Tuesday, 15 April 2014

PocketQube Power!

I thought the it was about time that I put something out there about the power subsystem for PocketQubes. As I've said previously, there aren't many COTS products available specifically for the PocketQube form factor. So to date, this means that everything from solar cells to EPS ( Electrical Power Systems) needs to be custom made. Cubesats are a different story. There are a myriad of vendors offering EPS boards with many different options, depending on the intended purpose and cubesat size. Clyde Space have a large range, starting from $3500.
Then there's solar cells. Once again, plenty of vendors. A common DIY cell is the Spectrolab TASC cell. This has been used in many Cubesats, including PhoneSat. The problem is that these aren't readily available outside USA

So back to a PocketQube sized budget. What are the options?

I'm sure someone will say "It depends on a range of factors........" but I like to simplify. The surface area of PocketQubes is fairly small, which seriously limits the amount of solar power that can be harnessed. So we're looking at fairly low power scenarios.

Take $50Sat for instance. The EPS board was built from regular COTS components. It is based on four LTC3105 max power point controllers, one for each solar panel, and some current measuring components etc. The solar panels themselves are TASC cells, with 6 on each of the 4 sides of the satellite. One of the other smaller sides is used by the antenna, and the other left vacant. Now I'm assuming that they are using pairs of the cells in series to generate approx 4.4V ( I could be wrong here, so don't take this as gospel), and 3 pairs in parallel. Looking at the datasheet, and based on the Max Power Point values, we're looking at 84mA, which totals approx 370mW per side. ( and yes, that's not based on the irradiance levels expected in space. Most datasheets use the terrestrial measure of 1000W/m2)

The place to look for power components here is the "Energy Harvesting" categories of the various chip makers. This is where you have to trawl through datasheets. Here's some that I've come across.

The LTC3105 is from Linear Technology. They have a few IC's in this area, but the LTC3105 seems to be a good match for PQ's in terms of input voltage range, and power efficiency. It has MPPT charging for maximum charging efficiency.
Then there's the SPV1050 from STMicroelectronics. It is also an MPPT design. It does seem limited in battery charging current though, so it may not be suitable.
Texas Instruments have a few in this area. The BQ25505 looks promising, and has a cool feature in that it can switch to a primary ( non rechargeable battery) if the rechargeable one is fully drained.
Spansion have MB39C831 . It is a little larger package than the others listed.

Ideally, you want to design the system to maximise efficiency. The diminutive size of PocketQubes

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