Low Vsat transistors ?

[MrAl]

Hello again,<p>Anyone know where to get low Vsat transistors?
These are typically 0.2 volts (or less) @ 1 amp
or so. I already have Zetex on the list,
but im looking for a second source. Any ideas?<p>Thanks,
Al

[Stan]

This is probably not what you have in mind, but if your junk box has germanium transistors, low Vsat is one of their strong points. .1v is typical, and the heavier ones can handle 1 amp. Just keep it cool and watch the Vceo max. Also you have to tolerate some leakage current in "off" state. (in .5 mA range for heavier transistors, but you can select for as little as .05 mA)

[MrAl]

Hello Stan,<p>Thanks for the idea, but i need very fast
switching too in order to maintain a decent
efficiency. It's for a battery operated
power supply for a 350ma white LED and
im looking for 85% minimum efficiency.<p>--Al

[Chris Smith]

Have you considered designing around a Fet instead? Resistance can be .2 ohms and even less, current to the 100s of amps, and using a driver circuit if necessary to make it very sensitive?

[L. Daniel Rosa]

Saturation voltage varies with collector current, base current, temperature, and coffee brand. I could interpret The Semiconductor Data Book (1968, Motorola) on 2N4001s. Several curves are shown, one crosses 0.2V at 100mA Ic and about 2 mA Ib. At 10mA Ic, the curve crosses 0.2V at about 150uA. If you stay away from the device's limits by an order of magnetude, the engineering should be easier. On the down side, this does take more space for multiple units. If space is of great concern, there are MOSFETs available in TO-92s.

[MrAl]

Hello,<p>Thanks for all the ideas.<p>The circuit is a dc to dc converter that takes
0.8 to 3 volts input and drives a white
350ma LED at a little over 3 volts.<p>The reason it has to be a bipolar NPN is
because the driver chip is made only to
drive that kind of device, and the input
can be as low as 0.8 volts dc, although
it will typically be higher, like around
2 volts. As you probably already know, the
gate drive for the MOSFET has to be typically
at least about 5 volts or higher, so it
wont turn on with only 1 volt gate drive.
Also, it's a minimum parts count design also,
meaning adding a handfull of parts for a drive
circuit or pre-boost circuit cant be tolerated.
All the parts are SM parts and the design right
now fits on a pc board as small as 15mm x 15mm.
The reason for the small size is so that it can
fit in the smallest flashlights, such as the
AA MiniMag made by Maglight. The circuit has to
power a 350ma LED at about 3.1 volts output so
that means running from 2 volts the transistor
will see more then that, and in practice it
actually goes as high as 1.5 amps for a very very
short period of time (typically 1.6 us or less).
The transistor has to turn on and off quick
so it has to be silicon.
If you are interested in the circuit, it appears
here:<p>http://www.candlepowerforums.com/cgi-bi ... =forum&f=3<p>Click on "Zetex ZXSC300 single-cell LED driver"
to view the thread.<p>Take care for now, and thanks again for the ideas
and suggestions.<p>[ January 23, 2002: Message edited by: MrAl ]</p>

[Ron H]

Mr. Al, I hate to be a wet blanket, but are you going to get enough base drive for the current you want? The ZXSC300 base drive current is 1.5ma minimum, 3.6ma maximum. Where are you going to find a transistor with the beta you need? Also, the guaranteed drive voltage is [Vcc-0.3v]. Worst case, you won't even get your tranny to turn on when Vcc=0.8v.<p>On a brighter note, Philips has some low Vce(sat) transistors.<p>Ron H

[Chris Smith]

I can see your size problem, but turning on a fet, no problem. At worst a small inductor will store the low voltage and when turned off will preform way above the input voltage. Buck boost. A lot of fets cant handle 5 volts at their gate, and they dont draw power at the gate, just the mere presence of a voltage at that gate sets it off.<p>[ January 23, 2002: Message edited by: Chris Smith ]</p>

[L. Daniel Rosa]

I once had a stereo that ran on a single AA until it was dead as a doornail. The cell wouldn't even power a clock for more than a few weeks. After a cell is discharged to less than 0.9V with a light load, there's not much left to it. So with a two cell torch, any performance below 1.8V is a bonus. I don't know about the feasibility of running at 0.8V, but a complementary beta multiplier should give some push at 1.0V. I see that something like that is already in one of your driver circuits. Can it be assumed that for high packing density you're already planning on wire-wrapping or SMT?

