Get 13.8 V / 15 A out off a PC Power Supply

The major disadvantages of usual linear power supplies are high power dissipation, the size and the appropriated weight. When looking for an alternative solution, I decided to use a switch mode power supply (SMPS). The efficiency of such power supplies is around 70 % to 90 % at a power density of 0.2 W / cm³. Because homebrewing was out of the question due to lack of time, I tried the modification of a PC switch mode power supply. The later are mass-produced goods and available for less than 50 DM.

Depending on the PC model, these are rated anywhere between 150 and 240 W. For supplying socket 7 main boards they have four different output voltages of +5 V, +12 V, -12 V and -5 V. They are mainly primary switching power supplies with power switches arranged in a half-bridge configuration. The outputs can drive the usual 20 A (+5 V), 8 A (+12 V) and 0,5 A (-12 V, -5 V). At approx. 205 W output power and a typical efficiency of 75 % this means a dissipation of only 68 W. I had acquired an unbranded PC power supply, measuring 140 x 100 x 50 mm (W, D, H) and weighing 350 g. Most power supply units are designed according to the same principle (half-bridge configuration) and hence the following described modification should be applicable also to power supplies from other producers.

Several protection circuits are included in the original power supply. Excessive primary current due to a very high secondary current leads to a high alternating voltage at the T3 output. If this voltage is above a fixed threshold the TL494 stops immediately generating cyclically pulses and changes to the intermitted mode (on / off). The circuit and the load are protected likewise against over-voltage at the +5 V output or short-circuit at the -12 V and -5 V outputs. Switching off is executed via H-signal to the IC1 protection input (pin 4) too.
If you see a KA7500 or IR3MO2 PWM regulator IC on the board, each one is a pin compatible second source to the TL494CN. IC3 is a dual comparator from LM339 type. Some power supplies are not equipped with this IC, but with a two transistor discrete monitoring circuit, offering the same functionality.

Get 13.8 V / 15 A out off a PC Power Supply 

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