Searching \ for '[EE]: Transistor maximum current vs max power diss' in subject line. ()
Help us get a faster server
FAQ page: www.piclist.com/techref/power.htm?key=power
Search entire site for: 'Transistor maximum current vs max power diss'.

Exact match. Not showing close matches.
'[EE]: Transistor maximum current vs max power diss'
2006\09\21@224804 by

Can someone explain to me what defines the maximum current via a transistor?
Since Imax * Vce != Pmax...

Vitaliy wrote:
> Can someone explain to me what defines the maximum current via a transistor?
> Since Imax * Vce != Pmax...
>

Maximum current thru a transistor is when Vce is saturated (full
conducting).  While Pmax is achieved when it is half-conducting and
dissipating the power used as heat.

regards,

Reggie
http://www.microelektronics.com
Regulus Berdin wrote:
> Vitaliy wrote:
>> Can someone explain to me what defines the maximum current via a
>> transistor?
>> Since Imax * Vce != Pmax...
>
> Maximum current thru a transistor is when Vce is saturated (full
> conducting).  While Pmax is achieved when it is half-conducting and
> dissipating the power used as heat.

Unfortunately, this doesn't answer my question. If Pmax is what defines
Imax, why then when the transistor is saturated, Imax * Vce < Pmax ?

If the maximum power dissipation is not the limiting factor for the maximum
current in a saturated transistor, what is?

Generally, you cannot exceed any of them. When the transistor is in
saturation, the max current may not get to Pmax. When the transistor is
cut off, Vce is at max, but dissipation is very low. The real dissipation
appears when the transistor is not in saturation and there is considerable
current.
> Can someone explain to me what defines the maximum current via a
> transistor?
> Since Imax * Vce != Pmax...
>
> -
Harold Hallikainen wrote:
[top posting fixed]
>> Can someone explain to me what defines the maximum current via a
>> transistor?
>> Since Imax * Vce != Pmax...
>
> Generally, you cannot exceed any of them. When the transistor is in
> saturation, the max current may not get to Pmax. When the transistor is
> cut off, Vce is at max, but dissipation is very low. The real dissipation
> appears when the transistor is not in saturation and there is considerable
> current.

Perhaps I'm not phrasing the question correctly?

I'm not asking how to use a transistor. I understand that exceeding or even
approaching the maximum ratings is a no-no. I understand that when the
transistor is in saturation, the power dissipated (assuming Imax) is lower
than when it's operating in the linear region, etc, etc.

I'm just trying to understand why in a saturated transistor the product of
the C-E voltage and maximum allowed current through the transistor is
considerably less than the maximum dissipated power.

As one moves from TO92 to SOT23, the maximum dissipated power decreases (as
expected), but the maximum current stays constant.

Does anyone know why? <:-|

Vitaliy

> I'm just trying to understand why in a saturated transistor the product
> of the C-E voltage and maximum allowed current through the transistor
> is considerably less than the maximum dissipated power.

I'll try here.

The Power dissipation has little to do with the current and/or voltage
at saturation, or at fully off.

Let's assume you're driving a 1 Ohm Load (for ease of math).  Let's also
ignore things like the fact that a transistor is rarely fully on or
fully off.  I.E. assume a perfect transistor.  Let's also forget the
base current.  Let's also assume you are using 1 Volt (for ease of math).

If a transistor is fully on, the resistance across the transistor is
nearly zero.  Thus,  the current through the circuit is 1V across 1 Ohm
or 1 Amp.   The transistor is seeing 1 amp @ zero ohms, or zero volts.
Thus, the transistor is dissipating (P=EI) 1A*0V = 0 watts.   The load
is dissipating 1A*1V = 1W.

With the transistor fully off, the resistance across the transistor is
nearly infinite.  Thus, the curent through teh circuit is 1V across an
open circuit, or no current.  The transistor is seeing no current @
infinite ohms.   Becase the effective resistance of the load is only one
ohm, there is essentially zero volts across the load, and 1 V across the
transistor.   Still no amps * 1 Volt = no watts.

So if a perfect transistor is fully on, or fully off, it isn't
dissipating any heat.   A low on-reisistance FET is pretty close to this
ideal, and can switch some serious current and voltage without
dissipating much heat, as long as the FET doesn't spend much time switching.

Between fully on and fully off is where you dissipate heat.  Using that
same example, let's assume the transistor is biased such that it is
effectively 1 Ohm across the CE junction.   At this stage, there would
be 0.5 Volt on the CE junction, 0.5 volt on the load, and a total
current through the circuit of 1V/2ohms=0.5A.   But, how much heat is
the transistor dissipating?  Well, 0.5A*0.5V=0.25W.   So, the transistor
is dissipating 0.25W, and so is the load.

