MOSFET Driven Power Supply Channel Combination Circuit - Page 1
Forum section: electronics / beginners

author: trini8ed
author_rank: Newbie
timestamp: May 02, 2020, 02:32:23 am
content: Hi everyone, I've been working on a two channel isolated power supply and am currently stuck on how to combine the outputs in either independent mode, series mode, or parallel mode. I have created the logic with mosfets to act as switches for connecting certain wire configurations together seen here in the picture. Note the grounds for each supply are isolated from each other allowing them to be combined in parallel or series, or independent operation.MOSFET Circuit:Digital Logic Decoder:Digital Logic Calculations:I've come to realize that the VGS gets messed up here from the current configuration, but at least I know the digital logic works. I suppose I was thinking of the design in terms of relays where it was isolated allowing 0v to turn it off and 5v to turn it on and forgot that VG is dependent on VS when it comes to turning these on or off. Since all i'm trying to do is short 2 wires together making them the same potential minus the voltage drop across RDSon. If there anyway for me to use perhaps a pmos in conjunction with an nmos or some sort of configuration allowing for my 5V 0V control logic or perhaps using optocouplers to some extent or just using relays is needed here.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 02, 2020, 03:42:59 am
content: Simple -- if they're isolated, then they're isolated, there's nothing you can do that's common ground. Use back-to-back MOSFETs (you can't buy any that don't have body diodes), and an isolated supply to drive the gate (typically a PV opto). All three are integrated in AC/DC SSRs, if you prefer.Now, that's a lie, there are two ways you can connect MOSFETs in anti-series: source to source (and gate to gate, as above), or drain to drain. In the latter case, you need independent gate drives, referenced to each source. If you want to put level shifting and logic local to each supply, this might not be a terrible deal. It'll take more parts though, and I don't think it can come out cheaper. (It can be faster -- those photoMOS relays are dreadfully slow, in the 3-10ms ballpark. That can be a problem, but it should be perfectly acceptable for something like a power supply, no?)Tim

author: trini8ed
author_rank: Newbie
timestamp: May 02, 2020, 04:32:11 am
content: That definitely can work creating my own solid state relay in a sense by connecting the sources together with nmos and pmos. My only concern is the voltage drop im assuming the voltage drop is just based on the RDSon values? For instance 3A of current going through at 24v at some example RDS value of 0.02 ohms means we have 60mV drop across the resistor. Perhaps I should move that sensing circuitry after the MOSFETS and adjust according there. The resolution im trying to achieve is 10mV increments. Also note that the signals being sent are from user input to setup in series parallel and independent mode the delay isn't much of an issue as long as the delay between all MOSFETS are happening at the same time is important and thank you I appreciate the help. Also want to avoid having not super big fets but I realize this comes with the lower RDS values in general the bigger they are.