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Power Quality in Military Applications

Thu, 04/21/2011 - 7:17am
Ron Demcko, Application Engineering Manager at AVX Corp
Military electronics, whether land, air or sea based all require increased amounts of power due to functionality and mission profile expectations. An easy-to-relate example of this is the 20-pound battery load a soldier might carry for a three-day mission. Power conversion efficiency and battery type consolidation is a welcomed topic to that soldier. A more complex scenario might be those of aircraft and spacecraft power trends. Commonly, spacecraft technology and trends ultimately tend to trickle down to avionic and earth based applications. Two interesting trends that design engineers are encountering in military power designs include size, weight and power (SWAP) optimization, and distributed power concepts. Both trends are driving designers, their sub system architectures and ultimately the development of new passive component devices designed to meet increasing power demands in military applications.

Size, Weight and Power Optimization
The SWAPconcept is a desire to create small size power sources with excellent weight and maximum power levels. The SWAP concept has been embraced by ICs as well. Digital ICs attempt maximum processing power for a given power load in packaging that gets smaller and lighter. Examples include the embedded computer application support ICs used in radar processing, sonar and signal intelligence work. Likewise, power ICs have progressed by offering reduced losses and higher power operation in smaller and lighter packages.

The efficiency of semiconductors is compounded by improved passive components and, as a result, system configuration/designs benefit greatly. Aircraft manufacturers must drive systems with SWAP techniques mercilessly. For example, civil aircraft are expected to have improved passenger entertainment, more spacious and functional galleys and improved passenger environment (lighting, audio and temperatures); and military aircraft must perform more complex flight maneuvers (sometimes computer controlled) at faster speeds and a wider altitude envelope -- and these maneuvers must occur with increased sensors and weapon system loads.

Distributed Power Concepts
Instead of one power supply within a given unit to power all sub assemblies, the trend is for smaller, lighter, more flexible and efficient power supplies to be placed right at the load. The more electric aircraft (MEA) has a goal to distribute power all around the aircraft in an attempt to electrify systems that were once hydraulic, pneumatic and mechanical. Expectations of MEA are that all aircraft type would become more reliable, with improved flight performance and efficiency.

The Impact on Passive Components
Designers demand passives with reduced parasitic parameters, wider operation environment components (ones capable of higher G/vibration scenarios across higher temperatures). Most importantly passive components with increased values and voltages are required.

Among example of new component types are stacked tantalums, wet tantalum modules, vertically oriented ceramic capacitors ‘stacks’ and mixed component stacks such as multilayer varistors (MLVs) and multilayer ceramic capacitors (MLCCs).

Mixed Component Stacks
Newly introduced mixed component stack MLVs and MLCCs offer dramatic EMI and transient suppression performance advantages in a very small footprint package. Multilayer varistors are used to clamp incoming transients in a sub nanosecond turn on time in a bi-directional manner. The capacitor is used to provide broadband EMI filtering across a range expected from the wide range of MLCC values available in the industry. Commonly, MLCCs feature temperature stable X7R dielectrics. These devices are typically manufactured using high melting point temperature solder (268 solidus/290c liquidus) allowing for standard reflow processing during board level assembly 

Figure 1. An example of multilayer ceramic capacitor EMI response.

Figure 2. An AVX multilayer ceramic capacitor.

Novel Stacked MLCC Lead Configurations
Switch Mode power supply capacitors are used for their extremely low ESR, large value and low inductance. A modification to the traditional lead frame associated with SMOs can dramatically improve the impact of the already efficient SMO on power quality.Both standard and modified lead frame SMOs are shown in Figure 3.

The standard configuration SMO uses a drilled via for each and every pin on its two side terminations. These drilled vias are connected to the Vcc and ground lines by traces.

The new SMO lead configuration has fewer pins for the drilled vias. As in the traditional design, each and every pin of the new lead frame are soldered into drilled vias however the new SMO lead frame has less pins (since center pins eliminated). The power and ground lines are actually configured in such a manner that power (and ground) enter one set of SMO lead frame pins. The pcb power (or ground) trace is then stopped, leaving the side termination of the SMO to route power (and ground) across the SMO to the other set of leads which enter the PCB drilled vias and the continuing PCB power (ground trace). See Figure 3.

Figure 3. Standard versus alternate low inductance SMO lead frame.

An equivalent model of the reduced and transformed inductance is shown in figure 4. 

Figure 4. Equivalent model comparison of standard SMO and low inductance SMO configuration.

When an SMO is used in a low inductance configuration, the frequency response is improved so greatly that designers may find output inductance filter values can be reduced significantly -– potentially translating into smaller, lighter cheaper inductors.

Conclusion
Military electronic applications require more efficient power conversion, transient attenuation and EMI control, placing new demands upon passive components. Novel components such as MLV and MLCC stacks as well as low inductance SMO configurations can greatly affect circuit size, performance and cost. Additional stacked developments are expected shortly, providing higher voltage and temperature operation output filters, snubbers and modules.
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