Isolating power supplies - Page 1
Forum section: electronics / projects

author: beandigital
author_rank: Contributor
timestamp: June 13, 2018, 09:05:21 am
content: What would be the best way to create an isolated supply? I have a design that needs a analog supply that is isolated from the input power. Really I want the analog ground to be isolated from the input ground. Do I need to use some form of transformer isolation or is there a simpler solution? Thanks

author: tszaboo
author_rank: Super Contributor
timestamp: June 13, 2018, 09:08:47 am
content: There are many 1-2W isolated power supply manufacturers. Tracopower, Recom for example. These are quite noisy though, so might want to use something, like a 5V input, 9V output isolated DC-DC and then connect it to an LDO, like a 7805. So for example Traco TME 0509S.

author: JS
author_rank: Frequent Contributor
timestamp: June 13, 2018, 09:10:13 am
content: Depends on the demands of the power supply, it does need a transformer of some kind but it could be simpler than you think, or quite more complex, depending on specs... we need specs to help!!JS

author: beandigital
author_rank: Contributor
timestamp: June 13, 2018, 09:18:17 am
content: Yes, sorry I forgot the specs. The input is 12V and the analog supply is 7V. The output current is 0.2A.

author: capt bullshot
author_rank: Super Contributor
timestamp: June 13, 2018, 09:27:03 am
content: Quote from: NANDBlog on June 13, 2018, 09:08:47 amThat alone doesn't solve the huge common mode noise some of them spit out.

author: David Hess
author_rank: Super Contributor
timestamp: June 13, 2018, 12:37:13 pm
content: A transformer is the easiest good way; this could be a 50/60Hz power transformer or something like a gate drive or pulse transformer used in a high frequency inverter. A transformer could also be used as part of an isolated switching regulator.Sometimes a capacitive charge pump is suitable.

author: IDEngineer
author_rank: Super Contributor
timestamp: June 13, 2018, 12:58:15 pm
content: Quote from: capt bullshot on June 13, 2018, 09:27:03 amSo true... and there's also the spectrum emitted by those DC-DC modules. Most linear voltage regulators (like the 78xx series) don't filter/reject noise much above 1KHz but those DC-DC modules often have incredible conducted and radiated noise in the 10's or even 100's of KHz which passes right through a downstream linear regulator. To fight this, the spec sheets that even acknowledge the problem recommend various LC filters involving ferrite beads and the like. It's a deep, dark rabbit hole.

author: tggzzz
author_rank: Super Contributor
timestamp: June 13, 2018, 01:07:44 pm
content: Quote from: beandigital on June 13, 2018, 09:05:21 amIf by "input power" and "input ground" you mean mains power supply connections, then I'm concerned thatbecauseyou ask this question you might not have sufficient appreciation of the nuances of how safety can be significantly compromised. A decent answer would require knowing far more about your application and how the equipment can be used and misused.I have no intention of attempting to give brief guidance on a forum such as this; there are too many ways any such brief guidance could be misleading, misunderstood, and simply dangerous.