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A project log for PQ60 - EPS

An Electronic Power System in the PocketQube 60 femtosatellite form factor

Pinski1Pinski1 07/06/2015 at 21:060 Comments

So a few months back the PQ60 Standard group published a new PQ60 standard that redefined the board sizes and pin maps. This of course meant I needed to redesign my board so as to maintain comparability.

This time, with specific figures for how much each rail needed to be able to source I could design my EPS to fully met the specification. Of course to control the 6 switched power rails and collate the telemetry I'm adding a micro-controller to the EPS. I'm still somewhat against having separate switch-able rails. I feel that a satellite won't need the 6 switch-able rails and they become wasted pin space. On the other end of the scale the satellite needs more switch-able rails and so needs to come up with a creative way of sourcing new ones.

Regardless I now need to fit 8 current limiting switches and have 6 of them individually controllable. To fit these 8 switches with 2 levels of current limiting I've chosen the TPS2553. This chip is available in a tiny surface mount package and the current limit can be set with an external resistor.

I'm keeping the battery charger chip (MAX8606) the same but I've found a TI chip (BQ24072)

that has most of the same functions. I'm also keeping the Boost-Buck converter (TPS63001) the same as it can supply the required current.

A big change I've made is to move the Maximum Power Point Tracking (MPPT) circuitry to the top of the board. I decided that it would be too hard to solder the complex circuitry on both sides of the PCB. To do this I also combined two of the channels as on a 6 sided satellite only 3 faces can ever be illuminated at once so by connecting opposite faces in parallel more efficient use of the MPPTs can be made.

The biggest change is to add a micro-controller to the EPS. This causes a lot more work as now code must be written, and tested, and verified etc. But it does allow me to build in better features improving the safety and ease of interfacing. I've chosen the Feescale KL05 as it is cost effective, I have some experience of using it and it is mbed compatible.

The next step is to actually order the boards & parts and get it made up. Then I will need to start testing it!

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