Secondary Analysis
Contents
This power supply uses four Schottky rectifiers on its secondary, however they are connected on a way that is different from other power supplies.
Nowadays on power supplies with four rectifier packs we usually have two of them connected in parallel rectifying the +12 V line, one rectifying the +5 V and another rectifying the +3.3 V line. This reflects the current usage of power supply, where most of the power is pulled from the +12 V outputs. In the past most of the power was concentrated on the +5 V outputs.
On this power supply, however, the two rectifiers that are connected in parallel are in charge of the +5 V outputs and they are used, at the same time, for helping the rectification on the +12 V line. This is the first time we’ve seen such exotic configuration. To help you to understand this configuration, we have drawn a simple schematics of the secondary from this power supply in Figure 13.
Figure 13: The secondary.
All four Schottky rectifiers are the same: DF40S4. Each pack can handle up to 40 A at 106° C (20 A per internal diode). Because of this different design, calculating the maximum theoretical currents is not so easy.
The maximum theoretical current each line can deliver is given by the formula I / (1 – D), where D is the duty cycle used and I is the maximum current supported by the rectifying diode (or by the freewheeling diode, whichever has lower current limits). Just as an exercise, we can assume a typical duty cycle of 30%.
Since the +3.3 V doesn’t share rectifiers with the other outputs it is the easiest one to calculate: 20 A rectifying diode, 29 A maximum current [20 A/(1 – 0.30)] and thus 94 W maximum theoretical power.
From what we understood the +5 V output is produced by the two rectifying diodes drawn on the bottom of the schematics with the two freewheeling diodes drawn in the middle of the schematics. This would give us a 40 A rectifying diode (20 A x 2), 57 A maximum current and thus 286 W maximum power.
Calculating the maximum theoretical values for the +12 V output is hard, and we may be wrong. We will consider only one rectifying diode (the top one), which would give us a 29 A maximum current and thus 343 W maximum power.
Figure 14: Secondary rectifiers.
Figure 15: Secondary rectifier and -12 V voltage regulator.
This power supply uses a thermal sensor on its secondary heatsink (see Figure 16; to take this picture we removed the secondary heatsink), which is used to control the fan speed according to the power supply internal temperature.
Figure 16: Thermal sensor.
This power supply uses a PS223 monitoring integrated circuit (see it in Figure 16), which is in charge of the power supply protections, like OCP (over current protection). This IC also
provides over voltage protection (OVP), under voltage protection (UVP) and over temperature protection (OTP), but not over power protection (OPP).
The active PFC capacitor is Japanese from Chemi-Con and rated at 85° C, while the electrolytic capacitors from the secondary are rated at 105° C.
