This article is mainly about the related introduction of ATX power supply, and focuses on how to drive the LED with the 3.3V output of the ATX power supply.
ATX power supplyThe function of the ATX power supply is to convert the AC 220V power supply into the DC 5V, 12V, 24V power supply used inside the computer.
Features of ATX power supply: Compared with AT power supply, ATX power supply has increased "+3.3V, +5VSB, PS-ON"
"Three outputs. The "+3.3V" output is mainly used for memory, while the "+5VSB" and "PS-ON" outputs reflect the characteristics of the ATX power supply. The main feature of the ATX power supply is that it does not use traditional The mains switch is used to control whether the power is working, but the combination of “+5VSB, PS-ON†is used to turn on and off the power. As long as the change of the signal level of “PS-ON†is controlled, the power on can be controlled. And turn off. "PS-ON" turns on the power when it is less than 1V, and turns off when it is greater than 4.5V. Unlike the AT power supply, the ATX power supply has made some improvements on the line. The most important difference is that the ATX power supply itself is turned off. It does not completely cut off the power, but maintains a relatively weak current. At the same time, it uses this current to add a power management function called Stand-By. It allows the operating system to directly manage the power supply. Through this function, Users can directly realize soft shutdown through the operating system, and also realize networked power management. For example, when the computer is turned off, a signal can be sent to the computer's Modem through the network, and then the monitoring circuit will send out a unique ATX power supply +5V SB activates the voltage to turn on the power to start the computer, so as to realize remote boot.
There are two main versions of ATX power supply, one is ATX1.01 version, and the other is ATX2.01 version. The ATX power supplies of version 2.01 and version 1.01 have different activation currents except for the position of the cooling fan. Version 1.01 only has 100mA, and version 2.01 has 500mA～720mA. This means that the 2.01 version of the ATX power supply will not be as "allergic" as the 1.01 version, and it will often start the computer on its own under the influence of external voltage fluctuations.
Core circuit
The core circuit of the ATX power supply: The main conversion circuit of the ATX power supply is the same as the AT power supply, and it also uses a "two-tube half-bridge external excitation" circuit. The PWM (Pulse Width Modulation) controller also uses the TL494 control chip, but the mains switch is eliminated. . Since the mains switch is cancelled, as long as the power cord is connected, there will be a +300V DC voltage on the conversion circuit. At the same time, the auxiliary power supply also provides working voltage to the TL494 to prepare for the start-up power supply. The characteristic of ATX power supply is to use the “dead drive control†function of pin 4 of the TL494 chip. When the voltage of this pin is +5V, there is no output pulse on pins 8 and 11 of TL494, so that both switching tubes are cut off, and the power supply is in standby mode,
No voltage output. And when the 4th pin is 0V, TL494 will have trigger pulse provided to the switch tube, and the power supply will enter the normal working state. One output of the auxiliary power supply is sent to TL494, and the other output gets two signal voltages "+5VSB" and "PS-ON" through a voltage divider circuit, both of which are +5V. Among them, the "+5VSB" output is connected to the "power monitoring component" of the ATX motherboard. As its working voltage, the "+5VSB" output is required to provide 10mA of working current. The output of the "power monitoring component" is connected to the "PS-ON". When the trigger button switch (non-locking switch) is not pressed, the "PS-ON" is +5V, which is connected to the positive phase input of the voltage comparator U1 The voltage at the negative input terminal of U1 is about 4.5V, so the output of the voltage comparator U1 is +5V, which is sent to the "dead drive control pin" of TL494, so that the ATX power supply is in a standby state. When the power monitoring trigger button switch of the motherboard is pressed (installed on the panel of the main box), "PS-ON" becomes low level, the output of the voltage comparator U1 is 0V, and the power of the ATX mainframe is turned on. Press the trigger button switch on the panel again to make "PS-ON" change to +5V again, thereby turning off the power. At the same time, the program can also be used to control the output of the "power monitoring component", so that the "PS-ON" becomes +5V, and the power is automatically turned off. If a shutdown command is issued under the WIN9X platform, the ATX power supply will be automatically turned off.
