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'[EE]: low sat motor chip'
2000\12\18@081951 by Roman Black

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Hi, still looking for a h-bridge chip with
low-saturation voltage at 2A but preferably
3A to 4A. I also would like high package power
dissipation if possible.

The L298 is 5v sat at 2A, about 4 times worse
than I need. Surely there is a good mosfet (etc)
h-bridge chip out there?? :o)
-Roman

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2000\12\18@130540 by Dan Michaels

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Roman wrote:
>Hi, still looking for a h-bridge chip with
>low-saturation voltage at 2A but preferably
>3A to 4A. I also would like high package power
>dissipation if possible.
>

I wonder about this, Roman. I've been looking around at
specs, and seems MOSFET sat voltage is a direct function
of current pulled, so difficult to get low-sat.
Eg, .2v/5A = 0.04 ohms. Awfully small Rds. Maybe have
to stay with bjts.

- danM

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2000\12\19@150538 by Russell McMahon

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>Roman wrote:
>>Hi, still looking for a h-bridge chip with
>>low-saturation voltage at 2A but preferably
>>3A to 4A. I also would like high package power
>>dissipation if possible.
>>
>
>I wonder about this, Roman. I've been looking around at
>specs, and seems MOSFET sat voltage is a direct function
>of current pulled, so difficult to get low-sat.
>Eg, .2v/5A = 0.04 ohms. Awfully small Rds. Maybe have
>to stay with bjts.


MOSFETs are indeed specced interms of on resistance rather than asaturation
voltage.
I'm not aware of the maximum off voltage here but if it is low then FETs
wirgh Rdson well below tis are readily available.
FETs rated at 20 to 30 volt are available with Rdson of under 10 milliohms.
These are typically TO220 package with a rated current in the 50 to 100 amp
range.
Prices here are probably equivalemt to $US2 to $US4 each.

Alternatively, again depending on voltage, look at the Zetex E-line (TO92
like) standard (bi-junction) transistor high current devices - 1 to 5 amps
continuous , up to 20A peak and good saturations. Betas typically are 100+
at 1A !!

EVERYONE needs to know about Zetex products :-)



Russell McMahon

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2000\12\20@070206 by Roman Black

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Dan Michaels wrote:
>
> Roman wrote:
> >Hi, still looking for a h-bridge chip with
> >low-saturation voltage at 2A but preferably
> >3A to 4A. I also would like high package power
> >dissipation if possible.
> >
>
> I wonder about this, Roman. I've been looking around at
> specs, and seems MOSFET sat voltage is a direct function
> of current pulled, so difficult to get low-sat.
> Eg, .2v/5A = 0.04 ohms. Awfully small Rds. Maybe have
> to stay with bjts.
>
> - danM


Hi Dan, I've been working on some power drivers and did
some testing with transistors I had lying around. Again
came back to my BD203 (20 year old technology), how's
this for low sat from a 50c US part:

BD203 NPN (BD204 PNP)
8A 60v 60w TO-220

Ib      Ic      Vce
12mA    1A      0.21v   (PIC compatible)
12mA    1.5A    0.53v   (PIC compatible)
20mA    1.5A    0.26v   (PIC compatible)
20mA    2A      0.60v   (PIC compatible)

Note! all of these above will stay cold with NO HEATSINK.

Higher power:
BD203
123mA   2.5A    0.36v   (no heatsink)
193mA   4A      0.72v   (small heatsink)

TIP3055 (common, cheap, larger pack)
17mA    1A      0.22v   (PIC compatible)


I tested a heap more parts including some power MOSFETs,
but unless you must have really high Beta the FETs
don't cut it for price and size.

Anyway, I can build some very efficient low sat drivers
with these parts and others, (I have been doing exactly
this for years) but it involves more work in construction.
I have heard there are some MOSFET h-bridge driver ships,
but can't find any good details??

I am also aware of low R MOSFETs, I have used these for
years too, but many times more $$ for similar sat voltages
and gate driving is not always easier with the MOSFETs.

Any low sat chips?? Please???
:o)
-Roman

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2000\12\20@071448 by Roman Black

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Russell McMahon wrote:
{Quote hidden}

Thanks for the Zetex tip Russell! Web site??

