Looking Under the Hood of a Military Power Supply
- Friday, 01 February 2013
When it comes to design of military power supplies, NAVSO P3641 is the premier reference. First introduced in 1999, this comprehensive set of guidelines, subtitled “More Power for the Dollar,” details the best manufacturing practices for military power supplies. It includes provisions allowing the use of COTS (commercial-off-the-shelf) power supplies in applications like telecommunications, computing, or air traffic control where benign conditions do not exceed 0° to 70 C° limits. It also details requirements for designing and building power supplies for the more rugged -40° to +85° C conditions encountered in the field.
These guidelines suggest the use of metal clad printed circuit boards, surface mount packages for integrated circuits, gold plated interconnects to reduce the effects of corrosion, physical design parameters that impact cooling, electromagnetic interference management techniques including shielding and physical layout, and many more design details. It also gives detailed design guides for high and low voltage DC/DC converters, as well as AC/DC supplies, inverters and uninterruptable power supplies. In addition, environmental stress screening parameters and procedures, including highly accelerated stress screening and testing, are outlined.
Various branches of the military have published detailed sets of requirements that establish what will be expected of power supplies from all types of military systems deployed in the field. These Military Standards — or MIL-STDs — focus on performance issues as well as environmental conditions that can impact reliability.
Input voltage conditions for tactical military applications cover electromagnetic compatibility and input levels. The former is spelled out in MIL-STD-461 which details the amount of conducted RF energy the device must be able to withstand and still operate properly. This is typically 40 dB or more of attenuation from the internal power supply switching frequencies all the way out to several megahertz. Commercially available filters that meet FCC requirements for commercial noise suppression cannot achieve this level of signal rejection. So each military power supply application must be approached individually to construct filters and maintain proper impedance matching characteristics to eliminate radiated noise at the input.
Meeting the full range of input voltages required for military applications could be a challenge for off-the-shelf commercial products. For example MILSTDs generally detail three input voltage ranges for each device. The first would be normal operation; the second “abnormal” like when a malfunction or failure has occurred and protection devices are functioning to correct the situation; the third is emergency where the main generating equipment has failed and a limited independent source is powering vital systems. Examples for aircraft and vehicular standards are shown in the accompanying table.
Note that in all cases the minimum accepted input is lower for military power supplies than is generally found in specifications for COTS devices. This means that COTS supplies would shut down at higher voltages while military grade devices keep on powering systems. This is primarily because designers of military compliant devices tend to optimize performance of their DC/DC designs by trading off maximum output current for a wider input voltage range. High line ratings present another problem for COTS DC/DC converters, since their maximum rated line capability is less than applicable MIL-STDs. Here military compliant modules utilize clipping and pre-regulation schemes to prevent damage and allow for reliable operation across the entire full voltage range.
Harmonic noise and distortion can also be an issue with most commercial power supplies in certain environments. For example, in shipboard applications, MILSTD- 1399 specifies that single harmonic distortion be limited to 3 percent and total harmonic distortion to below 5 percent. Commercial units would require large passive components to meet these levels, whereas military designers accommodate these conditions as a matter of course.
Transient protection is required for virtually all military power supply input stages and varies from application to application. A typical specification can be found in the MIL-STD-1275B requirements for military vehicles. Here the input for a 28VDC DC/DC converter must be able to survive a spike of up to ±250V that is 50 μsec wide in a burst of 1 msec duration with 15 mJ maximum energy content per spike. Surges of up to 100V for 50 msec from a 0.5 Ohm source impedance repeated 5 times per second must also be accommodated. Most commercial units can meet neither specification, but well designed military units can. In addition military units must often be designed to survive lightning strikes and this would increase spike and surge protection requirements by several orders of magnitude.