3rd generiation HEXFET vs. 5th generation HEXFET

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Fifth generation units are generally optimized for minimum R_DS_on and fast switching.

This means that they have typically have highly non-linear input capacitance and a lot of it.

In general I would say that 3rd generation units are better suited for audio work.

5th generation HEXFET


Actually the fifth generation HEXFET's that I have seen typically have less inherent capacitance than the previous generation HEXFET's. Just looking at the IRFP044N shows that its worst case input capacitance is very near the best case input capacitance for the IRFP044. In addition, the linearity of the input and output capacitances of the 044N is also better given a change of less than 1500 pF vs. the 044 with a change of slightly more than 1500 pF. The difference on the output side is even more significant.

If I were basing my decision strictly on capacitance characteristics I would choose the 044N. However, there are plenty of other factors to consider before making the choice of transistor for your circuit.

Fair winds,
Not so fast.

It depends on your application. CRSS is useful if you have voltage gain, but usually you are using such output devices in an output follower. I am assuming of course that you are talking about output devices. Look for devices that have relatively low input capacitance, which does not vary all that much (check out curves). The devices mentioned are both high capacitance devices, and the "N" usually denotes minor product change. I am using IRFP240. For reference, check the progression of 510, 520, 530, 540 and you will see what I am talking about.

Anything above 530 would be considered less useful for audio if we are to generalize.

For input devices IRF 610 seem quite useful. Nice and high (!) Rds on means that it very likely is good capacitance wise. This time it fits.

If you are looking for ultimate devices, pick up a copy of MJ Audio Musings (in Japanese) and see which devices are being sold for clues.

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