"transconductance may limit current"

Guest
If I want this mosfet to conduct several hundred amps by shorting a car
battery briefly, what is the meaning of the note about transconductance
on fig 5, Peak Current Capability and how do I know whether that's an
issue in this application?
http://www.fairchildsemi.com/ds/RF/RFP30N06LE.pdf
 
On 22 Jan 2005 19:18:47 -0800, kellrobinson@yahoo.com wrote:

If I want this mosfet to conduct several hundred amps by shorting a car
battery briefly, what is the meaning of the note about transconductance
on fig 5, Peak Current Capability and how do I know whether that's an
issue in this application?
http://www.fairchildsemi.com/ds/RF/RFP30N06LE.pdf
Click to expand...

It means that the fet could stand 400 amps for a microsecond, but it
won't necessarily conduct 400 amps. The transfer and saturation curves
indicate what the typical current might actually be.

John
 
100 amps is enough. And I can drive the gate from a little cap to
avoid the voltage droop in the battery.

Now for the really important question: why did Google put my email
address there?
It never did that before.
 
kellrobinson@yahoo.com wrote:
If I want this mosfet to conduct several hundred amps by shorting a
car battery briefly, what is the meaning of the note about
transconductance on fig 5, Peak Current Capability and how do I know
whether that's an issue in this application?
http://www.fairchildsemi.com/ds/RF/RFP30N06LE.pdf
Click to expand...
Your FET is too wimpy for the high currents you want to conduct. 47mOhms
means that with 100A you have already 4.7V across the transistor, which will
now develop 470W of dissipation, which instantly will increase the channel
resistance and heat up even more. I suggest a couple of paralleled IRL3716
which have only 4.8mOhms Rds.
http://ec.irf.com/v6/en/US/adirect/ir?cmd=catProductDetailFrame&productID=IRL3716:
These have a very high gate capacitance, so your gate driver should be able
to source/sink a couple of amps to avoid long switching times.
--
ciao Ban
Bordighera, Italy
 
kellrobinson@yahoo.com wrote:
100 amps is enough. And I can drive the gate from a little cap to
avoid the voltage droop in the battery.

Now for the really important question: why did Google put my email
address there?
It never did that before.
Click to expand...
Was it the new Google Groups Beta? You have to set a "nickname" for _every_
group by subscribing to _every_ group you post in. You can't just set one
default that works everywhere. Very annoying ....
 
Andrew Holme wrote:
kellrobinson@yahoo.com wrote:

Now for the really important question: why did Google put my email
address there?
It never did that before.

Was it the new Google Groups Beta? You have to set a "nickname" for _every_
group by subscribing to _every_ group you post in. You can't just set one
default that works everywhere. Very annoying ....
Click to expand...
Evidently designed for sock puppets.
--
John Popelish
 
why did Google put my email address there?
It never did that before.
kellrobinson

Was it the new Google Groups Beta?
You have to set a "nickname" for _every_ group
by subscribing to _every_ group you post in.
You can't just set one default that works everywhere.
Andrew Holme
Click to expand...
A tempest in a teapot, I think.
After I subscribed to the 1st group,
the subsequent forms were pre-filled-in.
I just clicked once to get the Subscribe page
and once more for OK. No sweat.

The reformatting of posts (fonts & leading whitespace) still sucks;
ASCII diagrams and sigs are severely munged.

I've figured out ways around most of the new quirks;
*show options*, *Reply to Author* takes care of ~80% of them.
 
I can get the IRL3716 in the TO-220 from arrow.com
Thanks for the info, Win. Just one thing I didn't understand:
"and examining the single pulse curve, we see that this corresponds to
a maximum pulse duration of about 500us"
I couldn't figure out which graph you meant on the IR data sheet for
the IRL3716.
 
according to their website, arrow.com has IRL3716 in the TO-220 package
Thanks for the info, Win and everybody.
Kell
 
kell wrote...
I can get the IRL3716 in the TO-220 from arrow.com
Thanks for the info, Win. Just one thing I didn't understand:
"and examining the single pulse curve, we see that this corresponds
to a maximum pulse duration of about 500us" I couldn't figure out
which graph you meant on the IR data sheet for the IRL3716.
Click to expand...
That's the Maximum Effective Transient Thermal Impedance curves,
fig 11, the bottom plot, for a single pulse. Also, the suggestion
for using an IRL3716 is due to Ban, of Bordighera, Italy.

