17-09-2019, 09:24 PM
Ask yourself why the P4 caused the move from 5V being the main rail to 12V. Hint: You're forgetting about current...
The same logic is what fuels the suggestion for even higher voltages. But there is a limit on how high to take it, largely based on MOS-FET ratings - which of course are getting better all the time. 48V is more feasible today than it ever has been, but Schottky diodes aren't so good at those sorts of voltages, leading to other, less efficient choices.
The drive towards 12V single-rail is primarily about energy efficiency.
A PC PSU making several rails from a single full or half bridge (or occasionally flyback) converter is always going to be an uncomfortable compromise that needs help from mag-amps or similar to achieve half-reasonable cross-regulation. Whereas a well-optimised converter that only has to make a single rail will be more efficient and inherently better regulated.
Adding a series of buck converters to step 12V down to 5V and 3.3V gives better efficiency and better performance than an "all in one". If you look at the "80-Bronze" PSUs out there, they tend to be the multi-rail compromises, whereas the "80-Gold" power supplies tend to use separate buck converters. These buck converters can run at much higher frequencies than the forward convert runs at, so component sizes can be minimised - and as they're working at very low voltages, very fast MOS-FETs with minimal switching losses can be used. Where "all-in-one" converters need Schottky diodes for rectification, buck converters can use synchronous rectification - at low voltages and high currents, the difference in efficiency is huge.
All the "12V Only" concept does is move these buck converters from the PSU to the motherboard. That can only be a good thing - the converters can be optimised for the actual load, which is more of a known quantity to the motherboard designer than the designer of a generic PC PSU, who can only guess at what provision each rail might need (and that's not at all easy today). There are already converters on the motherboard anyway - putting them all together makes several things easier, such as EMI. Since there's no longer a "northbridge" on motherboards, most board area on a motherboard is power supply anyway.
Basically, from every angle, single-rail wins.
Let's not confuse this with "AIO" form factors. The choice of form factor is actually irrelevant here - single-rail is more efficient than multi-rail, whether it's a laptop or a PC. There's no shortage of testing being done out there if you need convincing - just look at what these guys get up to: https://www.tomshardware.co.uk/how-we-te...33160.html
The same logic is what fuels the suggestion for even higher voltages. But there is a limit on how high to take it, largely based on MOS-FET ratings - which of course are getting better all the time. 48V is more feasible today than it ever has been, but Schottky diodes aren't so good at those sorts of voltages, leading to other, less efficient choices.
The drive towards 12V single-rail is primarily about energy efficiency.
A PC PSU making several rails from a single full or half bridge (or occasionally flyback) converter is always going to be an uncomfortable compromise that needs help from mag-amps or similar to achieve half-reasonable cross-regulation. Whereas a well-optimised converter that only has to make a single rail will be more efficient and inherently better regulated.
Adding a series of buck converters to step 12V down to 5V and 3.3V gives better efficiency and better performance than an "all in one". If you look at the "80-Bronze" PSUs out there, they tend to be the multi-rail compromises, whereas the "80-Gold" power supplies tend to use separate buck converters. These buck converters can run at much higher frequencies than the forward convert runs at, so component sizes can be minimised - and as they're working at very low voltages, very fast MOS-FETs with minimal switching losses can be used. Where "all-in-one" converters need Schottky diodes for rectification, buck converters can use synchronous rectification - at low voltages and high currents, the difference in efficiency is huge.
All the "12V Only" concept does is move these buck converters from the PSU to the motherboard. That can only be a good thing - the converters can be optimised for the actual load, which is more of a known quantity to the motherboard designer than the designer of a generic PC PSU, who can only guess at what provision each rail might need (and that's not at all easy today). There are already converters on the motherboard anyway - putting them all together makes several things easier, such as EMI. Since there's no longer a "northbridge" on motherboards, most board area on a motherboard is power supply anyway.
Basically, from every angle, single-rail wins.
Let's not confuse this with "AIO" form factors. The choice of form factor is actually irrelevant here - single-rail is more efficient than multi-rail, whether it's a laptop or a PC. There's no shortage of testing being done out there if you need convincing - just look at what these guys get up to: https://www.tomshardware.co.uk/how-we-te...33160.html