Remember that the Pmax figure is an average over a certain amount of
time, so if you only switch a load at a slow frequency, but switch from
fully off to fully on very quickly you can generally switch something
close to C-E volts * total amps, even if it is far in excess of the Pmax
- because with the transistor fully on or fully off the transistor isn't
dissipating anything.

One last note.   For the description above, I assumed a perfect
transistor.  Remember that a Bipolar has at least a transistor-junction
worth of voltage drop, so even if a transistor is fully on, there's
going to be at least a little voltage drop causing heat.  Let's say you
have an imaginary transistor which is fully on, but has a voltage drop
across the C-E junction of something like 0.7V.  If you're running 10A
through it, you're still going to be dissipating 7W just because of the
transistor junction not being a "perfect" zero volts.    This also
applies to the base current - if you are trying to run some horrendous
base current through a bipolar (say to be able to turn on some large
power transistor with a crappy beta) you need to pay attention to the
dissipation on the base-emmitter current path as well.

Because of the whole Pd issue, I have generally switched to FET's of
some sort unless I have to high-side-switch for some reason.  Because
there's no base (gate) current, you can ignore this, and generally I
drive everything either off or on, and with Ron's in the very low
category, I generally don't have to worry about Pd issues.  If you're
just using the transistors to switch something which doesn't matter if
you switch the high side (positive) or low side (negative), try a FET

-forrest

> I'm just trying to understand why in a saturated transistor
> the product of
> the C-E voltage and maximum allowed current through the transistor is
> considerably less than the maximum dissipated power.

I can imagine that the I max is determined by the maximum dissipation in
some local resistive part of the transistor.

Wouter van Ooijen

-- -------------------------------------------
Van Ooijen Technische Informatica: http://www.voti.nl
consultancy, development, PICmicro products
docent Hogeschool van Utrecht: http://www.voti.nl/hvu

>-----Original Message-----
>From: piclist-bouncesmit.edu [piclist-bouncesmit.edu]
>Sent: 22 September 2006 03:48
>To: piclist
>Subject: [EE]: Transistor maximum current vs max power
>dissipation question
>
>
>Can someone explain to me what defines the maximum current via
>a transistor?
>Since Imax * Vce != Pmax...

In a bipolar device Imax is usualy determined by the size of the emitter area, and to a lesser extent the the size of the bond wires.

Regards

Mike

=======================================================================
This e-mail is intended for the person it is addressed to only. The
information contained in it may be confidential and/or protected by
law. If you are not the intended recipient of this message, you must
not make any use of this information, or copy or show it to any
received this e-mail, and return the original to us. Any use,
forwarding, printing or copying of this message is strictly prohibited.
No part of this message can be considered a request for goods or
services.
=======================================================================

On Thu, Sep 21, 2006 at 09:16:32PM -0700, Vitaliy wrote:

> Regulus Berdin wrote:
> > Vitaliy wrote:
> >> Can someone explain to me what defines the maximum current via a
> >> transistor?
> >> Since Imax * Vce != Pmax...
> >
> > Maximum current thru a transistor is when Vce is saturated (full
> > conducting).  While Pmax is achieved when it is half-conducting and
> > dissipating the power used as heat.
>
> Unfortunately, this doesn't answer my question. If Pmax is what defines
> Imax, why then when the transistor is saturated, Imax * Vce < Pmax ?
>
> If the maximum power dissipation is not the limiting factor for the maximum
> current in a saturated transistor, what is?
>
I think there there is a physical limitation at the weld connecting the lead to the
wafer.
Ray Warren
Michael Rigby-Jones wrote:
> In a bipolar device Imax is usualy determined by the size of the emitter
> area, and to a lesser extent the the size of the bond wires.

Do you know this for sure? What are the physics involved? If I exceed Imax,
the bond wires will start to melt or something?

This is not for any particular project, just something I recently bumped
into -- but it bothers me that I don't understand why it happens and that I
can't find the answer either on the 'net or in my transistor books.

Vitaliy

I'm interested in the mini-FET tutorial :o)

many thanks

Ken

{Quote hidden}

> --
Vitaliy wrote:
> If the maximum power dissipation is not the limiting factor
> for the maximum current in a saturated transistor, what is?