Failure analysis
If a computer system with ATX power supply fails, it must first be analyzed from several aspects such as CMOS settings, ACPI settings in Windows, power supply and motherboard. In terms of hardware, in order to distinguish whether the fault lies in the load or the power supply itself, the power supply can be disassembled and used as a fake load with a used equipment (such as a hard disk, etc.) to avoid no-load protection. Connect the PS-ON signal line (green) to the ground. Connect a 100-150Ω resistor between the wires to make the signal low. If the power supply can work, it means that the fault is on the motherboard or the power button, otherwise the fault is on the power supply itself.
According to the principle of "soft first and then hard" in computer maintenance, the user must first check whether the BIOS settings are correct, and eliminate false faults caused by improper settings; the second step is to check whether the auxiliary power supply and the main power supply in the ATX power supply are normal;
The third step is to check whether the mainboard power supply monitoring circuit is normal. Below, this article introduces the analysis and processing methods according to the different manifestations of the fault.
Repair method
Open the upper box of the power supply and observe the inside of the power supply.
A. There is no explosion phenomenon in the component. If the fuse is burnt black, it means that the primary circuit has a short circuit. Check the rectifier diode, standby power tube, half-bridge double triode, and see if there is any breakdown.
B. The component does not explode, turn on the power, use a meter to measure the green and purple wires in the 20-pin, whether there is +5V voltage, if not, check the standby circuit, and focus on measuring the power-on resistance. Generally, the power-on resistance is several hundred K, it is prone to increase resistance and open circuit phenomenon. Check whether the small transistor connected to the standby power tube is short-circuited or open.
C. The green and purple wires in the 20-pin have +5V voltage, and then use the wire to short the green and black wires to force the boot, and see if it can boot. If not, see if the power pin of TL494 (7500B) has voltage (12 The pin is the power supply), if not, check the line connected to the secondary circuit of the standby circuit. The power pin of TL494 (7500B) has voltage and cannot be turned on. Check whether the dead zone control pin (4) is 5V or 0V. If it is 5V, the circuit is generally protected. Check whether the three double-diode rectifiers are short-circuited.
Through the above three items, 70% of faulty power supplies can be repaired. In the repair, it was found that there was very little IC damage. The TL494 was damaged. The LM339 has not been damaged.
How to drive LED with 3.3V output of computer ATX power supplyBecause the damage of the south bridge generally has obvious heating. But you can't make such arbitrary judgments. You can run the circuit with a multimeter to see if the 3.3V connected patch capacitor or resistor is damaged. The approximate order of judgment: first remove the problematic filter capacitors connected to pins 1, 2, and 11, and pins 1, 2, and 11 are still short-circuited to ground; at the same time, the two pins of the pad that are vacated by the capacitor are also short-circuited. . Remove the IO, network card chip, clock IC, BIOS, power supply IC and other chips that need 3.3V power supply and the problematic filter capacitors around them, and the power supply 1, 2, and 11 pins still fail. The MOS tube that will power the memory and the 8-pin chip with a 1.25V pull-up power supply are also removed, and the power supply 1, 2, and 11 pins are still faulty. At the same time, it is good to measure other resistances and diodes on the board. Check that there is no foreign matter in AGP, PCI, and memory slots, and no signs of burnout due to excessive current. After countless repairs, there are roughly three reasons. One is that there is a short circuit caused by metal shavings in the slot, and the other is that the power supply circuit of the motherboard is broken. It may be that a capacitor in the loop is broken down (shorted) or damaged. The third is the damage of the south bridge. If it is true, unless your board is a high-end motherboard, there is basically no need for repair if it is not. The replacement of the South Bridge also requires a professional BGA machine, which is very troublesome to plant the ball, and the probability of failure is very high.
Computer ATX power control circuit and principle
The control circuit of the ATX power supply is shown in Figure 1. The control circuit uses TL494 (some power supplies use KA750B, and its pin functions are the same as TL494, interchangeable) and LM339 integrated circuits (hereafter referred to as 494 and 339). 494 is a dual-row 16-pin integrated circuit with a working voltage of 7-40V. It contains a +5V reference power supply output by the {14} pin, the output voltage is +5V (±0.05V), the maximum output current is 250mA; a frequency adjustable sawtooth wave generating circuit, the oscillation frequency is connected to the {5} pin with an external capacitor and { 6} is determined by an external resistor. When the {13} pin is at a high level, the {8} and {11} pins output dual inverted (ie push-pull working mode) pulse width modulation signals. This example is this working mode, so connect the {13} pin to the {14} pin. The comparator is a kind of operational amplifier. The symbol is represented by a triangle. It has a non-inverting input terminal "+"; an inverting input terminal "-" and an output terminal.
