Current mainstream module power supply technology and development

Module power supplies are widely used in communication fields such as switching equipment, access equipment, mobile communications, microwave communications, optical transmission, routers, and automotive electronics, aerospace, etc. As the use of modules to build power supply systems has the characteristics of short design cycle, high reliability, and easy system upgrades, module power supplies are increasingly widely used. In particular, in recent years, due to the rapid development of data services and the continuous promotion of distributed power supply systems, the growth rate of module power supplies has exceeded that of primary power supplies. With the extensive use of semiconductor processes, packaging technologies, and high-frequency soft switches, the power density of module power supplies is increasing, the conversion efficiency is increasing, and the application is becoming simpler.

Development trend of module power supplies

From 1999 to 2004, the global market for block power supplies is expected to increase from US$3 billion to US$5 billion, with data communications as the main market growth point, of which the proportion of 5V output has dropped from 30% (1999) to 11% (2004). The following trends in the development of module power supplies are worth noting:

1) The demand for high power density, low voltage output (less than 3.3V), and fast dynamic response drives the development of module power supplies.

2) Non-isolated DC-DC converters (including VRM) grow faster than isolated ones.

3) Distributed power supplies develop faster than centralized power supplies, but centralized power supply systems will still exist.

4) The proportion of DC-DC converters with standard designs will increase.

5) The design of module power supplies is becoming more standardized, and control circuits tend to use digital control methods.

Key technologies of module power supplies

At present, the main suppliers of module power supplies in the domestic market are VICOR, ASTEC, LAMBDA, ERICCSON and POWER-ONE. In order to achieve high power density, quasi-resonance and multi-resonance technology were used in the circuit in the early days, but this technology had high device stress and was frequency-modulated control, which was 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, and the general PWM can reach more than 500kHz. The volume of magnetic devices is greatly reduced and the power density is improved.

Circuit topology development trend

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, increase its power density, and improve dynamic response, the switching frequency of small power DC-DC converters will be increased from the current 200-500kHz to more than 1MHz, but high frequency will also produce new problems, such as: increased switching loss and loss of passive components, high-frequency parasitic parameters and high-frequency EMI problems.

Soft switching: In order to improve efficiency, various soft switching technologies are used, including passive lossless (absorption network) soft switching technology, active soft switching technology, such as: ZVS/ZCS resonance, quasi-resonance, constant frequency zero switching technology, etc., to reduce switching loss and switching stress to achieve high efficiency and high frequency. For example, the DC-DC high-frequency soft-switching converter developed by VICOR in the United States has an output of 48/600W, an efficiency of 90%, and a power density of 120W/in3. The Japanese LAMBDA company uses active clamping ZVS-PWM forward-flyback combined conversion and synchronous rectification technology to make the efficiency of the DC-DC conversion module reach 90%.

Low-voltage output: For example, the VRM voltage of a modern microprocessor will be 1.1-1.8V, and the output voltage of the DC-DC converter of a portable electronic device is 1.2V. The characteristics are large load changes, and 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 over the entire range of load changes. b) Low output voltage (the loss of the CMOS circuit is proportional to the square of the voltage, and the circuit loss is small when the supply voltage is low). c) High power density. This module adopts the packaging form of an integrated chip.

Development direction of module power supply technology

Reducing thermal resistance and improving heat dissipation: In order to improve heat dissipation and increase power density, medium and large power module power supplies mostly use multiple printed circuit board stacking packaging technology. The control circuit uses ordinary printed circuit boards on the top layer, and the power circuit uses boards with excellent thermal conductivity on the bottom layer. Early medium and large power module power supplies used ceramic substrates to improve heat dissipation. This technology has developed into direct copper bonding technology (Direct Copper Bond, DCB) to meet the needs of high power. However, because the ceramic substrate is fragile, it is difficult to install a heat sink on the substrate, and the power level cannot be very large. Later, this technology developed into directly etching the circuit with an insulated metal substrate (Insutalted Mental Substrate, IMS). The most common substrate is an aluminum substrate, which directly applies an insulating polymer on an aluminum heat sink, and then applies copper on the polymer. After etching, the power device is directly soldered on the copper. In order to avoid thermal mismatch caused by directly mounting chips on IMS, aluminum plates can also be used as substrates. The control circuit and power devices are respectively welded on multi-layer (greater than four layers, used as transformer windings) FR-4 printed boards, and then the side with power devices welded is bonded to the formed aluminum plate through thermal conductive adhesive for fixed packaging. Many module power supplies are compressed and sealed for better heat conduction, moisture resistance and shock resistance. The most commonly used sealing material is silicone resin, but polyurethane rubber or epoxy resin materials are also used. The latter two methods have good insulation performance, high mechanical strength and good thermal conductivity, which have become one of the development trends of module power supplies in recent years and are key technologies for improving module power density.

