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Old 23rd November 2003, 04:20 PM   #1
Bensen is offline Bensen  Belgium
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Default What's wrong with those MOSFET's

Hi all,

I,ve just build two Crescendo amps using the 2sk1530 and 2sj201 Toshiba FET's as output devices. Now I was planning to parallel the output FET's.
So, I started to measure the VGS on the normal way to match output devices. I used a resistor of 37ohm and a supply voltage of 15V.
Normally the VGS should be around 4V if I'm not mistaking. But I measured on both the 2sj201 devices, approx 1.85Vdc?? BTW, after one hour it still was 1.85V (0.35A).

Those FET's just came out off perfectly working Crescendo amps.

So, wat the hell is wrong with the FET's?
Do I have to buy new one's?

Cheers

Ben
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Old 23rd November 2003, 04:36 PM   #2
Netlist is offline Netlist  Belgium
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Could this help you?: http://doc.semicon.toshiba.co.jp/pdf...23/e001264.pdf
http://doc.semicon.toshiba.co.jp/pdf...23/e001355.pdf

Have a look at the state of the art mosfet matching devive from Nelson Pass too:
http://www.passdiy.com/pdf/mos.pdf

/Hugo
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Old 23rd November 2003, 04:42 PM   #3
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Default Re: What's wrong with those MOSFET's

Quote:
Originally posted by Bensen
[snip]
So, I started to measure the VGS on the normal way to match output devices. I used a resistor of 37ohm and a supply voltage of 15V.
Normally the VGS should be around 4V if I'm not mistaking. [snip]Ben
Maybe you are mistaken? Maybe it is normal that they carry a few 100mA with a Vgs of less than 2V? Did you check?

Jan Didden
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Old 23rd November 2003, 04:53 PM   #4
Rudy is offline Rudy  Belgium
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its very normal for these devices, they are very very linear
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Old 23rd November 2003, 04:56 PM   #5
UrSv is offline UrSv  Sweden
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Vgs of 4 V is normal for IRFxxx FETs bit for these Toshibas it is 0.8 to 2.8. Everything is just fine and as expected.

http://www.schuro.de/Daten/Japanhalbleiter/2SJ201.pdf
http://www.schuro.de/Daten/Japanhalbleiter/2SK1530.pdf
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Old 23rd November 2003, 05:00 PM   #6
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Rudy, Usrv,

So several people took the trouble to look it up in the datasheets, which Mr Ben apparently is too lazy for. I wouldn't have given him the answer on a platter.
Another candidate for my kill-file. Parasite.

Jan Didden
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Old 23rd November 2003, 05:01 PM   #7
Bensen is offline Bensen  Belgium
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Thanks guys,

I think I've found it. When I take a look at the graphs in the datasheet off the 2sj201 I found the attached graph. But I'm not shure if this graph can be used in this case.
But, anyway...

I'm glad the FET's are OK.

One more question. Am I wright when I say that it's best to match the FET's at a current the same as the quiescent current used in the amp?

Ben
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Old 23rd November 2003, 05:04 PM   #8
Netlist is offline Netlist  Belgium
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Quote:
Originally posted by janneman
I wouldn't have given him the answer on a platter.
Another candidate for my kill-file. Parasite.
I assume he didn't think of looking it up. Probably happens to everyone now and then.

/Hugo
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Old 23rd November 2003, 05:06 PM   #9
Bensen is offline Bensen  Belgium
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Janneman,

I dit take a look at the datasheets. But I never printed out more than the first page off it. All those graphs are a little strange for me.
I'm just out off shool, and I studied Electromechanics and not Electronics. So...

Ciao

Ben
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Old 23rd November 2003, 05:10 PM   #10
UrSv is offline UrSv  Sweden
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janneman, it does take a little bit of experience with components to know that "all FETs are not created equal" as the Bible says. Until some familiarity is ganined then some help is not bad. I just happened to know them fairly well since I have a box full and Bensen already contacted me about buying some. Secondly I think that on a forum where the Pass products come in huge numbers and all of them use the same type FETs as well as people listing their Vgs values almost every day it may very well be an easy misconception to expect 4 V everywhere...
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