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 often 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) for 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.

　　许多电源系统设计人员将稳压的 DC-DC 转换器视为整体设计的关键。但将合适的电压提供给负载点稳压器，不一定都需要 PDN 稳压，或者对于中间配电母线电压而言 PDN 稳压并不那么重要。考虑这一点时，电源系统工程师应该考虑应用固定比率 DC-DC 转换器，它可显著的提升PDN 的整体性能。

　　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 high voltage operation time 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?

　　Fixed ratio converters work similarly to transformers, but instead of performing AC-AC conversion, they perform DC-DC conversion, with the output voltage being a fixed ratio of the DC input voltage. Like transformers, these converters do not provide output voltage regulation, and 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: Working principle of a bidirectional fixed ratio converter. A buck converter with K=1/16 can also be used as a boost converter with K=16/1.

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

　　Fixed Ratio Converter Voltage Class

　　LV48V, 28V or 24VHV380V, 270VUHV800V, 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.

　　

　　Figure D: 48V power supply of the original system

　　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.

　　在 400V/48V 及 800V/400V 应用中，由于功率要求高，可有效地使用具有高功率密度、效率在 98% 以上的隔离式 K：1/8（400/48） 及 K：1/2 （800/400） 固定比率转换器并联阵列。稳压可在固定比率转换器级前面或者后面提供。未稳压的功率密度及效率提升，不仅在这一极高功率应用中的这个位置效果显著，而且还可简化热管理。

　　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

　　另一项需要隔离的高电压应用就是系留无人机。系留无人机的电源线长度可能会超过 400 米，无人机必须将其提起并保持，才能达到其飞行高度。使用 800V 等高电压，可显著缩减这些笨重电源线的尺寸、重量和成本，从而可实现性能更高的无人机。使用板载固定比率转换器（一般 K=1/16）转换至 48V，可提供非常高效的极小供电解决方案，充分满足机载电子产品及视频有效载荷的需求。

　　5G Communications

　　现在，全世界的都在提升4G 无线电和天线塔为之前 4G 设备5倍的最新 5G 系统。4G PDN 为 48V，通过线缆从地面电源系统提供。新增 5G 设备，功率级显著提升，如果 PDN 要保持在 48V 电压下，那直径就会非常大，电线就会很重。电信公司正在研究使用 380VDC PDN 的优势，以显著缩小线缆尺寸。在升压模式下使用双向 K 1/8 固定比率转换器，地面 48V 电源系统可向塔顶提供 380V 的电源（K：8/1）。4G 和 5G 系统在塔顶使用 380V 至 48V 稳压转换器，不仅可获得 48V 稳压电源，而且还可通过 380V 细小电线实现更低成本的供电。

　　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.