Seasonic X-Series 750W: Potent for the Price

X750 - Resonant Topology

Steep square-wave impulses are the standard output of integrated circuit voltages, steered by a PWM-chip. While the DC-to-DC converter improves efficiency on the DC side, there are still some switching losses on the AC side. Depending on various influences like operating time during a cyclic duration, switching frequencies, voltage, and current the level of loss varies. In order to prevent this loss effect, you can use resonant switching discharge with a resonant circuit. In order to understand the procedure, we have to clarify the significance of the term resonance first. Resonance means an increase in amplitude of a signal induced by an external influence that has a frequency close to the natural frequency of a system. To negate resonance, the current is forced to run in the opposite direction and clamped to a low voltage level during transfer.

Doing this requires two switches; the first one produces one half of an oscillation while the second creates the other half in reverse, with the resulting output being very close to a sine oscillation. The moment voltage crosses its zero point, where the following current still affects within the minus range, transistors are switching. The procedure can be described with ZVS (Zero Voltage Switching) and also exists in a reverse variant with Zero Current Switching (ZCS). Since the two physical values in the formula of the switching losses at the transistor play a role, the power dissipation is reduced by their minimization. One can operate almost free of loss. Another advantage is that the smooth oscillations (softer transitions) also lower electromagnetic interference; this was a problem with classical topologies, commonly seen in the first 80 Plus Gold PSUs.

Seasonic uses a traditional PWM controller during low load (which results in higher ripple and noise) since the resonance converter isn't easy to run at a high efficiency level at such loads. The integration of a special companion chip is crucial for this assembly. However, Enermax also works with frequency modulation to get higher efficiencies on larger loads. Enermax calls their design a DHT, Dynamic Hybrid Transformer. Dynamic might be overstating things a bit, as they still use fixed frequencies in several stages; a constantly varying modulation would be fully dynamic but it may not be necessary or ideal.

Manufacturers can often reach 80 Plus Gold level with a DC-to-DC converter, and that's exactly what Seasonic and other manufacturers do. However, there is still the possibility of selecting faster switches with an H-bridge design to make improvements, which brings us back to the EMI problem caused by the hard switching. It is much simpler to realize this solution than to implement a completely new technology. As usual, there's a balancing act and manufacturers try to keep the specific details of their designs secret; how they achieve Gold certification can bring disadvantages in some cases.

Regardless, we should note that getting one of these "green", high efficiency PSUs isn't going to save the environment. There are lots of wastes and pollutants created during production, and if you're replacing an old PSU you're still contributing to the land fill. Additionally, it's difficult to save money in the long term as the purchase price is often very high. The true benefit of power supplies like the X-Series is found in the deeper details. For example, higher efficiency means lower PSU temperatures, which affects the ambient temperature and may result in a system that lasts longer with fewer problems.