Introduction
Sustainability initiatives, green energy and environmental regulation are part of everyday life both in a domestic setting and in the wider world. Commercial aviation has been at the forefront of efforts to use lightweight materials, improved aerodynamics and more efficient engine technology to respond to commercial pressures and tighter emission targets.
Electrical efficiency has become increasingly important as aircraft shift from traditional air bleed systems and hydraulics to electrically-powered actuators, pumps and compressors to drive a multitude of systems.
On the Boeing 787, ice protection relies on electro-thermal heater mats to replace hot air bleed from the engines, further reducing load and improving fuel economy and thrust. These developments have seen onboard electrical power demand escalate over three decades from around 300kW in the B767 to more than 1.4 megawatts in the B787, making the efficiency of power conversion more critical than ever before. The military are also following the same direction of travel, striving to make systems more energy efficient, reducing weight, size and heat generated, and thereby trimming down acquisition and operating costs.
Selecting components for use in military and aviation equipment includes all the usual design trade-offs and compromises, and in addition, is constrained by tough specifications for equipment survivability and rigorous qualification for use in harsh environmental conditions. Furthermore, corporate policies for component selection sometimes limit the types of products available to those pre-approved by component engineering and re-use of proven standard circuit blocks is encouraged.
It is therefore desirable to have products that lend themselves to multiple applications through innovative architectures and clever chip design such that fewer qualified or approved part types are in use. The problem is that even in a relative narrow area such as power conversion, applications and requirements vary widely. However, as we will see, some products such as the recently released LT8705 are versatile enough to make them candidates in these scenarios.
High Efficiency Power Conversion
The LT8705 is a very high efficiency (up to 98%) synchronous buck-boost DC/DC controller that operates from input voltages above, below or equal to the regulated output voltage. Four feedback loops are integrated to regulate the input current, input voltage, output current and output voltage. The input current and voltage feedback loops prevent overloading of energy limited sources (i.e. solar cells) while the output current loop provides a regulated output current for a battery charger or current source.
The LT8705 operates over a wide 2.8V to 80V input voltage range and produces a 1.3V to 80V output, using a single inductor with 4-switch synchronous rectification. The high input voltage simplifies transient protection as it can withstand the surge voltage requirements specified in MIL-STD-704 and DO-160. With the addition of a surge suppressor such as LT4363, this can be further enhanced to meet the surge requirements of MIL-STD-1275.
Output power up to 250W can be delivered with a single device, making it suitable for a range of loads by sizing the power switching MOSFETs accordingly. Higher output power can be achieved by arranging multiple circuits in parallel.
The operating frequency is selectable between 100kHz and 400kHz and can be synchronized to an external clock. The LT8705 employs a proprietary current-mode control architecture for constant frequency operation in buck or boost mode and has powerful onboard quad N-channel MOSFET gate drivers. The user can select among forced continuous, discontinuous and Burst Mode® operation to maximize light load efficiency.
Additional features include servo pins to indicate which feedback loops are active, a 3.3V/12mA LDO, adjustable soft-start, onboard die temperature monitor and ±1% reference voltage accuracy over an operating junction temperature range of -40°C to 125°C. The LT8705 is available in a 38-pin 5mm x 7mm QFN and also a 38-lead TSSOP package with additional pin spacing for high voltage operation at altitude. Both lead-free and tin-lead terminal finish are available.
Applications
The versatility of the LT8705 with its “anything in, anything out” flexibility lends itself to a variety of applications. In figure 1 the LT8705 is configured as a Voltage Stabiliser , with a nominal 28V input and 12V output to an intermediate bus it achieves >95% efficiency.
Super capacitors can be used to supply large peak currents required by powerful servo systems and actuators used in military hardware or to act as a temporary hold up circuit if power is interrupted. In this scenario the LT8705 can be used as a single component Bidirectional Supercap Charger to replace two traditional pulse width modulated supplies, in one direction to charge the supercaps and in the other to deliver energy to the load. No additional power routing/management are needed, reducing the parts count and power losses incurred by the additional circuitry.
Forward operating bases want to utilize solar energy to reduce the consumption of diesel fuel in electrical generators and the LT8705 can provide a Solar Panel Battery Charger solution. The solar cell produces current in proportion to the illumination level, while the open circuit
voltage remains relatively constant (see Figure 2). By using the input voltage regulation loop and EA3 (see Figure 3), the LT8705 can extract maximum power, so called maximum power point tracking. On the output side, the LT8705 can run as a battery charger using the output current regulation loop and EA1 to provide a constant charging current until a predetermined voltage is reached, where the output voltage regulation loop can take over (not shown)
Finally, a Portable Power Supply for systems powered from battery packs, the LT8705 operating initially in buck mode to step down the battery voltage can then automatically transition through to boost mode to provide extended run time as the battery voltage reduces.
For more information on these applications please refer to the LT8705 datasheet.
Protecting Against Faults
There is a need to protect high reliability systems from the propagation of faults both in the upstream direction to avoid damaging the main power bus and in the downstream direction to protect expensive electronics from faults in the power conversion chain.
The LT8705 activates a voltage lockout sequence if conditions for proper operation such as minimum VIN are violated or if overcurrent or overvoltage conditions are detected on input or output. After the fault condition has been removed and a predefined timeout period has ended, the converter will restart at a rate dependent upon the capacitor value assigned to the soft start pin.
Thermal management is also assisted by a secondary function of the CLKOUT pin, its duty cycle being linearly proportional to the die temperature. The maximum specified junction temperature is 125ºC but an internal thermal shutdown protects the reliability of the chip, at approximately 165ºC the power switch and the internal regulators are turned off. After the chip temperature drops by approximately 5°C, the part will re-initialize and perform a soft-start before resuming normal operation.
Conclusions
Designers of military and aviation applications are often encouraged to reuse proven circuits and constrained to a narrow range of products that are approved. Therefore, it is desirable that such components are as flexible as possible so that they can be deployed in multiple applications.
One product that matches these requirements is the LT8705 Synchronous 4-switch buck-boost controller. With up to 98% efficiency, wide input and output voltage range and four control loops, it provides a versatile solution for many military and aviation applications.
Linear Technology UK ltd
Tel: 01628 477066
email: uksales@linear.com
