Aircraft ground power units must satisfy two distinct electrical supply requirements simultaneously: 28V DC for battery charging and DC-powered systems, and 115/200V 400Hz AC for avionics and AC-powered systems. The engineering challenges in each are different.
For the 400Hz AC output, the conversion from mains frequency (50 or 60Hz) or from DC bus involves generating a clean 400Hz waveform with low total harmonic distortion (THD). MIL-STD-704 specifies maximum harmonic distortion limits — the fundamental 400Hz component must dominate, with harmonics kept below specified percentages of the fundamental. Inverter-based designs using PWM (pulse width modulation) with appropriate output filtering achieve this. The output voltage regulation must hold within ±2% of nominal (115V phase or 200V line) across the full load range, with transient recovery within the MIL-STD-704 envelope for step load changes.
For the 28V DC output, the rectifier must provide regulated voltage with current limiting appropriate for the battery type being charged. Aircraft NiCd batteries (common in military aircraft) require careful charging current profiles — excessive charging rate causes thermal runaway, while too slow a charge rate wastes ground time. The charger control loop must implement the correct constant-current/constant-voltage (CC/CV) profile and detect full charge by the appropriate method for the battery chemistry. Sensing the battery voltage at the battery terminals rather than at the output of the charger (four-wire sensing) compensates for cable resistance and ensures accurate voltage regulation at the battery itself.
Protection requirements include: output overcurrent limiting that prevents damage to the charger if the load is shorted; overvoltage crowbar protection that disconnects the output if the voltage rises above a safe threshold (protecting aircraft systems from a regulator failure); and soft-start logic that ramps the output up gradually rather than stepping to full voltage, preventing inrush current transients that could disturb aircraft systems already powered from another source.
Environmental design for military mobile units adds requirements for operation across -20°C to +55°C ambient, ingress protection against dust and rain, and vibration tolerance during transport. MILSPEC component selection and conformal coating of PCBs extend the unit's service life in harsh environments.
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