LM2596-ADJ CC CV Supercapacitor Charger - Page 1
Forum section: electronics / projects

author: UPI
author_rank: Regular Contributor
timestamp: April 12, 2016, 01:54:57 am
content: I am building a circuit that relies on Supercapacitors to run a stepper motor for a couple of hundred milliseconds when power fails. I have designed (mostly plagiarized) the attached LM2596 buck converter set at 14V out that is current limited to ~.5A. C8, C9 and C10 are the 5.4V supercapacitors that will be charged to ~13.5V.The first half of the LM358DT is used to limit the current to ~500mA. The second half of the LM358 op amp is designed to light an LED when the voltage reaches ~13.5V as the supercapacitors are almost at their fully charged voltage of 13.5V (supercapacitors are after the ~.5V diode drop).We have prototyped the circuit using quality components from Mouser and it all appears to work well in our quick bench testing. As of this evening we have not done a full characterization but the output is clean (~5mV ripple), the current is limited, the voltage is relatively stable under load and the fully charged indicator is working.Main parts used:Bourns SRR1208-221KL 220uh 1.1A InductorTexas Instruments LM2596SX-ADJ/NOPB Adjustable Buck ConverterVishay VSSB410S-E3/52T 4A Schottky DiodeSTMicroelectronics LM358DT Op AmpNichicon UHV1V221MPD 220uf 35V CapacitorsTT Electronics LRMAM0805-R02FT5 .020 ohm current sensing resistorTaiwan Semiconductor BZT55C15 15V .5W ZenerPowerStor / Eaton PHV-5R4H505-R 5F 5.4V SupercapacitorsDoes anyone see any glaring issues or have any suggestions for improvement?

author: UPI
author_rank: Regular Contributor
timestamp: April 12, 2016, 02:08:28 am
content: See attached Eagle 6.6 file.

author: UPI
author_rank: Regular Contributor
timestamp: April 12, 2016, 09:55:19 pm
content: We found some major noise in the output voltage during the constant current mode (above 500mA).We have since changed the design as shown in the attachments. It is stable whether in CC (above ~500mA) or CV (14V) mode.EDIT: Please see revised schematics attached here and further down the message chain (Thanks zlymex!)

author: zlymex
author_rank: Frequent Contributor
timestamp: April 13, 2016, 12:28:25 am
content: Quote from: UPI on April 12, 2016, 09:55:19 pmThere is no DC path for Ib- of U1B.

author: UPI
author_rank: Regular Contributor
timestamp: April 13, 2016, 12:45:07 am
content: Great catch! It was lost in the translation from the bench to the schematic.Please see the revised schematic.

author: UPI
author_rank: Regular Contributor
timestamp: April 14, 2016, 03:33:49 am
content: Please see the attached schematic for the entire cap charging and 5V out section.Q100 enables the 5V out stage once the caps are charged to ~10.5V (on machine boot) and Q11 allows me to latch this 5V section on even when the cap voltage drops below 10.5V (to keep the 5V until I have finished moving to a safe motor position).Any suggestions are much appreciated.

author: UPI
author_rank: Regular Contributor
timestamp: April 14, 2016, 05:32:22 pm
content: We had to add an extra 220uf cap to the 5V output to stop a large 5kHz ringing under load. A ~470uf through hole cap would be appropriate according to the datasheet. We are limited in capacitor height so we chose to parallel two 220uf caps.Please see attached Eagle and PDF schematic.

author: Mosaic
author_rank: Regular Contributor
timestamp: July 06, 2021, 02:56:39 pm
content: Interesting but too complex for my app.I used a couple BJTs which seem to work well....in the prototype.