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900W Power Distribution Unit

900W Power Distribution Unit

The 900W Power Distribution Unit converts unregulated battery voltage down to a set of regulated outputs suitable for distribution within small to medium-sized UAVs.

The 900W PDU provides three high-power outputs (Avionics, Servo and Payload), and one low-power output (+5V). The Avionics, Servo and low-power outputs are fully duplicated internally to maximise system reliability through redundancy.

The 900W PDU has dual battery inputs with automatic and glitch-free switchover between the two. An umbilical input ensures that batteries are not depleted during pre-flight checks prior to take-off.

The 900W PDU was developed in partnership with VISIONAIRtronics of Austria.

• Four independent power supply systems:
  • Avionics: 12 - 28 VDC, 5 Amps (100W max.)
  • Servo: 6 - 28 VDC, 25 Amps (300W max.)
  • Payload: 12 - 28 VDC, 18 Amps (500W max.)
  • Low-power: +5 VDC, 0.5 Amps (2.5W max.)
• Avionics, Servo and Payload outputs are user-configurable for voltage.
• Avionics, Servo and low-power outputs have 100% redundancy (i.e. the outputs are the result of combining two identical and independent power supplies, each of which is capable of supplying 100% of rated load).
• Two battery inputs. Battery voltage may range from 24 to 55 VDC. Supported battery types include:
  • LiPo: 10 - 12S
  • LiS: 15 - 20S
  • LiFe: 10 - 14S
• Umbilical input: 28 - 60 VDC.
• RS232 and CAN monitoring interfaces provide extensive reporting of voltages, currents and internal temperature.

The 900W PDU is distributed internationally from Europe by VISIONAIRtronics. It is available off-the-shelf in either an IP67 or slimline enclosure, but the enclosure and connectors can also be fully customised to suit your requirements. Contact VISIONAIRtronics to discuss your requirements, obtain further technical information, or request a quote.

The 900W PDU is manufactured in Australia in an ISO9001:2015 compliant facility using lead-free components and processes. All boards are subjected to AOI (Automated Optical Inspection) to maintain control of build quality. Completed PDUs are tested and given a unique serial number to maintain traceability. All PDUs are shipped with a final assembly and test report.

Functional description

Internal architecture of the 900W PDU

The 900W PDU takes power from 3 possible sources and creates 4 regulated output rails as shown in the following table:

The high-power outputs are tailored to suit the Avionics, Servo and Payload demands of unmanned aircraft, and are named accordingly. There is also a +5V output intended for low power devices. For enhanced reliability, the Avionics, Servo and +5V outputs are driven by redundant pairs of power supplies. The individual power supplies making up the redundant pairs can each deliver the full rated power of their associated output.

The Avionics, Servo and Payload outputs are all user-configurable for voltage. The Payload output can be shed (turned off) automatically when the batteries become depleted, if configured to do so.

The PDU draws power from whichever input has the highest voltage. This means that as long as one input meets the minimum input voltage requirements, all power supplies will be regulated to their correct output voltages.

The PDU includes 2 shutdown inputs: a Payload shutdown input and a Master shutdown input.

The PDU has RS232 and CAN communications interfaces which are used for firmware update, configuration, monitoring and control.

Product resources

• Documentation:
  • Product brief: a short summary of the key features and specifications.
  • Product manual: detailed description of the PDU hardware - how it works, how to connect it up, electrical and mechanical specifications, etc.
  • RS232 protocol: describes what values the PDU can measure and transmit, what commands it can receive, and how this data is formatted.
  • CAN protocol: describes what values the PDU can measure and transmit, what commands it can receive, and how this data is formatted.
• Software:
  • Configuration utility: a Windows app for configuring the PDU and checking that it is all working correctly.

More resources are available here.

Technology - Fault Tolerance

The 900W PDU is a fault tolerant device and is able to deliver flight-critical power in the presence of many internal and external faults. It achieves this through input power source redundancy, internal power supply redundancy, internal circuit compartmentalisation and robust I/O protection.

Microcontrollers often represent a vulnerability to system reliability because of the risk of software bugs. With this in mind, the PDU has been compartmentalised such that failure of the microcontroller (due to hardware malfunction or software errors) does not affect any of the outputs. Thermostatic fan operation is accomplished by hardware and does not involve the microcontroller. The PDU will also power-up from cold and provide all power outputs without a functioning microcontroller.

The 900W PDU uses its microcontroller for firmware updates, configuration changes, monitoring, communications and miscellaneous features (load balancing, fan override, payload shedding and short-circuit management). Only these functions are lost if the microcontroller is non-functional. The default state for the Payload output with a non-functional microcontroller is on (not shed).

The PDU has redundant battery inputs. Failure – even to a dead-short – of either battery has no effect on the continued normal operation of the PDU as long as at least one source of power remains connected to the PDU.

The PDU has internal redundancy of Avionics, Servo and +5V power supply systems. This means that internally there are 2 identical power supplies which have their outputs combined with either a physical or ideal diode. Each of the power supplies is able to supply the full rated load on its own. Changeover is instantaneous; there is no transient dropout should one of the power supplies fail.

The Battery and Umbilical inputs are protected against:

• Reverse polarity, unlimited duration
• Short circuit to ground, unlimited duration
• Transient overvoltage

The Avionics, Servo and Payload outputs are protected against:

• Short circuit to ground, unlimited duration
• Transient overvoltage

Technology - Polyphase switching converter

Using a polyphase DC-DC switching converter has a number of advantages over using separate DC-DC switching converters.

Firstly, size is reduced. This is possible because the switching times of the converters are synchronised, each converter having its own unique phase offset. Input current demand is now evenly distributed over time, and so input capacitance sharing between converters can be realised, knowing that input currents can never be drawn simultaneously. Thus converter input capacitance volume can be reduced by almost a factor of 10 without incurring any performance penalty.

Secondly, the emitted noise spectrum is more predictable and can therefore be controlled more effectively. This is possible because the converters are all synchronised in time, thus ensuring that switching transients never sum unpredictably. This can be best appreciated by considering the system in the frequency domain, and noting that we no longer have multiple impulse spectrums mixing, we have just one. This represents a far simpler problem to solve.

These benefits do not compromise the independence of the individual converters. A load fault on one (or more) of the outputs does not propagate to the others; unaffected outputs retain full voltage and current authority.

For the UAV, a polyphase DC-DC switching converter means reduced size and weight, as well as reduced EMI.

The 900W PDU uses a custom 10-phase DC-DC switching converter.

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