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Author Topic: Mosfets High Current.  (Read 869 times)

The Old Fart

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Mosfets High Current.
« on: June 22, 2018, 08:10:41 PM »

I tried putting 2 IRF4905  P channel mosfets in parallel.
10k resistor source to gate.
npn opto isolator. gate to 0v.


Trying to switch on/off a 70A load. no pwm, just straight on/off.


Heatsink gets luke warm, then mosfets overload and fail.


Has anyone else managed to switch a high current using mosfets?


Need to switch positive side.










Brian60

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Re: Mosfets High Current.
« Reply #1 on: June 25, 2018, 02:03:16 PM »

I've never tried high current switching however are you using lipo's? I could be way off base here but, when switched they may be giving a sudden burst which is overloading the mosfets. Also do you have them attached to a heatsink of suitable size? and using thermal conductive adhesive to make the connection?


Failing that, can you change your circuitry to have your switching after the load and allow the N channel mosfets?



I feel your pain, P channel mosfets are expensive compared to their N channel counterparts!

The Old Fart

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Re: Mosfets High Current.
« Reply #2 on: June 25, 2018, 02:20:19 PM »

Hi Brian60,


Yes to lipo, but switched on at zero load, not until high current applied do they pop.
yes to paste, yes to heatsink,


No to using 'N. type, common 0 v rail through boat.


Mosfets are fully 'switched on' so theory is a very low resistance across the junction, < 0.01 ohm.

plastic

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Re: Mosfets High Current.
« Reply #3 on: June 25, 2018, 06:11:12 PM »

Are you sure the transistor is fully switched on? Under high currents, the supply voltage will collapse and you may end up in a balance where input voltage may cause it to oscillate at high frequency in its linear range so total power dissipation may be momentarily many times more than the device can dissipate - it may blow up so fast the heat never even gets to the outside of the case.You might want to beef up the wiring or replace the girlie TO220 packages with a proper switching block MOSFet.
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Brian60

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Re: Mosfets High Current.
« Reply #4 on: June 26, 2018, 07:43:46 AM »



No to using 'N. type, common 0 v rail through boat.



Same problem I had, however as mine was for many led circuits I was advised to drop the P channel mosfets and use an integrated circuit package which would do the low level switching I required. Absolutely of no help in your situation though.

C-3PO

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Re: Mosfets High Current.
« Reply #5 on: June 26, 2018, 09:25:12 AM »

Hi Dave

Any use - same device IRF4905

https://www.youtube.com/watch?v=AkwxrmDjZMY

see Julian's comments at the end of the video

and then maybe this can help

https://www.youtube.com/watch?v=ZZDdlAgZfvI

C-3PO
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malcolmfrary

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Re: Mosfets High Current.
« Reply #6 on: June 26, 2018, 10:44:27 PM »

What on earth needs 70A?
Having said that, and admitting that I have pretty much by-passed FETs, I did some googling and found this snippet -

If you do want to use multiple smaller mosfets then there is a little trick with the gate control that should be observed. The turn-on threshold voltage is a little different for each mosfet. If the gates are all shorted together, just like the sources and drains are, then the one with the earliest threshold will take the higher initial switching load.

So to keep the switching currents more evenly distributed it's advised to use a low value series resistor on each gate. Basically the same as what is done on a single mosfet but having a resistor on each individual mosfet gate.
from here - https://forums.parallax.com/discussion/146407/paralleling-two-mosfets-to-handle-more-current
It sounds a bit like the two MOSFETS are not sharing the load well - one wins and dies, leaving its mate to take the load that it can't carry.  The bit of theory that I took on way back suggested that a MOSFET under over current conditions should start to shut itself down, but that might mean that in practice, it just heats itself further and gets to the point where the heat is just too much inside the transistor, before the temperature makes it through to the heatsink.
Past experience with heat transfer paste says that the thinnest smear possible should be used.  While the stuff is better at conducting heat than air, it is not as good as metal to metal.  Its purpose is just to leave as much metal in contact as possible with no air involved.  And again from past observation, two heat generating devices on a common heat sink often results in one device acting as a heater for the other, and ends badly since the heatsink, rather than dissipating the heat, redirects it.
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The Old Fart

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Re: Mosfets High Current.
« Reply #7 on: June 27, 2018, 03:09:13 PM »

Hi Guys, thanks for reply's.


The high current is required to feed 2 ESC  that do not have an on/off lead.


The lipo-battery can handle surges of 160A, so more than enough reserve.


The gates are pulled low by a NPN transistor in a opto-isolator.


I will try separating the gates and switch on together.


Dave

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Re: Mosfets High Current.
« Reply #8 on: June 27, 2018, 03:13:55 PM »


https://www.youtube.com/watch?v=AkwxrmDjZMY

see Julian's comments at the end of the video
and then maybe this can help
https://www.youtube.com/watch?v=ZZDdlAgZfvI

C-3PO

( I'm a big Julian fan! Great videos & tutorials!  )

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The Old Fart

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Re: Mosfets High Current.
« Reply #9 on: November 21, 2018, 08:32:03 PM »

Wonder how much current these will really take.
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