Is it possible to connect both the 5VSB and 5V pins of an ATX PSU to provide +5V?
I would like to be able to initially boot using the 5VSB, which is always enabled. Once I've completed setup I would like to enable the ATX PSU. This would help with keeping certain devices from being powered until the appropriate pins are set to appropriate values.
What are the consequences of doing something like this? What alternatives are there?
simulate this circuit – Schematic created using CircuitLab
It's not a good idea to put the two outputs of the PSU into contention like this (shorting the outputs of the 5V and 5V-SB together).
When the PSU is off, you are back-feeding the output of the 5V supply which will draw current from the standby supply reducing the available useful current and possibly damage it over time. It may potentially draw sufficient current to short the output or dramatically reduce its output voltage.
When the PSU is running, you are putting two independent outputs in contention where they will fight for the exact final voltage of the line (also bad for efficiency/longevity). In some ATX supplies the 5V-standby is linearly regulated while the main 5V is switched. This can result in current spikes in the weak linear regulator resulting in failure.
You need to isolate the two supplies. Either use a wired-OR (two diodes, one on each 5V output, in a common-cathode configuration) or run your MCU from the standby supply full-time (if possible).
There are also active circuits (microchips/IC's) that are optimized for 5V standby power supply switching.
Various wired-OR configurations: LTC4411 LTC4413
Power switches: MAX14525 MAX1823B
Be careful with some of these power switches. For example, with the Max1823b you will need to make sure that you add logic to prevent both outputs from turning on at the same time.
Note: When following my links, click on "documents" to see datasheets and app notes.
