output Mosfet for Aleph2

sorry, one little question related to the first posting in the thread: whats against IRFP150? I just build yesterday the light bulb ZEN, and the IRFP150A was the only MOSFET i could get here fast and cheap (3 Euro). It´s 100V and i build a 85V DC 3,7 A powersupply with a 300W 220V bulb.
In wich points a IRFP240, 140,040,044 etc. (i´m not able to get here) are better? And whats about IRFP250 or IRFPC50 ? Thanks for answering this.
 

WayneS

Member
2002-03-24 11:44 pm
Dallas
Till,

Sorry, don't have time to go into everything on this. If I remember correctly, the 150 is a monster with huge current capability and rather "gross" properties, such as the gate capacitance, charge under the gate, etc. This means it's slow and needs a bunch more drive. You use a single 150 in a ZEN, but 6 '240s in an Aleph, for example.

The 240 has high voltage and relatively large current capabilities, and the parasitics that would slow it down are smaller, which means you don't need a honking huge driver stage and it's faster on transients. It is also readily available to US DIY'ers at Digikey - I don't know about foreign shipments on parts like that.

Get yourself the datasheets from the International Rectifier website, and just compare the parameters one by one until you see a transistor that suits your needs. For an Aleph 2, the 240 is your baby.
 
Thank you for answer. Shure, i studyed all this data sheets. Main differences seem to be IRFP150 :100V, Id 42A and IRFP240 : 200V and 20A. Ok, both i think is enough, more than we need. Output capacitance of 240 and 140 is about 1300pF, the 1900pF of the 150 is not much more (?) and to IRFP240 parallel is much more than one IRFP150. One different value i don´t know to judge about: What s forward tranconductance (Gfs)? its 6,9 S (Siemens?) at the 240 and the double (14 S) for the 150. what does this? I´m interessted in this diskussion about 240 (or 044 or so) versus the easy to get and cheap 150 type because i plan to buy some more for further projekts, and i searched the forum, others used the 150 too, but i found no comment about its quality versus the other types.
 
In Zen Variations-Part 1 Nelson Pass states:

"I have specified the IRFP240 MOSFET transistor in Fig.5, but a wide range of similar N channel devices work fine. The best performance is obtained with the IRF040 or IRFP044, but unfortunately they seem very difficult to get. An in-between alternative is the IRFP140."

"The lower voltage chips of the same type will give lower distortion, apparently due to their lesser sensitivity to voltage fluctuations on the Drain. They are generally the preferred parts".

Wouldn't IRFP140 be a better choice than IRFP240 then?