I'm trying to design a power current source. It has to have some features that make it a bit of a challange to design (for me at least...) it has to do the following:
- supply about 2-3 amps (no need for programmability, it can be preset to a value when constructed).
- have a low minimum voltage drop, of 1 Volt or less (0.5V would be better). That is, it should work even if Vcc=12V and the load needs 11V to keep the preset current. of course, it should also work with a higher Vcc.
- keep a reasonably constant current for both changes in the load, and (the bigger problem) for a changing Vcc.
- the load is returned to ground, so the source has to be on the "high" side.
- NO need for good precision, +/-5% is more than ok. the conditions are DC so bandwidth isn't an issue as well.
it seems that each design I find or make meets only some of the criteria above. for example, an LM317 or similar regulator in a current loop would have been fine, except the large minimum voltage drop (definitely not less than 2 or even 3 volts).
An important parameter is: How much higher? This will set the dissipation required in the pass element.
Obviously, "reasonable constant" is too vague to permit a design. How fast does the load change? How much current variation can be permitted as that change occurs?
Changing load and Vcc means "not DC".
You will find that a power PMOSFET with a current sense resistor in the source or drain, together with an op-amp circuit to control the gate in response to sensed current deviation from a setpoint, will likely do the job easily. A design will probably show up here shortly after you firm up your requirements.
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--Larry Brasfield
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I think I would use a big P-channel mosfet with a fraction of an ohm source resistor, with a rail to rail input opamp driving the gate. Add a voltage reference chip tied to the positive rail and you have something for the opamp to compare the source resistor drop against.
Fred, you need not waste any more bandwidth with such drivel. I will gladly assume your low opinion of me and my posts without further notice. And if you should change your mind, I would just as soon not learn about that unless you have undergone a total personality transformation.
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--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.
thank you for taking the time to look through my requirements.
as for the calrifications:
I'd say the maximum difference between Vcc and the load will be 3 to 4 volts. the minimum should be as low as possible, say 0.5 Volt. on 2 amps, that gives 8 watts of dissipation in the pass element. is it reasonble for to-220 and only a small heatsink?
as for the "reasonably constant current", I need about +/- 5%, as I specified later in the post.
you were right, since the load and Vcc are changing, the conditions should be more accurately called "nearly DC" I suppose. I approximate the load would only change at a max rate of 1-2% per second (because of heating). the voltage would also drop or rise slowly. the exception is that if this would be connected to a 12V source from a car, and the car starts, the voltage would rise from 12V to 13.8 or so pretty quickly (do you have an idea how fast, or how bad would transient spikes durning ignition would be?)
the idea of an opamp controlling a P mosfet sounds interesting. I suppose that for 0.5V minimum drop, I need a mosfet with Rds(on) of
0.25 Ohm. is that a reasonble requirement? (for a single digit price tag, that is). would the opamp work well if the drop on the sense resistor be only 100mV or so?
if you can give me a recommendation for a single supply rail to rail opamp, as well as a mosfet the will do, I'll be most grateful. I don't do much analog, so some pointers would save me lots of time.
There are a few subtleties in such a circuit, such as isolating the opamp from the high gate capacitance of the FET, and using a Kelvin connection for the low-value sense resistor, including the return for the reference divider chain. If the "12V" is a battery in an operating vehicle, provision should be made so "load-dump" spikes won't damage the opamp. Inductive loads may be applied, so we'll add a protective diode. Fairly high power may be dissipated in the MOSFET, so we'll add a heat-sink thermal cutout. And it's useful to switch the power at low current levels, so we'll add that too.
Although the concept is simple, a fully-fleshed-out circuit begins to show some signs of complexity. viz,
That helps, but you are still looking at 18 W dissipation under "normal" circumstances. (I rely on the 3 Amp output you mentioned in your earlier post and battery charging voltage of 16V.) During a load dump transient, that could briefly go to 75W. (The transient is brief enough that you need not size the heatsink for it, but it may mean that you want to keep the junction cooler to provide some headroom for the thermal transient.) Whether that can be managed with a small heatsink is questionable. I would rely on the heatsink vendor's datasheet.
So, the response to a voltage transient must stay within that bound. This makes the design a little more interesting and likely makes it a good idea to low pass filter the incoming supply for both the FET and op-amp. You will likely want to put a resistor in series with the integrator feedback capacitor to improve the transient response of the current feedback loop.
I was not trying to quibble about the meaning of "DC". Ignition transients are pretty easy to filter out before they hit the active part of your circuit. The load dump transients change much faster than you appear to suspect. You will need to research them.
Yes. Maybe.
I will not claim these are the most cost effective parts, but they will do the job:
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This op-amp, suggested by Mr. Hill, is well suited for this application due to its positive rail input range and capacitive load tolerance.
Just a few tips for your design:
Be sure to zener protect the op-amp supply. Transients on an automobile "12V" rail can be surprisingly large due to quickly changing loads, especially removed loads, and the way alternators are regulated. (This effect is often called "load dump".) I would use a 24 or 27 Volt zener diode, grounded at one end.
Unless you are willing to see an output current spike sometimes, the MOSFET output stage should get similar protection. I would use a power rectifier from the low end of the current sense resistor to the zener mentioned above. You should satisfy yourself that the zener's power rating is sufficient to absorb a load dump transient. (I would have to research this to know what that might be.)
Your op-amp circuit will be operating with inputs at the positive (filtered, protected) rail, and its output is referenced to that rail by the feedback condition, so be sure not to ground reference those parts of the circuit that influence the controlled current.
You're still welcome.
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--Larry Brasfield
email: donotspam_larry_brasfield@hotmail.com
Above views may belong only to me.
Our Roman X represents an upper-case chi (the first letter in the word pronounced "Christos" (Christ), as written in Greek).
So it's not the same as the x in xfrmr. But we can keep the x in xfmr, xducer, xmtr and so on. It could result in some obscure bumper stickers.
Best regards, Spehro Pefhany
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I read in sci.electronics.design that Jonathan Kirwan wrote (in ) about 'Current source design (tricky?)', on Thu, 10 Mar 2005:
I thought it was someone who used to work for Texas Instruments.
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I think I'd be worried about keeping the supply ABOVE 12V while starting that SUV on a cold winter morning with a 3 year old battery.
I think I'd look into some of the high-side current sensing ICs designed for laptops. Maxim maybe??? mike
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