Improving Power Supply Network Efficiency Using Fixed Ratio Converters

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Improving Power Supply Network Efficiency Using Fixed Ratio Converters [Copy link]

Most electromechanical or semiconductor loads require stable DC-DC voltage conversion and tight regulation for reliable operation. DC-DC converters that perform this function are usually called point-of-load (PoL) regulators and are designed with maximum and minimum input voltage specifications that define their stable operating range. The complexity of the power delivery network (PDN) of these regulators can vary depending on the number and type of loads, the overall system architecture, the load power level, the voltage level (conversion stage), and the isolation and regulation requirements.

　　Many power system designers consider a regulated DC-DC converter as critical to the overall design. However, PDN regulation is not always required to provide the appropriate voltage to the point-of-load regulator, or PDN regulation is not as important as the intermediate distribution bus voltage. When considering this, power system engineers should consider applying a fixed ratio DC-DC converter, which can significantly improve the overall performance of the PDN.

　　How to optimize the power supply network

　　PDN performance is usually measured in terms of power consumption, transient response, physical size, weight, and cost. A major design challenge affecting PDN performance is the voltage conversion ratio and high-precision line/load regulation. Engineers spend a lot of time dealing with a large number of different input/output voltage conversion ratios, dynamic regulation rates, and distribution characteristics to improve performance and reliability.

　　If the system load power consumption is in the kilowatt range, using a high voltage design with a large capacity PDN can reduce the current level in the system (P=V?I), thus reducing the size of the PDN, reducing weight and reducing costs (cables, busbars, motherboard copper foil power plane) (PLOSS = I2R). Maximizing the time of high voltage operation before converting to low voltage/high current, as close to the load as possible, is a major advantage.

　　But to get high-voltage, high-power PDNs close to the load, you need a DC-DC converter with high efficiency and high power density. If the input-to-output voltage conversion ratio is large, such as 800V or 400V to 48V, the most efficient converter is a fixed ratio converter that provides unregulated voltage. These high-efficiency converters not only provide higher power density, but also provide easier thermal management due to lower power consumption.

　　What is a Fixed Ratio Converter?

　　A fixed ratio converter works similarly to a transformer, but instead of performing an AC-AC conversion, it performs a DC-DC conversion, with the output voltage being a fixed ratio of the DC input voltage. Like a transformer, this converter does not provide output voltage regulation, the input-to-output transformation is determined by the device’s “turns ratio.” This turns ratio is called the K factor, and is expressed as a fraction relative to its voltage step-down capability. K factors range from K=1 to K=1/72, and are selected based on the PDN architecture and PoL regulator design specifications. 　　Figure A: Operation of a bidirectional fixed ratio converter. A K=1/16 buck converter can also be used as a K=16/1 boost converter.

　　Typical PDN voltages are low voltage (LV), high voltage (HV), and ultra-high voltage (UHV).

　　Fixed Ratio Converter Voltage Class

　　LV

　　48V, 28V or 24V

　　HV

　　380V, 270V

　　UHV

　　800V, 600V, 540V

　　Fixed ratio converters can be isolated or non-isolated and can achieve bidirectional power flow by converting the voltage in the opposite direction. For example, a K=1/16 fixed ratio converter that supports bidirectional functionality can be used as a K=16/1 boost converter.

　　Additional design flexibility includes easy paralleling for higher power supply requirements and the option to connect the converter outputs in series, effectively changing the K factor to provide higher output voltages.

　　Figure B: BCM converters can be easily paralleled to meet higher power requirements.

　　Power delivery networks are undergoing a major transformation as power demands in many end markets and applications are rising dramatically. Higher PDN voltages, such as 48V, are being used in electric vehicles, mild hybrids, and plug-in hybrids due to the addition of new features and increasing performance levels. 48V meets the safety electrical low voltage (SELV) standards required by many systems, and the simple power equation of P=V?I and PLOSS=I2R also explains why high-voltage PDNs are more efficient.

　　Figure C: BCMs with outputs connected in series to increase output voltage allow for greater design flexibility.

　　For a given power level, a 48V system uses 1/4 the current and 1/16 the line power compared to a 12V system. At 1/4 the current, cables and connectors can be smaller, lighter, and less expensive. A 48V battery for a hybrid vehicle has four times the power of a 12V battery, and the increased power can be used for powertrain applications to reduce CO2 emissions, improve fuel mileage, and add new safety and entertainment features.

　　The addition of artificial intelligence (AI) in data center racks has increased rack power requirements to over 20kW, making the use of 12V PDNs cumbersome and inefficient. Using 48V PDNs provides the same benefits as hybrid vehicles. In automotive and data center applications, it is best to retain the original 12V load and POL common buck regulators to minimize the need for modification.

