Isolated Switching power supply with analog voltage control? - Page 1
Forum section: electronics / projects

author: jmole
author_rank: Regular Contributor
timestamp: September 19, 2014, 09:52:09 pm
content: I've been looking around for something like this for a while. Have a project that has 8 internal modules, and a controller module, all with a common ground across a signal cable (16 pin 30AWG, 8 are ground).Each module has separate power and ground inputs, but because of the common ground in the signal cable, if one of the power grounds that goes to a module fails in some way, current will return through the signal ground, which I definitely don't want. (This is assuming that we use a single PSU to power the whole thing.)The solution of course is to use an isolated PSU for each module, so that it only sinks the current that it sources. This way, if we have a power ground connection fail, the module itself will stop working, but it wont damage the signal conductors.However, I can't seem to find an off-the-shelf PSU that allows for analog voltage control of the output voltage. Our target price is ~$40-80 per supply, each one rated for 15V at around 10A. I want analog voltage control, because the modules need to have the same output voltage (within 100mV or so), and I could pretty easily design a control circuit that would do this.I've run across this one:http://www.powergatellc.com/Featured-Products/1200-watt-modular-power-supply-in-ultra-miniature-6-x-6-x-16-footprint.htmlWhich fits our needs, in terms of analog control over output voltage, but Powergate is mandating NCNR terms, and I don't want to commit to something without at least trying it first.Anyone know of another solution out there? Is there some way to provide the protection I need with a single PSU?I was thinking that a low-side and high-side current shunt resistor on each power output pair, with an MCU for control might do the trick. If the voltage drops across the shunt show a large discrepancy, then a fault has occurred, and we could shut off power for that particular output. What do you guys think?

author: moffy
author_rank: Super Contributor
timestamp: September 20, 2014, 12:02:28 am
content: I can't quite visualise your problem, perhaps if you drew it as a schematic? You could buy one of the PSU's and see if it works, not a large work investment considering wages, delays etc.

author: jmole
author_rank: Regular Contributor
timestamp: September 21, 2014, 01:14:59 am
content: Here's a quick block diagram.

author: ajb
author_rank: Super Contributor
timestamp: September 21, 2014, 01:37:56 am
content: If you're only concerned with fault currents through the signal wires, might it be easier to isolate the signal interfaces instead?

author: CM800
author_rank: Frequent Contributor
timestamp: September 21, 2014, 01:39:55 am
content: How about on each module, have a shut resistor of say.... 0.01R in series with the negative rail and then have a comparator comparing it against a ref-voltage. The comparator triggers the fault that disables that module or whatever else. It seems like the simplest and most effective solution.

author: jmole
author_rank: Regular Contributor
timestamp: September 22, 2014, 05:32:18 pm
content: Quote from: TCWilliamson on September 21, 2014, 01:39:55 amCan't add anything or change the modules at this point, they're already built.

author: David Hess
author_rank: Super Contributor
timestamp: September 24, 2014, 09:10:20 am
content: The way I have done this sort of thing in the past is to isolate the signal grounds from the power grounds and use them as low current sense lines to detect ground at the power supply for the purpose of setting the analog control voltage. Power supplies with remote sense often do something similar to protect their sense lines which can have a series resistor added limiting fault current without affecting anything.In extreme cases, I drive the voltage control with a current and convert it to a voltage at the device with a shunt resistor to the signal ground which is again isolated from the power ground.You should be able to to what you need with any power supply that has remote sense by driving the remote sense lines to control the output voltage.

author: oldway
author_rank: Super Contributor
timestamp: September 24, 2014, 05:03:45 pm
content: We do not generally provide protection against the interruption of the power connections because it is very unlikely to happen.But there must be a diode in the + output of each power supply to prevent other supplies from feeding an internal short circuit. (The power supplies do'nt have an output fuse)There should also be a way to share currents to avoid that some supplies should be overloaded and working almost constantly in current limit.One of the easiest ways to share current is to provide a slightly sloping curve. (voltage dropping with current increasing)Another reason why the output diodes are needed is to ensure the proper operation of the OVP protection.Without the diodes, if an overvoltage occurs, OVP will shut down the non-defective supplies.