Current Limited Constant Voltage Source - Page 1
Forum section: electronics / projects

author: gcp6ca
author_rank: Contributor
timestamp: August 04, 2012, 03:54:12 am
content: I am trying to build a control loop to set an output voltage and limit the current. I got the voltage control done so I tried to add the current limiting control and in theory it seemed like it would work but the current limiting acted like a comparator - max current or none at all. I decided to test out the high side current sense difference amplifier and it did not work (output voltage unpredictable) but in the low side configuration it works well. I am having difficulty trying to figure out why that is since I did not think it matter as long as the difference in voltage is the same.I have attached the schematic of the overall power source as well as the current amplifier that I tried. I tested it on a breadboard but I do not think that would cause the major discrepancies.If I do not have a large load capacitor then the circuit oscillates, is there a way to reduce that value and bring to closer to the range that linear regulators require?Also, some other information about the components:Resistor Tolerance: 5%Op-Amp Vos Max (µV): 250Op-Amp CMRR Min (dB): 75Op-Amp Rail-to-Rail: OutOp-Amp Unity Gain Stable: Yes

author: Niklas
author_rank: Frequent Contributor
timestamp: August 04, 2012, 08:10:15 am
content: From the datasheet of MCP607:Common ModeCommon Mode Input Range VCMR VSS – 0.3 VDD – 1.1The opamp handles voltages between VSS and VDD (rail to tail) only on the output, but not on the inputs. If the input voltages are above VDD - 1.1 V, in your case 3.9 V, you will get problems. The rightmost opamp in the current sense schematics will get:5V * 100/122 = 4.098 VYou can either try to change the value of the resistors R1 to R4, but that will also affect the opamp's gain. Or swap the opamps to ones that are rail to rail on both inputs and output. Microchip's MCP6002 could be a candidate.

author: Rerouter
author_rank: Super Contributor
timestamp: August 04, 2012, 10:54:08 am
content: or better yet use a current sense amplifier, sure they are a bit more pricy, but you dont have to worry about non linear gain due to voltage dividers,alternativly, divide both in half, buffer each with an op amp then use them to measure, with twice the gain,

author: gcp6ca
author_rank: Contributor
timestamp: August 04, 2012, 06:25:40 pm
content: Thanks for the help, I will try an op-amp with a rail-to-rail input/output and low offset voltage (MCP6024). I looked at current sense amplifier but I thought it would be cheaper to use extra op-amps. With the MCP607 I kept VDD at 5V and used 3.3V for the load. In this case VCMR should be around 2.71V but the circuit still did not work. I am reading the Microchip Application Note AN1332 and it seems like my problem is still with getting CMMR right.The application note gives the worst case CMMR as: CMMR = 20log((1+R2/R1)/4*Tolerance). MCP607 has a minimum CMMR of 75db so should the CMMR I calculate be above 75db or below?Edit: Actually it seems like it does not have to match the Op-Amp CMMR but rather it determines how much noise it will eliminate. So I think it would want it as high as possible.

author: gcp6ca
author_rank: Contributor
timestamp: August 06, 2012, 12:29:14 am
content: I still have not had luck with high side current sense so I will try current sense amplifiers and delta-sigma converters (final is goal is to use it battery charger/discharger with logging). For now I switched it to a low side sense configuration was able to get the Current Limited Constant Voltage Source working (schematic attached). The circuit seems to naturally oscillate until I add the massive RC filter on the main MOSFET gate - greatly reduces the capacitance needed on the load. When I do the oscillations amplitude is greatly reduced but it is still there. I was wondering if there is a better solution that can completely get rid of the oscillations.

author: Rerouter
author_rank: Super Contributor
timestamp: August 06, 2012, 02:25:25 am
content: put a moderate value say a few nF across the inverting and output pin of your comparator, it will slow its slew rate enough that it should stabilize, just be aware your current will overshoot, the larger the cap value the more overshoot,faced a similar problem myself with what i would guess is a very very similar circuit, however i was driving an npn in place of a mosfet to pull down the set voltage, so i was able to get away with only a few pF to stabilize the loop

author: gcp6ca
author_rank: Contributor
timestamp: August 06, 2012, 05:07:53 pm
content: I did not use any comparators, did you mean op-amp U1:B? I tried adding a capacitor there but it did not help. In voltage mode connecting the main P-Channel MOSFET (Q1) gate directly to to the op-amp (U2:B) and adding a 100pF between the negative and output removed the oscillation entirely. However, in current limit mode the massive oscillations exist and I am not sure where capacitors would help.