I'm giving her all she's got, captain!
Happy New Year to all readers of Out-of-this-World Design! I hope 2014 is a successful year for you, your designs and the space electronics industry. A number of new, enabling technologies will be released this year and others will achieve qualification.
Satellites are not launched with a long, dangling mains cable, but come with 'Batteries Included' and access to a large and hot wireless charging station 93 million miles away.
Satellites use solar arrays to generate power when exposed to direct radiation from the sun or the albedo reflection from Earth. During each orbit, energy is collected from the solar panels, stored in batteries and distributed to the spacecraft to power the various sub-systems.
When the satellite passes through the shadow of the Earth, the energy harvested from the solar cells is insufficient to supply the spacecraft's systems and power is taken from the on-board batteries during this eclipse. During each orbit, the batteries are replenished when the spacecraft is directly exposed to solar radiation and discharged when the satellite enters the Earth's shadow.
The on-board Electrical Power System (EPS) is architected to manage the charge-discharge cycle of the batteries and ensure that the budgeted power is generated and distributed reliably and efficiently to the various sub-systems during all stages of a mission. The satellite's orbit determines how significant a task this is, but low-earth orbit (LEO) is unique because of its high eclipse-to-orbit ratio in that a thirty-minute heavy discharge must then be replenished in a period of less than one hour. All of these factors impact the choice of the solar cells, batteries and the design of the EPS.
The diagram below shows a block diagram of a CubeSat's power collection, storage and distribution system with the solar panels represented by their simplified equivalent circuit of current sources.
With so little power available on a CubeSat, the major objective of the EPS is to maximise the power collected by the solar arrays and then distribute this reliably and efficiently.
The Orbit Average Power for a 1U satellite in a sun-synchronous orbit with maximum eclipse is typically 2 W per panel, increasing to 8 W per panel for a 3U-sized CubeSat using seven series cells per panel. The Battery Charge Regulators (BCR) collect energy from the solar cells at their maximum operating point, maintaining the power-bus voltage with a changing battery-voltage characteristic during re-charge.
The diagram below shows Surrey Satellite's Power System 150 EPS designed to manage the solar arrays and the batteries producing an unregulated +28-V bus to power a payload, platform avionics, heaters and deployment devices. This EPS is capable of receiving up to 1.4 kW of input power from the solar panels.
For comparison, the Alphabus platform that provides the power bus for the Alphasat, GEO, telecommunication payload is capable of supplying up to 22 kW to the spacecraft (this bird has two very large wings!).
To compound the problem, the latest space electronics consume and dissipate more and more power, placing greater demands on the distribution network exposing the limitations of a centralised approach. Previously, this was made worse by the fact that the space-electronics designer had to choose from a limited number of suppliers, but today, many companies offer innovative options for our industry. To supply the latest devices, alternate topologies such as a distributed power architecture generating an intermediate bus voltage, exploiting the benefits of point-of-load regulation together with a combination of linear and switching DC/DC converters, delivers the most efficient and reliable way of powering the latest avionics.
For example, several of the latest space-grade FPGAs have a core voltage of +1 V drawing up to 20 A. Some devices consume up to 7 W after configuration but before clocking and actual operation, and the supply rails have to be very tightly regulated, sequenced, and have a fast transient response. An FPGA technology trend is shown below, and powering such devices will only become more challenging (and interesting) in the future.
Every type of satellite requires space-grade power electronics to harvest the energy collected by the solar panels and supply the payload and platform avionics, the antenna and the on-board heaters. There are now a number of key suppliers offering qualified solar diodes, regulators and point-of-load technologies, and some of the primes have developed proprietary hybrid and integrated solutions that offer them certain technical, commercial and political advantages.
I will describe the operation of linear and switched-mode DC/DC converters for space applications in a future blog including SEE mitigation, loop stability and simulation. LDOs with limited unity-gain bandwidth will not reject nor filter a preceding buck or boost switching at megahertz speeds, and such interference can affect the revenue-generating capability of a telecommunications operator.
The design of the power distribution network for the GAIA mission was particularly challenging in terms of efficiency and isolating it from the sensitive scientific instruments. In case you missed the launch last month, this, together with an animation describing the operation of the CCD cameras can be viewed below:
I'd like to hear about your spacecraft power generation and distribution challenges and the improvements required to passives, switching transistors, regulators or topologies to achieve efficiencies approaching 100%.