Harsh military environments demand high performance power conversion

Background

Military vehicles are the platform for increasingly complex electronic systems for command and control, communications, weapons management, surveillance and countermeasures. Size, weight and power represent a major challenge for designers trying to combine a myriad of systems with soldiers and their supplies into a small and cluttered space. In particular heat management and power distribution are critical and new high performance power conversion technology helps designers to develop smaller, more thermally efficient and reliable systems.

A further complication for designers of military vehicle electronics is the need to comply with MIL-STD-1275D or similar national standards which specify the steady state and transient voltage requirements of 28V power systems, which have a typical operating input voltage range of 16V to 40V. The MIL-STD-1275D specifies voltage surges to 60V and 100V, requiring operation at these higher voltage levels or needing an extra clamp circuit to maintain safe operating input voltage. To further complicate design considerations, the number of rails within an electronic system is also increasing. For example, a typical navigation system can have six or more different supplies including 8.5V, 5V, 3.3V, 2.5V, 1.8V and 1.5V. At the same time, as the number of components increase, space requirements continue to shrink. Therefore, high efficiency conversion to minimize power dissipation becomes more critical due to the space limitations and high temperature conditions.

Many military vehicle systems require continuous power to the onboard electronics even when the motor is not running. It is essential for these types of “Always-on” systems to have a DC/DC converter with low quiescent current in order to maximize the battery run-time when in sleep mode. In such circumstances, the regulator runs in normal continuous switching mode until the output current drops below a predetermined threshold of 30-50mA or so. Below this level, the switching regulator must go into Burst Mode® operation to lower the quiescent current into tens of micro amps, thereby lowering the power drawn from the battery in order to extend its run-time.

With 60V input DC/DC converters in short supply, designers have resorted to a transformer-based topology or external high side drivers to operate from up to 60V. Others have used an intermediate bus converter requiring an additional power stage. Both of the alternatives increase the design complexity and, in most cases, reduce the overall efficiency. However, the LTC3890 from Linear Technology is the newest member in a growing family of 60V input capable step-down switching regulator controllers that specifically addresses many of the key issues found in automotive, military vehicle and truck applications as outlined above. Figure 1 shows a schematic of the LTC3890 operating in an application that converts a 9V to 60V input into 3.3V/5A and 8.5V/3A outputs.