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Complex DC-DC power conversion requirements

Meet complex DC-DC power conversion requirements
In industrial applications such as test and measurement equipment or embedded computing, the system architecture of embedded DC-DC converter may be quite complex, and there are relevant requirements in many different aspects, such as output voltage and current, ripple, EMI and power on sequence. This paper mainly discusses the influence of converter power stage selection in DC-DC applications.
Requirements for advanced embedded DC-DC converters
Many industrial systems, such as test and measurement equipment, need embedded DC-DC converters because of the increasing computing power required by these applications. This computing power is provided by DSP, FPGA, digital ASIC and microcontroller. Thanks to the increasingly shrinking process geometry, such devices are making continuous progress. On the other hand, this also brings three requirements: first, the power supply voltage is getting lower and lower (of course, there are allowable voltage ripple and load change); Secondly, the power supply current increases gradually; Third, these ICs usually need to provide separate voltages for the core and i/o structures in an accurate order to avoid the occurrence of latch up.
The embedded DC-DC converter must have excellent efficiency. The available space of this kind of converter is very small, which is particularly challenging for thermal design, because the embedded converter mainly relies on the copper area around the components on the PCB to improve the system thermal impedance. As the power consumption is proportional to the square of the current, this situation will worsen with the increase of load current. Therefore, a power switch with low on impedance RDSON and low switching loss is required. However, the lower the on impedance RDSON of the device, the higher the parasitic capacitance and even the switching loss, and the higher the final power consumption, so it is necessary to make certain trade-offs. Another major requirement for embedded DC-DC converters is that EMI must be low. The noise generated by these converters will cause interference to the surrounding circuits, so it must be as small as possible. However, high-speed (to reduce switching loss) conversion of large current (if required by the load) will inevitably produce large switching noise, including conduction noise and radiation noise (mainly magnetic field). Therefore, special attention must be paid to the optimization of power stage component selection and layout, especially in the connection of drives. In addition, the PWM control topology also has some influence.
For example, use 0.09 μ The digital IC of M technology may require a power supply voltage of 1.2V ± 40mv. According to the data table of the DSP, the power supply current can be as high as 952ma Another example is a large-size FPGA manufactured by 65nm process. When the power supply voltage is 1.0V +/-50mv and the temperature is 85 ℃, 4.2 a idle power supply current is required. In the operating mode, the current can be increased to 18a according to the specific configuration, because the dynamic requirements are very high at high switching frequency.
It is quite common for these applications to include multiple different ICs. For example, a smaller microcontroller (when the power supply voltage is high) is responsible for all interface and host functions, and a larger DSP or dedicated hardware is used to perform computing intensive functions. In many cases, another set of high-performance a/d converters with voltage requirements are specially used to improve the noise performance, so as to truly make full use of the resolution and bandwidth of these converters. These trends have led to complex power management systems with many dependencies.
Design of modular control hoisting system
One application suggestion is to place the DC-DC converter as close to the load as possible. This can minimize EMI, reduce the board space occupied by wide high current traces, and improve the dynamic characteristics of the converter. "Distributed" power management systems have emerged, in which all converters are ideally connected to each other. An example of a controller that can work with other converters in the network is fd2004, as shown in the module diagram of Fig. 1.
Fd2004 is a member of the digital DC product family, integrating digital loop control and highly integrated power management functions. This controller and its products in the same series can be connected with the main controller and other DC-DC converters through SMBus (system management bus) to easily realize many different functions, such as the converter's system configuration, power on sequence, margin function, fault protection and system monitoring. All these functions help to shorten the time to market and, more importantly, improve the reliability of the system.
Fd2004 can work with external gate drivers (such as fd1505) and discrete MOSFETs, or single package drmos products with integrated drivers and MOSFETs. It can also be programmed by resistance in stand-alone applications - in particular, the maximum value of output voltage is set by resistance, and the maximum value of voltage set by software command shall not exceed 10% to protect the load. In applications requiring large current, such as multi-phase converter, the selected architecture can realize current sharing of up to 8 phases, and can realize phase shedding when the output power is low to maintain high efficiency. The controller is based on a digital control loop with adaptive performance algorithm and loop compensation and supports switching frequencies up to 1.4mhz. Clock synchronization helps improve EMI performance. Fd2006 is also a good choice for applications that require both integrated drivers and discrete external MOSFETs.
For system voltage with low output current, integrated DC-DC converter is recommended. At this time, PCB area and ease of use are the most important considerations. Digital converters, such as fd2106 (6a max), like other products of the digital DC series, have communication functions and can be used with discrete MOSFET or drmos based converters that can provide greater current. For stand-alone applications, since there is no need to connect with other converters in the system, integrated converters (such as fan2106 of Fairchild Semiconductor) can also be used.
The controller and converter chain of the digital power management system can be controlled through the graphical user interface, and it is easy to correct all parameters and system performance monitoring. The software runs on a PC and is connected to the controller via a USB interface. When all the parameters are good, they are stored in the nonvolatile memory of the controller, so that the PC no longer needs to run the system.

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