Bus Converter Maximizes System Efficiency, Minimizes Heat Losses

March 1, 2011
Small size, high efficiency, and heat management are valuable attributes for any element in a computer system, but the elements that comprise the system depend to some extent on the system architecture

Find a downloadable version of this story in pdf format at the end of the story.

Over the past quarter century or so, businesses - banking, insurance firms, dispensers of online music, search engines, and everyone else - have become ever more reliant on the computer. Data centers or server farms have been proliferating worldwide to satisfy the demand, but, of greater interest here, they are becoming more massive. Because they need increasingly more power, which, of course, is more costly, efficient operation has become a high-priority. Since 100% efficiency is not achievable, the management of the heat generated is also a high priority.

Electricity is generated somewhere and it's distributed to the user (to the data center, say) where it is further distributed, usually in a number of steps. Somewhere along the line, it is changed from AC to DC for final consumption by arrays of processors at increasingly smaller voltages and higher currents. Efficiency drops at every conversion (AC to DC, a higher voltage to a lower voltage), so it is desirable to keep the number of conversions low. Much more power is wasted when it's distributed at lower voltages (and higher currents) because of I2R losses, so it's desirable to distribute at high voltage.

High voltage from the power line is converted to a lower voltage by some means, whether it's done in a silver box, which is typical of computer type applications, or whether it's done in a central office application. These applications typically start with AC power from the utility. In order to get DC the AC voltage needs to be rectified, and it's typically line reference to ground; therefore, it needs to be isolated and down converted.

The VI BRICK BCM (bus converter module) Array is a high efficiency (typically 95 percent), high-power vertically-mounted BCM array (Fig. 1) that provides isolation and conversion from 380 V to 12 or 48 V for low-voltage distribution near the point of load. It incorporates the superior technical attributes of VI Chip technology in a robust package that facilitates thermal management.

The new VI BRICK BCM Arrays are ideally suited for server applications using a PFC front end that require relatively high power levels with challenging thermal issues. The offline power can be bussed to the motherboard and converted to either 48V or 12V, which minimizes distribution losses, reduces conversion steps, improves efficiency, and reduces overall cost. These products can be used in a wide variety of applications that require high efficiency, high power density, improved thermal management, low noise, fast transient response, and overall design flexibility.

Ideal for PFC front-end applications providing the capability of a high voltage bus with minimal distribution losses, the VI BRICK BCM Array provides a highly efficient solution for applications using point-of-load (POL) converters to provide output voltages. They are available with 384 and 352 nominal input voltages and output voltages of 11, 12, 44, and 48 Vdc. The efficiency and compact size of these modules yields power density up to 290 W/in3 and fast transient response.

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Less capacitance is required for energy storage near the load, which equates to space and cost savings. Due to its fast response time and low noise, the need for limited life aluminum electrolytic or tantalum capacitors is reduced - or eliminated - resulting in savings of board area, materials and total system cost. In addition, the BCM power train has a unique capacitance multiplication feature: the input capacitance normally located at the input of a regulator can be located at the input of the BCM. Since the K factor of a BCM array is 1/8, that capacitance value can be reduced by a factor of 64x.

These models provide output power up to 650 watts in a board space of less than two square inches in a 1U high package that measures 3.54 × 0.56 × 1.13 in. (89,9 × 14,2 × 28,7 mm).

The high power and compact size of the BCM array modules yield power densities up to 290 W/in3, resulting in a very small footprint on the PC board.

A very important attribute is that VI BRICK BCM arrays are easily paralleled. Each array has up to 650-Watt output capability. If more power is needed, multiple arrays can very easily be put together to create a larger array for higher power such as for a server or in telecommunications. When connected in an array with other BCMs (all with the same K factor), the BCM module will inherently share the load current with parallel units, according to the equivalent impedance divider that the system implements from the power source to the point of load. It is important to know that, when started, BCMs are capable of bidirectional operation (reverse power transfer is enabled if the BCM input falls within its operating range and the BCM is otherwise enabled). In parallel arrays, because of the resistive behavior, circulating currents are never experienced because of the energy conservation law.

A couple of examples demonstrate the distribution of a high offline voltage (including the AC to HVDC “silver box” for rectification, EMI and inrush current protection, and power factor correction) to the motherboard for final distribution on the board. Incidentally, the silver box is simpler at this high voltage stage, reducing its size by more than 50%.

The example in Fig. 2 shows the post “silver box” 380 Vdc distributed directly to the blade, which virtually eliminates distribution losses because the I2R loss is 0.1% of the loss that would have been incurred if the distribution had been done at 12 V. Obviously, at 380 V, the wiring and connector sizes and costs are much less. The 380 V to 12 V conversion is done on the blade, leaving minimal distance to the voltage regulators.

The example in Fig. 3 also shows the post “silver-box” 380 Vdc distributed directly to the blade with the same high-voltage distribution benefits. In this case, however, the conversion on the blade is 380 Vdc to 48 Vdc. The voltage regulators have been replaced by PRM-VTM pairs, which convert the 48 V to 1.x V at the highest efficiency and the smallest conversion package at the load. This arrangement minimizes on-blade distribution loss.

Both the VI BRICK PRM and VTM can achieve higher than 96% efficiency. Overall efficiency for a power system - including the combination of a PRM and a VTM - operating from an unregulated DC source and supplying a low-voltage DC output - typically ranges from 90% to 95%.

In many cases, it's possible to achieve overall efficiency exceeding 92% even at full load. With higher efficiency comes lower total heat dissipation, another important consideration in power systems design.

Download the story in pdf format here.

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