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FET's of same brand, but wont work!

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OiD

Fri Jul 31 2009, 07:54PM

Registered Member #2150 Joined: Tue Jun 02 2009, 08:33PM
Location:
Posts: 29

Hi all.
I dug up an old project, and I needed to buy 8 IRFZ44N FET's.
This isn't the first time this has happened, the new FET's wont work in the circuit.
I don't understand it;
Old: International Rectifier IRFZ44N (331G AC 36) and IRFZ 44N PHm (0449 C7 8434)
New: International rectifier IRFZ44N (514L 5T 34)

This is the third time I have bought new FET's and not one of them worked. A cording to the shops they tested them and they were ok.

The FET's are controlled by a PIC (5V) to control some LED strips (12V).

Could someone explain what could be happening?
Or a simple circuit to test the FET's?

Thanks!

Conundrum

Fri Jul 31 2009, 08:08PM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061

they could be counterfeit, this has happened to several people who publish articles in "Television" and one in EPE.

best bet is to check one to see if they behave as expected out of circuit, ideally on a Peak Atlas or other transistor analyser.
-A

OiD

Fri Jul 31 2009, 08:52PM

Registered Member #2150 Joined: Tue Jun 02 2009, 08:33PM
Location:
Posts: 29

Hi Conundrum, I buy the FET's at a shop in the city, I don't know what you mean by "EPE".

According to the data sheet the gate voltage can be +/- 20v from source.

Could a simple circuit test if the are working or not?
Common ground and applying +5v to gate?

Thanks. This is really driving me crazy. I had to put the project aside last year. All the parts for the project are from the same shop.

Conundrum

Fri Jul 31 2009, 09:36PM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4061

what sort, enhancement or depletion mode?

Most FETs begin to turn on around 2-5V gate voltage, a simple LED and resistor from drain to +V will work

Arcstarter

Fri Jul 31 2009, 09:39PM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

Could a simple circuit test if the are working or not?
Common ground and applying +5v to gate?

Yes.

First, hook the gate and the source to common ground, and +12 volts to an LED with a current limiting diode, and then connect that to the drain. The light should not come on. Now hook the +12 straight to the gate, as well as through the LED and resistor to the drain. Hook ground to the source. This time, the LED should turn on.

The MOSFET may need 12 volts on the gate, though as long as it is low frequency i cannot see this being too big of a problem.

MinorityCarrier

Sat Aug 01 2009, 12:40AM

Registered Member #2123 Joined: Sat May 16 2009, 03:10AM
Location: Bend, Oregon
Posts: 312

The threshold voltage, Vt, and the RDSon are two parameters you can check against the data sheet for the part. For Vt there is a minmum drain-source current spec for a specific gate-source voltage. RDSon is spec'd often at 25 deg. C, so don't run a lot of current through the device when checking that.

The AC parameters for running a light bar are not demanding, so I wouldn't focus on device dynamic characterstics.

There is a big problem with counterfeit power MOSFET parts spewing out of China, my company has had it's fair share of dealing with this.

OiD

Sat Aug 01 2009, 04:10AM

Registered Member #2150 Joined: Tue Jun 02 2009, 08:33PM
Location:
Posts: 29

what sort, enhancement or depletion mode?

Don't know, I can't find it in the data sheet. :(

First, hook the gate and the source to common ground, and +12 volts to an LED with a current limiting diode, and then connect that to the drain. The light should not come on. Now hook the +12 straight to the gate, as well as through the LED and resistor to the drain. Hook ground to the source. This time, the LED should turn on.

I've built a simple test circuit, my old FET's work, and a variety of other power FET's I have. The new ones do nothing :(

There is a big problem with counterfeit power MOSFET parts spewing out of China, my company has had it's fair share of dealing with this.

Ugh. Sounds like I've been getting counterfeits... but I guess the shop would get complaints, as they seem to sell and restock them.

Could they be dead by ESD?
Thanks all.

MinorityCarrier

Sat Aug 01 2009, 05:02AM

Registered Member #2123 Joined: Sat May 16 2009, 03:10AM
Location: Bend, Oregon
Posts: 312

Enhancement mode N-ch.

If the gate is not shorted or leaky to the source, probably not ESD damage. Judging by your circuit, with Vgs of 12V and source at Vrtn, kinda says that everything is open.

You can pull up the data sheet on International Rectifiers' Hexfet site

Maybe you should order it from International Rectifier, Newark or Digikey or someone you trust. Meanwhile I suggest you break open one of these suspect parts and see if there is even a die in there.

Mads Barnkob

Sat Aug 01 2009, 06:55AM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

OiD wrote ...

Don't know, I can't find it in the data sheet. :(

You can find almost any datasheet at alldatasheet.com

OiD

Mon Aug 03 2009, 11:02PM

Registered Member #2150 Joined: Tue Jun 02 2009, 08:33PM
Location:
Posts: 29

Thanks all, I took them back and have managed to scrounge up a heap of odd FET's.
The shop said they will look into it and tell me.

Thanks again!

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