The aim of this project is to eliminate battery from the NavComm NC-55A airband radio by providing a regulated voltage supply directly through the battery contacts of the radio.

• Radio uses 2 cell Li-Ion battery used -> 7.4 V nominal working voltage
• Radio current consumption: 1 A for Tx, 70 mA for Rx per manual
• Input voltage range 8-28 V of aircraft power bus (12 V nominal)
• Good input voltage noise filtering is needed - spikes on engine startup, etc.

• The mechanical part (holder) of the project was designed in a way that it can be clicked to the radio back in similar way as the battery
• The holder serves as a battery replacement and in the same time as a mounting point of the radio to the aircraft panel
• A simple connector to accomodate 2 jacks needed by the radio (mic, headphones, PTT) is included in the design files
• Design files can be found in 3d folder or on my Onshape profile

• Polarity protection using SBR3U40S1F diode
• Transient protections using SMDJ28CA 28V 3kW TVS diode
• Shorts protection using a fuse - assuming 1 A output and 80 % efficiency at minimum working voltage, 1.3 A hold current at 80°C is needed. The C1Q 1.25 will be used.

A simple SMD ferrite bead followed by 100 nF and Buck converter input capacitors provides input filtering to reduce amount of noise going from the DC/DC supply to the aircraft power bus.

As the voltage difference is quite high (worst case 28 - 7.4 = 20.6 V / 1 A output), the LDO is not a good choice here, switching supply is needed.

• Buck topology
• ~8-30+ V input voltage range
• more than 1 A output current
• Large output current range (light load mode is possible)

The AP63200WU-7 was selected as it matches all requirements and requires a minimum amount of external parts.

• 2 A output current limit
• 3.8 - 32 V input voltage
• 500 kHz switching frequency with spread spectrum to reduce EMI
• integrated MOSFETs
• PFM mode for high efficiency on low output currents
• Overvoltage and overtemperature protection

The feedback voltage reference is 0.8 V, a 75k and 9k1 5% resistors will be used, giving output voltage (worst case) between 6.77 and 8.09 V which is well within input voltage range of the radio, a 1 % resistors would be better though.

The inductor will be (assuming 50 % ripple current of 2 A - 1 A):

$$ L = \frac{V_{out}(V_{in}-V_{out})}{V_{in}\Delta I_L f_{sw}} = \frac{7.4 (28-7.4)}{28_1_500000} = 10.9 \mu H $$

The datasheet recommended maximum inductance od 10 $$\mu H$$, so let's stick with this. The recommended RDC is 100 $m\Omega$ and saturation current 35 % higher than peak one. The SRR6028-100Y was selected.

Output is fed through a ferrite bead to a parallel combination of small ceramic capacitor and a large capacitor with higher ESR to provide some dumping.

The output of the switching regulator is separated from radio by a fuse and a TVS diode to protect the circuit from ESD when radio is not connected.

• Measured at 13 V input voltage (usual value when engine is running in 12 V based aircraft power system)
• Radio consumption pulses between 20 and 44 mA in Rx while battery saving is enabled, the coil makes annoying buzzing noise
• With battery saving off -> 44 mA constant current, the coil makes very weak high pitch sound
• With backlight on, consumption is 90 mA, no audible noise from coil
• When transmitting, the consumption is around 700 mA