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250W Power Management Unit

250W PMU photo

Millswood Engineering 250W PMU

The Millswood Engineering 250W PMU provides up to 250 Watts of on-board electrical power generation for small to medium-sized UAVs.

The 250W PMU simplifies UAV power distribution by providing multiple battery-backed power outputs which are individually user-programmable for voltage. Dual battery support with lossless switchover is included as standard.

The PMU connects to a suitable brushless DC electric motor, which is in turn driven by the aircraft's primary power plant, usually an internal combustion engine.


Advantages of on-board electrical power generation

Minimising the mass that must be carried by an airframe is central to maximising performance and range. Batteries can constitute a significant part of the overall mass of small to medium-sized UAVs, and so storing the necessary electrical energy in an alternative form is one way of reducing the battery mass.

The gravimetric energy density of liquid hydrocarbon fuels remain more than an order of magnitude better than the best battery technology (12 kWh/kg for aviation fuel versus 0.35 kWh/kg for Li-S). Even allowing for the relatively poor efficiency of internal combustion engines, onboard electric power generation remains a highly effective way of reducing the battery mass.

Contact us for further technical information. See the Purchase page for where to buy.



Block diagrams

Block diagram of a typical generator system

Typical UAV generator system block diagram

Internal architecture of the 250W PMU

Internal architecture of the 250W PMU

Comparative weight

Weight is one of the most important considerations in the design of a UAV. The 250W PMU has the highest power output per unit weight in its class. Here's the data:

Manufacturer Power rating Mass Specific power
Millswood 250W 290g 0.86 W/g
Competitor A 100W 190g 0.53 W/g
Competitor B 200W 350g 0.57 W/g
Competitor B 325W 650g 0.50 W/g

Technology - Active rectifier

Comparative power loss

Comparative power loss of various rectification strategies

The first step in turning high-voltage AC into regulated DC is rectification. This process is traditionally performed using a diode bridge, which is an inefficient device that wastes some potentially useful power as heat. Active rectification replaces the diodes with FETs, which have lower loss than either Silicon or Schottky diodes.

As can be seen from the graph on the left, there is up to 90% reduction in the power lost in the rectification process when an active rectifier is used. This translates into improved overall efficiency, particularly at low rpm where the diode drop is a significant fraction of the rectifier's total voltage.

For the UAV, active rectification means reduced heating and heatsinking requirements and therefore smaller enclosed volume, as well as operation to lower rpm.

The 250W PMU uses a 3-phase active rectifier.

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 5 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 5 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 250W PMU uses a custom 5-phase DC-DC switching converter.

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