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Why MOSFET is used as a power transistor on the aim-tti ql355tp

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exe:

--- 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.

--- End quote ---

I believe modern 2n3055 equivalents are not as bad at all. Definitely more than enough for a linear power supply, which needs a closed-loop bandwidth of some tens (hundreds at most) of kHz. I've built a prototype of a linear power supply using sziklai with 2sta1943 and db139 (afaik) and it was fast (blackdog's power supply). In my measurements, 2sta1943 and tip3055 show pretty much identical performance (measured at 2A collector current), with the later slightly better. That's why I conclude tip3055 is a viable choice.


--- Quote from: namster on July 16, 2020, 07:13:17 pm ---so the mosfet has an advantage for the precision of current measurement throught shunt , the QL355 have a resolution of 1mA ,such prĂ©cision can't be reached with a BJT , in such Sziklai  or darlignton configuration a base current is summed to load current ! , What do you think ?

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Depends where to put shunt and where's the load. I don't see problems if shunt is in series with the load. With a mosfet it's possible to have accurate current measurements even if the mosfet is in-between the load and the shunt because drain current is the same as source current. With bjt one have to put the load and the shunt somewhere after emitter.

David Hess:

--- Quote from: namster on July 16, 2020, 07:13:17 pm ---so the mosfet has an advantage for the precision of current measurement throught shunt , the QL355 have a resolution of 1mA ,such prĂ©cision can't be reached with a BJT , in such Sziklai  or darlignton configuration a base current is summed to load current ! , What do you think ?
--- End quote ---

Like exe says, put the current shunt on the output lead.  For increased AC stability reasons, it is advantageous to have resistance in series with the output capacitor and with some clever circuit design, the output current shunt can be this resistance.

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