Today, desktop PC components become obsolete very quickly. The only exception is the power supply unit (PSU). The design of this device has not undergone major changes over the past 15 years, when the ATX form factor PSUs appeared on the market. The principle of operation and the schematic diagram of the power supply for a computer are not much different for all manufacturers.
Content
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Structure and working principle
- Rectifier and PWM controller
- Converter output stages
- Pinout of the main connector
- Load sharing and possible malfunctions
Structure and working principle
A typical ATX computer power supply circuit is shown below. By design, this is a classic pulse-type power supply unit based on the TL 494 PWM controller. The signal to start this element comes from the motherboard. Until the control pulse is formed, only the standby power supply remains active, giving out a voltage of 5 V.
Rectifier and PWM controller
To make it easier to understand the structure of the computer's power supply and the principle of its operation, you need to consider the individual structural elements.
It's worth starting with a network rectifier.
The main task of this unit is to convert an alternating mains electric current into a constant one, which is necessary for the functioning of the PWM controller, as well as the standby power supply. The block includes several main parts:
- Fuse F1 - needed to protect the power supply from overload.
- Thermistor - it is located in the "neutral" line and is designed to reduce the surges in electric current that occur when the PC is turned on.
- Noise filter - it includes chokes L1 and L2, capacitors C1-C4, as well as Tr1, which have a counter winding. This filter allows you to suppress interference that inevitably arises during the operation of a pulsed power supply unit, which can negatively affect the operation of television and radio equipment.
- Diode bridge - located immediately behind the noise filter and allows you to convert an alternating electric current into a constant pulsing one. A capacitive induction filter is provided to smooth out the ripple.
At the output of the network rectifier, voltage is present until the power supply unit is disconnected from the outlet. In this case, the current flows to the standby power supply and the PWM controller. It is the first structural element of the circuit that is shown in the figure.
It is a pulse-type low power converter. It is based on the T11 transistor, whose task is to generate supply pulses for the 7805 microcircuit.
After the transistor, the current first passes through an isolation transformer and a rectifier based on diode D 24. The microcircuit used in this PSU has one rather serious drawback - a high voltage drop, which, under heavy loads, can cause the element to overheat.
The basis of any pulse-type converter is a PWM controller. In this example, it is implemented using the TL 494 microcircuit. The main task of the PWM (Pulse Width Modulation) module is to change the duration of the voltage pulses while maintaining their amplitude and frequency. The resulting output voltage on the pulse converter is stabilized by adjusting the duration of the pulses that the PWM controller generates.
Converter output stages
It is on this structural element that the main load falls. This leads to serious heating of the switching transistors T2 and T4. For this reason, they are mounted on massive radiators. However, passive cooling does not always allow you to cope with strong heat generation, all power supplies are equipped with a cooler. The output stage circuit is shown in the figure..
In front of the output stage, there is a power supply switching circuit based on a T9 transistor. When the power supply is started, a voltage of 5 V is supplied to this structural element through the resistance R 8. This happens after the formation of a signal to start the PC on the motherboard. If there are problems with the operation of the standby power supply, then the power supply unit may turn off immediately after starting.
Now all manufacturers use almost the same power supply circuits for computers. The changes they make do not have a serious impact on the principle of operation of the device.
Pinout of the main connector
At first, an ATX power supply unit was equipped with a 20-pin connector for connecting to the motherboard. However, the improvement of computer technology has led to the need to use an additional 4 contacts. Modern power supplies can be equipped with a 24-pin connector in a single housing or have 20 + 4 pins. All connector pins are standardized and here are the main ones:
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+3.3 V - power supply for the motherboard and central processor. - +5 V - voltage is necessary for the operation of some nodes of the motherboard, hard drives and external devices connected to the USB ports.
- +12 V - controlled voltage used by HDDs and coolers.
- -5 V - not used since ATX 1.3.
- -12 V - is used extremely rarely today.
- Ground - mass.
Load sharing and possible malfunctions
The voltage supplied by the power supply is for different loads. Thus, depending on the configuration of a particular PC, energy consumption in each power supply circuit may vary. That is why the technical characteristics of the power supply unit indicate not only the total power of the device, but also the maximum electric current consumption for each type of output voltage.
Keep this fact in mind when upgrading your PC hardware. For example, installing a powerful modern video accelerator leads to a sharp increase in the load in the 12V circuit. For the PC to work correctly, you may need to replace the power supply. Most often, problems with the operation of the power supply unit are associated with the aging of its structural elements or a significant lack of power.
Do not forget that overheating of the output stage can be associated with the accumulation of a large amount of dust inside the power supply. Electrolytic capacitors installed in the mains rectifier and output stages are more prone to aging than other parts.
First of all, this concerns the products of little-known brands that use cheap components. In fact, it is the element base and the quality of the parts that distinguish good devices from cheap ones. Only a person with a certain set of knowledge in the field of electronics can repair a power supply unit on his own. However, modern devices made by well-known brands are highly reliable. Subject to the rules of PC maintenance, problems with them arise very rarely.
