Isolated Voltage Regulator Circuit - Page 1
Forum section: electronics / beginners

author: dlad87
author_rank: Newbie
timestamp: November 09, 2020, 03:22:56 pm
content: Hello,I'm looking to do a project where I need multiple isolated regulated outputs. I understand how to wire the regulator but am unsure how to isolate them from each other. They need to be isolated for groundloops. I've attached a portion of a schematic for illustration of what I think may work. Am I on the right track with the diodes? Or should I do something different?I've searched for similar circuits with "isolation" but all I get is simple wiring circuits of the V reg's.Thanks

author: ledtester
author_rank: Super Contributor
timestamp: November 09, 2020, 03:59:30 pm
content: What's the application?How much isolated power do you need?

author: dlad87
author_rank: Newbie
timestamp: November 09, 2020, 04:07:53 pm
content: Each regulator will have less that 200mA draw.

author: David Hess
author_rank: Super Contributor
timestamp: November 09, 2020, 04:08:35 pm
content: No, that will not work to isolate the outputs. You need to start with floating supplies for the 7805 regulators. Typically this is provided by separate floating secondaries on the transformer.

author: ledtester
author_rank: Super Contributor
timestamp: November 09, 2020, 07:51:19 pm
content: I've seen a lot of hobby projects use the B0505S-1W module which provides 1W of 5V isolated power.Available from aliexpress/ebay. Otherwise look for devices such as:https://www.ti.com/tool/PMP6813

author: dlad87
author_rank: Newbie
timestamp: November 10, 2020, 06:30:33 pm
content: Would one of these Meanwell SPBW06F-12 isolated dc-dc converters ahead of the regulators work. I would use one of these converters to supply each regulator. The converters would be powered by the transformer secondary after the rectifier and filter. I think it would allow me to use a transformer with one secondary and still maintain isolation of each regulator output from each other. Is my understanding correct?https://www.digikey.com/en/products/detail/mean-well-usa-inc/SPBW06F-12/7707012

author: TimFox
author_rank: Super Contributor
timestamp: November 10, 2020, 07:29:43 pm
content: Yes, the spec sheet indicates 1500V isolation from input to output. Note that 7805 and similar regulators have good rejection of mains-related ripple (50 to 120 Hz), but may be insufficient to reject the higher-frequency ripple of the switching module: you may need to add passive filtration between the switcher output and the regulator input at each device.At 580 kHz switching, the ripple will be in the MHz neighborhood; the data sheet specifies 100 mV pk-pk in a 20 MHz bandwidth, with an external 47 uF capacitor.

author: dlad87
author_rank: Newbie
timestamp: November 11, 2020, 11:33:46 am
content: I've attached an updated schematic with low pass filters between the converters and the Vreg's. I think it should filter down to 3.4kHz. Do you think this would be sufficient for the regulators to reject?Thanks

author: TimFox
author_rank: Super Contributor
timestamp: November 11, 2020, 02:41:55 pm
content: It's the appropriate topology. The devil is in the details on the passive filters, so you will need to try it and specify what ripple and noise is acceptable at the regulator outputs. One change I would make is to change the tantalum capacitors at the input of each 78xx to 0.47 uF X7R ceramic.

author: dlad87
author_rank: Newbie
timestamp: November 11, 2020, 02:45:42 pm
content: Thanks, I’ll give it a shot. I got the .33 Tantalum from the datasheet for the regulator. What it the reason for changing it to a smaller ceramic cap? Just so I know.Thanks

author: TimFox
author_rank: Super Contributor
timestamp: November 11, 2020, 04:33:07 pm
content: Since you have potential high frequency noise at that node, the X7R ceramic should have lower impedance at 1 MHz. When the 78xx series was announced, the data sheets required a minimum 0.33 uF capacitor at that node to prevent oscillation. When one of my colleagues omitted that, the result was an oscillation at roughly 100 kHz. Personally, I have always used the 0.47 uF X7R with good results.

author: dlad87
author_rank: Newbie
timestamp: November 11, 2020, 05:08:02 pm
content: Thanks, I’ll build it with the .47 ceramic and see how this circuit measures.

author: mariush
author_rank: Super Contributor
timestamp: November 11, 2020, 05:33:15 pm
content: How many isolated outputs? And what's your budget?Because if you have more than 2-3, you may want to consider getting a small toroidal trasformer with your desired VA rating, and then unwinding the secondary and rewinding it with multiple secondaries.There's also chips like Adum5000 but they're expensive and not very efficient :https://www.digikey.com/catalog/en/partgroup/adum5000-series/15568... adum5000 is only 33% efficient and can only do 100mA ... and you probably want to add a tiny ldo after it to do some extra filtering (ex take 5v out from adum5000 and output smooth 4.85v or something like that for your components)