I know about the FETs and NPN specs, I have been testing
some of our stock here over the weekend. I really want a
low-sat h-bridge chip! Otherwise I'll just build good
discrete circuits like always. Surely someone makes a
decent chip? Amazing.
:o)
-Roman

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2000\12\20@072523 by Roman Black

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> BD203 NPN (BD204 PNP)
> 8A 60v 60w TO-220
>
> Ib      Ic      Vce
> 12mA    1A      0.21v   (PIC compatible)
> 12mA    1.5A    0.53v   (PIC compatible)
> 20mA    1.5A    0.26v   (PIC compatible)
> 20mA    2A      0.60v   (PIC compatible)
>
> Note! all of these above will stay cold with NO HEATSINK.

Forgot to add, when driving the BD203 with
5mA base current and 40mA collector current
the Vce was 0.019v! That's not a misprint.
For a while there I thought I had a faulty
transistor, but a few tested the same.
This may be of use for someone who needs
almost total saturation, PIC driven load
at fairly low currents. :o)
-Roman

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2000\12\20@162152 by Peter L. Peres

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>0.040 ohm MOSFET on

This is a garden variety Rdson for good make MOSFETs (like, BUK... from
Philips). I have used one in a 100W SMPSU at 27A pk and it needed a very
small heatsink (under the circumstances). Check out some datsheets for
recent power MOSFETs... Sometimes I wonder seriously how these low Rdson's
are achieved. By using standard MOSFET calculations the required doping
densities appear to be obscenely large. Scary, in fact.

Peter

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2000\12\20@175213 by Roman Black

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Peter L. Peres wrote:
>
> >0.040 ohm MOSFET on
>
> This is a garden variety Rdson for good make MOSFETs (like, BUK... from
> Philips). I have used one in a 100W SMPSU at 27A pk and it needed a very
> small heatsink (under the circumstances). Check out some datsheets for
> recent power MOSFETs... Sometimes I wonder seriously how these low Rdson's
> are achieved. By using standard MOSFET calculations the required doping
> densities appear to be obscenely large. Scary, in fact.
>
> Peter


Which explains:
1. why very high current (low Rds) MOSFETS have such
low voltage limits.
2. Why we see such high failure rate of high current
MOSFETs in inverters and the like compared to similar
high current low-sat bipolar transistors. One of the
reasons I will always choose NPN over a FET, and the
NPN is normally cheaper for the similar spec, if
you discount drive current. I'd prefer to sacrifice
drive current for reliability and ruggedness anyday.
Many new OEMs agree with me too I think! :o)
-Roman

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2000\12\20@211153 by Dan Michaels

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Peter Peres wrote:

Sometimes I wonder seriously how these low Rdson's
>are achieved. By using standard MOSFET calculations the required doping
>densities appear to be obscenely large. Scary, in fact.
>

More likely, parallel channels.

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2000\12\20@232007 by Roman Black

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Roman Black wrote:
>
> Dan Michaels wrote:
> >
> > Roman wrote:
> > >Hi, still looking for a h-bridge chip with
> > >low-saturation voltage at 2A but preferably
> > >3A to 4A. I also would like high package power
> > >dissipation if possible.
> > >
> >
> > I wonder about this, Roman. I've been looking around at
> > specs, and seems MOSFET sat voltage is a direct function
> > of current pulled, so difficult to get low-sat.
> > Eg, .2v/5A = 0.04 ohms. Awfully small Rds. Maybe have
> > to stay with bjts.
> >
> > - danM


Dan, i've just found a low-sat motor driver chip for
a fairly reasonable price.

LMD8201 (national semiconductor)

3A cont (6A peak) 55v 25w
($7.40 US in 250q)
DMOS power stage 0.33 ohm Rds (2v total sat at 3A)
Similar package to L298 chip

Has internal clamp diodes and a few other nice
power driver thingies. This is one of the nicest
little mid power h-bridges i've seen.
However it's only one h-bridge, not two like
in the L298.

