Power supply and virtual midpoint - Page 1
Forum section: electronics / beginners

author: Katanara
author_rank: Contributor
timestamp: March 01, 2022, 10:42:30 am
content: Hello there! I have three boards that need to be powered with different voltages: +12V, -12V, GND. I also have a 500W 0-24V power supply (324W without fan). Сonsumption is quite high: the boards have circuits of power amps (up to 3 TDA 7294 and 8 TDA2050). The exact output load consumption is still unclear. One of the boards is a control board with a microcontroller. The -12V wire needs only for TDA7294. I'm going to use a virtual midpoint of the psu as the ground and GND of the psu as the -12V.I roughly imagine a scheme for getting the midpoint. The question is how to implement the necessary voltages with sufficient output current.I need your wise advice! Thank you!

author: Ian.M
author_rank: Super Contributor
timestamp: March 01, 2022, 01:51:18 pm
content: If GND of the PSU is actually grounded (i.e. to mains PE ground) you are setting yourself up for a whole world of grief as sooner or later something that expects ground to be ground will get connected (whether that's an input source, your scope or a debugger/programmer for the MCU is a matter of luck) and will get fried by 20A of 12V dumped down the Gnd wire of its interconnect!With that caution out of the way, to do ay you plan, you need a high power 'rail splitter' (the general name in english speaking electronics for a circuit that provides a virtual ground supply rail). Unfortunately common linear rail splitter circuits are most suitable when you only need a few mA or tens of mA sink/source capability from your virtual ground, and as your load could be highly asymmetric, you need to plan to source/sink somewhere in the 10A to 20A range, which is too much even for rail splitters based on Class D amplifier chips, so you'd need a fully custom switched mode rail splitter.Personally, I doubt its worth the grief, and if I *HAD* to use that PSU, I'd use a high current buck converter module, configured as an inverting buck converter to get -12V, and another wired conventionally to get +12V. Both direct from the 24V rail, and the PSU no longer needs to be floating as 0V/Gnd can be true ground. Some caution is needed - it will need beefy Schottky diodes across the outputs to prevent either output rail getting dragged too far the wrong side of 0V during startup or shutdown, and a protection circuit that monitors the rails, shuts down the negative side buck if the positive side buck's output collapses, and holds the TDA7294 chips in shutdown if the rails aren't both in spec. will probably also be needed.I'd *rather* get it at least one 12V output PSU. Whether or not to then use an inverting buck for the -12V rail needs careful consideration.

author: Katanara
author_rank: Contributor
timestamp: March 01, 2022, 02:32:16 pm
content: Thanks, Ian! I'll try to abandon this dubious idea.

author: TimFox
author_rank: Super Contributor
timestamp: March 01, 2022, 02:56:57 pm
content: Rail splitters, mentioned above, are handy when the current in the positive and negative voltage loads are close to each other (such as powering op-amp circuitry with low output power).There is a pass element (such as the output stage of an op amp) that needs to handle the difference between these two currents, and if that difference is large (always or sometimes), it will have a high power dissipation.