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Equalising HEXFETs(IRFP240 )

2017-08-03 10:38  
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This article briefly describes Equalising HEXFETs (IRFP240). This principle is easy to understand, but also very practical. Depth understanding of circuit elements, can be more effective to grasp this principle. Do you know the circuit, for example, can understand and buy these components: IRFP240 , IRF9240.

When experimenting with audio output stages featuring multipleHEXFETs it quickly becomes apparent that the total power is not divided equally among the individual transistors. The reason for this lies in the wide part-to-part variations in gate-source voltage, which in the case of the IRFP240 (or IRFP9240) can be from 2 V to 4 V. Source resistors in the region of 0.22 Ω as commonly seen in amplifier circuits (see example circuit extract) help to counteract this, but usually not to a sufficient extent. One possible solution to this problem is to ‘select’ the transistors used so that their gate-source voltages match as closely as possible.

Figure:1 Circuit diagram

Figure 1 Circuit diagram

For building prototypes or very short production runs this is feasible, but requires additional manual effort in testing the components, and, of course, more transistors must be ordered than will ?nally be used. The circuit idea shown here allows differences in gate-source voltage between pairs of transistors to be compensated for by the addition of trimmer potentiometers: the idea has been tested in simulation using Simetrix. The second circuit extract shows the required changes.



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