Continue to Site

Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Paralleling MOSFETs to spread and lower the heat dissipation.

Status
Not open for further replies.

David_

Advanced Member level 2
Joined
Dec 6, 2013
Messages
573
Helped
8
Reputation
16
Reaction score
8
Trophy points
1,308
Location
Sweden
Activity points
12,217
Hello.

I am reading about paralleling MOSFET's and all the application notes is entirely focused on using MOSFETs to allow the circuit to pass higher currents, but I have a application in which the amount of current can be handled by a single device.

But I would want to use 2 MOSFET's instead of 1 to lower the heat dissipation and to split the heat source into two locations on the PCB (it is SMD devices), but I wonder if this use is viable?

The current to be passed may be as high as 40A and the RDSON of the MOSFET is approximately 1,43mΩ(I am not at all sure at what temperature the circuit will operate since my application is quite odd and the converter will not operate continously, but that 1,43mΩ is based on an assumed junction temperature of 100°C).

There are aspects of the circuit that makes it impossible for me to calculate the temperature but because the lower the temperature the better I am considering to use 2 such 1,43mΩ MOSFET's instead of 1 but I am not sure that this is a valid action to take.

The single MOSFET will dissipate(if operating the circuit at it's maximum) 2,22W and if we assume that the cost of another MOSFET isn't a problem then will I gain anything from using 2 instead of 1 MOSFET?

I think is is at least plausible that 1 MOSFET is sufficient to the point that another MOSFET is simply overkill but I would like to ask someone whom has any experience of this sort of things.

Regards
 

If both devices are still mounted on the same board, then the temperature rise of the board will not change significantly, since the board-to-ambient thermal RBA impedance has not changed. However, the temperature rise of the junctions relative to the board will decrease by up to one half since the overall junction to board temperature RJB is being reduced. The overall junction temperature rise will depend on how much of the total junction-to-ambient thermal impedance RJA is composed of RBA and RJB.
 
  • Like
Reactions: David_

    David_

    Points: 2
    Helpful Answer Positive Rating
Hi,


in ideal case when two MOSFETs are used.... each of them sees half the current and thus each of them dissipates only 1/4 of the power of a single MOSFET (R_ds_on dissipation).
Now you have two MOSFETs ... thus the overall dissipated power (heat) is 1/2 of the power dissipation with one single MOSFET.

Conclusion:
It will reduce the MOSFET temperature significantely and it will also reduce the overal board temperature.

***
Mind that this is true for R-ds-ON calculations.
This is not true for heat disspation caused with MOSFET internal body diodes.
And it is not true for switching loss in the MOSFETs.

Maybe you need a more detailed calculation of the dissipated power.

Klaus
 
  • Like
Reactions: David_

    David_

    Points: 2
    Helpful Answer Positive Rating
I have continued to read about MOSFETs and thermal design and it seems to me that any calculation that would actually tell me anything valid is way over my head, I would love to be able to simulate my board with some thermal simulation software but 1, I don't have one and can't afford to buy one, and 2, I am not sure I would be capable to collect all the necessary information needed for something like that.

Some documents give a perspective that says that I can't really find out anything without implementing a prototype, though I wasn't really looking for answer for at what temperature my circuit will run. I think I was simply trying to figure out if doubling the number of MOSFETs would be beneficial.

Assuming(no matter if it is true or not) that switching losses doesn't heat the MOSFETs enough to be considered I think that what Klaus writes make sense, after all the resistance of 2 identical MOSFETs is half of the resistance of 1 such MOSFET. If one would compare two situations in which situation 1(#1) has a single MOSFET with 2mΩ RDS(on) and situation 2(#2) has 2 MOSFETs with a combined RDS(on) of 2mΩ then I could imagine that mtwieg would be correct.

But in this situation it would be the case of halving the RDS(on) from approximately 1,43mΩ to 0,715mΩ, I don't think that this is really necessary for my circuit to work so I will let it be decided by the board layout. If I have space enough to use 2 parallel MOSFETs in my synchronous buck converter then I will use parallel MOSFETs because it might very well make other things easier due to a decrease in the temperature variations of the board.
 

Status
Not open for further replies.

Similar threads

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Back
Top