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BJT or MOSFET for Current Source Circuit

Started by panfilero February 5, 2014
I'm looking to make a "Current Source" I'd like to supply a maximum of 10A, I'm thinking about putting a transistor at the output of an Op-Amp and using the transistor to pull the 10A through my load.

My question is, would I be better off using a MOSFET or a BJT for this (darlington or maybe even IGBT) it really just has to sit there and handle 10A... does this point towards which transistor has a better SOA profile?

much thanks!

On Wed, 05 Feb 2014 09:03:25 -0700, panfilero <panfilero@gmail.com> wrote:

> I'm looking to make a "Current Source" I'd like to supply a maximum of > 10A, I'm thinking about putting a transistor at the output of an Op-Amp > and using the transistor to pull the 10A through my load. > > My question is, would I be better off using a MOSFET or a BJT for this > (darlington or maybe even IGBT) it really just has to sit there and > handle 10A... does this point towards which transistor has a better SOA > profile? > > much thanks! >
Actually, to make a BROADBAND current source into the 10's of MHz, it's better to use a good resistor. Something like 100 ohms would even be better than an active component. Go look at the schematic for HP's 8552 plug-in, part of the Spectrum Analyzer line and you'll find such a resistor. albeit a bit high wattage. [End of tongue-in-cheek.] I'd preference towards the IGBT, as yielding a bit better control for those 'unexpected' scenarios, ...that ALWAYS happen. Be sure to match a passive network to make the output Z stay relatively constant over your spectrum, else the capacitance starts lowering it dramatically as you go up in frequency. You're starting to get into 'component' characteristics, maybe Jim will jump in with a definitive answer instead of an opinion.
On Wednesday, February 5, 2014 11:03:25 AM UTC-5, panfilero wrote:
> I'm looking to make a "Current Source" I'd like to supply a maximum of 10A, I'm thinking about putting a transistor at the output of an Op-Amp and using the transistor to pull the 10A through my load. > > > > My question is, would I be better off using a MOSFET or a BJT for this (darlington or maybe even IGBT) it really just has to sit there and handle 10A... does this point towards which transistor has a better SOA profile? > > > > much thanks!
Wow.. 10A current source.. no matter what you choose you'll need a monster heat sink. How about using a current regulated power supply. You can get a 10 Amp 30V Mastech for less than $200. (I was looking the other day.) Do you need to do something fancy with the 10A? like modulate it or something? George H.
On Wed, 5 Feb 2014 08:03:25 -0800 (PST), panfilero <panfilero@gmail.com> wrote:

>I'm looking to make a "Current Source" I'd like to supply a maximum of 10A, I'm thinking about putting a transistor at the output of an Op-Amp and using the transistor to pull the 10A through my load. > >My question is, would I be better off using a MOSFET or a BJT for this (darlington or maybe even IGBT) it really just has to sit there and handle 10A... does this point towards which transistor has a better SOA profile? > >much thanks!
Mosfet. Better SOAs, no base current error, easy to drive. Post your schematic and we'll check it for you. -- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation
On Wednesday, February 5, 2014 10:28:46 AM UTC-6, John Larkin wrote:
> On Wed, 5 Feb 2014 08:03:25 -0800 (PST), panfilero wrote: >=20 >=20 >=20 > >I'm looking to make a "Current Source" I'd like to supply a maximum of 1=
0A, I'm thinking about putting a transistor at the output of an Op-Amp and = using the transistor to pull the 10A through my load.
>=20 > > >=20 > >My question is, would I be better off using a MOSFET or a BJT for this (=
darlington or maybe even IGBT) it really just has to sit there and handle 1= 0A... does this point towards which transistor has a better SOA profile?
>=20 > > >=20 > >much thanks! >=20 >=20 >=20 > Mosfet. Better SOAs, no base current error, easy to drive. >=20 >=20 >=20 > Post your schematic and we'll check it for you. >=20 >=20 >=20 >=20 >=20 > --=20 >=20 >=20 >=20 > John Larkin Highland Technology Inc >=20 > www.highlandtechnology.com jlarkin at highlandtechnology dot com =20 >=20 >=20 >=20 > Precision electronic instrumentation
Thanks, it's just to control a heater, just a resistive load. Here's a sket= ch of what I'm thinking. http://i555.photobucket.com/albums/jj477/panfilero/CurrentSource.jpg I was looking at the STY139N65M5 Mosfet, thinking it would need minimal hea= tsinking It has thermal resistance of 30C/W (j-a), if I double RDSon from 17mOhm to = 34mOhm, at 10A that's 3.4W, which at 30C/W is 102C above ambient... which i= s very hot but I think the part should survive right? I'll be heatsinking i= t anyway. 10A is my max which I probally wont use muc. much thanks!!
El 05/02/2014 18:22, panfilero escribi&#2013265923;:
> On Wednesday, February 5, 2014 10:28:46 AM UTC-6, John Larkin wrote: >> On Wed, 5 Feb 2014 08:03:25 -0800 (PST), panfilero wrote: >> >> >> >>> I'm looking to make a "Current Source" I'd like to supply a >>> maximum of 10A, I'm thinking about putting a transistor at the >>> output of an Op-Amp and using the transistor to pull the 10A >>> through my load. >> >>> >> >>> My question is, would I be better off using a MOSFET or a BJT for >>> this (darlington or maybe even IGBT) it really just has to sit >>> there and handle 10A... does this point towards which transistor >>> has a better SOA profile? >> >>> >> >>> much thanks! >> >> >> >> Mosfet. Better SOAs, no base current error, easy to drive. >> >> >> >> Post your schematic and we'll check it for you. >> >> >> >> >> >> -- >> >> >> >> John Larkin Highland Technology Inc >> >> www.highlandtechnology.com jlarkin at highlandtechnology dot com >> >> >> >> Precision electronic instrumentation > > Thanks, it's just to control a heater, just a resistive load. Here's > a sketch of what I'm thinking. > > http://i555.photobucket.com/albums/jj477/panfilero/CurrentSource.jpg > > I was looking at the STY139N65M5 Mosfet, thinking it would need > minimal heatsinking > > It has thermal resistance of 30C/W (j-a), if I double RDSon from > 17mOhm to 34mOhm, at 10A that's 3.4W, which at 30C/W is 102C above > ambient... which is very hot but I think the part should survive > right? I'll be heatsinking it anyway. 10A is my max which I probally > wont use muc.
Your calculations are wrong, the STY will dissipate Vcc*10-(RL+Rshunt)*10*10 watts. -- Saludos Miguel Gim&#2013265929;nez
For just a heating element I would switchmode it. It's such a piece of cake=
 and won't use a ton of aluminum. You don't even need any inductance. Any f=
requency that doesn't bother your ears should be fine.