It can be seen at their page (with pricing and datasheet)
http://www2.national.com/pf/LM/LMD18201.html

:o)
-Roman

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2000\12\21@063606 by Russell McMahon

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>This is a garden variety Rdson for good make MOSFETs (like, BUK... from
>Philips). I have used one in a 100W SMPSU at 27A pk and it needed a very
>small heatsink (under the circumstances). Check out some datsheets for
>recent power MOSFETs... Sometimes I wonder seriously how these low Rdson's
>are achieved. By using standard MOSFET calculations the required doping
>densities appear to be obscenely large. Scary, in fact.


Zillions of small MOSFETs in parallel.

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2000\12\21@064429 by Roman Black

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Russell McMahon wrote:
>
> >This is a garden variety Rdson for good make MOSFETs (like, BUK... from
> >Philips). I have used one in a 100W SMPSU at 27A pk and it needed a very
> >small heatsink (under the circumstances). Check out some datsheets for
> >recent power MOSFETs... Sometimes I wonder seriously how these low Rdson's
> >are achieved. By using standard MOSFET calculations the required doping
> >densities appear to be obscenely large. Scary, in fact.
>
> Zillions of small MOSFETs in parallel.

Wow! That's even scarier! I'm yet to come across an
electronics repairer who has anything good to say
about MOSFETs. :o)
-Roman

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2000\12\21@175032 by Peter L. Peres

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Roman Black <fastvidspamKILLspamEZY.NET.AU> wrote:
>Which explains:
>1. why very high current (low Rds) MOSFETS have such
>low voltage limits.

Uh, it has nothing to do with it. It has to do with the gate oxide
thickness. You can make MOSFETs with 100V and more insulated gates but it
will likely not go together with low turn-on control voltage. In fact such
things exist and there are a couple of switching mode supplies that use
this to good effect. But they are not consumer equipment.

27A is not 'very high current'. The part I used was rated 55A, 50V, in
TO220.

>2. Why we see such high failure rate of high current
>MOSFETs in inverters and the like compared to similar
>high current low-sat bipolar transistors. One of the
>reasons I will always choose NPN over a FET, and the
>NPN is normally cheaper for the similar spec, if
>you discount drive current. I'd prefer to sacrifice
>drive current for reliability and ruggedness anyday.

The state of the art in switching FETs off the shelf seems to be at about
1500V and 7A or so, in a modified TO220 (insulated). It is about as easy
to do these in as a bipolar. The part choice for current consumer OEM lies
with cost and supply, not technology. The turnover of this equipment is so
fast, that nobody in a sane mind cares what will happen in 5 years. They
apply statistical fault modelling, do some HALT, and dump the thing into
production if it passes what they set as criteria. I can't see very much
use in an ultra reliable over-engineered SMPSU in an all plastic VCR that
will self destruct by munching its own gears and bushings to dust within
about 3 years of semicontinuous domestic operation.

Peter

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2000\12\22@090404 by Olin Lathrop

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> I can't see very much
> use in an ultra reliable over-engineered SMPSU in an all plastic VCR that
> will self destruct by munching its own gears and bushings to dust within
> about 3 years of semicontinuous domestic operation.

Not to mention most consumers buy on price.  Even if you were to produce a
"brick outhouse" VCR that lasts 20 years, very few people would buy it
because it would cost $100 more than the cheap model right next to it with
all the same features.  Reliability is seldom listed as a feature in
consumer gear, and when it is it's hard to know what is marketing hype and
what's real.

In other words, it usually makes sense to produce a $50 gizmo that may
destruct or need a $100 repair in two years than to produce a $100 gizmo
that will last 5 years.  This is especially true when based on rapidly
moving technology so that nobody would want it at any price in 3 years.


*****************************************************************
Olin Lathrop, embedded systems consultant in Devens Massachusetts
(978) 772-3129, EraseMEolinspam_OUTspamTakeThisOuTembedinc.com, http://www.embedinc.com

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2000\12\27@215841 by Mitchell D. Miller

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> MOSFETs are indeed specced interms of on resistance rather than saturation
> voltage.

Could someone explain saturation (as it applies to this thread) to me?