[MrAl]

Hello there,<p>RonH:
Wet blanket? hee hee.
It's not a question of whether the circuit works or not, as there
are several of us who have been testing this circuit out and
mailing each other parts and stuff so we can get similar results
for the past few months now All of the circuits function
fairly well now that we went through several design phases and
parts swapping and such. A typical transistor that meets the
specs pretty well for two cell apps is the Zetex FMMT617 transistor.
There are others too, but they are all Zetex, that's why i was
looking for a second source.
I think the circuit makes a good general purpose dc / dc boost
converter too and i'll probably use it for that as well as
the LED light. The size is down to 13mm by 13mm and i should have
a pc board that size by the weekend. The pc board im using now is
several times larger then that, but it was made for experimenting
with different transistors and chokes and all, so size wasnt an issue
way back then. This size makes it possible to modify a Mag Lite
Mini Mag 2 cell AA flashlight with a Luxeon Star white LED and
possibly fit in the optics too. That's why size became an important
issue now. Right now all of the parts are available from Digikey too,
but id very much like to have a second source on the transistor.<p>Chris:
Thanks for the inductor idea. In many apps though you cant get
away with just an inductor, as you end up needing other drive
circuit components also. Also, keep in mind that the mosfets'
gd and gs capacitances require a pretty hefty current drive
in order to turn the FET on fast, typically 500ma or more.
Once turned on, it's true that the device will stay on
with almost no current, but then turning it off fast also
requires more current sink.
It is possible to turn it on and off with a much higher impedance
source, but the switching time will suffer greatly. Keep in
mind that the gd capacitance acts as negative feedback during
the switching transition, so while the drain voltage is falling
the gate sees the current though the gd capacitance tending to
keep the device in the linear mode eating up efficiency.
I'll keep the inductor drive idea in mind though, perhaps i'll be
able to use it with a circuit that can stand to have a few more
parts. Also, the chips' drive output is only very few milliamps
as RonH has pointed out. Although we may in fact move to
a different chip even maybe by a different manufacturer, right now
we are sticking to this chip because of it's simplicity and very
small size using all SM parts (except for the toroid hand wound
inductor).<p>L. Daniel:
The complementary transistor idea is good too, and Zetex uses that
with one of their chips: the ZXSC100 chip. With the 300 chip im
not sure if it would work or not, and the most important point
is the saturation voltage, not really the current output.
The kind of saturation voltages we are talking about here are
much lower then can be obtained with a run of the mill transistor
being overdriven or something, and turns out to be an intrinsic
characteristic of the specific transistor itself.
Most other transistors i've seen drop at least 0.4 volts while
conducting 1 amp or so, while the Zetex transistors do 0.2v or
better. This is why i was wondering if there even is a second
source or not. I'll check out the Phillips site too.
And yes, all the parts are SM parts except the inductor.<p>---------------------------------------------------------------<p>Although the original question was to find a second source for a
low v sat transistor i thought the other ideas were quite interesting
also, so thanks to all those who took the time to reply. I'll be
reading other posts for good ideas like these too.<p>Sincerely,
Al

[Chris Smith]

You seem to be doing a lot of research. When you get time check into all of the new "FET" types of new transistors. ZETEX has a whole slew of them on one of their sites under lighting. I was doing some research on them for Fluro lighting and after more than a weeks reading, I hadnt even got half way through the data. There are hundreds of new designs, values, etc, while some of the newer designs have a pre "driver" circuit built right in. Its worth the time when you get a chance. There is some amazing new stuff out there being designed right now because of some of the set backs you have been fighting over.

[MrAl]

Hi again Chris,<p>The new FET's your talking about sound
very interesting and i'll have to take
a look on the Zetex site for the ones
with the driver built in. That sounds
very useful to me. I also wonder how
they compare in price to the Zetex
bipolar low sat transistors.<p>Take care for now,
Al<p>Added later:
Couldnt find any of the pre-driver mosfets or
ics, what is the part number you were talking
about Chris?<p>--Al<p>[ January 24, 2002: Message edited by: MrAl ]</p>

[CLEVERHOMES]

from charlie:<p>i did not have time to read all of the answers but i would definitely entertain a MOSFET for you can get turn-ons with Vgs less than 1.5 volts. <p>trick here: get one with a low Vd rating and an adequate current. digikey has a lot. The low Vd accompanies a low Vgs turn-on.<p>option: use power transistor in inverting mode -<p>limits voltage in and out to around 7 volts or less and the Hfe is very small - say - 2 or 3.

[Chris Smith]

?Dont know? I was on the R&D site. Some are brand new, some may be available now, some will be on the market in the near future? I read for about a week, so numbers and specs? I'll see if I still have the R&D address. Its Big. Also there are a few other Companies on one site lumped together under lighting. I dont know how far back my "History" holds the addresses but I'll check. Its set for 200 days but lately it seems to cut off after about a week? I think its still in the files though unless NORTON tosses it automatically every few days? They dont call them "pre-Driver" they have Prefix before the "FET" like xxxfet, like they did with MosFet and IgFet. Its new so I dont remember those prefix letters.<p>[ January 30, 2002: Message edited by: Chris Smith ]</p>