Vast Improvements on second DC/DC Power Supply - Page 1
Forum section: electronics / projects

author: LoveLaika
author_rank: Frequent Contributor
timestamp: March 23, 2022, 06:12:09 pm
content: I once made a DC/DC power supply as part of a project. It took a DC source (from 12-20 volts), and it used an LM2673 and an LM2679 as a buck converter and a buck-boost inverter to generate a fixed positive/negative voltage. It worked well for the time, but I wanted to see if I could improve on it. Thus, I created version 2 (at least as a prototype in a schematic simulation). I wanted to kind of show it off here, but I was also wondering if I could get some critique on my improvements.Previously, I used the LM2673 and LM2679 ICs, using the former as a buck-converter and the latter as a buck-boost inverter. Originally, I needed to get at least +10/-10 volts max with a current of 3 amps each, so an adjustable voltage circuit would be beneficial here. The current limitations were arbitrary that I set upon myself; though the use applications were not going to draw that much current, I wanted to give myself plenty of room just in case. Since I read that using the buck converter as a buck-boost inverter would involve more current draw, I used the LM2679 for it, leaving the LM2673 as the buck converter. I also added a small current-monitoring circuit using the INA293 IC to convert current through a shunt resistor to voltage (in a 1-to-1 ratio). That voltage was compared against a reference voltage (acting as a current reference of sorts) which turns on an LED if the current gets too high.The end result of circuit 1 was that....it works, though not without its problems. I had initial issues with start-up current when a small load was already connected when I tried to start it up. That was solved by using a beefier 13.7-V DC supply that was capable of supplying lots of current. Plus, the current comparator circuits didn't do anything except light up as a warning for going over the current limit. Not much of a safety feature. I wanted to take in these issues and try to address them in my second version.In this second version, I wanted to be able to adjust the output voltage to reach +20/-20 volts from a 13.7 volt DC supply. Through use cases with my old board, being able to reach those voltages would be beneficial in op-amp circuits that relied on +15/-15 volts. However, in order to do that, I couldn't use a buck converter. After researching, I went with a SEPIC circuit and a buck-boost circuit using the LT3579 ICs. Lucky that I could use the same IC rather than needing to use two different ones. Learning from my mistakes, I added a small reverse-voltage protection circuit as well as a slow-starter circuit using Power PMOSs in order to limit the in-rush current. Now, with regards to the current-sense circuit, it more or less stayed the same as before, using the INA293 to convert the shunt current to a voltage. However, this time, rather than powering it from an independent power supply, I used a Zener diode to drop my regulator's output voltage to a lower value to remove the independent supply. The outputs from the INA293s go to their comparators, the LT1719. That is referenced against an adjustable reference voltage like before. So, when the shunt current exceeds a certain threshold, the comparator output will go high. That output voltage goes to a simple S-R latch circuit, which controls an SPDT relay to cut off the load from the DC-DC voltage and short it to ground. This can be reset with a physical push-button.So, that's my overview of how version 2 works. I would like some feedback on my improvements. Personally, I feel this is a big step up from version 1, though this is not without issues.The first thing that comes to mind is the current measuring circuit in how it is powered by essentially the DC/DC converter. The circuits won't work at low voltage outputs like 1 volt. So, looking at the minimum voltages that could be used, I thought to limit the minimum voltage to a +3/-3 volts so that the current limiting circuit will work. This would also endow a max current limit to 3 amps (in terms of measuring), but I'm not too concerned about that. Also, on that note, while it is the output of a DC/DC converter, I have my doubts about powering circuits like that without going through a linear regulator.My next concern is with my choice of a relay to cut the voltage. The relay switching times seem rather slow. I tried experimenting with faster transistor elements, but I couldn't get them to work. When the current is within limits, there's a significant difference between the load voltage and the DC/DC output even though there's very little resistance from transistors. Using a relay was a sure-fire way to work, though I wonder if it is the best way.