Tell us more about your goals in the auto battery testing. I'm not
sure 100A is high enough to test a well-functioning car battery.


--
Thanks,
- Win
 
Also, low Junction to Case resistance is a good indication of good
thermal mass and large chip size. 1/2 degree C/W or so is pretty big.

boB


On 23 Jan 2005 11:58:08 -0800, Winfield Hill
<hill_a@t_rowland-dotties-harvard-dot.s-edu> wrote:

Ban wrote...

kellrobinson@yahoo.com wrote:

If I want this mosfet to conduct several hundred amps by shorting
a car battery briefly, what is the meaning of the note about
transconductance on fig 5, Peak Current Capability and how do
I know whether that's an issue in this application?
http://www.fairchildsemi.com/ds/RF/RFP30N06LE.pdf

Your FET is too wimpy for the high currents you want to conduct.
47mOhms means that with 100A you have already 4.7V across the
transistor, which will now develop 470W of dissipation, which
instantly will increase the channel resistance and heat up even
more. I suggest a couple of paralleled IRL3716 which have only
4.8mOhms Rds.
http://www.irf.com/product-info/datasheets/data/irl3716.pdf

A logic-level FET is a good choice if one wants to use the FET's
transconductance to establish a constant current electronic load.
An IRL3716 would be a good choice for a 100A load, because 100A is
close to the current at which it has zero transconductance tempco,
with about a 2.9V gate voltage for Vds = 15V, see fig 3. I'd add
a source degeneration resistor to better establish the current at
different drain voltages. For example, a 0.02-ohm low-inductance
resistor would drop 2V at 100A, and a 5.0V gate drive would bias
the IRL3716 to sink about 90A for 8V to 100A for 15V on the drain.
One can adjust the gate pulse voltage to trim the 100A current.

These FETs may be hard to get in the TO-220 version (the surface-
mount versions, which have less thermal capability, are in stock),
so an IRF IRL1404 or IRL2505 (Vgs = 3.8V), or a Fairchild FDP7045L
(Vgs = 3.3V) can be considered instead.

Done this way, with the battery current dissipated mostly in the FET,
each battery-test pulse has to be short, limited by the thermal mass
of the MOSFET. The thermal mass parameter isn't given directly on
the datasheet, but for a quick part search one can eyeball the FET's
maximum Pd spec, which is usually on the front page. A low thermal
resistance is required for a high Pd, and this usually implies a high
thermal mass. To complete the calculation for the selected FET, one
refers to the Maximum Effective Transient Thermal Impedance curves,
e.g., fig 11 for the IRL3716. For example, let's assume our FET has
about 10V across its D-S terminals during our 100A pulse, which would
be 1kW dissipation. Assuming a 150C junction temp rise, we calculate
a maximum allowed Thermal Response ZthJC = dT/P = 150C/1kW = 0.15C/W,
and examining the single pulse curve, we see that this corresponds to
a maximum pulse duration of about 500us. This is consistent with the
figure 8 Maximum Safe Operating Area plots.

These have a very high gate capacitance, so your gate driver should
be able to source/sink a couple of amps to avoid long switching times.

Right, we're talking Ciss into the 5nF territory, which requires a
0.25A gate current for a 0.1us switching time (for a 5V pulse). A
wimpy 10mA gate-drive capability, as from a CMOS 555 timer, could
result in a rather slow 2.5us to 5us switching time. That's 1 to 2%
of a say 250us test pulse.
Click to expand...
 
boB wrote...
Also, low Junction to Case resistance is a good indication of good
thermal mass and large chip size. 1/2 degree C/W or so is pretty big.
Click to expand...
Yes, Pd and Rth on the power MOSFET datasheet are directly related.

Pd = (Tj-Tc)/Rth, where for maximum Pd or Ptot, Tj is a high maximum
junction temperature like 150C or 175C, and Tc is assumed to be 25C.

