Article

DC-DC converters for reliable power supplies in network switches

Reliable power supplies are a key factor in network devices providing high reliability and availability.

By Yutaka Mizutani, Senior Field Application Engineer

The rapid expansion of cloud computing services has fueled a dramatic surge in data traffic. This trend is expected to continue with a CAGR (Compound Annual Growth Rate) of approximately 20% from 2024 to 2033. Network devices, including routers and network switches, are essential for accommodating this rapid growth and play a critical role in facilitating internet data flow.

To guarantee uninterrupted service and low latency for large data volumes, network devices require high reliability and availability. The power supply system is a cornerstone for achieving this goal. While 48V_DC is the legacy standard input for network infrastructure, network devices operate on positive voltages. Therefore, isolated DC-DC converters are employed to convert the negative voltage input to a positive voltage output. Redundancy of these converters is essential to maintain system uptime in case of failures (Figure 1).

Power system configuration in network switch applicaitons image

Figure 1: Redundancy is an important reliability aspect of the typical network switch power configuration rails with voltage and current requirement.

A stable 12V output across a wide input voltage range

Network switches are typically powered by an external negative –48V_DC supply source. However, this input voltage can fluctuate significantly, with a range of –36V to –75V. The DC-DC converter has to maintain a stable 12V output voltage despite these wide input voltage variations.

Some power modules may struggle to maintain stable output across this entire range, even though they specify an input voltage range from –36V to –75V. These modules may fail to provide a consistent 12V output voltage when the input voltage reaches its lower limit.

In contrast, Vicor DCM™ modules excel at maintaining a stable 12V output voltage across the entire –36V to –75V input voltage range. The Vicor DCM3623 achieves this through a primary side, proprietary Vicor controller IC that actively regulates the output voltage. The relationship between input and output voltage for DCM3623 can be visualized using the Product simulators tool.

The Vicor DCM3623E75H13C2T00 is a high-performance DC-DC converter ideally suited to meet these demanding requirements. Its high-power density and parallel operation capabilities make it an excellent choice for power systems in high-density network switches, particularly in large-scale data centers. This article will delve into the specific features of this product.

Power scalability without additional components

To ensure continuous service operation for essential information technology assets, redundant power supplies are essential. When multiple power supplies operate in parallel, power modules typically monitor output voltage and current, communicating this information to each other to balance the output current. Additionally, ORing diodes can be added to the output to prevent circulating currents caused by voltage differences between the power supplies. Generally, parallel operation of power supplies requires additional components.

Nevertheless, Vicor DCM power modules can operate in parallel without additional components (Figure 2). This is achieved due to the DCM’s built-in droop mode function. This enables current sharing and simplifies the design.

Parallel array configuration for scalable redundant output power image

Figure 2: Parallel array configuration for scalable redundant output power.

Droop characteristics in parallel arrays

The droop characteristic is a control function that dynamically adjusts the output voltage of a power supply in response to changes in module conditions, such as load current and internal temperature.

The output voltage is given by the following equation:

V_OUT = 12V + 0.6316 x (1 - I_OUT / 26.67) - 1.600 x 0.001 x (T_INT - 25) + ΔV_OUT-LL

Note: The equation applies only to DCM3623E75H13C2T00. To calculate V_OUT for other DCMs, please refer to the relevant DCM datasheet.

Defining terms (see Figure 3):

V_OUT = output voltage

I_OUT = output current

T_INT = module internal temperature in °C

ΔV_OUT-LL = additional output voltage during light load boosting

The module outputs a nominal voltage of 12V at full load (I_OUT = 26.67A) and 25°C.

Under constant temperature conditions, the DCM exhibits a negative-slope load-line relationship of –5.26%, resulting in a lower output at full load (I_OUT = 26.67A) compared to no load (I_OUT = 0A). This voltage difference for the load-line is 0.61V (= 12V x 0.0526).

The control IC within the DCM monitors the internal current and regulates the output voltage. In parallel array operation, modules carrying lower currents output higher voltages, while those carrying higher currents output lower voltages.

Furthermore, changes in module temperature also have a slight influence on output voltage, affecting current sharing.

If a module is loaded more than others, its relative temperature tends to rise, causing its output voltage to decrease. Since the output voltages of the other parallel DCMs match that of the incrementally loaded DCM, they will adjust their output voltages to share the load more evenly. Under full load condition, the DCM outputs a lower voltage at 120°C compared to –40°C, with 0.256V difference between –40°C and 125°C.

This droop characteristic, the combined effect of load line and temperature coefficient, enables current balancing among the DCM modules without additional circuitry.

Calculating the droop voltage as a function of load line and temperature coefficient (DCM3623) graph image

Figure 3: Calculating the droop voltage as a function of load line and temperature coefficient (DCM3623).

Light load boosting

Lastly, some additional comments related to ΔV_OUT-LL in the V_OUT equation above.

Light load boosting is activated when the combined internal power consumption of the DC-DC converter and the external output load falls below the minimum power transfer per internal MOSFET switching cycle. This typically occurs when the load current drops below 10% of the rated value.

During light load boosting, the DCM powertrain cycles on and off repeatedly, reducing the switching frequency and significantly lowering the module's power consumption.

The extended off-time can potentially lead to an increase in output voltage (Figure 4). This voltage rise can reach 2.15V at maximum, particularly under no-load conditions. To mitigate this voltage increase, a bleeder resistor can be added to the output to draw approximately 10mA of current, effectively reducing ΔV_OUT-LL.

Impact of light load regulation on output voltage graph image

Figure 4: Impact of light load regulation on output voltage.

Stability and balancing load current are essential to effective power delivery in network switches

The Vicor DCM3623 achieves high output voltage stability under a wide range of input voltage variations through its proprietary control loop technology. Additionally, with its built-in droop management function, it can automatically balance the load current when multiple modules are connected in parallel without the need for external components and communication. This simplifies the power circuit design and enables stable power supply in systems demanding high reliability, such as data center network switches.

This article was originally published by Power Systems Design.

Yutaka Mizutani joined Vicor in 2021 as a Senior Field Application Engineer in Japan. He provides technical support and consulting for High-Performance Computing (HPC), aerospace and defense, industrial, and automotive power systems. Before Vicor, he worked at a semiconductor supplier, supporting a wide range of applications including automotive, industrial, and consumer electronics. Yutaka holds a B.S. degree in Electrical Engineering and an M.S. degree in Business Administration (MBA).