Electronics > Beginners
Transistor VS MOSFET for linear power supply
Zero999:
You're right, VBE doesn't need to be reversed, just the base current.
As far as the SOA of BJTs vs MOSFETs is concerned, it's a pain to find BJTs which can dissipate a significant amount power at high DC voltages, but with MOSFETs, it's much easier. Take the FQL40N50 for example. Its SOA at DC goes all the way up to 500V at 9A*. Now see if you can find a BJT who's SOA extends to even 90mA at 500V. Note that I found that MOSFET fairly quickly and there are many others with similar SOA ratings.
*Of course it won't be able to do that in real life, unless it's cooled with liquid nitrogen.
http://www.mouser.com/ds/2/149/FQL40N50-112136.pdf
Kleinstein:
I would net get too thrilled about the SOA curve of the FQL40N50. The SOA curve does not show any sign of thermal instability effects. It also shows a transient thermal response curve. There are quite a few MOSFET data-sheets that just use the transient thermal response curve to calculate a kind of pseudo SOA curve that ignores possible thermal instability. So not all SOA curves shown are real.
So finding a MOSFET with a promising SOA is easy, the trouble is to know if the curve is real FB_SOA and not just a different way to show P_tot for different puls length form transient thermal response.
The FQL40N50 might still work - from a quick look I would consider it boarder-line. So maybe worth a try for less power, but still be prepared for a failure.
Still it's my feeling that above some 200 V it's getting difficult with BJTs.
Zero999:
--- Quote from: Kleinstein on December 06, 2018, 05:29:30 pm ---I would net get too thrilled about the SOA curve of the FQL40N50. The SOA curve does not show any sign of thermal instability effects. It also shows a transient thermal response curve. There are quite a few MOSFET data-sheets that just use the transient thermal response curve to calculate a kind of pseudo SOA curve that ignores possible thermal instability. So not all SOA curves shown are real.
So finding a MOSFET with a promising SOA is easy, the trouble is to know if the curve is real FB_SOA and not just a different way to show P_tot for different puls length form transient thermal response.
The FQL40N50 might still work - from a quick look I would consider it boarder-line. So maybe worth a try for less power, but still be prepared for a failure.
Still it's my feeling that above some 200 V it's getting difficult with BJTs.
--- End quote ---
I agree about the FQL40N50.
There's lot of information in the thread linked below on MOSFETs for linear HV PSUs. The general consensus is at high voltages, they're better than BJTs and the older types are more likely to be good for linear operation, than the newer ones. T3sl4co1l tested the IRL740 in linear mode at 300V, with good results.
https://www.eevblog.com/forum/projects/looking-for-yours-opinions!-hv-stabilized-power-supply/msg1355890/#msg1355890
Before we get carried away, the original poster didn't state the voltage. Below 50V or so, the SOA is a less of an issue.
Wolfgang:
--- 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.
not1xor1:
--- 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.
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