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'[EE] 2N7002 P-channel equivalent?'
2008\07\28@230855 by Vitaliy

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We're looking for a simple high-side switching FET. A 2N3906 would work, but
for this design it's preferable to leave out the base resistor. Is there a
transistor similar to the 2N7002, in terms of characteristics, and cost?

Digikey and Google spew out hundreds of results for "p-channel fet", however
the cost is considerably higher.

Thanks in advance for your help.

2008\07\29@060949 by Forrest Christian

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I often do this type of research on Jameco..  The parts list is smaller,
and typically only the most common parts.

Just a quick glance indicates that the BSS84 might work for your
application.

-forrest

Vitaliy wrote:
{Quote hidden}

2008\07\29@072353 by Spehro Pefhany

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At 11:09 PM 7/28/2008, you wrote:
>We're looking for a simple high-side switching FET. A 2N3906 would work, but
>for this design it's preferable to leave out the base resistor. Is there a
>transistor similar to the 2N7002, in terms of characteristics, and cost?
>
>Digikey and Google spew out hundreds of results for "p-channel fet", however
>the cost is considerably higher.
>
>Thanks in advance for your help.

Just keep limiting the search parameters until the number of choices
becomes reasonable. It doesn't take much to get down to a couple of pages,
from which you can see several choices between 0.05 and 0.10 in reel lots.

You do run the risk of missing something that way without doing
a bit more work (eg. I limited to SOT-23 packages, ignoring other
packages such as SC-70). The BSS84 will show up from several makers.

Go back and check more deeply and you'll find stuff like the more expensive,
but  rather nice NTA/NTE4151 which is rated for 1.8V drive and has a nice low
Rds(on) in an SC-75/89 package.

Of course an MMBT3906 or 4403 will be considerably cheaper in parts cost
in volume (less than half).

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
spam_OUTspeffTakeThisOuTspaminterlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com



2008\07\29@073408 by Apptech

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FDN338 is nice and perhaps about as cheap as GOOD P channel
FETs get.
YMMV


>> We're looking for a simple high-side switching FET. A
>> 2N3906 would work, but
>> for this design it's preferable to leave out the base
>> resistor. Is there a
>> transistor similar to the 2N7002, in terms of
>> characteristics, and cost?

2008\07\29@081839 by Spehro Pefhany

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At 06:52 AM 7/29/2008, you wrote:
>FDN338 is nice and perhaps about as cheap as GOOD P channel
>FETs get.
>YMMV

Indeed. For some values of "good". If you're looking for low gate charge,
it's not a good choice, nor are most of the other new low-Rds(on) MOSFETs.

A BSS84 is around 5 times better, and cheaper.

I've had to use multiple older MOSFETs in parallel to get an appropriately
low gate charge (actually, input capacitance).

Of course for most general purpose low frequency switching applications, the
newer types are wonderful, especially as you get up in current into the
hundreds of mA or A.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
.....speffKILLspamspam@spam@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com



2008\07\29@084033 by Apptech

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The eg BSS84 is an OK 'small signal' P Channel.
For not vastly more (below price only an example) you get
something like the FDN338 which is very very substantially
more capable.
The '84 is specd at Vgs = 2.5V but the 338 has far superior
Rdson at this gate voltage.
If low price is the utter driver then the '84 may be OK.



       Russell


BSS84
$US0.05/3000
search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=BSS84ZXTR-ND
50V, 130 mA SOT23
Nominal Vth < -2V.


FDN338
$US0.11/1000
20V, 1.6A
SSOT-3 (super SOT23)
0.5V Vds at Vgs=2.5Vat 2A.



2008\07\29@085526 by Alan B. Pearce

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>Of course for most general purpose low frequency switching applications,
>the newer types are wonderful, especially as you get up in current into
>the hundreds of mA or A.

Your saying that, this lunchtime I was perusing the latest issue of
Electronics Weekly, where there is a press release from STMicroelectronics
for the STV250N55F3 Mosfet.

250A, 1.5mohm PowerSO-10 package rated for 300W @ 25C.

Suggested uses high current electric traction applications such as fork lift
trucks, wheelchairs and electric bikes ...

Impressive looking package in the pictures.

Press release at
http://www.st.com/stonline/stappl/cms/press/news/year2008/p2308.htm

Data at
http://www.st.com/stonline/stappl/productcatalog/app?path=/comp/stcom/PcStComRPNTableView.onClickOfRPNLink&code=185326
(link from press release)

Hmm $2.50 ea @ 10k quantity. Tempted to get some samples, just to have the
chunky package kicking around  ...

No curves on the datasheet but at a Vgs threshold of 4V max, and Vgs = 10V
to get <2.2 mohm, it would have a viable on resistance run at logic level
input.

