Power Supply Basics

(1) Overview

PC power supply It has been around for about 20 years since it appeared in the early 1980s and has been developing along with the evolution of PCs. Its basic function is to obtain energy from the power grid and then convert it into low-voltage DC power suitable for PCs, while completing the necessary safety isolation function.

PC power supply is a switching power supply, which adopts PWM switching conversion technology. The energy obtained from the power grid must go through the conversion process of rectification, filtering, chopping, voltage reduction, re-rectification, filtering, etc., and negative feedback technology is used to keep the output voltage stable. Compared with linear power supply, it has the advantages of small size and high efficiency.

In the development of PC power supply, there have been some different types and standards, from the early AT type to the ATX type, and then to the current ATX12V (P4) type, and the output voltage has increased from the original 4 groups to 6 groups. These evolutions are closely related to the diversified power management functions, diversified configurations and PC system power bus structure that have gradually developed in PC.

The power supply is a key component of the PC. Not only should the performance meet the corresponding standards and specifications, but its load capacity, reliability, and adaptability and compatibility with the motherboard and peripherals have a great impact on the reliable and stable operation of the entire system. Next, I will discuss with you from the standards and specifications that the power supply should meet, power supply reliability, power and load distribution, and compatibility of the power supply with the motherboard and peripherals.

(2) Standards and specifications that the power supply should comply with

There are many standards and specifications related to PC power supplies. They put forward different requirements for power supplies from different angles and are important criteria for judging whether the power supply is qualified. Here we divide them into two categories for a brief introduction:

One type of standard is a mandatory standard, which is a standard that the power supply must meet.

Electrical safety: GB 4943-2001 "Safety of Information Technology Equipment (including Electrical Business Equipment)" (equivalent to IEC 950-1986). Products must not only meet the requirements of this standard, but also be approved by an authoritative organization before they can be produced and sold, which is commonly known as safety certification. Product safety is an issue that every country and region attaches great importance to, because it is directly related to human life safety. Domestic safety certification is called Great Wall Certification, and the China Electrical Product Certification Committee (CCEE) specializes in electrical product safety certification and related qualification certification activities. Electromagnetic compatibility: GB 9254-1998 "Limits and Measurement Methods of Radio Disturbance for Information Technology Equipment" (equivalent to CISPR 22:1997). This standard mainly sets limits on the conducted interference and radiated interference generated by the product. Its purpose is to require that the product should not interfere with the normal operation of other equipment when in use.

In terms of harmonic current: GB 17625.1-1998 "Limits of harmonic currents emitted by low-voltage electrical and electronic equipment (equipment input current per phase ≤ 16A)" (equivalent to IEC 61000-3-2:1995). This standard is formulated for the impact of products on the power grid, which is called power pollution. The problem of harmonic current is also an issue that is receiving more and more attention. Europe has begun to enforce the standard of harmonic current limitation since 2001. China has promulgated the corresponding standard since 1998, but it has not yet been enforced. The technology for suppressing harmonic current is also customarily called power factor correction technology (PFC).

All mandatory standards have a trend of being merged together for certification. Relevant documents have been issued and will be implemented in 2003. After implementation, the certification name will be called CCC certification.

Another type of standard is non-mandatory standards, also called recommended standards.

Electromagnetic compatibility: GB/T 17618-1998 "Limits and methods of measurement for the immunity of information technology equipment" (equivalent to CISPR 24:1997). This standard and GB 9254-2001 "Limits and methods of measurement for radio disturbances of information technology equipment" are actually two aspects of the electromagnetic compatibility of products. GB9254 focuses on the interference emitted by the product, while GB 17618 is the anti-interference ability that the product should have. Only when these two requirements are met at the same time can it be considered a perfect product, and it can ensure that different devices will not affect each other when used at the same time. However, these two aspects are different in severity, and interference will cause more serious problems than anti-interference, so GB 9254 is a mandatory standard and GB/T 17618 is a recommended standard.

Comprehensiveness: GB/T 14714-1993 "General Technical Requirements for Switching Power Supplies for Microcomputer System Equipment" has no corresponding standard in the world. It is a guiding standard specially written by my country for computer power supply products. Its content involves product performance, environment, manufacturing, testing, packaging, transportation, etc. Although it is not a mandatory standard, the content it contains is relatively comprehensive and has good reference value and guiding significance.

