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Circuit for MosFets in parallel for extra current capacity.(Solved)
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spec:
Hi Jim-0000

Just read through your thread above- sounds like your project will be great fun.

You say that you have a load of FQP50N06 NMOSFETs and implied that one option would be to use those, so I did some quick calculations to see what would be needed:

Four FQP50N06 NMOSFETs in parallel would be needed so the nominal maximum current through each NMOSFET would be 25A.

The maximum RDSS of  FQP50N06 is 22mR, so at 25A this gives a static dissipation of 13.7W which  will be OK.

But you only get that RDSS with a gate drive of 10V or more (maximum limit = 25V), so I would advise a 12V gate drive.

That takes care of the static conditions in theory, but there are the physical conditions to attend to as well. Briefly, large heatsinks would be required and the wiring of the parallel MNMOSFETs is critical, especially the drain wires, which must have the same resistance to aid current sharing between the four NMOSFETs. 

Then there are the dynamic conditions to consider.

If you know all this, just skip it, but here goes.

MOSFETs do not switch on and off immediately, mainly because of the massive parasitic capacitors that they have, and this is especially the case with high-power MOSFETs, like the FQP50N06. As a consequence, while switching they dissipate a lot of power because they have a voltage across them and a current flowing from their drain to their source (power= I * V). For this reason, it is important to turn the NMOSFEts on and off as fast as possible, especially in pulse width modulation (PWM) applications where the MOSFETs would be continuously turning on and off.

The bottom line is that you need to hit the MOSFET gates with a relatively high voltage (12V) and have a large current available to charge and discharge the effective gate capacitances. In short, you would need a gate driver chip for each NMOSFET (in practice driving a MOSFET gate is quite complicated and a bit of wave shaping is required, but forget that for the present).

Just a word about power transistor characteristics on the data sheet. The first thing is that the power rating is a sales figure and has no use in the majority of designs and, to a lesser degree, the same goes for the maximum current figure. The reason for this is that other factors constrain the performance way before these two limits.

The characteristics that are vitally important though are [figures for FQP50N06]:

VDSmax [60V] (good)
VGSmax [+-25V] (good)
Vthmax (Voltage where the MOSFET just start s conducting current from its drain to source) [4V]
RDssmax(resistance between drain and source when the MOSFET is fully turned on by a high specified GS voltage) [22mR]
TJmax (maximum junction temperature) [175 deg C] (good)
ThRJCmax (thermal resistance from junction to case) [1.2degCW] (not bad)
Safe operating area (SOA) [see data sheet]
Gate Charge (capacitance) [41nC] (a lot!)

https://www.onsemi.com/pub/Collateral/FQP50N06-D.pdf
MarkF:

--- Quote from: Jim-0000 on December 30, 2018, 02:54:15 am ---Mark,
I found the Mouser link formidable to navigate. However, I did find this data sheet:  https://www.st.com/resource/en/datasheet/stp160n75f

However, there seems to be some confusion about the max current capabilities.
Also, I am confused about the nomenclature (identification of these devices). Even on the above data sheet, it is not clear which ones are being rated. TO-247 or STB160N75N3?
Screenshot attached.

Jim.

Edit: I did find my way into the Mouser site. My search lead me to the data sheet on SUG90090E. For all I know, it is an equivalent, or similar to the device number you suggested above Mark.
I surely does seem to be a candidate for my application. Price is ok too.

--- End quote ---

Here is the datasheet for the IRFP064N for reference.
The SUG90090E you mentioned looks like a good candidate too.  Although twice the cost.

Better yet, the IRFP044N @ $1.63US
spec:
Hi again Jim-0000,

It will be important for us to know what type of drive you are planning on using. The options are low side NMOSFET, high side NMOSFET, high side PMOSFET. Technically, low side NMOSFET is far superior, and high side NMOSFET is the most complicated, but is still reasonably practical with the appropriate gate driver chip and power supply lines.
Jim-0000:

--- Quote from: spec on December 30, 2018, 05:27:48 am ---...................................
The maximum RDSS of  FQP50N06 is 22mR, so at 25A this gives a static dissipation of 13.7W which  will be OK.  ...
--- End quote ---

What is RDSS? Pardon my ignorance.


--- Quote ---But you only get that RDSS with a gate drive of 10V or more (maximum limit = 25V), so I would advise a 12V gate drive.
....................................
--- End quote ---

The onboard main power in the model aircraft this is planned for is 14.4 volts. So, that is no problem. I could just wire a supply off the main LiPo battery supply.


--- Quote ---.......................... [figures for FQP50N06]:

VDSmax [60V] (good)
VGSmax [+-25V] (good)
Vthmax (Voltage where the MOSFET just start s conducting current from its drain to source) [4V]
RDssmax(resistance between drain and source when the MOSFET is fully turned on by a high specified GS voltage) [22mR]
TJmax (maximum junction temperature) [175 deg C] (good)
ThRJCmax (thermal resistance from junction to case) [1.2degCW] (not bad)
Safe operating area (SOA) [see data sheet]
Gate Charge (capacitance) [41nC] (a lot!)

https://www.onsemi.com/pub/Collateral/FQP50N06-D.pdf

--- End quote ---

Thanks for the figures, and all the other details. The size and configuration of the heatsink might be a limiting factor. I will have to think further.
Jim-0000:

--- Quote from: spec on December 30, 2018, 06:37:17 am ---Hi again Jim-0000,

It will be important for us to know what type of drive you are planning on using. The options are low side NMOSFET, high side NMOSFET, high side PMOSFET. Technically, low side NMOSFET is far superior, and high side NMOSFET is the most complicated, but is still reasonably practical with the appropriate gate driver chip and power supply lines.

--- End quote ---

Spec,
Thanks for your question. However, I have no idea about this..........sorry!
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