Si Power Device|Basic



In this chapter, we have explained methods and procedures for determining whether or not a selected transistor is appropriate in actual operation. This final article ends with “Summary”.

The flowchart on the right and the list of procedures below were followed to check whether selected transistors are appropriate for the actual operating conditions, and to confirm operation under conditions for which sufficient reliability and safety are secured.


Below, key points indicated for each of the procedures in the list are summarized.

Measurement of the actual current and voltage waveforms

Key Points:

・In prototyping, it is essential to determine whether a selected transistor can be used in actual operation.

・For purposes of confirmation, the voltages and currents handled by the transistor are measured.

Confirmation that absolute maximum ratings are satisfied

Key Points:

・Correctly understand the definition and purpose of absolute maximum ratings, and learn to judge whether a component can be used.

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

Key Points:

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

・In essence, in a graph of the relationship between ID and VDSS, the SOA indicates 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.

Confirmation that Operation is within the SOA Derated at the Ambient Operating Temperature

Key Points:

・An SOA graph is for data at Ta=25°C, and so the SOA must be derated according to the temperature at which the transistor will actually be used.

・As the derating rate, the derating rate for allowable power dissipation is used.

⑤Continuous pulses (switching operation)

Confirmation that Average Power Consumption is within the Rated Power

Key Points:

・In the case of continuous-pulse (switching) operation, the average power consumption is determined, and it is confirmed that allowable power dissipation is within the rated value.

・A judgment will in the end depend on whether Tj exceeds the absolute maximum rating.

Confirmation of the Chip Temperature

Key Points:

・It should be confirmed that Tj ultimately does not exceed the absolute maximum rating.

・Tj is the sum of Ta or Tc and the heat generation (the product of the thermal resistance and the power consumption).

This concludes the discussion of “Confirmation of Suitability in Actual Operation and Preparations”. Executing each of the separate procedures is rather troublesome, but it should be understood that these processes are an unavoidable part of circuit design.

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Downloadable materials, including lecture materials from ROHM-sponsored seminars and a selection guide for DC-DC converters, are now available.