Electronics > Projects, Designs, and Technical Stuff
Why MOSFET is used as a power transistor on the aim-tti ql355tp
David Hess:
--- Quote from: namster on July 17, 2020, 03:48:52 pm ---@David Hess
the AC performance tend to be complicated for me , i only know that for a stable regulation circuit the phase margin have to be large , do you have any book or lectures about opamp AC performance ?
--- End quote ---
Operational amplifiers have difficulty driving the large input capacitance of a power MOSFET while a similarly sized bipolar power transistor will be used with a smaller transistor to drive it.
Linear power supply and regulator design is very closely related to audio amplifier design so check out Designing Audio Power Amplifiers (2011) by Bob Cordell and Audio Power Amplifier Design Handbook by Douglas Self.
H713:
I still have a hard time figuring out why the 2N3055 is still as common as it is, given that it isn't all that cheap and has pretty poor performance.
FETs can have superior SOA to BJTs in some applications. At high voltages, BJTs become increasingly useless in this regard. If it weren't for their stupidly high price, lateral MOSFETs would make excellent series pass transistors since they're pretty well immune to secondary breakdown. Modern switching FETs do suffer from something similar to secondary breakdown, however, using high voltage FETs with a high on resistance can help get around this. In my testing, the FQA8N90C-F109 did not exhibit any secondary breakdown at 75V, and they seemed to fail right around the 280W mark. That said, I have found the DC SOA charts in switching FET datasheets to be less than trustworthy.
namster:
@David Hess
i would be happy to read them thank you
@H713
the 2n3055 was cited as an example only , i agree that for high voltage and current the mosfet is the right chose as long as we drive it correctly
David Hess:
--- Quote from: H713 on July 18, 2020, 10:25:11 am ---I still have a hard time figuring out why the 2N3055 is still as common as it is, given that it isn't all that cheap and has pretty poor performance.
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The 2N3055 part number has become the "junk bin" of power transistors which did not meet higher specifications. If you want a more consistent part, then something like the 2N3771 through 2N3773 is more appropriate but it will cost more. Tektronix had at least three difference "classes" of 2N3055 which they used in different applications, and that was 50 years ago!
--- Quote ---If it weren't for their stupidly high price, lateral MOSFETs would make excellent series pass transistors since they're pretty well immune to secondary breakdown.
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Lateral MOSFETs have great performance but it comes with lower current density than even vertical MOSFETs and that extra die area directly adds to their cost. I was not aware that they were even produced anymore except as part of DMOS integrated circuits. Maybe we can get the audio guys interested in them to drive up production and lower price.
--- Quote ---That said, I have found the DC SOA charts in switching FET datasheets to be less than trustworthy.
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I have had the same experience. If the datasheet for a vertical MOSFET does not explicitly show the thermal instability region, then I assume it is just as bad if not worse than the secondary breakdown of a bipolar transistor, which they often are.
H713:
Exicon still manufactures lateral MOSFETs, but they are costly. They can be purchased through Profusion in the UK. As far as I know, there is no US distributor. The only thing driving that market it audio- they work great in power amps.
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