Secondary integration and packaging technology: In order to improve power density, module power supplies developed in recent years have all adopted surface mounting technology without exception. Due to the serious heat generation of the module power supply, the use of surface mount technology must pay attention to the thermal matching between the chip device and the substrate. In order to simplify these problems, MLP (Multilayer Polymer) chip capacitors have recently appeared. Its temperature expansion coefficient is very close to that of copper, epoxy resin filler and FR4 PCB board, and it is not easy to cause the problem of capacitor failure caused by rapid temperature change like tantalum capacitors and magnetic chip capacitors. In addition, in order to further reduce the volume, the secondary integration technology is also developing rapidly. It is to directly purchase bare chips, assemble them into functional modules, package them, weld them on the printed circuit board, and then bond them. This method has higher power density and smaller parasitic parameters. Because the same material substrate is used, the thermal matching of different devices is better, which improves the module power supply's ability to resist cold and hot shocks. CPES led by Professor Li Zeyuan is studying IPEM (Integrated Power Electronics Module) in terms of technology. It is a three-dimensional packaging structure, mainly for power circuits, replacing wire bonding technology.

Flat transformer and magnetic integration technology: Magnetic components are often the largest and highest devices in the power supply. Reducing the volume of magnetic components increases power density. In medium and large power module power supplies, in order to meet the standard height requirements, most professional manufacturers customize their own magnetic cores. Among the existing magnetic suppliers, only Philips can provide universal flat magnetic cores, and the winding production of this transformer is also difficult. The use of 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 magnetic integration technology at CPES three years ago. One of their prototypes is a half-bridge circuit. The output rectification uses the current doubler rectification technology, and the two inductors at the output end are integrated with the main transformer in an iron core. The final power density is 300W/in3. The current doubler rectification technology is suitable for occasions with large output current and high di/dt requirements. For example, this rectification circuit is often used in circuits that implement VRM.

Introduction to common module power brands in domestic and foreign markets

VICOR

The core of the module circuit technology of VICOR in the United States is zero current switching. The process uses a large number of secondary integration and customized devices, which enables the VICOR converter to operate at a frequency of more than 1MHz, with an efficiency of more than 90%, and a power density 10 times higher than that of ordinary converters, up to 120W per cubic inch. After the module is powered on, a quantized energy block is transferred from the input source to an LC resonant circuit composed of the inherent leakage inductance and capacitance elements of the primary coil of the transformer. At the same time, a current similar to half a sine wave passes through the power field effect tube switch. The switch is turned on when the current is zero, and the switch is turned off when the current returns to zero after half a sine wave. The VICOR module adopts this zero current switching principle to reduce switching losses and reduce the conduction and radiation noise levels. In order to ensure the stability of the system under different loads, the VICOR module uses frequency conversion technology to track the changes in load current to ensure that the module works in the best state under any circumstances.

NEMIC-LAMBDA, POWER-ONE, ASTEC, TYCO

The power modules of LAMBDA in Japan, POWER-ONE in the United States and ASTEC belong to the PWM partial resonant zero voltage switch. Under the condition of the pulse width modulation constant frequency converter circuit, the power field effect tube is allowed to resonate at the moment of opening and closing to achieve zero voltage switching, thereby greatly reducing the switching loss and radiation interference, increasing the operating frequency to 200-500kHz, and increasing the efficiency to 80%-90%. Because the frequency is basically constant and not too high, the requirements for the device are not very strict, and the circuit is not very complicated, so the cost is not very high. Compared with the full resonant converter, this converter is relatively low in price and has achieved a better performance-price ratio in the computer and communication fields.

ERICSSON

The Swedish ERICSSON power module is mainly a low-voltage input module with a power range of 5-200W. The characteristics of this module are that it mainly uses push-pull and half-bridge pulse width modulation field effect tube circuits with an operating frequency of 300kHz. The drive and control use patented circuits. The DCB and wire bonding technologies are used in the process, which greatly reduces parasitic parameters, ripples and improves heat dissipation.

Domestic modules

The main suppliers are Mornsun, Delus, Yihongtai, Xinleineng, Disai, 24 Institute, etc.

Xinleineng and Disai come from the same source, so the products of the two companies are roughly the same. Most modules use mixed assembly processes of patch and plug-in. The modules developed in the early stage use a large number of aluminum electrolytic capacitors and homemade potting glue, which is low in cost, but poor in process and reliability.

ZTE module power supply

ZTE Power Supply Product Department began to develop module power supply at the end of 1999. At present, there are 5 series and 31 varieties of modules that have been completed and are under development, with power levels ranging from 1.5W to 300W and output voltages ranging from 3.3V to 48V. The research and development of ZTE module power supply is based on a careful analysis of the failure modes of similar module power supplies and absorbs the research and development experience of alternative modules.