If the level of the non-inverting terminal of the comparator is higher than the level of the inverting terminal, the output terminal outputs a high level; otherwise, it outputs a low level. There are four comparison amplifiers in 494. For the convenience of description, they are represented by lowercase letters a, b, c, and d in Figure 1. Among them a is the dead time comparator. Because two transistors for inverter work are connected in series to the +310V DC power supply, if the two transistors are turned on at the same time, it will form a short circuit to the DC power supply. The simultaneous conduction of two triodes may occur when one tube turns from cut-off to conduct, and the other tube turns from conduction to cut-off. Because there is a time delay in the conversion of the tubes, the cut-off tube has already been turned on, but the conducting tube has not been completely turned off, so the two tubes are both turned on and form a short circuit to the DC power supply. In order to prevent such things from happening, 494 sets the dead time comparator a. As can be seen from Figure 1, a "power supply" is connected in series to the inverting input terminal of comparator a, the positive pole is connected to the inverting terminal, and the negative pole is connected to pin {4} of 494. The sawtooth signal input from the non-inverting terminal of A comparator has only the part greater than the "power supply" voltage. During the period when the transistor is turned off and turned off to turn on, that is, the dead time, there is no pulse output in 494, so avoid A short circuit to the DC power supply. The dead time can also be controlled by the external level of pin {4}. When the level of pin {4} rises, the dead time becomes wider, and the output pulse of 494 becomes narrower. If the level of pin {4} exceeds The peak voltage of the sawtooth wave, 494 enters the protection state, and the {8} and {11} pins no longer output pulses.
There are three two-input AND gates (represented by 1, 2, 3), two two-input NAND gates, inverters, T flip-flops, and other circuits inside the 494. The AND gate is such a circuit, only when all input terminals are high level, the output terminal can output high level; if one input terminal is low level, the output terminal outputs low level. The function of the inverter is to isolate and amplify the input signal and then invert the output. The NAND gate is equivalent to a combination of an AND gate and an inverter. The function of the T flip-flop is: every time a pulse is input, the level of the output terminal changes once. If the output terminal Q is low level, after inputting a pulse, Q becomes high level, and then inputting a pulse, Q returns to low level. The output waveforms of the comparator, AND gate, inverter, T flip-flop, sawtooth oscillator and pin {8} and {11} are shown in Figure 2. 339 is a four-comparator integrated circuit. Set the four internal comparators as A, B, C, D comparators according to the order of the pins. 494 and 339 cooperate with other circuits to jointly complete the voltage stabilization of the ATX power supply, generate a PWM signal and various protection functions.
1. Generate PWM signal
The PC host requires that the power supply of each channel is stable before working to protect the components from damage due to voltage instability. Therefore, the PWM-OK signal (about +5V) is set, and the host starts to work after obtaining this signal. When the power is turned on, the PW-OK signal is required to be generated hundreds of milliseconds later than the ±5V, ±12V, +3.3V power supply. The PW-OK signal should disappear hundreds of milliseconds before the DC power supply when shutting down, so that the host will stop working first. The head of the hard disk returns to the landing zone to protect the hard disk.
After the ATX power supply is connected to the mains, the auxiliary power supply works immediately. On the one hand, it outputs +5VSB power supply, and at the same time provides more than ten volts to more than twenty volts of DC power to the {12} pin of 494. 494 outputs +5V reference power from pin {14}, and the sawtooth oscillator also starts to vibrate. If the host is not turned on, the PS-ON signal is high, and the B comparator {6} pin of 339 is also high via R37, because the resistance R37 is smaller than R44, and the level of pin {6} is higher than pin {7} Level, B comparator output terminal {1} ​​pin outputs low level, after the clamping effect of D36, the inverting terminal {4} pin of A comparator is also low level, and its level is lower than the non-inverting terminal {5 } Pin, the output pin {2} is high, the {4} pin of 494 is high through R41, so the dead time comparator a inside 494 outputs low level, and the AND gate 1 is also high. Therefore, the output low level and the AND gate 2 and AND gate 3 output low level, which blocks the output of the oscillator. There is no pulse output at pins {8} and {11}, and the ATX power supply has no ±5V, ±12V, +3 ．3V power output, the host is in standby state. Because the +5V and +12V power output is zero, the {1} pin level of 494 is also zero through resistors R15 and R16, the output of pin {3} of the output terminal of the c comparator of 494 is also zero, and the output of pin 339 is also zero through R48. Pin 9} is also at zero level, so the output pin {14} of the 339 C comparator is at zero level.
