Tying isolated flyback primary and secondary grounds together - Page 1
Forum section: electronics / projects

author: techy101
author_rank: Unknown
timestamp: February 01, 2018, 03:05:53 pm
content: I know this comes across as an odd question, but the purpose is for BOM reduction and circuit re-use.I am working on a battery-operated device that needs to have up to four fully isolated high voltage (80V+) DC outputs. To achieve this, I am using Linear Tech Micropower No-Opto Isolated Flyback Converters. The design is modular (can be configured with 1, 2, 3, or 4 outputs), but will always have at least one output. The need for isolation is not a safety concern, but instead to maintain discrete control of the return current paths. Digital and analog signals from each isolated channel will be going through digital isolators and isolation amplifiers to get to/from the microcontroller.Since the device is battery operated and will always have at least one channel I want to use the main 3.3V rail to power all devices for channel 1, but still use the proven flyback design. As mentioned before, isolation is not necessary for single channel operation. Assuming proper selection of the ground tie point on the PCB, is there any problem tying the secondary Vout- from the flyback converter to the Vin ground? I realize this defeats the purpose of an isolated flyback, but would really like to avoid the need for isolated digital and analog signals as well as isolated 3.3V power for the active devices.The reference LTSpice simulations do share grounds, but my understanding is that LTSpice needs to have one common ground to operate correctly so I'm not sure if that is representative of real life.This is my first foray into the world of flyback converters and I’m not sure if this question is totally clear; if not I can try to be more specific.Thanks!

author: David Hess
author_rank: Super Contributor
timestamp: February 01, 2018, 06:32:01 pm
content: No, there is no reason you cannot do that and the result is basically the same as a boost converter with auxiliary windings to provide additional isolated outputs. Using a transformer in this way is convenient for isolating return currents as you suggest and is sometimes done even in non-isolated converters.

author: ocset
author_rank: Super Contributor
timestamp: February 03, 2018, 10:15:51 am
content: Yes, as OP says isol is not needed per se.So you could just tie them all to the same ground and make it easier.I mean, each secondary would have loop currents returning back to itself, so you would not get return currents mingling, so to speak....and its nice to avod the problem of one isolated output getting induced up to a high voltage.Also, even if you do have isolation, remember, its not perfectly isolated, noise jumps from pri to sec through interwinding capacitance.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 03, 2018, 10:26:02 am
content: Better, because EMI current is shorted out this way. Incidentally, don't forget that a capacitor may be required in the same position (i.e., between grounds), for the other (isolated) channels!You can also draw power off the switch directly (as a boost converter), but if you need lower voltage, not higher, this probably isn't too helpful. Hmm, you could make a current multiplier (by analogy with the voltage multiplier circuit, but series-parallel, inductor-capacitor transformed). The cost of the extra inductors and diodes would probably kill that though.Tim

author: techy101
author_rank: Unknown
timestamp: February 07, 2018, 03:02:02 pm
content: Thank you very much for the comments! I tied the two sides of my demo kit together yesterday and it worked a treat.Like I said, flybacks are new to me and thus far the process has been somewhat finicky. It turns out getting an isolated 100V+ out of four AA batteries isn't quite as simple as whacking a simple boost converter in (at least for the beginner).

author: John Heath
author_rank: Frequent Contributor
timestamp: February 07, 2018, 05:01:42 pm
content: Quote from: techy101 on February 07, 2018, 03:02:02 pmJust wondering what the larger picture is. What do you want to do or measure with these isolated high voltage sources?

author: techy101
author_rank: Unknown
timestamp: February 07, 2018, 05:04:01 pm
content: As it's a work project I can't go very far into detail, but the general picture is a multi-channel constant-current source. Isolation is needed because multiple channels will be used on the same article.