Design of power adapter for high-speed communication products

　　High-speed communication products (such as ADSL, ROUTER, etc.) usually require one or more low-voltage power supply adapters, such as 3.3V power adapters, 2.5V power adapters or even 1.8V power adapters. Because the MCU or DSP has a high processing rate, it consumes a lot of current. Very large, for example, the 3.3V power adapter of the 16-channel ADSL central office board requires a power adapter of up to 8A, while the 1.8V power adapter requires a larger power supply current (up to 10A). Although the traditional switching power adapter module can meet the above requirements , But the cost, volume, heat loss and other aspects still bring great pressure to the power adapter manufacturer. Therefore, this section introduces several actual circuits that optimize the design of the power adapter

　　1. Utilize the switching power supply adapter module

　　Traditional communication products require a small number of power adapters, and usually +5V is the main power adapter. Switching power adapters are a good choice. But with the emergence of high-speed and broadband communication products, the power supply required by DSP or MCU The voltage is getting lower and lower, and the core voltage has dropped to 3.3V, 2.5V or even 1.8V. In addition, in order to be able to interface with external chips, such as ELASH, SDRAM and other peripheral devicesIt also needs 5V and 3.3V power supply voltage. For this kind of products that require multiple sets of power adapters, the power adapter design faces large size, expensive, low-voltage and large-current output, especially when the output is multiple, the efficiency is low, etc. Challenges. If the power adapter module is completely used, the product cost will increase, the system power supply pressure will increase, and more importantly, the occupied circuit board area will be larger, which will make the system PCB layout difficult. Therefore, the power supply must be reasonably designed. The adapter module is combined with the DC/DC conversion chip to optimize the design of the power adapter

　　2. Use linear regulator to obtain low voltage output

　　Use a linear regulator to obtain the required 3.3V, 2.5V or 1.8V voltage from the 5V or 3.3V power adapter using a step-down method. When the low-voltage power adapter current required by the system is small (such as several hundred milliamps), Using the circuit shown in Figure 8-40 is a better low-cost solution. Not only that, because the linear power adapter has the advantages of low interference and low output noise, it can also provide a very stable voltage for the DSP or MCU core. However, if the core requires a large low-voltage current, for example, some 16-channel ADSL may require a 1.8V power adapter to provide 10A output current, and a Gigabit Ethernet switching system may require a 3.3V power adapter to provide 8A current. For the former, if The 1.8V voltage is obtained from the 3.3V power adapter using the linear power adapter step-down method, then the power consumed by the power adapter is P1=(3.3V-1.8V)×10A=17W, and the conversion efficiency is only Pou/(P1+Pout) =18/33=54%. In addition, in order to ensure normal operation, the power adapter needs to occupy a large PCB area to dissipate heat. At the same time, the load needs to keep a certain distance from the power adapter, otherwise the system performance will be too high due to temperature rise. High and affected.

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