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Seasonic X-750 (SS-750KM), 750 W

The star of this review is the new 80+Plus Gold certified Seasonic X-750. Apart from that certification, the X-750 is interesting for having been developed from scratch. Thus, it is going to be completely different from the various modifications of the 12th series (S12, M12, M12-II and M12D).

Exterior Design

The PSU has in a black case with a punched fan grid. The rest of the Seasonic models have a wire fan grid.

Every cable is detachable although I don’t think anybody would ever want to unplug the mainboard cable. The PSU has as many as 11 connectors and the mainboard cable is plugged into two of them at once. The connectors differ in size, so you can’t mismatch them, but it is not quite easy to connect cables because the connectors are too close to each other.

Interior Design

The X-750 is dismantled in an unusual way:

Instead of a standard U-shaped cover, you remove the top panel only.

After that, you can take off the front and side panels by unfastening a few screws.

It is easy to see in the last two photos that the X-750 has little in common with traditional computer PSUs. It has only one large heatsink instead of two or three, and even that heatsink stands aside (this must be the reason why it is so large; it just doesn’t stand in the way of the air flow).

The heatsink carries the power components of the high-voltage section of the PSU.

The largest heatsink in most PSUs is meant for the low-voltage components which heat up a lot due to the high output currents of modern models. The X-750 is different. The output rectifier is only cooled by two slim aluminum plates which can be seen in the photo above.

The explanation is simple. This PSU employs a high-efficiency synchronous rectifier. Such rectifiers use field-effect transistors instead of diodes. When open, the transistors have a resistance of only a few thousandths of an ohm, so there is a very small loss of power on them. The rectifier’s transistors are placed on the reverse side of the PCB and are additionally cooled by the bottom panel of the case through heat-conducting pads.

There is only one rectifier on the main PCB. It is for 12 volts. The Seasonic X-750 features dedicated DC-DC converters which generate the necessary voltages out of 12 volts. Each DC-DC converter is in fact a small switching power supply with a dedicated PWM controller, switches, a choke and a rectifier. These converters, for +5 and +3.3 volts, are placed on a small separate card on the back panel of the PSU.

By the way, it is due to this design that the mainboard cable has to be attached via two connectors: one connector supplies only 12 volts and is located on the main PCB and the other connector, with +5 and +3.3 volt pins, resides on the additional card. This solution is not handy when it comes to assembling a computer, but simplifies the design of the PSU and lowers the loss of power provoked by extra cables and connectors.

Another interesting feature of this PSU is that it uses a resonance converter, like the recently reviewed Enermax MODU87+.

You can see two greatly simplified schematics of an ordinary flyback converter and of a resonant converter. In the first schematic, when the transistor Q1, managed by a PWM controller, opens, the double-wound choke TR1 begins to accumulate energy and the current in its primary winding rises up quickly, reaching some maximum value. Then, the transistor turns off and the TR1 discharges across the diode into the load (the smoothing capacitor C1 and the load proper). This design is good from every aspect save for one: the rapid growth of electric current when the transistor turns on and the subsequent turning off of the transistor at the maximum level of current increase the loss of power due to the switching and the high-frequency noise in the circuit.

The resonant converter circuit adds two components: a choke LR and a capacitor CR whose ratings are low, so they are rather small and cheap (compared to the main choke TR1). This addition improves the circuit dramatically. First, the choke LR limits the speed at which the current grows up when the transistor opens. Second, the inductances LR and TR1 and the capacitance CR are selected in such a way that the current drops back to zero by the moment the transistor turns off (you can read more about that on page 217 of Power Semiconductor Applications: Switched Mode Power Supplies).

Thus, only two extra components help reduce the loss of power on the high-voltage converter and suppress high-frequency noise. Although developing and setting up resonant converters is somewhat more complex than that of ordinary PWM converters, this design seems to be ready to occupy the market of high-efficiency PSUs.

One peculiarity of resonant converters is that they are controlled through pulse-frequency modulation rather than pulse-width one. As the resonant frequency of the LC circuit is constant and such that the current through the switching transistor rises up and falls back to zero during the transistor's open period, the duration of that period cannot be changed. Thus, PWM cannot be used to control the switch. As a result, it is the frequency rather than duration of pulses that is adjusted in resonant converters (however, there exist combined converters that switch into PWM mode at low load when the power loss on the transistor is going to be low anyway).

I guess I’ve given you enough theory. After all, end-users are more interested in how good the particular PSU is rather than how it is designed.

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