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Confirming the Suitability of a Transistor in Actual Operation

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.

170525_si_01

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.

graf07_171207

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.

・The SOA is only based on a single pulse (in this example, three types of pulse widths are presented).
 If a transistor is driven by repetitive pulses (that is, continuous switching), it is necessary to check that all
 the pulses stay within the SOA, and also the average applied power (calculated using ⑥) is not greater
 than the rated power.
・The temperature condition is nearly always Ta=25℃.
・These are essentially reference values, and not guaranteed values.
・Results will differ depending on transistor mounting conditions and the like.

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.

graf08_171207

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

Key Points:

・The SOA (Safe Operating Area) is information used to confirm that a transistor is operating under safe conditions.

・In essence, the SOA indicates in a graph the safe area of operation with respect to the rated voltage and current and the allowable power dissipation (heat generation).

・The SOA conditions should be checked carefully, and differences with actual usage conditions should be considered, prior to device use.


Power Supply Design Technical Materials Free Download

Power Supply Design Technical Materials Free Download

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