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I am looking for a drop-in replacement for an IRF540 MOSFET with a more robust transistor. I want to find a MOSFET with a larger, Safe Operating Area because it is the pass element in a linear 14-volt, 6-amp DC power supply. An op-amp voltage regulator will drive the MOSFET.

The raw rectifier voltage ranges from 23 volts with no load to 20 volts at full load. Under normal operating conditions with a 6 amp load, the PD should be roughly 40 to 50 watts continuous. The circuit does have a foldback current limiter of 3 amps, placing PD to 60 watts during a worse-case fault.

It has been long since I last designed and searched for MOSFETs. I know most MOSFETs are optimized for switching applications, and I need help locating MOSFETs optimized for linear applications with a large SOA. When I look at Mouser or Digi Key, I do not know what filters will help me identify a device. The worst-case numbers, I believe, are Vds = 20 volts and Id = 6 amps.

So, how do I search for or give me a model or two to consider, please?

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  • \$\begingroup\$ A 40-100V MOSFET with 100A would be a much better choice :-) IRF540 is rated 100V 22-28A, 145W power dissipation. What exactly you don't like here? \$\endgroup\$ Commented Jun 29 at 1:20
  • \$\begingroup\$ @Ale..chenski well, dynamics with a 100A transistor might be awful; maybe that's as simple as compensation changes, but something similar to the original would minimize changes. Dereck: thermal management includes much more than the device itself; what heatsinking is present, what operating temperature, allowable rise, etc.? \$\endgroup\$ Commented Jun 29 at 1:28
  • \$\begingroup\$ @TimWilliams, not necessarily, all depends on gate-whatever capacitances/charges and recovery times. And on a driver. Plenty of choices there. The "original design is a home-grown mock-up with plenty of questions. electronics.stackexchange.com/q/621987/117785 \$\endgroup\$ Commented Jun 29 at 1:41
  • \$\begingroup\$ How about this one to start, infineon.com/dgdl/Infineon-IPB048N15N5LF-DS-v02_00-EN.pdf ? \$\endgroup\$ Commented Jun 29 at 1:48
  • \$\begingroup\$ @Ale..chenski, thank you. It looks OK, but I'm not sure exactly what I'm looking for. The SOA looks larger; correct me if I am wrong. I can run roughly Id=15 amps @ Vds = 20 volts? With an Op Amp driving the MOSFET, I think low input gate capacitance is essential. I know the IRF540 works, but I see some failures when the power supply output is shorted and turned on for an extended time. So I am looking for a dop-in replacement. Again thank you. \$\endgroup\$
    – Dereck
    Commented Jun 29 at 2:12

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This is dangerously close to a shopping question, but in any case there are some MOSFETs specifically targeting linear operation, so I suggest looking for those.

Anecdotally, the SOA numbers for MOSFETs aimed at switching applications (most of them) are calculated rather than measured and may be 'less than conservative'.

IXYS (Littelfuse) has a couple series that are said to be so-optimised, for example this one:

![enter image description here

Note that the total gate charge is quite a bit higher than an IRF540 so I would expect you'd need to change your compensation.

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    \$\begingroup\$ FYI, all(?) the old classics are rated for linear use -- or were at one time(!), so use new versions at your own risk, but, they are likely to still work. Personally, I've tested a IRF740 to destruction, at voltage, to a fair margin better RthJC than the minimum given. IRF(P) "HEXFET" (mostly 3rd and 5th generation), and 2nd source equivalents, are most likely candidates. Later generations ("advanced" HEXFET, trench, etc.) tend to suffer from 2nd breakdown. AFAIK, ye olde planar haven't gone away -- they've just rebranded as "linear" types and priced at a premium! \$\endgroup\$ Commented Jun 29 at 5:25
  • \$\begingroup\$ I apologize if this appears to be a shopping question. It's not so much asking for a fish as teaching me how to fish. I am a retired electrical engineer, and discrete analog circuits are not my specialty; they are a hobby. Some 20 years ago, I designed a heavy-duty 12-volt 5 to 50-amp power supply, mostly for amateur amateur radio operators. Now I have some time and want to upgrade components. I upgraded the voltage reference, the op-amps, and the series pass elements. When I go to Mouser and search, I do not know what filter to use, to isolate linear optimized devices with a larger SOA. \$\endgroup\$
    – Dereck
    Commented Jun 29 at 17:48
  • \$\begingroup\$ @Dereck The parametric search functions at distributors can be very useful, but as always the most accurate and (usually) best categorized data will be found by going to individual manufacturers. For example, I was looking for high voltage (like >20kV) resistors the other day and Digikey does not list the voltage rating. You can infer that from value and power rating to some degree, but they've deemed it unimportant (which it is for 99% of their stock under leaded resistors) compared to, say, failure rate. Sometimes you just have to put the work in. \$\endgroup\$ Commented Jun 29 at 17:57

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