• • • ELECTRIC VEHICLES • • •
Enabling higher power delivery network performance with fixed-ratio converters
The vast majority of electromechanical or semiconductor loads require stable DC-DC voltage conversion and tight regulation to operate reliably, says Phil Davies, corporate vice president of global sales and marketing at Vicor Corporation
he DC-DC converters that perform this function are commonly called point-of-load (PoL) regulators and are designed with a maximum and minimum input voltage specification defining their stable operating range. The power delivery network (PDN) to these regulators can vary in complexity based on the number and type of loads, overall system architecture, load power levels, voltage levels (conversion stages), as well as isolation and regulation requirements.
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Many power system designers consider regulated DC-DC converters as essential to their overall systems design. However, PDN regulation is not always necessary for providing the right level of voltage to the point-of-load regulators or imperative to an intermediate distribution bus voltage. With this in mind, power system engineers should consider implementing fixed-ratio DC-DC converters, which can offer significant advantages to the overall performance of the PDN.
How to optimise your power
delivery network PDN performance is commonly measured in terms of power loss, transient response, physical size, weight and cost. One major design challenge impacting PDN performance is the number of times the network needs voltage conversion and tight line/load regulation. Engineers spend a great deal of time optimizing bulk power voltage conversion, dynamic regulation and distribution characteristics to deliver high performance and reliability.
If system load power is in the multi-kilowatt range, designing the bulk PDN with a high voltage reduces the current that is distributed in the system (P= V•I), and therefore the size, weight and cost (cables, bus bars, motherboard copper power planes) of the PDN itself can be reduced (PLOSS = I2R).
Maximising the length of the high-voltage runs as close as feasibly possible to the load before converting down to lower voltages and consequently higher currents is, therefore, a major advantage. However, bringing a high-voltage and high-power PDN close to the load requires a DC- DC converter with high efficiency and high power density. If a large step for input-to-output voltage conversion is required, such as 800V- or 400V-to- 48V, the highest efficiency converters would be
Typical PDN voltages range from low voltage (LV) to high voltage (HV) to ultra-high voltage (UHV). Fixed-ratio converters can be isolated or non- isolated and also capable of bidirectional power flow with reverse voltage conversion. For example, a K = 1/16 fixed-ratio converter with bidirectional capability can be operated as a boost converter with a K of 16/1.
Fixed-ratio converter voltage categories LV 48V, 28V or 24V
fixed-ratio converters that do not have the limitation of providing regulation.
Because of their high efficiency figure of merit, these converters offer better power density and easier thermal management due to lower power dissipation.
What is a fixed-ratio converter? A fixed-ratio converter operates much like a transformer but instead of AC-AC conversion, performs DC-DC conversion where the output voltage is a fixed fraction of the DC input voltage. As with a transformer, the converter provides no output voltage regulation, and the input-to-output voltage transformation is defined by the “turns ratio” of the device. This turns ratio, referred to as the K factor, is expressed as a fraction relative to its voltage step-down capability. K factors can range from a K = 1 to as low as K = 1/72 and are selected based on PDN architecture and the PoL regulator design specifications.
Power delivery networks are undergoing significant changes due to the soaring power demands within many end markets and applications. As new features are added and performance levels advance, higher PDN voltages such as 48V are being used in EVs (electric vehicles), mild hybrid and plug-in hybrid vehicles. 48V meets the SELV (Safety Electrical Low Voltage) standard required by many systems and the simple power equations of P = V•I and PLOSS = I2R, explain why higher-voltage PDNs are more efficient. For a given power level, the current is four times lower at 48V than in a 12V system and has 16 times lower losses. At ¼ of the current, the cables and connectors can be smaller, lower weight and cheaper. The 48V battery used in hybrid vehicles has four times the power of a 12V one, and the added power can be used in powertrain applications to reduce CO2 emissions, improve gas mileage and add new safety and entertainment features.
Additional design flexibilities include ease of paralleling to meet higher power demands and the option of connecting converter outputs in series to provide higher output voltages by in effect changing the K factor.
HV 380V, 270V
UHV 800V, 600V, 540V
28 ELECTRICAL ENGINEERING • FEBRUARY 2022
electricalengineeringmagazine.co.uk
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