation occurs, and if the allowed temperature of the channel is exceeded, thermal runaway can result in a failure.

Next time, we will explain ④ Confirmation that operation is within the SOA derated at the ambient operating temperature.