- The LaST Upgrade -

PART 36 - EXXOS PSU 2018

exxos 2012 - Last updated October 8, 2018

The latest PSU from exxos. The "red edition". Featuring highly efficient high power modern switch mode chips from one of the leading manufacturers, Texas Instruments.

PCBs optimised for maximum cooling and uses heavier weight copper than most general PCBs. This not only keeps resistance down which helps increase efficiency and reliability. It also greatly aids in the cooling of the switch mode controllers as the heat can be transferred away much more efficiently with a heavier weight copper. As the PCB is used as a heatsink, there is no need to bolt the regulators onto the metalwork like my previous design.

I have done away with all the electrolytic's apart from the large bulk capacitance. This means there are practically no components to fail or degrade over time. I now use ceramic capacitors which give much better performance and they are practically indestructible.

I have moved over to the much larger flat coil inductors. These is practically no voltage drop as the resistance is near zero and this increases efficiency and reduces heat output. While these inductors are a little overkill in this design, manufactures does not currently make a smaller version. However while these are rated as 30amp parts, the inductance only remains stable up until about 10amps. This of course still gives the design a good amount of headroom as the max output on the 5V rail is 6amps. These are certainly not "toy inductors" which just about every PSU I have seen uses. The inductance will remain ultra stable and not impact reliability or stability of the power supply regardless of load conditions.

Another good feature about this design is the operating frequency is a lot higher so we can have much tighter control over regulation and use smaller value inductors and capacitors. A huge advantage of this is that lower inductances is that the reaction time of current surges is reduced dramatically. Generally we are forced to use higher value inductors which inherently give better regulation figures, but they also become more sluggish to instantaneous power requirements (such as floppy motor powering up etc). So this power supply manages to get the best of both worlds.

The four leftmost holes are 5Volts output.
The six middle holes are 0V.
The two rightmost holes are 12V output.

The 4 PSU mounting holes (each corner of the PCB) have been increased in diameter to help aid in fitting to various metal power supply bases.

As the power supply is more efficient it can run higher amperage easier and more efficiently . Typically 5V rail can deliver up to 6 Amp RMS, the 12V rail can deliver to 3 Amps RMS. However the total output VA is not exceed 40VA.

For example:

3A x 5V = 15va
2A x 12V = 24va
15 + 24 = 39va.

4A x 5V = 20va
1A x 12V = 14va
20 + 14 = 38va.

5V can deliver 6amps max. (30va)
12V can deliver 3amps max. (36va)
Total output must not exceed 40va.

Note: A typical STF/Falcon pulls about 1.8amps on the 5V rail and generally almost no load on the 12V rail.. For example 5V at 1.8amps is only 9va and we have 40va available. So there is plenty of headroom to power just about anything.

(PSU must be operated with minimum 1amp load else fuse may blow due to the peak voltage charging of the main reservoir capacitor)

The power supply has many modes of protection. It has short circuit and overcurrent protection where it will rapidly enter hiccup mode where there is a cycle by cycle current limit until the fault is resolved. The chips will also shut down upon seeing any under or over voltages on any of the inputs or outputs.

Secondly we have the classic crowbar protection circuit which monitors the 5V,12V rails and the main DC supply coming from the transformer typically 15VDC (20V max). Should either the 5V or 12V rail increase more than approximately 0.5V it is assumed there is some problem and the crowbar activates which blows the fuse and immediately cuts power to the whole board. So equipment attached to this power supply have maximum protection possible.

While this power supply does not have a ripple filter which is normally customary like a my previous design, this new power supply topology does not require a ripple filter and can achieve extremely good regulation like my previous design.

Test was done using a x10 probe with 4.5amps loading on the 5V rail.

Typically 2mV (0.002V) ripple which is as good as things are likely ever going to get. Noise figure is approximately 18mV (0.018V). To put it in perspective the original Atari power supplies were easily over 200mV (0.2V) noise. Note that this was under heavy load, so results here are considered "worst-case" figures.

So while my previous design was totally awesome, I have managed to make this power supply even more awesome by increasing efficiency and amperage output while also reducing the amount of electrolytic's used and increasing performance.

PSUs will be available towards the end of 2018 in my webstore.