The principle behind the computer ATX auxiliary power supply circuit is crucial for maintaining the normal operation of both the microcomputer and the ATX power supply. The auxiliary power supply plays a pivotal role in ensuring that the system functions correctly. Firstly, it provides a +5VSB standby voltage to the power monitoring circuit on the motherboard of the microcomputer. Secondly, it delivers approximately +22V DC working voltage to the internal pulse width modulation chip of the ATX power supply and the primary winding of the push-pull transformer. This means that even if the microcomputer isn’t actively running, the auxiliary power supply keeps essential circuits active for standby control and controlled startup. The auxiliary power supply operates in a high-frequency, high-voltage self-oscillating or controlled oscillating state. However, due to the lack of perfect voltage regulation and overcurrent protection in some parts of the circuit, it tends to have a higher failure rate compared to other components in the ATX power supply.
Let’s take a look at three domestic ATX switching power supplies currently used in modern computers. By examining their auxiliary power supply circuits, we can better understand how they function. For instance, consider the Galaxy Silver Star-280B ATX power supply auxiliary circuit (as shown in Figure 1).
The rectified 300V DC voltage is driven by the current-limiting resistor R72 and the starting resistors R76 and T3, which apply the voltage to the primary winding L1 of the transformer and the b15 poles of the Q15 oscillation tube, turning Q15 on. The feedback winding L2 induces a potential that is applied to the Q15b pole via positive feedback loops C44 and R74, accelerating Q15's conduction. The T3 secondary windings L3 and L4 produce a negatively positive induced potential, turning off the rectifiers BD5 and BD6. As the C44 charging voltage rises, the base current injected into Q15 decreases, causing Q15 to exit saturation and enter the amplification state. The oscillating current of the L1 winding then decreases. Since the current in an inductor cannot change instantly, the L1 winding induces a reverse potential. The inverting induced potential of the L2 winding charges C41 through the R70, C41, D41 loop, grounding the positive pole of C41 and negatively charging the negative pole. This process turns on ZD3 and D30, quickly pulling the base of Q15 to a negative potential, cutting it off. During the cutoff period, the C44 voltage discharges through the windings of R74 and L2. As the discharge voltage of C44 decreases, the base potential of Q15 rises. Once it exceeds 0.7V, Q15 turns on again. During the conduction period, C41 discharges through R70. If the time constant of the C41 discharge loop is much longer than the oscillation period of Q15, a forward voltage of 0.7V forms at the base of Q15, reducing the Q15 turn-off loss. D30 and ZD3 form a base negative bias cutoff circuit. R77 and C42 are resistance-capacitance absorption circuits that suppress the peak resonance pulse generated by the collector when Q15 is turned off.
The auxiliary power supply lacks controlled adjustment voltage protection circuits. Common faults include increased resistance or open circuits in R72 and R76, breakdowns of Q15, ZD3, D30, and D41, and short circuits, along with rectifier tube breakdowns in the AC input rectifier filter circuit and blown fuses. A concealed fault might involve the C41 being close to the Q15 heatsink, getting overheated and losing its capacitance. This causes the rectified output voltage of the secondary winding BD6 to rise sharply when the ATX power supply is connected to the mains, reaching up to 80V, often burning out the DBL494 pulse width modulation chip during power-on. Such faults are hard to detect, leading to many repaired Galaxy ATX switching power supplies failing to identify the root cause, and subsequently burning new components.
Next, let’s examine the Senda Power98 ATX power supply auxiliary circuit (as seen in Figure 2). It operates similarly to the Galaxy ATX switching power supply but includes an overcurrent regulating tube Q2 in the T3 driving transformer primary winding oscillating circuit. Q1 self-oscillates and is regulated by Q2. When the rectified input voltage of T3's primary winding rises or the secondary winding load is too heavy, the oscillating current through L1 winding and Q1 ce and e pole increases, raising the voltage drop across the R06 overcurrent detection resistor. This is transmitted via R03 and R04 to the Q2 b pole, pulling the Q2 b potential above 0.7V, turning Q2 on. The base potential of Q1 is pulled low, shortening Q1's saturation conduction time and reducing the energy storage from electrical to magnetic energy in the primary winding. The secondary winding rectified output voltage drops. When Q1's self-excited oscillation is normal, the Q2 adjustment tube remains off.
This circuit enhances the auxiliary power supply's reliability somewhat. However, when the commercial power rises, the rectified input voltage increases, or the T3 secondary winding load is too heavy, and if the Q2 adjustment action lags, the R01, R02, Q1, and R06 components may still burn out, sometimes along with ZD1, D01, and Q2 components.
Finally, there is the Technology Exhibition 200XA ATX power supply auxiliary circuit (as shown in Figure 3). The primary winding side is similar to the previous two circuits; however, the secondary winding side adds an overvoltage protection circuit.
The working principle is as follows: If the output voltage of the secondary winding of T3 rises, the voltage is divided by R51 and R58, increasing the potential of the Q12 reference terminal of the precision voltage regulator and lowering the potential of the Uk of the control terminal. This turns on the LED of IC1 and the phototransistor c and e poles, allowing the output current to flow into the base of the adjustment tube Q17, turning Q17 on, which in turn turns off the oscillation switch tube Q16, providing overvoltage protection. D27, R9, and C13 form the Q16 peak resonance pulse absorption loop, while C29, L10, and C32 form a filter loop to eliminate the ripple voltage of +5VSB.
In conclusion, understanding the auxiliary power supply circuit is essential for diagnosing and repairing ATX power supplies effectively. Each design has its unique features and potential pitfalls, and recognizing these differences can help technicians perform more accurate troubleshooting and repairs.
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