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Old 27th November 2004, 04:55 PM   #1
andey is offline andey  Germany
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Question Replacement mosfets for car amp

Hello out there from Germany,

i blew up my JBL GTO 6000 car amp last week. A heavy box slit into the acrylic window, pushed it in (not broken) and all 10 MOSFETS from the PSU blew.

I still don't know why, the PCB did not touch the case bottom metal.

Mosfets were Fairchild FQP50n06, which are hardly to get here (20$ one!!!)

A TL494 PWM controller is steering 4 small transistors that switch the gates of 4 Mosfets for the normal channels of the amp and 6 mosfets that supply the sub amp. 2 transformers, a bigger one at the 6 and a smaller one at the 4 mosfets.

I replaced TL494, output transistors, mosfet gate resistors and added 4 bigger mosfets just for testing. it still works.

now my question to the electronic skilled:

can i replace these original MOSFETS:
http://www.fairchildsemi.com/ds/FQ/FQP50N06.pdf

with these i can easily get for 60 cent:
http://www.irf.com/product-info/data...ta/irfz48n.pdf


the data seems to fit except the "Avalanche Current" which is 50A on the original, and only 32A on the replacement device, although the replacement's Ids current is higher.

What do you think?

THANKS!!
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Old 27th November 2004, 05:21 PM   #2
sss is offline sss  Israel
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it should work , check also on and off times of the mosfets , u can use only 2 "new" mosfets and see if its working under small load .if its ok then u can put the others in .
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Old 27th November 2004, 05:35 PM   #3
djQUAN is offline djQUAN  Philippines
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the higher Ids means higher dissipation on the mosfets. you might not be able to push the amp as hard as you did with the original mosfets. basically, they would work though not as reliable as before.
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Old 27th November 2004, 05:36 PM   #4
andey is offline andey  Germany
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Hi there,

the IRFZ is switching faster than the original FQP.

I read in another thread, that as higher the number of an IRF Transistor is, as more unsuitable it is for audio applications.
Why?
Should i go for the IRFZ46N with less current?

I knew about only one mosfet per polarity is needed per transformer. Saying i added 4 test mosfets, this was 2 for each PSU.

Can you imagine why they blew, and why _all_ of them blew as they are in 2 separate PSUs? Could a failure of the TL494 have caused this?

thanks!
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Old 27th November 2004, 05:39 PM   #5
djQUAN is offline djQUAN  Philippines
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Quote:
I read in another thread, that as higher the number of an IRF Transistor is, as more unsuitable it is for audio applications.
it doesn't matter as the mosfets are used in the switching PSU as they are either completely on or completely off.

audio applications means that the mosfets are run in a linear region.
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Old 27th November 2004, 05:40 PM   #6
sss is offline sss  Israel
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if u got a scope u chould check the voltage at the output of the driver transistors
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Old 27th November 2004, 06:25 PM   #7
andey is offline andey  Germany
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mh, no editing allowed.

a question that i forgot: Is the Gate resistor ok with 22 Ohms?

See the attached pic for gate circuit details

:edit: picture removed at member's request
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Old 27th November 2004, 06:25 PM   #8
andey is offline andey  Germany
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djQuan - i could also get IRFZ 44 or 46, they have 49 / 53 Amperes. Should i take one of these?
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Old 27th November 2004, 06:37 PM   #9
sss is offline sss  Israel
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i see u got no driver transistors , the pnp transistor is there to forse the mosfet to shot down only . if u got 10 mosfets then u need a buffer between the pwm chip and the power transistors
anyways
imho 22ohm is too much ,a bit less then 10ohm is more common
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Old 27th November 2004, 06:53 PM   #10
andey is offline andey  Germany
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the fact i am new and under moderation totally scrambles the order of my posts.


the question to djQuan was referring to
Quote:
the higher Ids means higher dissipation on the mosfets. you might not be able to push the amp as hard as you did with the original mosfets

sorry, i am really having a problem with no editing. I always get sth in mind i forgot to post.
So, in addition to the post with the picture:
I replaced the 1266 transistors with BC640 ( http://www.semiconductors.philips.co..._638_640_5.pdf )

can that stay that way? are these ok for the job? 1266s are not available here.

thanks so much for your fast responses.
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