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Drain voltage and mosfet paralleling

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maicael

Member
If a MOSFET has a VDs of say 55v and I parallel two then VDs should increase to 110v right?
Just as current capability increases.
 

dougy83

Well-Known Member
No. VDS will still be 55V for said parallel MOSFETs
 

maicael

Member
No. VDS will still be 55V for said parallel MOSFETs
You mean for parallel connection like gate to gate , source to source , and drain to drain?
What am trying to ask is can I now connect a higher voltage say 60v since both MOSFETs act like one whole giant mosfet?
 

MikeMl

Well-Known Member
Most Helpful Member
Not easy. Simple answer: get a higher voltage NFET. Operating a FET within a couple of volts of its maximum rating is asking for trouble; I look for at least a 50% safety margin; 100% even better...
 

dougy83

Well-Known Member
You mean for parallel connection like gate to gate , source to source , and drain to drain?
Sure.
What am trying to ask is can I now connect a higher voltage say 60v since both MOSFETs act like one whole giant mosfet?
You would normally get a MOSFET with a higher VDS rating. While it is possible to use two MOSFETs in series, you'd require some crazy drivers to make sure that neither is ever subjected to a greater than its specified VGS, and that the upper MOSFET gate can be driven with a voltage up to 65V; it's cheaper, easier and more reliable to just use the appropriate MOSFET to start with.
 

maicael

Member
OK
Now I get it.
One more thing between irf3205 and irfp260n which is better.
Irf3205 has 55v VDS with 110A current and 8millohm RDS while 260n has 200v VDS with 50A drain and 44milliohm RDS
 

dougy83

Well-Known Member
OK
Now I get it.
One more thing between irf3205 and irfp260n which is better.
Irf3205 has 55v VDS with 110A current and 8millohm RDS while 260n has 200v VDS with 50A drain and 44milliohm RDS
Well, if you need more than 55 VDS, then the one with a 200 VDS spec. is better.
 

audioguru

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Most Helpful Member
You cannot connect transistors or Mosfets directly in parallel to increase the current handling because they will not be exactly the same. The more sensitive one will take most or all of the load current and be destroyed.
 

ronsimpson

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You cannot connect transistors or Mosfets directly in parallel to increase the current handling because they will not be exactly the same. The more sensitive one will take most or all of the load current and be destroyed.
Yes and no.

We are talking MOSFETS.
The IRF3205 has a typical resistance of 4.9m ohm and a max of 6.5m ohm. If a typical and a max part are parallel, yes they don't share equal but one will not take all the current!
Look at the graph below. If one part takes more current it will heat more. This heat will cause the RDSon to climb. This will shift the current more to the other part.
The parts will never share equally but they will share.
upload_2015-8-26_21-30-11.png
 

audioguru

Well-Known Member
Most Helpful Member
I wanted to point out that you cannot buy matched transistors or Mosfets. Although the OP was confusing voltage with current, he talked about doubling the load on the Mosfets.
If typical and max RDSon Mosfets are paralleled then the one with the lowest resistance takes more current, gets hotter and burns out first. Then the other Mosfet becomes overloaded and also burns out soon.
 

simonbramble

Active Member
Talking slightly off topic here, but you can parallel MOSFETs if you are driving them hard on (like in a switched mode power supply). If you are driving them in their Ohmic region (between fully off and fully on) then they do not share current very well. Differences in VGS threshold alone can ensure that one FET takes all the current while the other is nearly off. Also, the tempco is only positive when in the fully enhanced region enabling a level of thermal regulation, but not so in the Ohmic region.
 

AnalogKid

Well-Known Member
Most Helpful Member
If a typical and a max part are parallel, yes they don't share equal but one will not take all the current!

AND, the channel resistance of a MOSFET has a positive temperature coefficient, so as one FET carries more current than the other, it heats up more and it's Rds increases, decreasing its current. Not enough to balance the load sharing, but better than nothing and way better than bipolars.

ak
 
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