The current limitations of semiconductor junction.
You can't pass more than certain amount of people through a door
regardless of pressure you are applying on them without destroying the
door.
You can't pass more than certain amount of electrons or holes through
the junction without destroying the semiconductor structure. Number of
electrons (holes) is limited as well as their speed.

MS
Vitaliy wrote:
> Unfortunately, this doesn't answer my question. If Pmax is what defines
> Imax, why then when the transistor is saturated, Imax * Vce < Pmax ?

Because Pmax / Vce is only one limit on Imax.  In fact that is not even a
"limit" in the sense that the transistor will limit the current.  It is a
limit that you must make sure is not exceeded if you wish the transistor to
function as specified in the data sheet.

The transistor itself will limit Ic to Ib * Hfe.  Hfe is the transistor's
current gain at the particular operating point.  Circuits are usually
designed to work with a minimum Hfe, so that is what is guaranteed in the
data sheet.  This is therefore not useful as a guaranteed current limit
since Hfe could in theory be infinite.  Good bipolar transistor circuits are
designed so that Hfe can be from some specified value to inifinity.

If it's important that Ic be limited in your circuit, you need to
specifically address this.  A simple and common means is a emitter resistor
with a guaranteed maximum base voltage.  There are other means of course
too.

******************************************************************
Embed Inc, Littleton Massachusetts, (978) 742-9014.  #1 PIC
consultant in 2004 program year.  http://www.embedinc.com/products
Vitaliy wrote:
> Perhaps I'm not phrasing the question correctly?
>
> I'm not asking how to use a transistor. I understand that exceeding or
> even approaching the maximum ratings is a no-no. I understand that when
> the transistor is in saturation, the power dissipated (assuming Imax)
> is lower than when it's operating in the linear region, etc, etc.
>
> I'm just trying to understand why in a saturated transistor the product
> of the C-E voltage and maximum allowed current through the transistor is
> considerably less than the maximum dissipated power.

Because heating isn't the only limiting factor on transistor operation.  The
holes and electrons in the semiconductor material can only move so fast and
there can only be so many of them.  Most of the useful operating modes of
semiconductor devices comes from "starvation" or careful control of the
minority carriers.  Another way to put it is that if you have too many holes
and electrons banging around in a transistor, you cease to have a
transistor.

Other secondary (usually) limitations come from other parts of the overall
package.  The dissipation in the bond wires and the junctions between the
bond wires and the semiconductor material is a function of current thru the
device, not on how much the semiconductor device dissipates itself.  In
practise beefing up the bond wires and connections is relatively cheap
compared to beefing up the transistor, so in most real cases the limits they
impose are higher than the limits of the semiconductor device itself.

There are various independent physical processes going on, and each impose
their own limits.  The allowed operating region is the union of the
operating regions imposed by each of the separate limits.  In some parts of
the operating range you will bump up against power dissipation, in other
parts against inherent maximum current carrying capability, and voltage
stresses in other parts.

> I'm just trying to understand why in a saturated transistor the product of
> the C-E voltage and maximum allowed current through the transistor is
> considerably less than the maximum dissipated power.

The Vce rating has more to do with the off state, it's the voltage you need
to stay under to prevent avalanching.

As one moves from TO92 to SOT23, the maximum dissipated power decreases (as
> expected), but the maximum current stays constant.

The physical limits of the bond wires and metalization of the die, and
maximum current density haven't changed. Just it's ability to get rid of
heat.

--
Feel the power of the dark side!  Atmel AVR
Christian,

Would a book on semiconductor theory help me
understand more on what you just said? I would like to
have a deeper understanding. Any recommendation?

Thanks,
John

--- Ken Walker <ken.walkermanchester.ac.uk> wrote:

> I'm interested in the mini-FET tutorial :o)
>
> many thanks
>
> Ken
>
> > {Original Message removed}
On 9/22/06, John Chung <kravnusyahoo.com> wrote:
>
> Christian,
>
> Would a book on semiconductor theory help me
> understand more on what you just said? I would like to
> have a deeper understanding. Any recommendation?

Horowitz and Hill's "Art of Electronics".  Highest recommendation!
John Chung wrote:

>  Would a book on semiconductor theory help me
>understand more on what you just said? I would like to
>have a deeper understanding. Any recommendation?
>
I have found "The Art of Electronics", plus the student manual, to be an
excellent reference when I'm not quite getting something.  It's
definately worth the \$100ish that both of these together would cost
new.   They are available used, but make sure you get the latest edition
if you buy them from a used source.

BTW, these are "Search inside"-able at amazon.com.   You might want to
search the book for some topics of interest to you and look at the pages
which result to see if the book provides useful knowledge to you.

