Electronics > Beginners
Operation in This Area is Limited by R DS(on)
Jwillis:
--- Quote from: T3sl4co1l on January 01, 2020, 12:32:19 am ---Linear operation is not safe to parallel, so source resistors or individual controllers are used in linear circuits.
Tim
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Is this the kind of circuit to control parallel Linear Mosfets. Since I need at least 40V I should have at least a 40V supply to each op amp .Is that right?
Circlotron:
No, a 10-15V supply for the opamps would be plenty. The drain of the mosfets with 40V or whatever on them is completely separate from the opamps. Just make sure you use an op amp that has an input common mode range that extends down to zero volts, assuming that the negative supply rail of the opamp is also connected to zero volts.
Jwillis:
--- Quote from: Circlotron on January 09, 2020, 04:12:54 am ---No, a 10-15V supply for the opamps would be plenty. The drain of the mosfets with 40V or whatever on them is completely separate from the opamps. Just make sure you use an op amp that has an input common mode range that extends down to zero volts, assuming that the negative supply rail of the opamp is also connected to zero volts.
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I have TL081 with CM voltage range of -12V to 15V and OPA604 with CM voltage range of ±13V .I was leaning towards the OPA604 because it won't require any offset.
http://www.ti.com/lit/ds/symlink/tl082.pdf
https://www.ti.com/lit/ds/symlink/opa604.pdf
Circlotron:
^^ Nup, no good. Gotta have input CM range down to ground. A humble LM358 or LM324 would probably be good enough.
pdf.datasheetcatalog.com/datasheet/stmicroelectronics/2163.pdf
tszaboo:
--- Quote from: magic on December 31, 2019, 09:04:12 am ---You have two MOSFET vendors who pretty much admit that their SOA ratings at DC are plain bullshit. International Rectifier even blew one of their parts at 16V / 4A, which was presumably well withing the part's documented dissipation and temperature limits.
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The DC rating is typically 10-100ms rating. Otherwise you get hotspotting. You gotta understand, that a modern FET is a lot of smaller FETs in parallel, sometimes hexagons shaped.
If you have a tiny FET which gets hotter, then the VGS of that cell will decrease, so that cell will conduct more current. And it gets even hotter, leading to thermal runaway. Same reason, why you cannot put two FETs in parallel, one gets hotter.
The SOA should be replaced with FBSOA, which is forward bias safe operating area. Most FETs dont specify this, some from IXYS or Microsemi do. Or de-rate the FET by a lot.
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