FET & Max. Amp for BL speed controller - RC Groups
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Sep 01, 2005, 09:43 PM
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FET & Max. Amp for BL speed controller


As my bl speed controller burnt out, I cannot find the same FET to replace. Howerve, I can find different FET in my local with slightly different spec.
So, I have a question about how to rate the max. amp for my speed that used new FET. Anyone counld give suggestion to approximate for this?
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Sep 03, 2005, 10:29 AM
-plancks only-
Franck's Avatar
If you want a decent answer to your question you should give more info on this. What controller, FET replacement etc etc.

I have had very little succes with replacing burned out fets, even when replacements were of the same type.


Sep 04, 2005, 09:22 PM
Registered User
Thanks for your suggestion.

The original pFET is SI4425 in Jeti 30Amp.
Rated for SI4425: Ron @ 4.5v 0.019Ohm, Id = 9.1Amp

But I have IRF7424
Rated for IRF7424: Ron @ 4.5v 0.022Ohm, Id = 8.8Amp

In the controller, it use 3 FET for each phase.

One more question, It will has any effect for different FET spec. in each phase.
Sep 05, 2005, 04:34 AM
Registered User
Better not mix up different types of FETs in parallel. Unless the characteristics are very similar, the current will not be distributed in all the parallel branches uniformly. The branch sinking more current might burn out.

If your FET has nearly the same threshold and Rds ratings, you could however replace all FETs of that type with the new one. You also have to make sure gate charge value is not very different either.

-- Rajesh
Sep 05, 2005, 06:26 AM
-plancks only-
Franck's Avatar

I have a strong feeling that the FET's on the controllers are matched, so replacing just one FET even if it is the same type will probably mess things up.

I had very little succes replacing single FETs on a number of controllers.
Sep 05, 2005, 09:32 AM
Registered User
Many Thanks for suggestions.

How I can estimate the max amp for new replancement FET?
Sep 05, 2005, 11:42 AM
Registered User
In ESC applications where heatsink is small or not present at all, the current is limited by the power dissipation, not the max current rating. The actual power dissipation will depend on the load type and rise time, but I have found a thumb rule of about 0.4W works well for SO8 package. Actually this might be conservative since my designs barely gets warm.

I assume your controller has a uC driving the mosfets working off a 5V supply as usual. Find the Vds vs Id graph for Vgs = 4 or 4.5V(covering for uC output stage losses). Your max practical Id will be the value where Vds*Id = 0.4W along the graph.

-- Rajesh
Sep 06, 2005, 11:30 PM
Registered User
Thanks Rajesh,


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