It is called the VRM ( Voltage Regulator Modules ). It takes input from +12V, step it down to 3.3V and 5V. Principle of operation is similar to CPU VRM.
5V and 3.3V are locally regulated by the VRM controller. Can achieve very tight regulation and high efficiency with this methods ...

The conventional secondary output for 5V and 3.3V is as shown in the above figure. +5V and +3.3V share the same secondary winding but, with different control methods.

+5V and +12V are group regulated and the cross regulation greatly depends on the design of coupled transformer L1.

The +3.3V, on the other hand, uses magnetic duty ratio controller for independent regulation. Whenever system draws big current from +5V and +3.3V, there will be loss across D1, D2, D3, D4, L1, L3, and small amount on L2, L4. The forward voltage drop of schottky diode D1 ~ D4 is proportional to forward current. Therefore the loss is high as current increases.

By using VRM, we can eliminate diode D1 ~ D4 and L1, L3. VRM draws power from +12V lines and step down to +5V and +3.3V using high side MOSFET and low side MOSFET. The MOSFET on impedance is very low ( in miliohm range ). Hence, the efficiency is higher and tighter regulation is possible because it is locally regulated by VRM controller. VRM output requires low ESR capacitors as it affects the output ripple voltage. The entire VRM circuits is on the modular connector PCB. 2 toroidal coils are the output inductor. All capacitors are chemicon PSA series.

This post has been edited by kisuke_77: Feb 6 2010, 08:53 PM

It costs about S$265 in Singapore ... That would be around RM650 !

Understanding how it works is easy... But, designing a PSU requires experience and skill. I am not up to that level

Both are good PSUs, base on Synchronous Rectification Forward Converter ...

But,
(1). Seasonic uses Sanyo Denki PWM Fan ... HX750 - Yate Loon
(2). Seasonic uses all Japanese Capacitors ... HX750 - some are non-Japanese capacitors

VRM ouput capacitors in HX750:


It it up to individuals ...

This post has been edited by kisuke_77: Feb 7 2010, 09:39 AM