Detailed analysis of technical analysis of ATX power supply

At present, ATX power supply is widely used in computers. Compared with AT power supply, it is more in line with the "green computer" energy-saving standard. Its corresponding motherboard is ATX motherboard.

1. Features of ATX power supply

Compared with the AT power supply, the ATX power supply has three outputs of "+3.3V, +5VSB, PS-ON". The "+3.3V" output is mainly for the CPU, 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 the traditional mains switch to control whether the power supply works. Instead, the combination of "+5VSB, PS-ON" is used to turn the power on and off. Just control the "PS-ON". The change in signal level controls the power on and off. When "PS-ON" is less than 1V volts, the power is turned on, and when it is greater than 4.5 volts, the power is turned off.

2. 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. It is also a "double-tube half-bridge-excited" 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 line is connected, there will be +300V DC voltage on the conversion circuit, and the auxiliary power supply also supplies the working voltage to the TL494, ready for starting the power supply.

The characteristic of ATX power supply is to use the "dead drive control" function of pin 4 of TL494 chip. When the voltage of this pin is +5V, there is no output pulse on the 9th and 11th feet of TL494, so that both switches are cut off, the power supply is in Standby state, no voltage output. When the 4th pin is 0V, the TL494 has a trigger pulse to be supplied to the switch tube, and the power supply enters a normal working state. One output of the auxiliary power supply is sent to the TL494, and the other output is divided by the voltage divider circuit to obtain the "+5VSB" and "PS-ON" signal voltages, which are both +5V. Among them, the "+5VSB" output is connected to the ATX motherboard. The power monitoring component, as its operating voltage, requires a "+5VSB" output to provide 10mA of operating current. The output of the “Power Monitoring Unit” is connected to “PS-ON”. When its trigger button switch (non-locking switch) is not pressed, “PS-ON” is +5V, which is connected to the positive input of voltage comparator U1. Terminal, and the voltage of the U1 negative phase input terminal is about 4.5V, so the input of the voltage comparator U1 is +5V, and is sent to the "dead drive control pin" of the TL494, so that the ATX power supply is in the standby state. When the power monitor trigger button switch of the main board is pressed (installed on the panel of the main unit), "PS-ON" goes low, the output of the voltage comparator U1 is 0V, and the ATX main power is turned on. Press the trigger button switch on the panel again to turn "PS-ON" to +5V again to turn off the power. At the same time, the program can also be used to control the output of the "Power Monitoring Unit", so that "PS-ON" becomes +5V and the power is automatically turned off. For example, under the WIN9X platform, the ATX power will be automatically turned off when the shutdown command is issued.

3. Failure analysis of the motherboard can not be powered

Since the opening of the ATX power supply is subject to the power monitoring component of the motherboard, when the ATX host fails to power up, it cannot immediately determine whether the fault is the power supply itself or the "power monitoring component" of the motherboard, which brings certain difficulties to the maintenance.

According to the above analysis, we can connect a resistor of about 100 OHM between the “PS-ON” output and the ground, so that “PS-ON” can be turned low to start the ATX power supply, so that the fault location can be distinguished. At the same time, it also reminds us that if the "power monitoring component" of the ATX motherboard fails, because it is difficult to repair, we can skip the "power monitoring component" and directly control the voltage of "PS-ON" to turn it on or Turn off the host. Of course, the automatic shutdown function of the host is not available at this time.