How do I change output Voltage on this SMPS - Page 1
Forum section: electronics / beginners

author: FriedMule
author_rank: Frequent Contributor
timestamp: January 20, 2020, 10:23:49 pm
content: I have found a PWM SMPS circuit in the Micro-cap spice program, it uses some sort of generic PWMVM chip. I have tried to change values on evry component but it keeps outputting 16V.What chip shal I try to change the PWMVM with and what component combination will change the output Voltage?Thanks in advance:-)

author: MagicSmoker
author_rank: Super Contributor
timestamp: January 20, 2020, 10:54:27 pm
content: Look at the voltage on the FB pin during steady state and then change the ratio of Rupper to Rlower to change the output voltage. Note that an isolated flyback like this can accommodate a fairly wide range of output voltage without changing the turns ratio of the transformer, but at some point youwillhave to change it.Also note that changing Rupper affects the frequency compensation (but so does changing the output voltage - as we Yanks say, "you can't win for losing").

author: FriedMule
author_rank: Frequent Contributor
timestamp: January 22, 2020, 06:26:26 am
content: Quote from: MagicSmoker on January 20, 2020, 10:54:27 pmSorry for my slow answer, but I was trying to make sure that it was not me who just made a stupid mistake. But for some reason nothing helps, so I have decided to ask all of you about a different design of a power supply.I have looked for sollutions on google and other plases but I am not sure if any of them meets these criteria:Ac pwm flyback buck smps 220V-240V AC in 5V-10V 1A-2A DC outI do not need the rectifier part, just the AC-DC part and the PSU do not have to be variable. All I need is the possibility to change some resistor values to change the voltage..Please if you have or can point toward a schematic, it would be extremely helpful:-)

author: Gyro
author_rank: Super Contributor
timestamp: January 22, 2020, 09:20:02 am
content: Hopefully you're just using that circuit as a simulation learning example.You cannot use it in practice because it doesn't have galvanic isolation between the 220-240V AC input and the output (due to the feedback network).Just checking.

author: FriedMule
author_rank: Frequent Contributor
timestamp: January 22, 2020, 10:35:16 am
content: Quote from: Gyro on January 22, 2020, 09:20:02 amYes for the moment, but I hope to end up making the unit.About the feedback, I think I have seen where a feedback were made via an optocoupler?

author: MagicSmoker
author_rank: Super Contributor
timestamp: January 22, 2020, 11:25:56 am
content: Quote from: FriedMule on January 22, 2020, 06:26:26 amI don't have Microcap; like most everyone else here I mainly use LTspice (hint-hint).If changing Rupper and/or Rlower doesn't affect the output voltage then also try adding a load resistor to one or both outputs. Note that the resistors in series with the output capacitors are strictly for modeling ESR (LTspice lets you specify that within the capacitor itself, so much cleaner looking) and so the current schematic shows the outputs unloaded (once the caps charge up, anyway).Quote from: FriedMule on January 22, 2020, 06:26:26 amI don't generally play safety cop or tell people what they are capable of, but in this case I would strongly recommend getting a commercially available 12V/2A SMPS to feed one of those cute little buck regulator modules you can buy for $5. It will be far easier (and safer) to modify the buck module to limit the adjustment range than it will be to make your own variable output offline SMPS from scratch.

author: Gyro
author_rank: Super Contributor
timestamp: January 22, 2020, 12:02:47 pm
content: Quote from: FriedMule on January 22, 2020, 10:35:16 amYes, that's right.P.S. The other issue is going to be sourcing a ready made transformer with proper characteristics and safe isolation. That is a challenge for one-off designs.

author: nigelwright7557
author_rank: Frequent Contributor
timestamp: January 23, 2020, 06:15:15 pm
content: Quote from: Gyro on January 22, 2020, 09:20:02 amIf someone swapped live and neutral the output would become live......