OCZ ProXStream 1000 W Power Supply Review

Secondary Stage

As we have already explained, the main transformer and the entire secondary section from this power supply are located on its upper printed circuit board. We have already posted a picture of this board in Figure 7, but in Figure 13 you can take another look of this board with all its heatsinks removed.

Figure 13: Overall view from the upper printed circuit board.

This power
implements a synchronous topology on its secondary. On this topology the diodes are replaced by power MOSFET transistors. The idea, at least in theory, is to increase efficiency. This is achieved by reducing the waste produced by the rectifying components: while Schottky diodes have a typical voltage drop of 0.5 V (i.e., voltage wasted by the component), power MOSFET transistors have a typical voltage drop of 0.1 V or even less. Unfortunately, however, this unit provided a not so good efficiency, as we will discuss in more details later.

This power supply uses nine FDP047AN08A0 power MOSFET transistors for the synchronous rectification (two for each main positive voltage), each one being able to drive up to 15 A at 25° C in continuous mode or 80 A in pulsating mode (which is the mode used), also rated at 25° C. This equals to 73 A at 50° C, 62 A at 85° C or 56.5 A at 100° C, calculated using the formula present on the datasheet from this transistor.

As you can see, the maximum current a semiconductor can deliver varies with its working temperature. This is why it is so important to know at which temperature the manufacturer labeled their power supplies. When not specified usually the power supply is rated at 25° C, a temperature the power supply will never work under (a typical working temperature is around 40° C), meaning that the maximum labeled power will only be reached on the manufacturer’s lab with the PSU internal temperature put at 25° C but never at your home, where your PSU will be running hotter.

By the way, we couldn’t find any mention to the temperature used to label this power supply on OCZ website, on the product box or on the product manual. We will discuss more about this in the next page.

The maximum theoretical current each line can deliver is given by the formula I / (1 – D), where D is the duty cycle used and I is the maximum current supported by the rectifying diode (which in this case is made by one transistor with a 56.5 A limit at 100° C – we will have to use the value at 100° C for a fair comparison because usually rectifiers are rated at this or greater temperature). Just as an exercise, we can assume a typical duty cycle of 30%. This would give us a maximum theoretical current of 81 A for each output, i.e. 969 W for the +12 V output, 404 W for the +5 V output and 266 W for the +3.3 V output. The maximum current each line can really deliver will depend on other components, in particular the coil used.

Other three transistors are used for other functions, so a total of nine power MOSFET transistors can be found on the secondary from this power supply.

Figure 14: Seven of the nine power MOSFET transistors used. You can also see the temperature sensor.

On this power supply the +5V and +3.3 V outputs use independent transformer outputs. Usually on high-end power supplies these two outputs use separated rectifiers but connected to the same transformer output, which limits the maximum current each output may reach.

Now let’s talk a little bit more about the actual power specs from this power supply.