Electronics > Beginners
Transistor VS MOSFET for linear power supply
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Wolfgang:

--- Quote from: not1xor1 on December 07, 2018, 08:30:35 am ---
--- Quote from: Wolfgang on December 06, 2018, 07:53:37 pm ---
--- Quote from: Prithul0218 on December 06, 2018, 06:32:00 am ---What are the advantage/disadvantage of using MOSFET/transistor as the main pass transistor of a linear power supply?

I see most professionally made power supplies use transistor, why do they avoid MOSFET? Can I just replace the transistor of a linear power supply with a MOSFET with similar ratings?

--- End quote ---

The story is quite complicated. As stated in other posts, its a SOAR issue. At higher voltages, bipolar transistors and *switching* MOSFETs show a hot-spot effect called second breakdown. To be on the safe side, there are linear MOSFETs from IXYS that are specified for use with high dissipation and high voltage simultaneously. They make good PSU pass transistors.
A lot of manufacturers try to cut corners and use old switching MOSFETs at a fraction of their power rating. Its not guaranteed to work, and the consequences of a broken down pass transistor in a PSU can be quite catastrophic.

--- End quote ---

AFAIK secondary breakdown concerns only BJTs, mosfets problems are different (thermal runaway in linear mode).

I had saved a few tips from IRF, regarding switching mosfets use in linear region. Unfortunately the links are dead.
In any case they suggested to select high voltage mosfets, with high Rdson and a slowly rising (i.e. not-steep - please suggest a more appropriate English term) Idrain-vs-Vgs curve.

--- End quote ---

Sorry for oversimplifying. I do know that the "second breakdown Term" only applies to bipolars. For MOSFETs its the "Spirito effect".
The practical outcome is the same, however; at high voltages and currents a hot spot forms on the chip that finally melts a whole into the die and that was it for your transistor, be it a MOSFET or bipolar type.
not1xor1:

--- Quote from: Wolfgang on December 08, 2018, 07:03:57 pm ---
--- Quote from: not1xor1 on December 07, 2018, 08:30:35 am ---AFAIK secondary breakdown concerns only BJTs, mosfets problems are different (thermal runaway in linear mode).
 [...]

--- End quote ---

Sorry for oversimplifying. I do know that the "second breakdown Term" only applies to bipolars. For MOSFETs its the "Spirito effect".
The practical outcome is the same, however; at high voltages and currents a hot spot forms on the chip that finally melts a whole into the die and that was it for your transistor, be it a MOSFET or bipolar type.

--- End quote ---

Actually I've just found another document mentioning secondary breakdown regarding MOSFETs so I was probably wrong  :D :
Using Trench Power MOSFETs in Linear Mode
Wolfgang:
Thanks,

yeah, I've seen another paper using "second breakdown" also for MOSFETs.

http://www.ixyspower.com/images/tech-papers/Article_Linear_Power_MOSFETs_2007.pdf

has some info about MOSFETs that are hardened against this type of failure.
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