-forrest
I already have that. Still does not give me the depth
I want on semiconductor theory:( Theory of the
behavior of the silicon for a given environment or
operation.

John

--- David VanHorn <dvanhornmicrobrix.com> wrote:

{Quote hidden}

> --
John Chung wrote:
> I already have that. Still does not give me the depth
> I want on semiconductor theory:( Theory of the
> behavior of the silicon for a given environment or
> operation.

Googlize with "semiconductor theory" OR "theory of semiconductors"

Results 1 - 100 of about 47,400 for "semiconductor theory" OR "theory
of semiconductors

MS
John Chung wrote:

>I already have that. Still does not give me the depth
>I want on semiconductor theory:( Theory of the
>behavior of the silicon for a given environment or
>operation.
>
I'm not aware of any book which specifically deals with that in
particular.  I'd envision that to be very difficult since there are so
many subtle variations.   I.E. a normal NPN transistor operates like
this except when x, y or z and b are true.    The trick is really
understanding what the limits are and staying inside them.

A good source for some "theory" are the manufacturers app notes, white
papers, data sheets, and other documents.   Onsemi (previously Motorola)
has some good papers on differnet failure modes.  See their AN1628-D
which is entitled "Understanding Power Transistor Breakdown Parameters"
for an example.   If you can get ahold of an old (circa '90 or so)
motorola FET data book, there are some cool examples and instructions on
how to use FET's within it.

Another book I've found to be both an entertaining read and very helpful
in learing why things do and don't work (the title is a bit of a
misnomer) is "Troubleshooting Analog Circuits" by Robert A. Pease.
Although it doesn't cover all of the theory you want, it does provide a
good peek into the ways that you can seriously break a design, and how
to fix them.   One example is a story in the book where Pease built a
test instrument containing a diode, which worked great until he picked
the unit up, at which point it started failing.   Put it back down, it
works again.   When he finally figured it out he found that one
particular diode had some of it's dark paint scraped off and as such
light was getting in the diode junction and causing photoelectric
effects to occur.

I'm sure there's others out there but these are the favorites I use when
I need to.

-forrest
There's "Advanced Semiconductor Fundamentals", which describes all the
atomic processes in a semiconductor. ISBN is 013061792X. It's ~\$40 at
nerdbooks.com .
- Marcel

On 9/22/06, Forrest W. Christian <forrestcimach.com> wrote:
{Quote hidden}

> -
At 9/22/2006, you wrote:
>When he finally figured it out he found that one
>particular diode had some of it's dark paint scraped off and as such
>light was getting in the diode junction and causing photoelectric
>effects to occur.

In the early days of EEPROM (late 70s) we had one that was sensitive to
florescent lights. It worked in full light and in darkness but not partial
light.   There were several vent holes on the case, the unit worked or did
not work depending on the orientation of the case under a desk.

Don Bowen              Awl Knotted Up                       KI6DIU
http://www.braingarage.com
My travel journal
http://www.braingarage.com/Dons/Travels/journal/Journal.html

I am omnivagant.
On 9/21/06, Forrest W Christian <forrestcimach.com> wrote:
>
> One last note.   For the description above, I assumed a perfect
> transistor.  Remember that a Bipolar has at least a transistor-junction
> worth of voltage drop, so even if a transistor is fully on, there's
> going to be at least a little voltage drop causing heat.  Let's say you
> have an imaginary transistor which is fully on, but has a voltage drop
> across the C-E junction of something like 0.7V.  If you're running 10A
> through it, you're still going to be dissipating 7W just because of the
> transistor junction not being a "perfect" zero volts.    This also
> applies to the base current - if you are trying to run some horrendous
> base current through a bipolar (say to be able to turn on some large
> power transistor with a crappy beta) you need to pay attention to the
> dissipation on the base-emmitter current path as well.

Better look at Vce(sat) for the transistor in question.  Depends on
collector current and 0.7 might be too low!

Orin.

On Sep 22, 2006, at 11:57 AM, Forrest W. Christian wrote:

>
>> I already have that. Still does not give me the depth
>> I want on semiconductor theory:( Theory of the
>> behavior of the silicon for a given environment or
>> operation.
>>
> I'm not aware of any book which specifically deals with that

Yeah, semiconductor theory tends to start deep in theory and doens't
quite get up to the "practical" level of things like power dissipation.
Manufacturer literature starts way up with the practical limits and
never gets down to their theoretical causes.

