Electronics > Projects, Designs, and Technical Stuff
Constant current mode for DIY power supply
aiq25:
I'm working on a home made power supply (for fun). I simulated the attached circuit and it seems to work. I don't remember if I came up with this circuit or if I got it from somewhere, I have been working on this project for a while, on/off redesigning it multiple times. The output of this circuit will feed into a LM317 for voltage regulation. This is a lower power, power supply, ~1.25V to 12V regulated output and I would like current adjustment. I'm going with a linear design for the voltage regulator because I already have all the parts and heatsink I can use and it will be easier for me to understand and analyze.
I would appreciate some feedback. I'm not sure if I will go with LM324 but that's what I put in the simulation. Also not sure of what MOSFET to use. I just choose one that is in TI-TINA.
Basically I'm looking for some feedback on anything that I should be careful with or drawback of this design.
T3sl4co1l:
How precise does it have to be?
In my experience, a "ring of two" style circuit, with a thermistor to compensate for Vbe tempco, performs at least as well as you'd need for a "precision" CCS upstream of an LM317 (which draws some current itself, so you can't expect the output current to be quite accurate to what your CCS is doing).
I suspect you'll have problems with compensation, for which a series resistor to U1 -IN and an R+C across U1 (OUT to -IN) will do. Values depend on T1 capacitance and T1, T3 and U3 gains.
LM324 just barely works as shown, either by accident, or by the SPICE model being oversimplified (which often happens -- don't trust your SPICE models, verify them when you can..). Namely, you need VCC >= VIN to deal with the input common mode range of LM324 (which is about 1.5V below VCC; the diff sense divider happens to read a common mode of 9/10ths of VIN, or exactly 1.5V below, a rather handy coincidence at this voltage :) ). Better options include TLV2372 (18V max, not really the best here), TL072 (includes +V rail, but not -V; beware phase reversal and output voltage range -- probably unsuitable for U1), or uhh, some other RRIO amps that I'd have to go shopping for (except for the OPA2192 that does come to mind, but you surely don't need the precision and cost of that one :) ).
Tim
David Hess:
The two operational amplifier functions can and should be combined into one to simply frequency compensation.
xavier60:
If the resistors in the balanced amplifier were all 10K, the op-amp's common mode voltage will be half of VIN.
If there is instability, local negative feedback can be added to U1 although it will cause some extra delay in limiting when a sudden overload occurs.
T3sl4co1l:
--- Quote from: xavier60 on February 18, 2019, 04:56:54 am ---If the resistors in the balanced amplifier were all 10K, the op-amp's common mode voltage will be half of VIN.
If there is instability, local negative feedback can be added to U1 although it will cause some extra delay in limiting when a sudden overload occurs.
--- End quote ---
Yeah, diff amps kinda suck if you can't get common mode to behave AND you need gain. :(
You can also pull down the inputs -- consider for example, 10k+10k from sense resistor to input to ground. Or to put that another way, take the circuit as shown, but add a pull-down from each input to ground. You get a common mode of ~1/2 VIN, and half the differential signal at the inputs. Put in feedback resistors as usual (including the resistor from input to ground, or its parallel-combined equivalent), and you get whatever gain you wanted -- but now your noise gain is doubled (which means input offset error, and input referred noise, are both doubled with respect to the output).
A ready-made solution, like an INA1xx current sense amp, gets attractive in this case. :)
Tim
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