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Registered Member #191
Joined: Fri Feb 17 2006, 02:01AM
Location: Esbjerg Denmark
Posts: 720
no, i mean the output of the opamp being clamped by the supply voltage.
also, if you run it maxed out like that, any change in the output voltage of your 5V flatpack will result in current drift.
What kind of IGBT are you using, I don't think you are suppose to use IGBTs in the active region. Something about their NTC and on die current hot spots, i think.
Laptop fans are 5V for 5V to 12V boost, PTN04050C.
Registered Member #99
Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637
Electroholic wrote ...
no, i mean the output of the opamp being clamped by the supply voltage.
also, if you run it maxed out like that, any change in the output voltage of your 5V flatpack will result in current drift.
What kind of IGBT are you using, I don't think you are suppose to use IGBTs in the active region. Something about their NTC and on die current hot spots, i think.
Laptop fans are 5V for 5V to 12V boost, PTN04050C.
Hello, I am not sure what you mean by the output being clamped by the supply voltage? I am running everything off the 5 volt flatpac. The op-amp's supply voltage is tied directly to the +5v and its ground to the negative. It's output is just under 5 volts max. Yea, the IGBT's are only temporary. I found some 55volt 75amp t0247 devices at digikey that I will use. I did notice the needle moving a little when I maxed out past 11amps. I backed it down to 10amps and it was solid, but heating was large on the sense resistors, so I backed down to 5 amps. Thanks! Matt
Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308
What's he's getting at is most mosfets and IGBTs have a linear range from like 3 - 10V typically. So your op-amp's output will only turn the mosfet/IGBT on half-way, limiting the maximum current you can draw. I assume the IGBT allows for more current because 5V is enough to nearly saturate it.
Registered Member #99
Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637
Uzzors wrote ...
What's he's getting at is most mosfets and IGBTs have a linear range from like 3 - 10V typically. So your op-amp's output will only turn the mosfet/IGBT on half-way, limiting the maximum current you can draw. I assume the IGBT allows for more current because 5V is enough to nearly saturate it.
Hmmm...So maybe using a 5 volt op-amp is a bad idea? Or, do I just need to find a transistor that will turn on at a much lower voltage? Maybe the FET's I have in mind (to replace the IGBT's) will not work well either due to this fact. Or maybe I should look into IGBT's for this? Or maybe think about using a 12 volt op-amp. This one seems to be very ideal except for its output Matt
Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308
Mosfets that are saturated at 5V exist and will be your best bet seeing as you want to use a 5V battery and the rest of the circuit is designed for it. The BUZ31L for example, or any logic level mosfet will work well.
Registered Member #99
Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637
Uzzors wrote ...
Mosfets that are saturated at 5V exist and will be your best bet seeing as you want to use a 5V battery and the rest of the circuit is designed for it. The BUZ31L for example, or any logic level mosfet will work well.
Hello, thanks for that tip! I have been scanning through datasheets at digikey and found the HUF75345G3... Datasheet: This one appears to be quite ideal to me...Can you guys find anything wrong with it? There is a chart that graphs gate voltage vs. current through the device. At 5 volts, the current is well over what I am going to need...Or, that is how I see things...
Thanks for all the help! This project is now coming along very well! Just need to work on: 1-Making the current stay put. I ordered some precision voltage refs. to try out... 2-Minimize heating 3-High current
Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308
It looks like it should work to me, if you have problems you could always build a small boost converter to get a little more voltage just for the logic section.
Edit: Another thing I thought of, the mosfets you had earlier might have had a rather large on-resistance. Even with say, 0.2 ohms if they are passing 7A that's a voltage drop of 1.4V across the mosfet leaving 3.6V for your diode. You should check it's forward voltage at various currents to make sure the voltage drop isn't too high for the current you want to pass. Not that it will matter as much with a 0.007 ohm device though.
Registered Member #99
Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637
Ok, so I got some new mosfets in the mail. They work great. Very little heating, even at high current. (5 amps or so). I also got a 5 volt fan to run off the 5 volt supply. I also added a precision 2.5 volt bandgap vref. I am still having trouble with the walking. Its more stable now, but still will drop off. Here is what it does: I will set it at some current. It will constantly go up and down by a few mA's and then begin to just fall off the radar. It will drop some 50mA before I get frustrated and turn it off. It does occasionally begin to climb back to the setpoint, but never gets significantly close to what I want it set at. The vref is rock solid, but it does jump around once current is being drawn. Any ideas about that? Thanks! Matt
Registered Member #99
Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637
Yup...Still the breadboard...But now I have some success! I removed some wires and replaced a few others. Minimized anything carrying current. (Mostly the ground wires). Probed around the board with my DMM and found "iffy" spots. Beefed them up with several wires. So far, the current has moved AT ALL! (Yea, pop the wine champaign bottles!). I will test it further before I have the board made. I have what I am about to call the final design. I havent tested the t0-247 fets i got, but they ought to work better than the t0-220 that I am using now.
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High Current Laser Diode PSU
