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 on Figure 7, but on Figure 13 you can take another look of this board with all its heatsinks removed.

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

The design used on the secondary is really unique. In fact, this is the first power supply we’ve seen using such design. Instead of using Schottky power rectifiers after the transformer, the rectification and regulation are done using power MOSFET transistors. This power supply uses nine FDP047AN08A0 power MOSFET transistors, each one being able to drive up to 15 A at 25º C in continuous mode or 80 A in pulsating mode (which is the mode used), also rated at 25º C.  This equals to 73 A at 50º C, 62 A at 85º C or 56.5 A at 100º C, calculated using the formula present on the datasheet from this transistors.

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 25º C, a temperature the power supply will never work under (a typical working temperature is around 40º C), meaning that the maximum labeled power will only be reached on the manufacturer’s lab with the PSU internal temperature put at 25º 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 on the next page.

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

Each positive output (+12 V, +5 V and +3.3 V) uses two transistors and the other three transistors are used on the control and protection circuits. 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.

Let’s make some math using the maximum theoretical current supported by each transistor, always having in mind that this is just an exercise, as the maximum current (and thus power) a power supply can actually deliver will depend on several other factors such as like the transformer, coils, capacitors, the PCB layout, the wire gauge and even the width of the printed circuit board traces.

Considering the transistor maximum current at 50º C (73 A), the +3.3 V output would have a maximum power of 480 W, the +5 V output would have a maximum power of 730 W and the +12 V outputs would have a maximum power of 1,752 W. That is really impressive, showing that at least in theory the transistors used on the secondary are working far below their maximum capacity – which is absolutely great.

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

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