DC Power Source from an AC Line Design - Page 1
Forum section: electronics / projects

author: SebG
author_rank: Regular Contributor
timestamp: April 23, 2013, 02:10:21 am
content: Hi. I'm planning on designing a precision, low noise, variable power supply and with current up to 3A. To reduce noise I decided to us a transformer with a diode bridge to get a DC voltage instead of a switching power supply. It will need to generate 3 outputs: 14-16V to power my op-amps, a 5V supply to power the microcontroller and some other stuff like 5V fan, LCD or LED displays and an unregulated high current output that will be regulated elsewhere.The image shows my design which includes a common mode choke, some filter caps and a MOV. So I would like some feedback on this design. Is this a good design or not? Also I've never designed anything that used a common mode choke so any comments on the AC Line filter are welcome.Hopefully not all feedback will subscribe to Dave's shirt's philosophy and will be positive.

author: c4757p
author_rank: Super Contributor
timestamp: April 23, 2013, 02:19:25 am
content: At 800mA with that kind of input voltage I can only imagine the heat sinking solution for the 7805 will involve liquid nitrogen. I suggest either a massive series power resistor to share some of the dissipation (the hamfist approach), or using a switcher for the +5.

author: jmole
author_rank: Regular Contributor
timestamp: April 23, 2013, 02:46:56 am
content: Check out the TPS7A4700 Regulator from TI.Also, I'll echo the other comment about power dissipation. Put in a switching regulator that drops the voltage down before the linear reg, and you'll be in a much better position there.

author: c4757p
author_rank: Super Contributor
timestamp: April 23, 2013, 02:57:32 am
content: Quote from: jmole on April 23, 2013, 02:46:56 amImpressive chip - I will add that to my own collection. But (no disrespect to SebG - sorry!) I think a chip only available in QFN might be a bit out of OP's league.

author: mariush
author_rank: Super Contributor
timestamp: April 23, 2013, 03:36:38 am
content: If you have two secondary windings, maybe just get a second bridge rectifier and power the 7805 from that separate input. 10-12v in to 5v 800ma is doable.

author: SebG
author_rank: Regular Contributor
timestamp: April 23, 2013, 04:34:39 am
content: I suspected that the 5V output would be the main issue. I was considering either using a DC-DC switching converter before the 7805 or just having another small transformer and diode bridge setup. The TPS7A4700 costs $6.33 in qty 1 which is as much as another transformer and diode bridge setup. I would like to avoid the extra cost if possible. I'm thinking of just inserting a dc-dc switching converter and the 7805 to get the 5V since it will only power digital ICs and a bit of noise shouldn't be a problem there.Is it possible for the noise on the 5V rail to affect the other power rails through ground or other means? And to what degree.Any other comments about the rest of the design?

author: jmole
author_rank: Regular Contributor
timestamp: April 23, 2013, 05:19:58 am
content: Another option is just to buy a bunch of high-current schottky diodes to drop the voltage to a reasonable value before the regulator.