Technology

Millswood Engineering invests a lot of time and resources in developing useful and innovative technologies that can be used in its products. This results in significantly better performance and specifications than are available from other manufacturers. Take the all-important parameter of weight, for example.

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

Below are just a couple of the key technologies developed for use in Millswood PMUs.

Active rectification:

Comparative power loss

Power loss of a typical 3-phase Silicon diode bridge, a high-performance
3-phase Schottky diode bridge, and our 3-phase active rectifier.

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 right, 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:

All Millswood Engineering PMUs use active rectification.

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:

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

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