2Amp CW Current Source, BJT or MOSFET?

[[email protected]]

Hi,

I am thinking about a 2Amp CW current source project. I prefer a
simple, working on the linear region current source. Op-Amp +
Transistor.

Does MOSFET have better performance than BJT at this point? Which one
has lower resistance in linear region at 2A?

Do you have recommendations for MOSFET? I tried SI9426DY, IRLR3105,
from the datasheet I can only guess the resistance at 2A in the linear
region.

Thanks!

 

[Eeyore]

Hi,

I am thinking about a 2Amp CW current source project. I prefer a
simple, working on the linear region current source. Op-Amp +
Transistor.

Does MOSFET have better performance than BJT at this point?

Define " better ".

Graham

 

[[email protected]]

at this point, I mean, 0-2A linear cw current driver, working in the
linear region with less power consumption on transistor.

 

[Rich Grise]

I am thinking about a 2Amp CW current source project. I prefer a
simple, working on the linear region current source. Op-Amp +
Transistor.

Does MOSFET have better performance than BJT at this point? Which one
has lower resistance in linear region at 2A?

This is one of the things you were supposed to have already learned from
your textbook or in class.

Hint: Ohm's law is involved.

Good Luck!
Rich

 

[Rich Grise]

Top-Post Repaired

Define " better ".

[/QUOTE]

at this point, I mean, 0-2A linear cw current driver, working in the
linear region with less power consumption on transistor.

What are you using "CW" to mean in this context?

And don't top-post - it's rude.

Thanks,
Rich

 

[[email protected]]

If better = less $$$ -> MOSFET.

Needs less (close to zero) drive power from the opamp while a BJT isn't
likely to have enough beta for that. And fat MOSFETs have come down to
about the price levels of fat BJTs. 15 years ago I'd have given a
different suggestion.

BJT might be Darlington. So back to the question, in the linear
region, which one has less resistance? BJT or MOSFET?
CW means continuous, i.e., charging MOSFET gate capacitance is not a
problem here.

I built several driver about 1A without forced air cooling. For 2A I
am doubted.

 

[[email protected]]

This is one of the things you were supposed to have already learned from
your textbook or in class.

Hint: Ohm's law is involved.

Good Luck!
Rich

BJT might be Darlington. So back to the question, in the linear
region, which one has less resistance? BJT or MOSFET?
CW means continuous, i.e., charging MOSFET gate capacitance is not a
problem here.

I built several driver about 1A without forced air cooling. For 2A I
am doubted. I heard some company make devices called linear mosfet,
anybody used it before?

 

[[email protected]]

To summarize several of the clarifications:

You're making a linear constant-current source, and you want to choose
the pass transistor that's going to burn up the least heat because you
want to avoid using heat sinks.

So you're going to drive 2A into some load, and that load is going to
respond with whatever voltage it needs across it for 2A to flow.  That
voltage is going to be fixed by the characteristics of the load and the
fact of the 2 amps.

Then you're going to power the transistor from some supply, whose voltage
is going to be determined by the 2A that the transistor is drawing.

So the voltage across the transistor will be fixed by the load and the
supply, and the current through the transistor will be fixed at 2 amps.  
Power = voltage * current.

Now, You tell Us how changing the transistor is going to changed the
power dissipation with a fixed voltage and a fixed current.

--http://www.wescottdesign.com

Yea, this is right. My mistake.

 

[[email protected]]

BJT heat feels like a different kind of heat, just like a wood stove

maybe I need to lower the supply rail or change the components working
in wider temperature range. Irlr3105pbf works from -55 to + 175C, but
high temperature will degrade the performance.

 

[Rich Grise]

BJT might be Darlington. So back to the question, in the linear
region, which one has less resistance? BJT or MOSFET?
CW means continuous, i.e., charging MOSFET gate capacitance is not a
problem here.

Resistance is irrelevant. The device will adjust its "resistance" to
whatever it needs to be to pass the regulated current.

You should ask, "Which is more efficient?".

Hope This Helps!
Rich

 

[Rich Grise]

BJT might be Darlington. So back to the question, in the linear
region, which one has less resistance? BJT or MOSFET?
CW means continuous, i.e., charging MOSFET gate capacitance is not a
problem here.

I built several driver about 1A without forced air cooling. For 2A I
am doubted. I heard some company make devices called linear mosfet,
anybody used it before?

I've answered this in another sub-thread. Resistance is irrelevant. The
device will adjust its own "resistance" to whatever it needs to be to
pass the needed (regulated) current.

You _did_ say a current source/sink, right?

Hope This Helps!
Rich

 

[Rich Grise]

BJT heat feels like a different kind of heat, just like a wood stove

And don't forget the clean, bright crispness of that BJT sound! ;-)

Cheers!
Rich

 

[whit3rd]

Hi,

I am thinking about a 2Amp CW current source project. I prefer a
simple, working on the linear region current source. Op-Amp +
Transistor.

Does MOSFET have better performance than BJT at this point?

Depending on the voltage source you use, the transistor
will have to dissipate 2A * V watts, and the usual "performance"
requirement is going to have a lot to do with heatsinking
and price, not much to do with the other transistor
properties. If you start with 20V unregulated power (use
a regulated 12V for the op amp and such), the transistor
will shed up to 40W.

TIP120 darlington transistor will do it: $0.49 each.
IRF520 NMOSFET will do it, too: $0.56 each.

The heatsink will be the expensive part.

 

[Robert Monsen]

Hi,

I am thinking about a 2Amp CW current source project. I prefer a
simple, working on the linear region current source. Op-Amp +
Transistor.

Does MOSFET have better performance than BJT at this point? Which one
has lower resistance in linear region at 2A?

Do you have recommendations for MOSFET? I tried SI9426DY, IRLR3105,
from the datasheet I can only guess the resistance at 2A in the linear
region.

Thanks!

The main deal here is drive current. With a bjt, you'll need 200mA.
So, use a darlington or a mosfet, whichever is cheaper. Either will
work. Use the one with the best heat transfer characteristics.

I would reference the output ground at the transistor's drain or
collector. Use a small shunt (maybe .1 ohm) and a single supply opamp.
An LM324 works well for low frequency loads, and you can't beat the
price.

Sizing the heat sink will mean predicting how much voltage will be
across the device at max power. If your load is a short to 100V, then
the mosfet + heatsink will need to dissipate nearly 200W at 2A. If the
load is a short to 10V, then they will only need to dissipate 20W at
2A.

Regards,
Bob Monsen