Some FETs have a 200C rating, which increases the Pd. When designing
a power MOSFET into a circuit, an engineer will pick his own Tj limit,
usually at a much lower temperature to improve reliability. The case
to heatsink thermal resistance further reduces the maximum in-circuit
Pd, often dramatically so, but it's still useful to quickly evaluate
power FETs by examining their datasheet Rth or Pd values.

The IRL3716's 0.72 C/W junction-to-case thermal resistance is pretty
low. Other TO-220 contenders are the BUK7510-100B at 0.5 C/W, the
FDP047AN08 and HUF76445 at 0.48, and the winners on my list, ta-da,
fanfair... are the IRF1405 and IRF1407 at 0.45 degrees per watt.

If the MOSFET's die is bigger than the TO-220 parts above, it needs
a larger package, such as a TO-247. But a larger package does not by
itself lead to a lower thermal resistance. For example, Fairchild's
FQP33N10L is a TO-220 with 1.18C/W, and their FQP33N10L is the same
die in a TO-247 package, with the same thermal resistance spec.

Examples of TO-247 parts with lower thermal resistance than the TO-220
parts are the IRFP2907 at 0.32C/W, the SPW47N60C2 and STW47NM50 at 0.3,
the IRFP3810 at 0.26, and Fairchild's powerful FDH44N50 at an amazing
0.20 C/W, which with leads to a Pd rating of 750 watts at Tj = 175C.

As mentioned, one cannot actually use all the apparent muscle, because
of the required lossy heat-sink thermal connection. However, consider
a power part like the IXFN44N50, which has a identical 500V 44A rating.
This FET comes in the SOT-227 package that includes an insulator, so
the disclaimer is muted. Here, the amazing 0.24C/W and 520W ratings
come close to what you can actually do with this single power MOSFET.


--
Thanks,
- Win
 
I intend to do battery reclamation/rejuvenation, not testing. Shorting
lead or nickel batteries gets rid of dendrites.
Also, possibly desulfation. I have a charger design in mind that will
use inductive pulsing for charging lead acid batteries, and thought I
would add a discharge pulse function.
 
kell wrote...
I intend to do battery reclamation/rejuvenation, not testing.
Shorting lead or nickel batteries gets rid of dendrites. Also,
possibly desulfation. I have a charger design in mind that will
use inductive pulsing for charging lead acid batteries, and
thought I would add a discharge pulse function.
Click to expand...
100A pulse testing can show the battery's series impedance, but
so can 10A or even 1A testing. To determine whether a battery
has a serious electrochemical capability, you'll want to remove
serious amounts of energy. This is most easily done with massive
thermal-mass structures, implemented with resistive elements. An
attempt to use a power MOSFET's thermal mass for this purpose will
no doubt be futile. Consider, 100A * 12V = 1.2kW, which for 500us
= only 0.6W-sec, a minuscule amount of energy. No doubt the most
effective approach is a massive power resistor in oil or water.


--
Thanks,
- Win
 
On Mon, 24 Jan 2005 17:09:16 -0800, kell wrote:

I intend to do battery reclamation/rejuvenation, not testing. Shorting
lead or nickel batteries gets rid of dendrites.
Also, possibly desulfation. I have a charger design in mind that will
use inductive pulsing for charging lead acid batteries, and thought I
would add a discharge pulse function.
Click to expand...
Commutated hockey-puck SCR's. :)

Good Luck!
Rich
 
On 24 Jan 2005 17:09:16 -0800, "kell" <kellrobinson@billburg.com>
wrote:

I intend to do battery reclamation/rejuvenation, not testing. Shorting
lead or nickel batteries gets rid of dendrites.
Also, possibly desulfation. I have a charger design in mind that will
use inductive pulsing for charging lead acid batteries, and thought I
would add a discharge pulse function.
Click to expand...

Applying a voltage to a sulfated flooded battery for a long time may
help it, but, if you need the battery now, and it is badly sulfated,
it's probably best to just recycle it and get a new battery. I think
that shorting the battery would do more harm than good. Trying to
charge it would probably be better.

Dendrites in NiCaDs can usually be burned out by briefly applying a
slightly higher voltage, good battery across its terminals.

boB
 
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