Intel: "ATX/ATX12V Power Supply Design Guide", "SFX/SFX12V Power Supply Design Guide". The last specifications to be introduced are these two power supply design guides from Intel. Although these two design guides are not standards issued by national institutions, nor are they specification documents in the strict sense, they are the most important product design references in the field of PC power supplies. Because Intel has been in an absolute leading position in the PC field for a long time, it has become the de facto industry "leader" and compatible standard. These two design guides give a very detailed description of PC power supplies, from the appearance structure, interface definition to the definition and setting of various input and output parameters, covering almost all the characteristics of PC power supplies. At present, the vast majority of PC power supplies in the world are designed, tested, and evaluated based on this guide.

(3) Power supply reliability

The power supply is processing energy, and its internal components have to withstand high voltage, high current, high power and heat loss. It is a component that is prone to failure in the whole machine, so its reliability is of great significance to the reliability of the whole machine. According to statistics, the reasons for equipment unreliability are design errors accounting for about 1/3, component quality accounting for about 1/3, and manufacturing, operation and maintenance accounting for about 1/3. In fact, the latter two aspects are also related to the lack of consideration in the design stage. In order to achieve high reliability, reliability issues must be considered from the design stage.

To obtain high reliability design is the most important, the design is mainly handled from the following perspectives:

Optimize the circuit. In circuit design, use as many standardized circuits or circuits with high reliability as possible, and use mature technologies as much as possible.

Circuit design follows the principle of simplification. Under the premise of ensuring the design functions and indicators, the design should be realized with the simplest circuit and the least number of components as much as possible. While reducing the number of components, the number of varieties and specifications should also be reduced. From the perspective of reliability, a large number of components cannot be added for a slight improvement in performance. There must be sufficient tolerance design and worst-case design. That is, the tolerance, drift and changes in environmental conditions of component parameters should be considered.

Select components correctly and perform reasonable derating design according to the working stress of components. The correct selection of component types is the first priority, which requires designers to have sufficient understanding of the types of each component. After selecting the type, derating design is performed. Derating means that the components work under stress conditions lower than their rated values. Derating designs are performed for temperature, voltage, and power according to the component type and derating curve.

In addition to good design, reliability can also be improved through experimental methods. The process is to continuously deteriorate the environmental conditions of the product (voltage, temperature, etc.) until failure. The weakest link can be found, and then this link is improved. Then the environmental conditions continue to deteriorate. This method is a relatively practical method.

Reliability is measured by failure rate or mean time between failures (MTBF), which are the reciprocals of each other. MTBF can be calculated or measured and verified through tests. In China, the test verification method is mostly used. According to GB/T 5080.7-1986, a timed (fixed number) truncation test scheme is used to determine whether the predetermined MTBF has been achieved.

Basic aspects of power supply performance evaluation 1. Technical specifications (including electrical performance, mechanical performance, safety regulations, electromagnetic compatibility, environment, etc.)

1) The power supply technical specifications should comply with the national mandatory standards (preferably also with the national non-mandatory or recommended standards). This standard is shown in Part 2.

2) The power supply technical specifications should comply with or refer to the industry specifications established by Intel. You can refer to Intel's industry specifications (design guidelines)

2. Reliability

Please refer to Part 3. The main thing is to leave a large margin for the electrical and thermal stress of components.

example:

The semiconductor junction temperature does not exceed 110°C (ambient temperature 50°C)

The temperature rise of transformers and inductors cannot exceed the requirements of safety regulations.

The capacitor case temperature cannot exceed 95% of the rated value

The voltage and current derating of components should be greater than 10%

The magnetic components must not saturate under any grid, load, starting or transient conditions.

3. Compatibility

Compatibility involves a wide range of issues. A good power supply product should not only have good technical specifications and reliability, but also have comprehensive compatibility, that is, the ability to adapt to various accessories. The power supply is the source of energy required by the entire computer system, so it is related to every component that needs to be powered by the power supply.

Common compatibility design points that should be noted:

1) Timing relationship between power supply outputs and Remote and PG signals

2) Anti-shake design of Remote signal and PG signal

Positive feedback, delayed reversal characteristics and peak absorption circuit are added to prevent false operation and ensure PG stability.

3) Power supply output stability, transient characteristics and protection functions

Example: Some motherboards will generate abnormal transient overcurrent, which will cause power protection or voltage drop.

4) Temperature stability of various electrical parameters (should work normally under various temperature conditions).

5) Leaving a margin (with an upper limit requirement) or centering (with upper and lower limit requirements) for each performance indicator can prevent deviation in fitting.