　　Solving Real-World Problems Using Non-Isolated Fixed-Ratio Converters

　　48V is SELV compliant, so non-isolated fixed ratio converters are ideal for the 48V to 12V DC-DC conversion stage, as current PoL 12V regulators can handle input voltage variations. Non-isolated, unregulated fixed ratio converters are the most efficient high-power bus converters, enabling lower power consumption, higher power density, and lower cost. This high density enables the latest distributed power distribution architectures for hybrid vehicles, where non-isolated fixed ratio converters can be placed next to the load, thereby maximizing the operation of smaller, more efficient 48V PDNs around the car. In blade servers, this small non-isolated 48V to 12V fixed ratio converter can be placed on the motherboard close to the buck regulator.

　　Many new AI accelerator cards, such as NVidia’s SXM and OAM cards from Open Compute Project (OCP) members, are designed with 48V inputs because AI processor power levels range from 500 to 750W. For cloud and server companies that still use 12V PDN backplanes in their racks to use these high-performance cards, a 12V to 48V conversion is required. Adding a bidirectional K=1/4 non-isolated fixed ratio converter as a 12V to 48V boost converter (K = 4/1) on these accelerator cards (or in higher power distributed 12V to 48V modules) can easily bring AI capabilities to older rack systems.

　　The Vicor NBM2317 can efficiently convert 48V to 12V and 12V to 48V because the NBM is a bidirectional converter. The bidirectionality allows the integration of legacy boards into a 48V infrastructure and the integration of the latest GPUs into legacy 12V racks.

　　How to meet the needs of demanding high voltage applications requiring isolation

　　Electric Vehicles

　　In electric vehicle applications, power requirements dictate that the battery voltage must be much higher than the 48V currently used in hybrid vehicles, and 400V is usually chosen. 400V is converted to 48V and distributed to different loads around the powertrain and chassis. To support fast charging, the 400V battery is charged by a charging station that provides a regulated 800V DC output through an 800V to 400V converter.

　　Figure E: A distributed 48V architecture places multiple smaller converters that consume less power close to the 12V load.

　　In 400V/48V and 800V/400V applications, due to the high power requirements, parallel arrays of isolated K:1/8 (400/48) and K:1/2 (800/400) fixed ratio converters with high power density and efficiencies above 98% can be used effectively. Regulation can be provided before or after the fixed ratio converter stage. The unregulated power density and efficiency improvements are not only significant at this location in this very high power application, but also simplify thermal management.

　　High Performance Computing

　　High Performance Computing (HPC) system rack power levels are typically above 100kW, so 380VDC is used as the primary PDN. In these applications, isolated fixed ratio converters of K:1/8 and K:1/16 are integrated in the server blades or on mezzanine cards that distribute power through the rack, providing 48V or 12V power to the motherboard. This is then regulated by an array of 12V multiphase buck converters or a higher efficiency advanced 48V to POL architecture. Once again, the density and efficiency of fixed ratio converters play an important role in implementing this type of PDN architecture to achieve high performance.

　　Tethered drone

　　Another high voltage application that requires isolation is tethered drones. Tethered drones can have power cables that are over 400 meters long and must be lifted and held to reach their flight altitude. Using higher voltages such as 800V can significantly reduce the size, weight, and cost of these bulky power cables, allowing for higher performance drones. Using an onboard fixed ratio converter (typically K=1/16) to convert to 48V provides a very efficient and extremely small power solution that fully meets the needs of onboard electronics and video payloads.

　　Today, the world is upgrading 4G radios and antenna towers to the latest 5G systems that are 5 times taller than previous 4G equipment. 4G PDN is 48V and is supplied via cables from the ground power system. With the addition of 5G equipment, the power levels have increased significantly and if the PDN is to be kept at 48V, the diameter would be very large and the wires would be heavy. Telecom companies are investigating the benefits of using 380VDC PDN to significantly reduce the size of the cables. Using a bidirectional K 1/8 fixed ratio converter in boost mode, the ground 48V power system can provide 380V power to the top of the tower (K:8/1). Using a 380V to 48V regulated converter at the top of the tower, 4G and 5G systems not only get regulated 48V power, but also achieve lower cost power delivery via thin 380V wires.

　　Fixed-ratio converters provide highly flexible PDN design for high-performance applications

　　The demand for high-performance power supplies is rising. Enterprise and high-performance computing advanced systems, communications and network infrastructure, autonomous vehicles, and a host of transportation applications are just a few of the high-growth industries that require more power. These markets have one thing in common: each has significant power demands, and they all benefit from small, high-power-density DC-DC converter solutions that save space and reduce weight. Power system engineers should consider fixed-ratio converters as an important and flexible solution to achieve higher-performance PDNs to gain a competitive advantage in overall system performance.