Discussion on the development trend and hot spots of DC module power supply

1. Development trend of DC module power supply

In order to meet the market's ever-increasing demand for power supply performance, DC module power supplies have begun to develop in the direction of high efficiency, high power density, low voltage and high current, low noise, good dynamic characteristics and wide input range. Thin, modular, standardized circuit topologies that are combined in a building block manner have been increasingly widely used. The following is an analysis of the key points.

(1) High power density and high efficiency

Modern communication products have higher and higher requirements for volume, which will inevitably require the module power supply to reduce the volume and increase the power density, and improving efficiency is complementary. The current new conversion and packaging technology can make the power density of the power supply reach 188W/in3, which is more than double the power density of the traditional power supply , and the efficiency can exceed 90%. The reason why these indicators can be achieved is that the development of microelectronics technology has enabled a large number of high-performance new devices to emerge, thereby reducing losses. The more typical one is the high-performance metal oxide semiconductor field effect transistor (MOSFETs), which replaces the diode used in the traditional design in the synchronous rectifier, reducing the voltage drop from 0.4V to 0.2V; power MOSFET manufacturers are developing devices with smaller and smaller on-resistance, whose on-resistance has been reduced from 180 mΩ to 18 mΩ; the high degree of silicon chip integration reduces the number of components by more than 2/3, and the compact structure and layout relative to discrete components reduce stray inductance and wiring resistance. High efficiency can reduce power consumption relatively, reduce operating temperature, reduce the required input power, and also improve power density.

(2) Low voltage and high current

As the operating voltage of microprocessors decreases, the output voltage of module power supplies has also dropped from the previous 5V to the current 3.3V or even 1.8V. The industry predicts that the output voltage of the power supply will drop below 1.0V. At the same time, the current required by integrated circuits has increased, requiring the power supply to provide a larger load output capacity. For a 1V/100A module power supply, the effective load is equivalent to 0.01Ω, and traditional technology is difficult to meet such a difficult design requirement. In the case of a 10mΩ load, each mΩ resistance on the load path will reduce the efficiency by 10%. The wire resistance of the printed circuit board, the series resistance of the inductor, the on-resistance of the MOSFET, and the die wiring of the MOSFET all have an impact on the efficiency.

The development of new technologies can integrate power semiconductors and passive components that are critical to the overall layout of the circuit to form a basic module with complete functions, reducing the resistance on the path to the load, thereby reducing power consumption and reducing size. Multi-phase design technology combined with basic modules has gradually been promoted. Since the output current per phase is reduced, smaller power MOSFETs and smaller inductors and capacitors can be used, which also simplifies the design.

The basic power module package that has appeared on the market is only 11mm×11mm in size, with a switching frequency of 1MHz. By cascading multiple modules and related components, an operating current of more than 100A can be obtained. Compared with other circuits using discrete components, its efficiency is increased by 6%, power loss is reduced by 25%, and the device size is reduced by about 50%.

(3) Designing power supply using software

In today's communication systems, the variety of DC voltages continues to increase, and the increase in power density and integration has also increased the difficulty of design. Traditional manual design and verification can no longer adapt to the rapidly changing market needs, so power supply auxiliary design software came into being. These software can guide component selection and provide material lists, circuit simulation and thermal analysis, shortening the power supply design cycle and improving power supply performance. Auxiliary design software can customize power supplies using a variety of parameters, including input and output voltage ranges, maximum output current, etc., to guide designers in device selection. It includes a complete transformer design, uses a variety of topological methods to synthesize circuits, optimizes by cost or efficiency, and outputs a component list.

Another function of the software is to evaluate the performance of the module power supply through simulation. It can comprehensively analyze the performance of the power supply in a stable state, display the waveform at any node to be detected, and calculate the efficiency with precise methods. In addition, thermal analysis can give a curve chart marked with different colors according to environmental parameters such as circuit board positioning, edge temperature, and airflow direction and speed, so as to help designers understand the heat distribution of the entire circuit board under stable conditions.

2. Discussion on Hot Issues

Today's market has put forward higher requirements on the performance of module power supplies. In order to adapt to the trend of market development, the industry needs to consider more than just the progress of design and production technology. The following is a discussion on hot issues of general concern.

