Roast my Adjustable Constant Current Circuit - Page 1
Forum section: electronics / projects

author: seancsnm
author_rank: Regular Contributor
timestamp: August 21, 2022, 05:09:46 pm
content: I'm working on a low-cost digitally adjustable constant current regulator for an application that emphasizes a low-cost and part count BOM over performance. My target requirements are as follows:38 V (+/- 2 V) output voltage capability12-36 V input voltage rangeCurrent range of 0-50 mA over resistive load that can vary from a couple hundred ohms to several thousand ohms (user must ensure V=IR limitations aren't exceeded, i.e. trying to run at 50 mA over a 5k ohm resistance with only a 40 V supply)Current resolution of 1 mA and accuracy of 5% over full current range and 0-1 Hz (meaning I don't care about high-frequency ripple, noise, or transients, only average current on a long time constant)No damage to user or circuit under sudden open circuit-> short circuit or short circuit -> open circuit conditionsI'll start with a general overview then add some details. The top half is the boost converter running on the MC34063. V12 is the input voltage and the output is dumped on Rs5. The bottom half is the current regulator centered around the feedback circuit created by M2 (the adjustable resistor), Rs3 (the current sense resistor), and U3 (drives M2 to make output current match the set current given by the Iset voltage divider). I'm mostly looking for high-level critique here. Is there a simpler/more stable circuit that can be used in place of something here? Is there a way to combine the current regulator with the boost converter while reducing cost? Stuff like that but I'll add some detail to provide my thought process.This boost converter provides the ~40 V source, seen at the top of the schematic. The chip is rated at 36-40 Vout (depending on the manufacturer) but it's readily available and cheap so I want to make it work even if it means adjusting design requirements a bit. I think I'll have to make some adjustments to ensure the switching voltage doesn't exceed that 40 V limit. Maybe reduce the voltage output to 30-35 volts. I also need to check to see whether I can use a higher voltage external transistor to increase that voltage limit. R1 is an over-current sense resistor (I haven't read the datasheet to see how it's used). I have a high level understanding of switching regulators but am no way comfortable using them so it's likely there are other issues as well.For the regulator circuit, S1 represents a short-circuit load to see how the circuit behaves when shorted. In typical use cases, this will be a variable resistive load that decreases in resistance over time. Rs5 is a dumb current limiting resistor to prevent excessive current surges in M2 under open-circuit->short-circuit conditions. Not the most elegant solution as it wastes a lot of voltage overhead under heavier loads. I haven't nailed a mosfet down yet but from my understanding, it just needs to be selected for low gate capacitance (to minimize switching time) while having a high enough Vds and Ids. The drive amp, U3, should be switched with an amp that can output a bit closer to the rails (at least down to below the mosfet threshold voltage) when using a single-ended supply. Iset is likely coming from a microcontroller PWM output that has a low cutoff RC filter to give an analog voltage. So high input impedance on the amp may be necessary. Otherwise the amp should have pretty lenient requirements and I should be able to use something pretty low-cost.

author: moffy
author_rank: Super Contributor
timestamp: August 22, 2022, 12:51:13 am
content: Just a few comments about items I noticed:A) Naming.1. Make Vaux- just your normal GND and use a RRIO opamp that can cope with at least a 12v supply.B) Other stuff1. You might also want to increase the value of C2. A minimum load across the output of the switching regulator might help also.