Auxiliary Power Supply in UAV

The use of unmanned aerial vehicles (UAVs) has developed rapidly in recent years, both in the civil and defense sectors. From small toys to professional devices that can carry several kilograms of payload to UAVs that can cover long distances, there is an almost countless number of different designs. The necessary energy can be supplied by built-in batteries, fuel cells, cables, or turbines with generators.

A customer was looking for low weight DC/DC converters to supply various payloads from 12 or 16 cell battery stacks in an UAV for defense applications. Voltage of LiPo cells can vary from 3V (discharged) up to 4.35V (end of charging cycle). Therefore, in this application the supply voltage range is 36 – 70V. 

Fuel cells used for UAVs have almost identical output voltage ranges and are increasingly being used, especially for longer flight times and devices with higher payloads and therefore higher power demands.

Figure 1 shows the basic power architecture used in UAVs. While the motors can be powered directly from the battery or fuel cell, the various electronic components require regulated supply voltages ranging from 3.3 up to 24V and for analog circuits dual voltages are required.

The following section describes some typical payloads for this specific UAV application example and how they are powered by P-DUKE converters.

Gimbal cameras (optical and thermal)

Many optical and thermal cameras are designed for 3s – 6s battery configurations and for higher supply voltages a DC/DC converter is needed. Highest power demand is 8W when an HD camera with 25 x optical zoom is mounted on the UAV. To avoid interference with the high current peaks of the propulsion motors or any other high frequency electronics, P-DUKE’s isolated converter RDL10-48S15W was selected. It provides an 15V/10W output from a 18 – 75V supply voltage and with only 7.2g it also meets the low weight requirements for this application

Data Transmission

Depending on operating range and data volume, various technologies are used for data transmission in UAVs.

WiFi can be used for short distances as it offers wide bandwidth. Power consumption of the WiFi module used is 3.3V/2W

RF transmitters can bridge distances of several tens of kilometers, and the device selected by this customer requires 5V/6W

GSM allows transmitting data over very long distances (out of sight operation) but needs a reliable GSM network across the complete flight area. The customer selected a GSM module compatible with a standard 5V USB port.  Power peaks during transmission can reach 1A and converters supporting 5W power peaks are needed.

Satellite communication, another way of transmitting signals over long distances, was not included in this design.

GPS module

To track the exact position and enable auto flight modes, a GPS module is built into the UAV. It needs a 3.3V supply voltage but maximum power consumption is 25mA

Sensors

For stable and safe flight operation various sensors are mounted on an UAV. The Inertial Measurement Unit (IMU) consists of accelerometers, gyroscopes and magnetometers to determine the exact position in all three axes. An air pressure sensor measures the altitude, a magnetic compass the orientation and short ultrasound or laser pulses are used for distance measurement. These sensors need either a 3.3 or a 5V supply voltage but power consumption is quite low.

P-DUKE recommended a very flexible and easy to implement design with a 5V bus generated by the RCD20-48S05W, an isolated 20W converter with 18 – 75V input voltage and housed in a small 1” x 1” package.

From this 5V bus a tiny PSR02, non-isolated converter in the LM78 package generates the 3.3V voltage and can deliver currents up to 2A.

Analog circuits   

While many sensors can operate with a single supply voltage, high-precision analog circuits with op-amps still require a positive and negative supply voltage with as little ripple and noise as possible. With the LDL 03 series, P-DUKE offers a 3W solution that can be powered directly from the battery (36–70V) and generates positive and negative voltages from ±5V up to ±15V with typical ripple and noise values as low as 5mVp-p.

The application described above covers only a fraction of the configurations possible in UAVs. In this case, four P-DUKE converters with a total weight of only 31.6 g were used which is a tiny fraction of the total weight of the UAV. Like many P-DUKE products all these converters are qualified for the stringent environmental conditions of MIL-STD-810F and therefore ideal for this demanding application.

Block diagram:

Products:

RDL10W: 10W isolated DC‑DC with 4:1 wide input; high efficiency and low no‑load current. Compliant with EN50155/EN45545‑2 for railway and harsh industrial systems.

RCD20W: 20W isolated DC‑DC with wide input range (9–160V); 3kV isolation, remote ON/OFF and adjustable output trim. Heat‑sink options available. EN50155/EN45545‑2 compliant for railway and industrial applications.

PSR02: 2A non‑isolated buck module with wide input up to 36V; high efficiency and fast transient response. Ideal for point‑of‑load power and compact designs.

LDL03: 3W isolated DC‑DC with 4:1 wide input; low ripple and high efficiency, plus short‑circuit protection with auto recovery. Suited for industrial control and communications.