In addition, the low level signal of pin {1} of 339 also makes pin {14} low level due to the clamping effect of D34, and pin {11} is also low level via R50 and R63. Therefore, the output pin {13} of the D comparator is low, that is, the PWM-OK signal is low, and the host will not work. When the host is turned on, the switch related to PS-ON is closed by manual or remote control, PS-ON is low, and the inverting terminal {6} pin of 339 is low via R37, B comparator {1} The pin outputs high level, D35 and D36 are reversely biased and cut off, and the output level of the A comparator is determined by the levels of the {5} and {4} pins. During normal operation, the level of pin {5} is lower than that of pin {4}, and pin {2} outputs low level, which is sent to pin {4} of 494 via R41, so that the level of pin {4} becomes low Level, sawtooth wave oscillation signal can output pulse signal from dead time comparator a. On the other hand, the oscillation signal is sent to the non-inverting input terminal of PWM comparator b. The width of the pulse signal output by PWM comparator is 494 The level of the {1} pin (that is, the size of the load) and the level of the {16} pin are determined. The pulse signal output by the PWM comparator is finally amplified by the buffer amplifier, and the pulse signal is output from the {8} and {11} pins, and the ATX power supply outputs ±5V, ±12V, +3.3V power to the host. This process has a delay of hundreds of milliseconds due to the charging of the C35, but it has no effect on the power-on of the host. The {1} pin of 494 gets the voltage from +5V, +12V through the sampling resistors R15 and R16, and its level is slightly higher than that of pin {2}, pin {3} outputs high level, and pin {9} of 339 is made by R48 Get the high level, its level is higher than the {8} pin level, so {14} pin outputs high level. This level is used to charge C39 through R50 and the reference +5V power supply through R64. After hundreds of milliseconds, { When the level of pin 11} rises to higher than that of pin {10}, D comparator {13} pin outputs high level, this level is fed back to pin {11} via R49, and pin {11} is maintained at a high level , So the {13} pin outputs a stable high-level PWM signal, and the host starts to work normally after detecting this signal.
When shutting down, the switch in the host makes the PS-ON high. At this time, the {6} pin level of 339 is higher than the {7} pin, and the {1} pin outputs a low level. Due to the clamping effect of the diode D34, { The 14} pin is low, C39 discharges the C comparator and the B comparator, and soon the {11} pin is low, and the {13} pin outputs a low level, that is, the PW-OK signal is low. When pin {1} of 339 is low, pin {4} is made low via D36, pin {2} outputs high level, and it is transmitted to pin {4} of 494 via R41, but the potential of C35 cannot Sudden change, after hundreds of milliseconds of discharge, the {4} pin of 494 is turned to high level, thereby blocking the output of positive and negative pulses, and the host enters the standby state. In the above process, both C39 and C35 are discharged when shutting down, but because of the different discharge time constants, C39 discharges faster, so the PW-OK signal turns to low level before each power supply, which satisfies the need for the host to shut down. In addition, it takes time for the electrolytic capacitors of each output power supply to discharge when shutting down, which also makes the PW-OK signal return to low level before each power supply.
The main difference between ATX power supply and AT power supplyThe difference between AT power supply and ATX power supply:
1. The AT power supply is a standard proposed by IBM when it launched the PC/AT machine in the early days, and it was able to provide about 190W of power supply at that time. There must be a power switch on the AT power supply to control the computer's switch.
2. ATX power supply is an industrial standard proposed by Intel in 1995. Since the original ATX1.0, the ATX standard has undergone many changes and improvements. At present, the two popular ones in the domestic market are ATX2.03 and ATX12V. Standards.
The main feature of the ATX power supply is that it does not use the traditional mains switch to control whether the power supply is working, but uses the combination of "+5VSB, PS-ON" to turn on and off the power, as long as the "PS-ON" signal is controlled. The level change can control the power on and off. In this way, the power of the computer can be turned off through the operating system.
Concluding remarksThis is the end of the related introduction about ATX power supply. Please correct me if there are any deficiencies.
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