Innovative power solutions for transportation systems
03 May 2018
Transportation systems can have input voltage ranges up to 14V (single battery automotive), 28V (dual battery trucks, buses, trains, airplanes), or even higher, as well as requiring one or more low voltage rails for digital systems. As a result, designers of such systems need to know how they can step down from high input voltages simply, efficiently and reliably.
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Figure 1 (see below) shows how the input voltage in an automotive environment can vary tremendously depending on its operating conditions, which can range from load dump to cold crank, and even reverse battery hook-ups.
When an application requires very high efficiency of power conversion, in order to minimise the heat generated from the power lost during the conversion process, a switching regulator solution can help.
Switching regulators can be monolithic in nature with MOSFETs integrated on chip – in either a synchronous or non-synchronous configuration. Or, they can consist of a switching controller that can drive external MOSFETs in single or multiple stage topology (multiphase) to deliver power levels from tens – up to hundreds – of Amps.
Figure 1. Typical automotive transients
As a result of this broad power range, Linear Technology offers an extensive array of switching regulator solutions that enable the user to select the most applicable device, based on the specific design criteria necessary for the end system. Accordingly, Linear’s switching regulators have very broad input voltage ranges, from 5V up to 150V; and output power levels, from hundreds of mille-amperes to greater than 1,000A.
An example of such a capability is the LTC3895: a 150V input-capable synchronous step-down converter that can be configured for multiphase operation, as shown in Figure 2. A commonly asked question in any transportation environment is: “How can I have a high step-down ratio and a compact solution footprint without impacting functional performance and conversion efficacy?” Until recently, there had not existed a solution that could deliver on all key performance matrices without having some level of compromise.
However, with the introduction of Linear’s LT86xx family of monolithic 2MHz plus synchronous step-down converters, all the necessary performance aspects can be delivered at once. A good example of this capability is the LT8609, a 2A, 42V input synchronous step-down switching regulator. A unique synchronous rectification topology delivers 93% efficiency while switching at 2MHz; this enables designers to avoid critical noise-sensitive frequency bands, such as AM radio, while providing a highly compact solution footprint.
Burst mode operation keeps quiescent current under 2.5µA in no-load standby conditions, making it well suited for always-on systems. The LT8609’s 3.0V to 42V input voltage range makes it ideal for automotive applications, which must regulate through both cold-crank and stop-start scenarios – with minimum input voltages as low as 3.0V, and load dump transients in excess of 40V. Its internal 3.5A switches can deliver up to 2A of continuous output current with peak load currents of 3A.
Figure 2. LTC3895 schematic and efficiency versus power loss curve
The schematic and corresponding efficiency curve for 2MHz switching are shown in Figure 3 below. Many transportation systems have a wide input voltage range due to the cold crank and load dump conditions commonly found in single or double battery vehicles. And, to complicate matters further, the desired output voltage can straddle this wide input voltage range. A system designer is therefore faced with the complex problem of having to design a solution that allows for a fixed output regardless of whether the input voltage is above, below or equal to the output voltage.
A common approach to solving this problem is to employ a SEPIC topology converter. However, this is a complicated design that requires two inductors and is usually not very space or conversion-efficient. As a result, Linear has designed an extensive family of 4-switch buck-boost controllers, which not only simplifies the design, but is both space and conversion-efficient, with power losses in the 5 to 7% range (depending on the input to output voltage range).
The LT8705 shown in Figure 4 (links to online magazine version) is an example of a 4V to 80V input-capable buck-boost controller, delivering a fixed 12V output that is commonly found in vehicular environments. An alternative approach to dealing with an automotive cold crank condition is to employ a boost converter, followed by a buck converter.
In this topology, the output of the boost converter from a single battery is set to a few volts above the battery’s nominal voltage, and then, it is stepped down with a buck converter to the desired operating voltage – as required by the downstream electronics. Although it requires two converters, Linear has developed a device that combines both a boost controller and buck controller that can be either used independently, or in boost-buck follower: the LTC7813 (Figure 5 illustrates how this is achieved).
Figure 3. LTC3895 schematic and efficiency versus power loss curve
Low noise power management
‘Electromagnetic radiation’ (EMR), ‘electromagnetic interference’ (EMI) and ‘electromagnetic compliance’ (EMC) are all terms that pertain to energy from electrically-charged particles and the associated magnetic fields, which can potentially interfere with circuit performance and signal transmissions.
With the proliferation of wireless communications, the plethora and pervasiveness of communication devices, and the growing number of communication methods, using more and more of the frequency spectrum (with some bands overlapping), electromagnetic interference is simply a fact of modern life. To mitigate the effects, many governmental agencies and regulatory organisations have set limits on the amount of radiation that can be emitted by communications devices, equipment and instruments.
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