Hardware Secrets tested the 750 W model from Corsair HX power supply series, which is based on a DC-DC converter, promising high efficiency.

It is very important to notice that Corsair HX750W, HX850W and HX1000W have nothing to do with other models from this same series. They use a complete different (and better, by the way) design using a DC-DC converter to generate their +5 V and +3.3 V outputs, like several other high-efficiency power supplies (PC Power & Cooling Silencer 910, Antec Signature, Antec TruePower New, Seasonic M12D and Cooler Master UCP series). These HX models are manufactured by CWT, while other models from HX series are manufactured by Seasonic. Why Corsair kept the same name is a mystery. In our opinion they should have used a different name so consumers would know they are facing a different product class, targeted to users looking a power supply with high-end parts and very high efficiency.

Corsair HX750W Power Supply Review