Meeting Military Aircraft Electrical Specifications

February 8, 2009
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MIL-STD-704* came into being to standardize aircraft electrical requirements for the armed services. Since 1959 there have been a number of modifications to this basic specification, with the latest, MIL-STD-704F, being introduced in 2004.

Initially MIL-STD-704 limited itself to two basic voltage buses, a three-phase 115Vac (typically 400Hz) supply with limited single-phase loading and a 28V DC system. Over the years there have been many changes in the surge and transient withstand levels, particularly for the 28Vdc rail and the introduction of a 270Vdc bus. Vicor manufactures parts to meet most of these requirements. However, a full understanding for each particular specification is essential to obtain an optimal solution: even now legacy aircraft are re-equipped with new avionics that reference older specs.

High Voltage Systems

The MI-AIM, in conjunction with either the MI-27x or MI-J7x DC-DC converter modules, can be used to produce a compliant MIL-STD-704 (all versions) PSU from the single-phase 115Vac rail. Because of phase imbalances this rail is often only used for relatively light loads in single phase. Recent airframes have introduced a 270Vdc bus, and Vicor’s MI-26x and MI-J6x modules provide compliance to MIL-STD-704D and later specifications because of their wide voltage operating range. Vicor modular filter solutions also provide MIL-STD-461 conducted emission and conducted susceptibility compliance (compliance versions do vary with the filter module). Furthermore, Vicor’s newer Maxi, Mini, Micro range of 300V (180-375V) input DC-DC converters can be used with the M-FIAM3 to gain MIL-STD-704E/F compliance. These parts achieve higher power densities and power handling and provide MIL-STD-461E conducted EMI compliance.

28 Vdc Systems

The 28 Vdc system (normally with battery back-up) has seen the most dramatic changes in quality and the levels of surges and transients. Moreover, the levels or surge and spike withstand characteristics are highest in MIL-STD-704A (circ. 1966), which requires a loci of surge withstand that includes 78V for 0.1 seconds and spikes up to 600V (for up to 10µs & 200V spikes for up to 50µs duration). These spikes can be simply removed by the addition of transzorbs or metal oxide varistors (MOV) suitably rated.

Whether operation during surges is required is dependent on the equipment specification. MIL-STD-704F, for instance, only requires operation during 50V surges for 12.5 milli-seconds and survival for 50 milli-seconds at this voltage level.

The table below lists some of the most important requirements and suitable Vicor parts for MIL-STD-704 compliance for the 28V DC system.

Version Key performance criteria
Vicor Product Compatibility
Vicor MI-J/200 product compatibility Maxi, Mini, Micro V•I Chip™
MIL-STD-704 80V for 50ms no spikes MI-A22 with MI-22x & MI-J2x operating M-FIAM5B & V24 Maxi, Mini, Micros survival (non-operating) M-FIAM7 & MPRM and MVTM chip set operating
MIL-STD-704A 80V for 70ms with 600V spikes MI-A22 with MI-22x & MI-J2x operating M-FIAM5B survival & recommended MOV M-FIAM7 & MPRM and MVTM chip set operating
MIL-STD-704B   Compliant No data Compliant
MIL-STD-704C   Compliant No data Compliant
MIL-STD-704D 60V for 10ms No spikes MI-22x & MI-J2x compliant M-FIAM5B survival M-FIAM7 & MPRM and MVTM chip set operating
MIL-STD-704E 50V for 12.5ms operating 50ms survival MI-22x & MI-J2x compliant V24 complaint with M-FIAM5B MPRM & MVTM chip set – compliant
MIL-STD-704F 50V for 12.5ms operating 50ms survival MI-22x & MI-J2x compliant V24 complaint with M-FIAM5B MPRM & MVTM chip set – compliant

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