IIRC, at least some of the current limits are related to things
like the carrier density, ohmic resistance of contacts, junctions,
and channels, and so on.  Some of that will be covered in solid
state physics texts; I see a bunch of stuff that I don't understand
any more in my college copy of "Solid State Electronic Devices" by
Streetman (c  1972, so...)  The practical aspects may be deep company
I.P., some secret, some patented.  High power mosfets seem to commonly
be a bunch of parallel devices in a single package, for instance.

If you think of the maximum current as being imposed by the
solid state physics of the internal device, while the power
rating is determined by needing to maintain a temperature where
the solid state physics still applies (even before wires melt,
I seem to recall that temperature has assorted effects on carrier
mobility and such), you won't be TOO far wrong.  It's also possible
that a device was fabricated and then measured, and what you see
in the spec sheet is the max current where the device behaved in
a way that the company wanted to take credit for (ie it is at least
partially derived experimentally.)

BillW
Thanks for the info Christian. Some of the book on
semiconductor theory covers too much physics on their
end. I doubt I would be counting their cos,sin and etc
formulas.

John

--- "Forrest W. Christian" <forrestcimach.com> wrote:

{Quote hidden}

> --
Solid State Physics. I will check it out. I believe it
is VERY heavy of theoratical stuff.

Thanks,
John

--- William Chops Westfield <westfwmac.com> wrote:

{Quote hidden}

> --
Olin Lathrop
> Because heating isn't the only limiting factor on transistor operation.
> The
> holes and electrons in the semiconductor material can only move so fast
> and
> there can only be so many of them.

This would imply that you won't be able to exceed the Imax (you would run
out of holes and electrons). Alas, the other day I saw 2A put through a 3904
with an Imax of 200 mA.

My point is, the number of minority carriers doesn't constitute a "natural"
limit where the current physically cannot go above a certain limit. You're
not supposed to exceed Imax, but it's possible -- just as it's possible to
exceed Pmax.

Vitaliy

On 9/22/06, Vitaliy <spammaksimov.org> wrote:
> Michael Rigby-Jones wrote:
> > In a bipolar device Imax is usualy determined by the size of the emitter
> > area, and to a lesser extent the the size of the bond wires.
>
> Do you know this for sure?

Absolutely sure.

Vasile
Vitaliy wrote:
> Olin Lathrop
>> Because heating isn't the only limiting factor on transistor operation.
>> The
>> holes and electrons in the semiconductor material can only move so fast
>> and
>> there can only be so many of them.
>
> This would imply that you won't be able to exceed the Imax (you would
> run out of holes and electrons).

No, it only means that the device characteristics may change to outside the
datasheet parameters.  Also, just like overclocking PICs, you can get away
with it most of the time, but it is not guaranteed operation.  The
manufacturer has to pick parameters that they can hit without excessive
fallout over the whole range of manufacturing variations.

> My point is, the number of minority carriers doesn't constitute a
> "natural" limit where the current physically cannot go above a certain
> limit. You're not supposed to exceed Imax, but it's possible -- just as
> it's possible to exceed Pmax.

I don't think I've ever seen a transistor data sheet that claimed the
transistor had a natural current limit at some value.  Rather Imax is the
value you must not exceed if you want the device to continue operating as
specified in the rest of the data sheet.

******************************************************************
Embed Inc, Littleton Massachusetts, (978) 742-9014.  #1 PIC
consultant in 2004 program year.  http://www.embedinc.com/products
John Chung wrote:
> Some of the book on
> semiconductor theory covers too much physics on their
> end. I doubt I would be counting their cos,sin and etc
> formulas.

John Chung wrote:
> Solid State Physics. I will check it out. I believe it
> is VERY heavy of theoratical stuff.

John,

Only very few highest educational institutions in the world teach
Solid State Physics seriously.
It heavily depends on very advanced topics of math analysis and math
physics, far beyond just "counting their cos,sin and etc formulas".

Good Luck,

MS
I was afraid of that.
www.amazon.com/gp/product/0201122952/ref=pd_rvi_gw_3/002-4525055-0239229?ie=UTF8
Semiconductor Fundamentals, Volume I (2nd Edition)

I found this book based on the feedback I got.
Hopefully it is *NOT* solid state physics. There are
other books that cover BJT and FET specifically.

Thanks,
John

{Quote hidden}

> --
I remember a textbook from college...
'Integrated Electronics' by Millman and Halkias.
McGraw Hill

I also remember it having a comprehensive chapter devoted to BJTs.

HTH,
Mohit.

{Original Message removed}

More... (looser matching)
- Last day of these posts
- In 2006 , 2007 only
- Today
- New search...