ATX switching power supply +3.3V power supply and +5V SB power supply - power supply circuit - Circuit diagram

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The +3.3V power supply of the popular ATX power supply. +3.3V power supply by pulse The 5V winding of the output transformer T1 is stepped down by the coils L5 and L6, rectified by the common cathode Schottky rectifier block D23, and then filtered by L1 and C28. The voltage drop of L5 and L6 is related to the current passing through it. The current has a small voltage drop and a high output voltage. The voltage at no load can reach 9.5V. When the current is large, the voltage drop is large and the output voltage is low. In order to ensure that the +3.3V power supply output voltage is not lower than +3.3V at maximum load, the inductance of the coils L5 and L6 should be properly designed. In this example, L5 and L6 use a core with an outer diameter of 12 mm, an inner diameter of 6 mm, and a thickness of 4 mm, and are wound around 8T with an enamel line of φ 0.93 mm. When the load current is 10 A, the unregulated output voltage is +3.5 V. . If the load current is required to be larger, the number of turns of the coil can be appropriately reduced. The Century Star ST-ATX320 power supply reduces the number of turns of the two coils to 7T, and the +3.3V power supply can output more current. Below the maximum load current and no load, the output voltage of the power supply will exceed +3.3V. In order to stabilize the output voltage of the +3.3V power supply, a voltage regulator circuit composed of TL43l and Q5 is set.

At this point, the no-load output voltage of the power supply is approximately equal to Vrefx (1+R26/R29). Vref is the reference voltage inside the TL431 tube, which is 2.44V-2.55V, which is generally 2.5V. The output voltage is approximately equal to 2.5×(1+4.7/13)=3.4V. If the output voltage rises for some reason, after being divided by R26 and R29, the potential sent to the control electrode R also rises, the potential of the cathode K of the TL431 decreases, the base potential of Q5 decreases with R17, and the current of Q5 increases. Large, that is, the current flowing through L5 and L6 increases. The voltage drop across it increases, so the output voltage of the +3.3V supply drops back, thus maintaining the output voltage stable. Diodes D30 and D3l are rectified to a -3.3V power supply. The current through Q5 includes a +3.3V supply and a -3.3V supply current, equivalent to the double +3.3V supply current in L5 and L6, extending the dynamic operating range. When the measured no-load, the current passing through Q5 is 57 mA, and the current passing through the 10 A load is 6 mA. The +3.3V power supply output voltage is +3.4V at no load and +3.28V at 10A load, which can meet the requirements of use. The advantage of this circuit is that the line is simpler and the output voltage can be adjusted. To increase the output voltage, connect the resistor in parallel with R29 (parallel a 150kO resistor to increase the voltage O.1V). To reduce the voltage, connect the resistor in parallel with R26. In the following circuit including the TL431, the output voltage can be adjusted in this way.
If one of the L5 and L6 coils is omitted, it becomes the +3.3V power supply of the Galaxy ATX. Since only one coil is used, the number of turns of the coil is increased from 8T to 11T. There are a variety of brands of power such as the Buffalo HPS-300S power supply, Great Wall ATX-300P4 power supply, etc. use this circuit.