How to design a modular DC DC system, Part 1: The 4 stages of design

“When designing a modular DC-DC system, the entire power supply system from the power supply to the load must be considered in order to achieve the required functions and performance. This series of tutorials will not only introduce the design process of modular DC-DC system, through these processes, to complete the design case of the power supply to the load.

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When designing a modular DC-DC system, the entire power supply system from the power supply to the load must be considered in order to achieve the required functions and performance. This series of tutorials will not only introduce the design process of modular DC-DC system, through these processes, to complete the design case of the power supply to the load.

In addition, this series will also introduce the overall system integration and support Circuits of the DC-DC module required for the design of the entire power supply system. This includes power supply and decoupling, filtering and stability analysis, output filtering and transient response, driving dedicated loads (ie pulse loads), safety and protection, current sharing and fault-tolerant arrays, and signal-level input and output considerations.

Advantages of modular design

The DC-DC power module not only simplifies the design process of the entire system, but also provides designers with three major advantages.

High performance-DC-DC module provides a pre-certified solution, which is characterized by high reliability and high efficiency, which is the primary problem of the current power supply system. A wide variety of module types are available, which provide numerous advantages in terms of power density, integration, and efficiency.

Modularity-Unlike discrete design, designers can use Modules to build complex power supply systems, and use high-performance power supply construction to easily implement distributed power supply systems. Such as Vicor Split Power Architecture™ or the earlier intermediate bus architecture. After development, these solutions can be easily expanded to meet different needs and different power levels. In addition, the changes in requirements that occur in the later stages of the system design will not completely disrupt the project: as the system requirements continue to develop, different modules can be replaced or added.

Speed-With the power module, it is possible to shorten the development time, and the risks in the entire process from the formation of the design concept to the final implementation can be minimized.

4 stages of design

Phase 1: Basic system requirements

The first stage of designing a power system is to determine the system requirements. Please ask the following questions at this stage to form a concept for the high-level definition of the system and its work:

Where does the power come from?

What are the characteristics of the power supply?

What type of load needs to be provided?

What architecture best meets the system requirements?

Next, list the operating voltage, current, and power levels of the system. The way to complete the list is to list the loads and organize them according to the required output voltage and load current requirements.

It is also useful to start thinking about dedicated functions that may be needed. In the following example, there is a 1.2V load point and the current requirement is 120A. This particular load has strict voltage regulation requirements, and peak currents of up to 200A may occur. The second example is a 2.2V load, such as an LED driver whose current must be adjusted.

Phase 2: System Architecture

Develop the power supply architecture, and start to select and finalize the power supply module that meets the system conversion needs. Draw the system block diagram and determine the architecture for supplying power from the power source to the load.

First list the system output, as shown on the far right in Figure 1. The load point range can reach 48V, 16A. A review of the power supply modules available on the market can determine which types of modules are suitable for providing these loads.

Use load characteristics and estimated efficiency to establish a basic system architecture for module requirements.

Next consider some physical limitations of the system. This may include the space provided for the implementation of the system, design considerations to meet isolation requirements, and the ability to take advantage of the private power supply network (PDN) architecture. As shown in the figure, the 48V bus is powered by a fractional power supply (architecture developed by Vicor) for the point load.

After the load and point-of-load regulators and power modules are deployed, the reverse engineering from the load side to the power supply can be started, and the current output demand of the upstream conversion module can be estimated using the data sheet efficiency estimates of each load branch. Through the calculation from the point load to the 48V bus, the system will need to provide at least 9.5A of current from the bus converter, thus clarifying the demand for the module.

It usually faces some necessary adjustments during design. For example, the 2.2V load current specification may be doubled. Correspondingly, the designer only needs to double the conversion module to meet the new demand. Next, adjust the estimated efficiency value and the load’s demand for the upstream bus converter to complete an overall system design.

Phase 3: Implementation

In this link, the module configuration and external wiring required for system integration will be finalized. A module or a group of modules is not a complete power supply system; to bridge the gap from power supply to load, many related problems need to be solved:

Design filters to meet relevant EMI standards

The protection range of the module is usually very narrow, limited to the protection of the module itself

Power supply and load characteristics determine decoupling requirements

High-reliability applications may have special loads or redundancy requirements

Connection with system controller and power sequence design

The main content of the design work is to determine the above problems. First check a typical example of an external Circuit, the following is a simplified external circuit of a DC-DC module power supply.

Examples of supporting circuits required to construct a complete system using DC-DC power modules.

Expanding work from the power supply module, designers must first reduce the noise characteristics of the switching power supply, including filtering of the input and output. Secondly, in order to ensure the stability of the system, it is necessary to analyze the impedance of the power supply and the power distribution line in order to properly decouple the power supply from the regulated power supply module. In addition, depending on the working environment, additional surge protection circuits may be required to meet safety requirements and protect the system from surges and peak voltage damage. Finally, the system design must also consider all special load considerations.

Phase 4: Module control and monitoring

The last stage of the design process is to interact with the module, including the control and monitoring interface of the module. Usually the module power supply has low-voltage analog or digital interface.

A typical analog power interface can be used to implement multiple functions, such as fine-tuning the output voltage. Used to connect the enable pin of the external controller, the monitor pin to indicate the power failure state, and the analog voltage pin to monitor the internal temperature of the module.

The digital power supply through the PMBus® interface can provide more functional control and monitoring methods. , Which can provide fine-tuning control of output voltage, enable/disable control and other configuration options. In addition, for some specific needs, the function of adjusting the fault protection point of the current limit point can also be provided. Not only that, the digital power supply can also identify the causes of faults such as undervoltage, overvoltage, and overcurrent through the fault status flag.

The voltage stabilization module usually provides remote sensing function, which can achieve more accurate voltage stabilization at the load position by compensating the impedance voltage drop of the distribution line. Remote sensing uses the Kelvin connection of the two sensor leads to directly monitor the voltage of the load terminal, so the controller can offset the voltage drop in the high-current power distribution system.

The analog and digital signal function of the power module.

Whether sending an analog control signal or a digital control signal, it is important to distinguish the ground network the signal refers to. If the low-power signal level connection and the power output of the module share a common ground return path, high-frequency switching noise may be coupled to the control signal through parasitic parameters on the path, resulting in unstable operation. In order to avoid the above situation, the signal ground must be connected to the power ground at a single point.

Summarize

This article introduces the design process of the DCDC power module system. The following series of tutorials will introduce the main considerations in the third stage of the design in more detail.