Thanks,
Mitch

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2000\12\27@222416 by Dan Michaels

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Mitch Miller wrote:
>> MOSFETs are indeed specced interms of on resistance rather than saturation
>> voltage.
>
>Could someone explain saturation (as it applies to this thread) to me?
>

Saturation is a region of operation of BJTs [bipolar junction
transistors], where the base is being driven hard enough that the
collector-emitter voltage drops to ~0.2v, by virtual of almost
all the supply voltage dropping across the load. Most easily
viewed in terms of a stock NPN inverter ckt.

                 +Vcc
                   |      +
                 Rload  ~Vcc
                   |      -
                   |
                   C     +
Vin-----Rbase-----B     ~0.2v
                   E     -
                   |
                  gnd

More strictly speaking, for normal BJT operation, Icoll = beta*Ibase,
while for saturation operation, Ibase >= Icoll/beta

For driving digital type loads, ie on-off, saturation is a nice
area to run the BJT in, as little energy is being lost in it, and
little heat is being dissipated by it, and most of the energy goes
to the load.

With MOSFETs, you think more in terms of the channel resistance
and the amount of voltage being dropped across it. If you want
to keep the dissipation low in a MOSFET and also run a lot of
current through it, you need to use a device with a very low
channel Rds value.

A bad power MOSFET would have, eg, Rds = 1ohm, so if you run 5A
thru it, you get a 5v drop and 25W dissipation. Very inefficient
- you lose all your power in the MOSFET --> smoke city. Better
to have Rds = .05 ohm, for example, so Vds = 5A*.05ohm = .25v
and Pds = .25v*5A = 1.25W. [even Roman could live with that
- till something better comes along].

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2000\12\27@225541 by Tom Messenger

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At 10:24 PM 12/27/00 -0500, you wrote:
>Mitch Miller wrote:
>>> MOSFETs are indeed specced interms of on resistance rather than saturation
>>> voltage.
>>
>>Could someone explain saturation (as it applies to this thread) to me?
>>
>

Bipolar Transistor Operation:

In the linear region, an increase in base current causes an increase in
emitter-collector current.  When the transistor is passing it's maximum
amount of collector current, further increases in base current do not cause
further amounts of collector current. At this point, the transistor is said
to be SATURATED.

Side note: complete filling of the base region with majority carriers (to
get to the saturation point) has a side effect: it takes a relatively long
time to remove the carriers to turn the transistor off.  Thus, to minimize
turn-off times (for fast operation), you must avoid saturation.  This is
exactly the premise behind ECL: emitter coupled logic. Ok. I'll stop now.

Tom M.>

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2000\12\28@090527 by Olin Lathrop

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> Could someone explain saturation (as it applies to this thread) to me?

As "on" as it gets, regardless of how it's being driven.


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2000\12\29@185604 by Peter L. Peres

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>explain saturation

Technically speaking, saturation occurs when all the possible charge
carriers in a semiconductor are busy carrying charge. This applies to all
the parts of the semiconductor device. F.ex. above a certain base current
the bipolar transistor's BE junction is saturated. Under these conditions,
at a certain (high) CE current the transistor's CB junction is also
saturated. The bipolar transistor acts as a controlled current source (or
sink). At saturation the sink is open all the way, and there will be no
further increase with base drive or CE voltage. The manufacturer specifies
these conditions as Vcesat and Isat at a certain Beta. This relates
directly to power dissipation under those conditions, which is roughly
Psat = Ib * Vbe + Vcesat * Ic. It is not a good idea to reach or exceed
these parameters for continuous operation. Usually the Ib * Vbe term can
be neglected (it is about 1W for a TO3 2N3055 with Beta 10 driving 10A Ic
at about 50W dissipation for example). A BUZ series FET in the same role
would dissipate zero watts on the gate and about 5W on the D-S channel.
This is for Roman <g>.

Collector and base saturation are responsible for the the peak in Beta
that characterizes every bipolar transistor. The peak occurs in the Ic vs.
Ib characteristic graph of the transistor.

FETs have somewhat different saturation mechanisms but in the end the
effect is the same, there is a current beyond which it is not good to go
(even though dissipation may still be within specs beyond that point). FET
saturation is given in terms of Rdson Idsmax and Pdmax. Usually Pdsmax <=
Rdson * Idsmax^2.

Peter

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