(1) Heat dissipation

Thermal performance is an important factor affecting the life of module power supply and should be given enough attention. To examine the thermal performance of the power supply, the cooling efficiency must be verified by measuring the key heat-generating components of the power supply, rather than simply measuring the ambient temperature. When using natural cooling, it should be ensured that there are enough vents on the top and bottom of the module power supply to form a cooling air flow. Adding heat sinks and arranging them vertically in the air can increase the heat dissipation area and effect. When using a fan, the airflow can force air cooling, greatly reducing thermal impedance. The airflow should also flow parallel to the surface of the heat sink. For a rectangular module power supply, the airflow blows along its long side, and the heat sink is parallel to the short side, so that the heat dissipation effect is best.

(2) Electromagnetic compatibility

At present, the world has established a complete electromagnetic compatibility standard and certification system. my country has also published a catalog of products that need to pass electromagnetic compatibility certification in batches, laying the foundation for the national industry to participate in international competition. The International Special Committee on Radio Interference (CISPR) is an electromagnetic compatibility standardization organization under the International Electrotechnical Commission (IEC). CISPR22 "Limits and Measurement Methods of Radio Interference of Information Technology Equipment" stipulates the electromagnetic interference limits of information technology equipment in the frequency band of 0.15 to 1000MHz. The Ministry of Information Industry has formulated YD/T983-1998 "Electromagnetic Compatibility Limits and Measurement Methods for Communication Power Supply Equipment" based on international standards. The above standards cover the electromagnetic compatibility test content and methods of module power supplies.

① Electromagnetic interference (EMI). Electromagnetic interference refers to the pollution to the environment caused by electromagnetic radiation propagation through space and electromagnetic energy conducted through signal lines and power lines. Electromagnetic interference cannot be completely eliminated, but it can be reduced to a safe level. According to the way it propagates, electromagnetic interference is divided into the following two types: conducted interference and radiated interference. Conducted interference is the noise generated by the system that enters the DC input line or signal line. Reasonable grounding and filtering of power lines and signal lines can reduce the conduction of electromagnetic interference. Radiated interference is directly propagated in the form of electromagnetic waves and can be weakened by metal shielding.

② Electromagnetic compatibility (EMC). Electromagnetic compatibility refers to the ability of electronic equipment and power supplies to work normally and reliably in a certain electromagnetic interference environment. It also refers to the ability of electronic equipment and power supplies to limit their own electromagnetic interference and avoid interfering with other surrounding electronic equipment. Improving electromagnetic compatibility can be done from the following three aspects: reducing the radiation of electromagnetic interference sources; shielding the propagation path of electromagnetic interference; and improving the anti-electromagnetic interference ability of electronic equipment and power supplies.

(3) Stability and reliability of module power supply

Stability and reliability have become key issues in power supply design, directly affecting the lowest cost for system manufacturers. Manufacturers must consider the performance of module power supplies under different temperature, airflow, humidity, and vibration conditions.

The high power density of the module power supply does not mean that its stability and reliability are high. There are many factors that affect the reliability of the module power supply, such as the airflow in the system and the direction of its flow on the module power supply, the input voltage and load requirements of the power module, the power supply required by the system and the temperature change conditions, among which the influence of temperature is crucial. The failure rate doubles for every 10°C increase in the module operating temperature. The module should have the ability to work at a higher temperature to ensure the safety and reliability of the system. In addition, in order to improve the reliability of the module, the component must work at 70-80% of its rated maximum junction temperature (Tjmax). Semiconductor device manufacturers are committed to increasing the maximum junction temperature of the device, so as to keep the operating junction temperature at a relatively low level under the condition of unchanged working conditions to improve reliability. At present, Tjmax is generally +150 oC or +175 oC, and the junction temperature of semiconductor devices should be maintained below +120 oC and +135 oC respectively.

(4) Standardization work

The trend of modular power products is becoming increasingly modular and standardized, and a distributed power supply system is formed with a building block structure. The packaged modular power supply is mainly based on the international industrial standard half-brick or brick structure. 50W, 75W, 100W and 150W are half-brick structures, and 200W, 250W, 300W and 400W are brick structures. Standardized pins bring plug-and-play convenience to designers and users, allowing designers to easily complete product design and facilitate power supply upgrades.

Nowadays, the role of standards in the power supply industry has been increasingly valued. Standardization can shorten the cycle of bringing products to market and reduce costs. However, most domestic companies currently use their own corporate standards for production and test according to their own test specifications. Various industry standards also have problems such as backward technical indicators and poor operability of test methods. As a result, the industry has no unified and complete design, production and testing standards. In order to promote the technological progress of module power supplies and provide a basis for domestic companies to control production quality, it is urgent to formulate scientific national standards.