Tying isolated flyback primary and secondary grounds together

Thread Starter


Joined Dec 1, 2017
I know this comes across as an odd question, but the purpose is for BOM reduction and circuit re-use. As this is a work project I have to be a little vague, but will provide as much information as I can.

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.

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.




Joined Mar 14, 2008
I don't understand what tying the input and output grounds together has to do with using common power for the rest of the circuit. :confused:
You can use a common ground for the analog and digital signals without having to tie the input and high-voltage output grounds together.