2008\07\29@093155 by Martin

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Rth junction->case 50C per watt!
At 50C per watt, even if you run it at the insane temperature of 175C,
you're only dissipating 3.5 watts. About 40 amps at Rds=2.2mOhm.
IXYS has much better data sheets showing you realistically what you can
expect from their parts, IMO. You pay for it though if you use IXYS.
-
Martin

Alan B. Pearce wrote:
{Quote hidden}

2008\07\29@093541 by Spehro Pefhany

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Quoting "Alan B. Pearce" <A.B.PearcespamKILLspamrl.ac.uk>:

{Quote hidden}

27A each for those tiny little leads? Not for long I suspect. The 75A test
rating is a bit more realistic at least for a short time (8A/lead).  
250A and 2.2m ohms is 138W! It also needs a pretty beefy gate driver  
(multiple A) to
get it to switch fast (100nC). Still, quite an incredible device-- and nice
high Vds (55V).

50 years ago you could have been burned as a witch for having technology
like that!


Best regards,
Spehro Pefhany
--
"it's the network..."                          "The Journey is the reward"
.....s...KILLspamspam.....interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com


2008\07\31@194005 by Vitaliy

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Apptech wrote:
> If low price is the utter driver then the '84 may be OK.

I think the BSS84 is what we're going to go with. Cost is really the only
important factor for this application (driving an LED). I thank everyone for
their feedback.

> BSS84
> $US0.05/3000
> search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=BSS84ZXTR-ND
> 50V, 130 mA SOT23
> Nominal Vth < -2V.

I couldn't find Vth in the Transistor Handbook, what is it?

Vitaliy

2008\07\31@210904 by Apptech

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> I think the BSS84 is what we're going to go with. Cost is
> really the only
> important factor for this application (driving an LED). I
> thank everyone for
> their feedback.
>
>> BSS84
>> $US0.05/3000
>> search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=BSS84ZXTR-ND
>> 50V, 130 mA SOT23
>> Nominal Vth < -2V.
>
> I couldn't find Vth in the Transistor Handbook, what is
> it?

Various names

Vgs-th or Vgs_th
Vth
V...

V gate to source threshold voltage.
= "Voltage at which FET 'just' conducts at some inadequate
current and a low transconductance compared to when fully
enhanced BUT an indication of how much gate drive you'll
need in real world applications"

   R


'[EE] 2N7002 P-channel equivalent?'
2008\08\14@181126 by William \Chops\ Westfield
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On Jul 31, 2008, at 4:36 PM, Vitaliy wrote:

> Cost is really the only important factor for this application  
> (driving an LED).

I've been wondering about using "tiny logic" for LED driving.  Price  
on something like a  NC7SZ14 is comparable to most small mosfets  
($0.06 in 1000, $0.13 in 100), input thresholds are nicely logic-
like, output drive is somewhat smaller but sufficient for LEDs...  
Probably abuse-able without current limiting resistors on the  
outputs, even.  Thoughts?

BillW

2008\08\14@192347 by Matt Pobursky

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On Thu, 14 Aug 2008 15:11:04 -0700, Chops\ wrote:
>
> On Jul 31, 2008, at 4:36 PM, Vitaliy wrote:
>
>> Cost is really the only important factor for this application (driving
>> an LED).
>>
>
> I've been wondering about using "tiny logic" for LED driving.  Price on
> something like a  NC7SZ14 is comparable to most small mosfets ($0.06 in
> 1000, $0.13 in 100), input thresholds are nicely logic- like, output
> drive is somewhat smaller but sufficient for LEDs... Probably abuse-able
> without current limiting resistors on the outputs, even.  Thoughts?

I use the TI "Microgate" logic series quite often for LED drive buffers and
logic level conversion. The LVC and AHC series are quite versatile. I
mostly use the single and dual gate packages.

The LVC series has 5V tolerant inputs for interface between 5V peripherals
and 3V chips that aren't 5V tolerant or just need a buffer. The SN74LVC1G17
Schmitt Buffer can source/sink 24mA at 3.3V Vcc, for instance. The LVC
series will run from Vcc 1.65V to 5.5V. The AHC series is similar with a
somewhat lower output drive capability (8 mA). Both series have Icc levels
around 10 uA so I use them in battery powered applications quite a bit as
well.

Never used them without output limiting resistors to drive LEDs (nor would
I. Frankly I've never understood the logic of that -- it's bad design no
matter how you look at it. I can think of better ways to save a sub-penny
in my designs).

Most parts are sub $0.10 in 1000's and available in SOT-23 or SC70
packages. Nice parts.

Matt Pobursky
Maximum Performance Systems

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