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

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David Hess:

--- Quote from: namster on July 16, 2020, 06:24:27 pm ---but is there any advantage in performance for using a more expensive transistor ?  :-// , i think that  a BJT power transistor like 2n3055 have better value for money in linear application ?
--- End quote ---

A power MOSFET has a speed advantage and does not suffer from gain and bandwidth droop at high currents but usually a faster bipolar transistor is fast enough when closed loop frequency response is taken into account and even if the bipolar transistor is current derated to avoid Ft and hfe droop, it is still less expensive.  Check out the design of high performance audio power amplifiers for a discussion on these issues when comparing MOSFET and bipolar power output stages.

The insignificant gate current of a power MOSFET means current sensing can be done on the drain side however in the highest performance applications, it is better to do the current sending on the source or emitter side anyway.  Even if collector side current measurement is required, a smaller MOSFET or JFET can be used to drive a bipolar output transistor.

Hydron:
The PSU in question is not a cheap item - I doubt the difference in cost between BJT and MOSFET comes into the equation, so they will have just used their preferred solution.

namster:
@Duak
the 10ms can give an idea of DC SOA , the mosfet can easly work with 35V Vds and 3Amp output current
@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  ?
 

duak:
@namster, you probably know that MOSFETs can suffer from Electro-thermal Instability when operated in linear mode.  It is probably why the device manufacturer does not show the DC SOA.  Jay Diddy & I (plus others) have posted on this subject: https://www.eevblog.com/forum/projects/please-let-me-know-if-my-very-high-power-load-has-a-fatal-flaw/msg3075882/?topicseen#msg3075882

The chosen device may have the DC SOA you have extrapolated from the data sheet.  My experience with data sheets is that extrapolation can be risky.  If the device manufacturer does not test for a condition or even show a predicted characteristic then a conservative designer risks failure by expecting that characteristic.  Sometimes we accept the risk and then compensate and test to see if there is a problem.  I have done that and have had very few failures.

Others have pointed out the speed advantages of MOSFETs.  I have pointed out some possible problems and what appears to be a solution.  We do not know what the designer knows and exactly why the circuit is the way it is.

namster:
@duak
from my point of view the SOA does not present a problem because the performance of the mosfet performance are much higher then  the work required , the max current of this PSU is 3A the manufacturer used two of these mosfet with current balancing so in the worst case the safe operating area will never be reached, I drew the operating area in red, maybe I'm wrong




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