Optimize the power supply design of high-speed Tongji products with high-efficiency switching power supply

    High-speed communication products (such as ADSL, ROUTER, etc.) usually require one or more low-voltage power supplies, such as 3.3V, 2.5V, or even 1.8V. Since the MCU or DSP processing rate is very high, the current consumption is also very large, such as 16 The 3.3V power supply of the ADSL central office end board requires a power supply of up to 8A, while the 1.8V power supply requires a larger supply current (up to 10A). Although traditional switching power supply modules can meet the above requirements, they still put great pressure on current designers in terms of cost, volume, heat loss, etc. Therefore, this article introduces several actual circuits for optimizing power supply design for reference.

1 Using switching power supply module

    As we all know, switching power supply modules have the advantages of simple use, high reliability, and low EMI noise. Therefore, they are deeply loved by product designers and have become the preferred solution for communication power supplies. Traditional communication products require a small number of power supplies, and usually use +5V as the main power supply. Switching power supplies are a good choice. However, with the emergence of high-speed and broadband communication products, the power supply voltage required by DSP or MCU 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 interface with external chips such as ELASH, SDRAM and other peripheral devices, 5V and 3.3V power supply voltages are also required. For such products that require multiple power supplies, the power supply design faces many challenges such as large size, high price, low voltage and high current output, especially low efficiency when using multiple outputs. If the power module is completely used, the product cost will be increased, the system power supply pressure will be increased, and more importantly, the circuit board area will be larger, making the system PCB layout difficult. Therefore, it is necessary to reasonably combine the power module with the DC-DC conversion chip during design to optimize the design of the power supply.

2 Use linear regulators to obtain low-voltage output

    Some designers use linear regulators to step down from the 5V or 3.3V power supply to obtain the required 3.3V, 2.5V or 1.8V voltage. When the low-voltage power supply current required by the system is small (such as a few hundred mA), the circuit shown in Figure 1 can be regarded as a better low-cost solution. Not only that, because the linear power supply has the advantages of small 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 supply to provide an output current of 10A, and a Gigabit Ethernet switching system may require a 3.3V power supply to provide an 8A current. For the former, if 1.8V is obtained from a 3.3V power supply using a linear power supply step-down method, the power consumed by the power supply is: P1=(3.3V-1.8V)×10=17W, and the conversion efficiency is only: POUT/( P1+POUT)=18/33=54%. In addition, in order to ensure normal operation, the power supply needs to occupy a large PCB area for heat dissipation. At the same time, the load needs to be kept at a certain distance from the power supply. Otherwise, the system performance will be affected due to too high temperature rise.

3. Use boost DC/DC switching converter

    If the 3.3V or 5V power supply current required by the system peripheral devices is small, such as less than 2A, and the 3.3V2 or .5V power supply current required by the DSP or MCU is large, such as more than 5A, then the circuit shown in Figure 2 has a higher Of course, the voltage reduction solution shown in Figure 3 can also be used for cost-effectiveness, but it will consume more heat and therefore occupy more PCB. Assuming that the +5V/40W power module and the +3.3V/35W module have the same camera price and the same conversion efficiency (85%), and assuming that the switching power supplies in Figure 2 and Figure 3 have the same conversion efficiency (90%), then The input power of the voltage circuit is 11.1W; while the input power of the buck circuit is 18.3W, so the total power required by the 5V power module is 37.3W input power, while the 3.3V module only requires 27.6W to meet the power supply requirements. Therefore, using the solution in Figure 2 can effectively reduce power consumption.

4 Use step-down switching power supply

    In addition to factors such as power consumption, price, and volume that must be considered when designing a power supply, the output noise of the power supply, especially the size of the output ripple, must also be considered. If the current consumed by the DSP or MCU core remains unchanged (such as 5A), and the operating voltage is reduced to 1.8V, the power supply requirement of the peripheral circuit is +3.3V/2A. If you continue to use the scheme in Figure 2 at this time, when the front-stage switching power supply module in the circuit directly converts the high input voltage (such as -48V) to the +1.8V/5A voltage required by the core, its output noise will usually exceed The allowable fluctuation range of the core voltage (±50mV ~ ±100mV) although adding a filter circuit will reduce the noise, it will occupy a larger PCB area; at the same time, because the conversion efficiency of the switching power supply with 1.8V output is higher than that of the switching circuit with 3.3V output Less efficient and therefore greater heat loss. In addition, it also needs to be boosted to provide 3.3V/2A power supply, which further aggravates the heating problem. If the buck circuit shown in Figure 3 is used, the DC/DC conversion module provides a 3.3V power supply. Since the minimum bias voltage of the MAX1714 in the figure is not less than 4.5V, a boost chip needs to be added to convert 3.3V into 5V, while the 5V power supply required by the MAX1714's internal control and bias circuits requires less than 40mA. Therefore, the problem can be solved by using the charge pump power supply MAX619 shown as the dotted line in the figure. Since the MAX1714 uses synchronous switching rectification technology, the conversion efficiency is 7 to 8 percentage points higher than that of ordinary fast-current converters, so the conversion efficiency of its power supply can be as high as more than 90%.

5 Using a charge pump to obtain negative power

    In SDH transmission equipment, a -5V power supply is usually required to power the clock disk. In the past, many engineers used the circuit shown in Figure 4 to design the required circuit, but now it seems that this is not the simplest and cheapest solution. If the circuit shown in Figure 5 is used, there is no need for an external inductor, which will greatly reduce the cost and circuit area. The power output ripple will be low and the electromagnetic noise interference will be smaller.

6 Conclusion

    High-speed and broadband communication products have higher and higher computing processing speeds due to DSP or MCU, lower and lower operating voltages, and higher consumption voltage. If multiple low-voltage and high-current power supplies are required, traditional open and parallel power modules and Linear voltage regulators are no longer sufficient, so today's high-efficiency, low-voltage switching integrated power supply technology needs to be combined to better solve the problem. Although the use of DC/DC switching power supply to design the required power supply requires external supporting components and certain design experience compared with the use of module power supply, it can greatly reduce the power supply cost, reduce the PCB area occupied by the power supply, and improve the conversion efficiency. , in the specific design, whether to use a power integrated power supply or a power controller needs to be considered based on the actual power supply of the system.