AX Series AX1200i Digital ATX PSU Technical Overview

By Jonny Gerow, on July 31, 2012


Today we're going to take our newest flagship power supply, the 1200 watt AX Series™ AX1200i digital ATX fully-modular PSU, and find out what makes it different — or dare I say, superior to every other PC power supply currently on the market.  Anyone can promise 80 PLUS® Platinum efficiency, superior voltage regulation, and minimal ripple and noise, but today we're going to crack one open and show you how we go well beyond that mark.

As we open our AX1200i PSU, the first thing we see mounted to the top of the is a 140mm ball bearing intake fan:



A closer look at the fan reveals two cables:



The small black cable is attached to a thermistor that measures the intake air temperature of the fan. The larger connector is the PWM fan connector. Most PSUs change their rotational speed by increasing and decreasing the DC power to the fan motor, which is very inefficient as there is power lost whenever the voltage is changed. It is also very hard to start a stopped fan if you only want to give the fan enough voltage to spin at a very low RPM. PWM, which stands for Pulse Width Modulation, increases and decreases the fan speed by pulsating one voltage on and off at different rates. This is more efficient because the actual voltage to the fan doesn't change, and it also allows the power supply to spin the fan at very low speed.



Overall, the AX1200i has a very unique appearance internally compared to conventional desktop power supplies. Let's take a look at the layout of this unit, and then we'll go over what each section does:


Everything starts in the upper right hand corner of the above diagram, where the power from your wall comes in and gets filtered. This part of the power supply is called the transient filtering stage.


The first stage of the AX1200i's transient filtering comes in the form of a one-piece EMI filter built into the AC input.


The AX1200i has the capacitors and coils required to filter any interference out of the incoming AC line, and also has MOV's (metal oxide varistors) that act as surge suppressors as well. The surge protectors in your house use MOVs similar to what's found in the AX1200i power supply, and this is an advantage as not all power supplies have an MOV as part of their transient filtering stage. That said, it is always recommended that you use a surge protector or UPS with your computer.


The transient filtering is continued on the main PCB. The Panasonic 105°C rated primary capacitors can also be seen in this photo.


All of the capacitors used in the AX1200i are Japanese, and are rated to perform normally at temperatures upwards of 105°C. Japanese brand capacitors, like Matsushita (Panasonic), United Chemi-Con and Rubycon, use a superior electrolyte that is resilient to higher temperatures. Japanese capacitors are also always properly and completley sealed to prevent the leaking of the electrolyte. The 105°C rating, as opposed to the more common 85°C rating, assures that the electrolyte will hold up even if the internal temperature of the power supply becomes incredibly hot.  Certainly the internal temperature of your power supply will never get as high as 105°C, or even 85°C for that matter, but by using such "over spec" components, we assure that the end product will have long service life.


All of the capacitors used in the AX1200i are 105°C rated, Japanese capacitors.


Your computer requires that your power supply delivers +5V even when the computer is "off". This power is called "stand by power", or "+5VSB" (five volt stand by). This allows your computer's components to maintain certain settings while it is "off" and keeps your USB ports "live" so your keyboard and mouse can tell your computer to come out of stand by or hibernation mode. The problem with stand by power is that it is always on as long as the PC is plugged in and it's sucking power from the wall; which is where the term "vampire power" comes from. Typically, when power supplies deliver this power, it is often very inefficient, requiring a good deal of AC power to be consumed in order to provide a small amount of DC power. This is because your typical power supply uses the same circuitry on the AC side for the +5VSB that it uses to produce all of the other voltages your computer needs when it's full up and running.


Hiding behind a couple of the primary switching MOSFETs on the right side of this picture is the +5VSB's circuit board. To the left of that, hiding behind the +12V transformers and +12V output, is the DC to DC board used to convert +12V to +3.3V.


The AX1200i uses a completely separate +5VSB circuit on its circuit board.  The +5VSB circuit board in the AX1200i acts as a separate power supply providing only +5V and consuming as little power as possible to do so.

Most modern computers, especially higher performance computers, use primarily +12V DC from the power supply for power. There are still many parts of a computer that use lower voltages, like +3.3V and +5V, but typically these voltages are provided by regulators on the component itself. Since these regulators can be engineered to only provide as much of the exact voltage a particular component requires, computers have become very efficient over the years. But a computer can only be as efficient as the power supply providing its power, and if a power supply is wasting energy converting +3.3V and +5V that the computer doesn't need, it's not working as efficiently as it could be.


The PCB in the foreground, off to the right, is the DC to DC circuit board that converts +12V to +5V. The PCB in the background is where the AX1200i's DSP resides.  The DSP also controls the PWM fan that cools the AX1200i.


The AX1200i PSU can provide all of its power to a +12V load (1204.8W or 100.4A). If a component in the PC requires +3.3V or +5V, the AX1200i uses DC to DC converters that take some of the +12V and converts it to lower voltages as needed. This is far more efficient than a power supply that converts AC power to +12V DC, +3.3V DC and +5V DC full time, whether all of these voltages are needed or not.

The part of the AX1200i that makes it unique versus other desktop power supplies is the absence of a PFC/PWM controller IC (integrated circuit) and a supervisor IC. Both of these parts have been replaced with DSP, which stands for "Digital Signal Processor". The AX1200i power supply still uses a resonant mode topology like a lot of modern day super-efficient power supplies, but typically in other PSUs a PWM controller IC changes the power supply's switching frequency based on an analog signal derived from the load measured at the transformer. This makes the power supply more efficient at a wider range of loads compared to other power supply topologies, but it can affect ripple and noise and voltage regulation. In some power supplies, a supervisor IC will monitor voltages via a "sense wire" at the load, in an attempt to improve voltage regulation. This information is sent back to the PWM controller via an analog signal, but this information moves relatively slow when compared to the ever-varying loads of your typical PC, so voltage regulation is still not as good as it can be.

The AX1200i improves on this analog system, by using its DSP to calculate measurements collected digitally from throughout the power supply (AC input, load on the transformers, the DC load and output voltage, etc.). This information is then analyzed by the DSPs microprocessor and adjustments are made to the different components of the power supply that improves efficiency while maintaining very tight voltage regulation and minimizing ripple and noise. Because the signals are digital instead of analog and a DSP is used to analyze the information as opposed to relatively simple ICs, adjustments can be made much faster than can be made with the typical set of ICs found in other desktop power supplies.

All in all, the AX1200i is so much more than your average 1200 watt power supply.  By taking an already highly efficient resonant mode topology, controlling it with DSP (a digital signal processor), using DC to DC for the non-primary rails (+3.3V and +5V), building it all up with top-notch quality components, and cooling it with a PWM controlled fan, the Corsair AX1200i is truly a no-holds-barred 80 PLUS Platinum power supply with superior voltage regulation and minimal ripple and noise.

Coming up next, I will be testing the AX1200i digital ATX PSU on automated test equipment that will load the power supply and take measurements so we can see just how superior the AX1200i is when compared to your typical 1200W desktop computer power supply.