Then your source resistor can become a small inductor for current feedback =
making the whole thing a tad more efficient.=20

I also question why to use a regulated current for a heating element. If it=
 is well insulated I guess that is a good method of temperature control but=
 if there are variations in ambieent that will affect it, then controlling =
voltage might be better because it will tend to self regulate as ambient ch=
anges. Of ocurse there are some applications that would require it, like fo=
r measurments or something. If that's the case and you're stuck with curren=
t mode control I think switching would still be much better.=20

Otherwise that FET may produce more heat than the heater. If you don't feel=
 ike designoing the PWM you could probably adapt someting that already used=
 a standard old (read cheap) IC like a upc494, something like that. Of cour=
se there are better ones now. With any luck someting will drive the FET jus=
t right and keep your component count down.
>"I was looking at the STY139N65M5 Mosfet, thinking it would need minimal h=
eatsinking " If by that you mean that it can dissipate 625 watts, don'e think you autoat= ically get all kinds of headroom. the maximum operating temperature is no h= igher so a 625 watt device needs as much heat sinking to dissipate 100 watt= s as does a 250 watt device. You simply don't gain that way. With it not being switched, the only time it will run col is all the way on= or off. Sure it will run cool switched but that is not what the drawning i= ndicates. If you use 24 volts DC in and want the current say, developed by = 12 volts at the element you let's say 7 amps at 12 volts. You need the heat= sink for 84 watts even if the FET can dissipate 10,000 watts.=20 A higher dissipation device may derate more slowly than a lower one, but th= e curve still ends up at the same place - zero watts at 200C or something l= ike that.
On Wed, 05 Feb 2014 08:03:25 -0800, panfilero wrote:

> I'm looking to make a "Current Source" I'd like to supply a maximum of > 10A, I'm thinking about putting a transistor at the output of an Op-Amp > and using the transistor to pull the 10A through my load. > > My question is, would I be better off using a MOSFET or a BJT for this > (darlington or maybe even IGBT) it really just has to sit there and > handle 10A... does this point towards which transistor has a better SOA > profile?
Whatever you use, read the data sheets very carefully. SOA profile has as much to do with packaging as with the basic technology, and most transistors these days, of whatever stripe, are made to be power switches, not current-passing devices. I'm going to go against J.L.s suggestion and say that maybe you should use 2N3055's, or other devices using that die. If you want the "no base current error" then use a MOSFET from collector to base in a Darlington configuration. Even if you use someone's xxx3055, though, you should still make sure it's not going to burn up. -- Tim Wescott Wescott Design Services http://www.wescottdesign.com
On Wed, 05 Feb 2014 09:59:52 -0800, jurb6006 wrote:

> For just a heating element I would switchmode it. It's such a piece of > cake and won't use a ton of aluminum. You don't even need any > inductance. Any frequency that doesn't bother your ears should be fine.
I second that. Please explain why you feel you need a controlled current! -- Tim Wescott Wescott Design Services http://www.wescottdesign.com