250 Watt PC power Supply

  
Up to 10 A continuous output current, or operation with 50 % ESD and 18 A peak current are possible without a fan if sufficient natural air flow is present and the ambient temperature does not exceed 30 °C. A small CPU fan (40 x 40 mm) should be used for more than 10 A continuous current. The heatsink surface is not large enough to keep the FETs junction temperature below the limit value ( Tj < 100 °C). With a CPU fan the heat sink temperature remains below 28 °C (Tu = 20 °C). The following table shows the measured and calculated power dissipation Pv of the basic components at 250 W output power. The differences to the modified PC power supply consist of the following items: 1) two power FETs are used instead of bipolar transistors as power switches, 2) a synchronous rectifier on the secondary side instead of power diodes, 3) the elimination of a switch driver stage (current-proportional control), and 4) a simpler over-current and over-voltage monitor.
250 Watt PC power Supply - schematic

250 Watt PC power Supply - img1

250 Watt PC power Supply - img2

The magnetic components of AT-style PC power supplies don't vary much. They are usually designed for a switching frequency of 25... 40 kHz and a power output of 200... 240 W. The transformers on the S.M.P.S circuit boards are to be found often in either a small, or a somewhat larger size. I am not able to say, whether the larger size brings more power or if it is only an older design. [Note: the larger size is usually found in flyback supplies. ja] For the new power supply I preferred the larger transformers because they have more space available for additional turns in all three transformers. The smaller transformers are completely filled with copper and insulation, and are therefore only marginally suitable for modification. This circuit section is uncomplicated. The common-mode choke Dr1 (mains filter) is followed by a NTC resistor for limiting the inrush current. Its resistance amounts to 5 ohms cold, and after few minutes the warm resistance is less than one ohm. The 230 Vac rectifier is generously specified at 4 A, so no cooling is necessary. The values of capacitors of C3 and C4 is determined by the allowable ripple voltage Ubr, and the number of mains-voltage half-cycles to be bridged [i.e., 'hold-up' time required]. For Ubr = 25 V and zero half-cycles, two 470 uF capacitors in series are sufficient. This specification applies to maximum load during low mains-voltage Umin = 230 Vac - 15%. FET power switches...



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