Module power supply is widely used in switching equipment, access equipment, mobile communication, microwave communication, optical transmission, routers and other communication fields, as well as automotive electronics, aerospace and so on. Because of the characteristics of short design cycle, high reliability and easy system upgrade, modular power supply is more and more widely used. Especially in recent years, due to the rapid development of data services and the continuous promotion of distributed power supply system, the increase of module power supply has exceeded that of primary power supply. With the extensive use of semiconductor technology, packaging technology and high-frequency soft switches, the power density of module power supply is getting larger and larger, the conversion efficiency is getting higher and higher, and the application is becoming simpler and simpler.
Development trend of module power supply
From 1999 to 2004, the global market of block power supply is predicted to increase from 3billion US dollars to 5billion US dollars. The main market growth point is data communication, in which the proportion of 5V output decreases from 30% (1999) to 11% (2004). The following trends in the development of modular power supply deserve attention:
1) the demand for high power density, low voltage output (less than 3.3V) and fast dynamic response drives the development of module power supply.
2) non isolated DC DC Converters (including VRM) grow faster than isolated DC DC converters.
3) distributed power supply is developing faster than centralized power supply, but centralized power supply system will still exist.
4) the proportion of DC DC converter with standard design will increase.
5) the design of module power supply is becoming more and more standardized, and the control circuit tends to adopt digital control mode.
Key technologies of module power supply
At present, the main suppliers of modular power supply in the domestic market are VICOR, astec, lambda, Ericsson and power one. In order to achieve high power density, quasi resonant and multi resonant technologies were used in the early stage of the circuit, but this technology has high device stress and frequency modulation control, which is not conducive to the optimization of magnetic devices. Later, this technology developed into high-frequency soft switching and synchronous rectification. Due to the use of zero voltage and zero current switching, the switching loss of the device is greatly reduced. At the same time, due to the development of the device, the switching frequency of the module is greatly improved. Generally, the PWM can reach more than 500KHz. The volume of magnetic devices is greatly reduced and the power density is increased.
Development trend of circuit topology
The main development trends of DC DC converter circuit topology are as follows:
High frequency: in order to reduce the size of the switching converter, improve its power density and improve its dynamic response, the switching frequency of the low-power DC DC converter will be increased from the current 200-500khz to more than 1MHz. However, high frequency will cause new problems, such as the increase of switching loss and passive component loss, high-frequency parasitic parameters and high-frequency EMI.
Soft switching: in order to improve efficiency, various soft switching technologies are adopted, including passive lossless (absorption network) soft switching technology and active soft switching technology, such as zvs/zcs resonance, quasi resonance, constant frequency zero switching technology, etc., to reduce switching loss and switching stress, so as to achieve high efficiency and high frequency. For example, the DC DC high frequency soft switching converter developed by American VICOR company has 48/600w output, 90% efficiency and 120w/in3 power density. Japanese lambda company adopts active clamp ZVS PWM forward flyback combined converter and synchronous rectification technology, which can make the efficiency of DC DC converter module reach 90%.
Low voltage output: for example, the VRM voltage of modern microprocessors will be 1.1-1.8v, and the output voltage of DC DC converter of portable electronic equipment will be 1.2V. It is characterized by large load change. In most cases, the operation is lower than the standby mode, and the long-term light load operation. The DC DC converter is required to have the following characteristics: a) high efficiency in the whole range of load change. b) The output voltage is low (the loss of CMOS circuit is proportional to the square of the voltage. If the supply voltage is low, the circuit loss is small). c) High power density. The module is packaged in the form of integrated chip.
Development direction of module power supply process
Reduce thermal resistance and improve heat dissipation - in order to improve heat dissipation and power density, the medium and high power module power supply mostly adopts the multi printed board superposition packaging technology, the control circuit adopts the ordinary printed board placed on the top layer, and the power circuit adopts the board with excellent thermal conductivity placed on the bottom layer. The early medium and high power module power supply used ceramic substrates to improve heat dissipation. In order to meet the needs of high power, this technology developed into direct copper bond (DCB) technology. However, because the ceramic substrates are fragile, it is difficult to install radiators on the substrates, and the power level cannot be very large. Later, this technology was developed to directly etch circuits with insulated mental substrate (IMS). The most common substrate is the aluminum substrate, which is directly coated with insulating polymer on the aluminum heat dissipation plate, and then coated with copper. After etching, the power devices are directly welded to the copper. In order to avoid thermal mismatch caused by direct mounting on IMS, aluminum plate can also be directly used as the substrate, the control circuit and power devices can be welded on multi-layer (more than four layers, as transformer winding resistance) FR-4 printed board respectively, and then the side welded with power devices can be bonded to the formed aluminum plate through thermal conductive adhesive for fixed packaging. In order to be more conducive to heat conduction, moisture-proof and earthquake resistance, many module power supplies are compressed and sealed. The most commonly used sealing material is silicone, but polyurethane rubber or epoxy resin is also used. The latter two modes have good insulation performance, high mechanical strength and good thermal conductivity, which have become one of the development trends of module power supply in recent years and are the key technologies to improve module power density.
Secondary integration and packaging technology - in order to improve power density, surface mount technology is adopted for all module power supplies developed in recent years. Due to the serious heating capacity of the module power supply, we must pay attention to the thermal matching between the chip device and the substrate when using the surface mount technology. In order to simplify these problems, MLP (multilayer polymer) chip capacitors have recently emerged. Its temperature expansion coefficient is very close to that of copper, epoxy resin filler and FR4 PCB board. It is not easy to cause the capacitor failure due to too rapid temperature change, such as tantalum capacitors and magnetic chip capacitors. In addition, in order to further reduce the volume, the secondary integration technology has also developed rapidly. It is to directly purchase bare chips, package them after successfully assembling functional modules, weld them on the printed board, and then bond them. This method has higher power density and smaller parasitic parameters. Because the substrate of the same material is used, the thermal matching of different devices is better, and the cold and thermal shock resistance of the module power supply is improved. CPEs led by Professor lizeyuan is studying IPEM (integratedpower electronics module) in terms of technology. It is a three-dimensional packaging structure, which mainly aims at power circuits and replaces wire bonding technology.
Flat transformer and magnetic integration technology - magnetic components are often the largest and highest components in the power supply. Reducing the volume of magnetic components will improve the power density. In the medium and high power module power supply, in order to meet the requirements of standard height, most professional manufacturers customize their own magnetic cores. However, only Philips, the existing magnetic supplier, can provide general flat magnetic cores, and it is difficult to manufacture the windings of this transformer. Using this magnetic core can further reduce the volume, shorten the lead length and reduce parasitic parameters. CPEs has been studying a magnetic integration technology. Professor Chen Wei of Fuzhou University studied the magnetic integration technology at CPEs three years ago. A prototype they made is a half bridge circuit. The output rectifier adopts the current doubling rectifier technology, and the two inductors at the output end are integrated with the main transformer in an iron core. Finally, the power density reached is 300w/in3. Current doubling rectifier technology is suitable for occasions with large output current and high di/dt requirements. For example, this rectifier circuit is often used in the circuit to realize VRM.