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Voltage controlled high current source problem with low value sense resistor

I've put together a voltage controlled current source for use at high currents (5 Amps) based on the desgin at

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It uses a single supply op amp (OPA340 or OPA703) driving a low gate threshold logic level MOSFET (RFP30N06LE). Load connects between drain and power source. Current sense resistor from MOSFET source to ground with that junction feeding back through 10 K to inverting input. Voltage control applied to non-inverting input.

Because of the high current and the desire to minimize power dissipation in the sense resistor and voltage drop across it, I am using a 0.1 ohm sense resistor. This is resulting in the current not following the V=(current / 0.1 ohms) equation. I observe 0.725 A (expect 1 A) at 0.1 control voltage and 3.5 A (expect 5 A) at 0.5 V. These large errors are even present at low currents. Things are better for higher valued sense resistors. For example, output current is only 5% low when a 1 ohm current sense resistor is used. Is some resistance not being compensated for or ?

Reply to
Steve
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Is it oscillating/unstable? ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
         You can never be too prepared for the REPRESSION!
Reply to
Jim Thompson

Reply to
Steve

Check for oscillation first, as Jim wrote. With the cap from OUT to IN- it shouldn't happen but depends on your load. Which opamp did you use? Does its input CM range include the negative rail? What's the offset spec?

Also, mind that cell phone signals and such can recitify in many opamp inputs, so if you have long wires there you might at least want to consider a cap from IN+ to ground. Hoping it's a ground plane ...

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Regards, Joerg

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Reply to
Joerg

Either your 0.1 Ohm resistor is high, or more likely, trace resistance hasn't been accounted for.

Maybe arrange PCB traces for Kelvin sensing? ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
         You can never be too prepared for the REPRESSION!
Reply to
Jim Thompson

The error seems to be mostly gain, not offset, so the problem is probably that the sense resistor doesn't behave like 0.1 ohms. It calculates to more like 0.14 or so. If it's a 2-wire resistor, resistor leads and pcb traces may be making up that extra 0.04 ohms.

You could add a resistor from opamp pin 2 to ground to correct for the error, or use a 4-wire sense resistor.

If you see an offset error near zero, you might consider using a chopper opamp. They have microvolt offsets.

John

Reply to
John Larkin

Thanks. Looks like it probably is unaccounted connection resistance. Rearranging things has eliminated most of the error. I'll solder it all up with heavy gage wire and try again.

I have had issues in other projects where I've used alligator clip jumper wires whose thin wire is simply folded over and held against the clip by the clip's crimp.

Reply to
Steve

Schematic looks ok and it's not unstable, so have you run all the critical voltage sensing wiring to a star point ?...

Regards,

Chris

Reply to
ChrisQ

Are you using Kelvin sensing or just the grunty single wire sensing?

Reply to
Robert Baer

...THAT would be the first thing to do. Convert from a grunty single wire feedback th true Kelvin sensing.

Reply to
Robert Baer

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