The project requirements are 5VDC, and 20VDC. The 5VDC is to power the microcontroller and 74HC14 IC and MCP3425. The 20VDC powers a 4-20mA loop transmitter. I'm not sure if it is essential to have an isolated power supply system or not. The supply is a 12VDC battery from a forklift, which is higher than 12VDC when the alternator kicks on i'm assuming. I chose to isolate it with a Recom RS6-1212S. After that converter I used another Recom (non-isolated) converter to step the 12VDC nom down to 5VDC. To get my 20VDC I used an MC34063 since I couldn't find a reasonable supply elsewhere. I tied the low outputs of the 5VDC and 20VDC together to form a ground on the circuit board, but it is still isolated from the forklift chassis ground/battery (-).
The coupled inductor, c5, and C6 were recommended in the Recom datasheet to reduce common mode noise. D1 was used to protect against reverse polarity when connecting the circuit to power. My problem is getting clean power, and the RS6 power supply is buzzing when I power the device up with my benchtop power supply. The buzzing is audible, and when I connect a dummy load (10k resitor) to the 20VDC output the buzzing lessens and the frequency seems to increase. I don't know what I'm missing.
couple thoughts come to mind: 1) The 500uH inductor might be too close on the board layout 2) Is the 20VDC SMPS backfeeding to the other two SMPS's 3) The SMPS need regulator circuits / filtering
I'm looking for help to try and fix this circuit, make it better, and any helpful considerations that can help me with this project. Also, I don't have access to software for multiple layers for board layout so I did the best I could with the software and limited knowledge of PCB layout that I have.