Prototype solar panel
The limited amount of electrical power available was (and remains) one of the greatest challenges in designing our satellite. Even if the power subsystem were optimized for peak power generation little more than a watt and a half would be available.The Stensat Group's CubeSat power system was designed to be simple, modular, and robust. Simple, to make it possible to design and implement in the limited time available. Modular, to permit upgrades to the design if time permits. And robust to increase the likelihood of at least minimal satellite function for a limited period, even in the presence of multiple hardware failures.The satellite possess four separate power domains: the Solar/Battery domain, the Main Power domain, the Transmitter Power domain, and the Payload Power domain. The power domains all share a common grounding system spanning the entire satellite; the high side of each domain is isolated from the others via various switching mechanisms.
The Solar/Battery domain (SBD) consists of 36 GaAs solar cells in six strings of 6 plus a schottky blocking diode in series; one string of six is on each of the six sides of the satellite. The six blocking diode outputs are ganged together, and connected across a 1.5 amp-hour rechargeable lithium ion battery and a shunt regulator. The cathodes of the six strings of cells, the battery negative terminal, and the regulator ground terminal are connected to the common satellite ground.The shunt regulator (an LM431) is set to prevent the voltage on the SBD from exceeding 4.1 volts, the voltage at which the internal battery protection circuitry disconnects the battery from its external load/charger. Similarly, the protection circuitry in the battery disconnects it when the bus voltage drops below about 3.3 volts. So the SBD voltage will range between these two values, based on the satellite illumination and the battery charge state.The solar panels are extensively instrumented, with voltage, current, and temperature monitored independently on each panel. Additional circuitry on the panels provides an analog sun sensor for coarse attitude determination and drive circuitry for a magnetorquer coil for attitude control. On four of the six panels an antenna release driver is also present. All instrumentation on each panel is controlled/interrogated via a serial network connection; the CubeSat has two such networks (the XYZ+ and XYZ- networks) for redundancy.The battery charge umbilical connection is on one of the six solar panels; the battery charger applies a fixed voltage to the SBD, appearing essentially identical to an illuminated solar panel. Note that this offers the ability to charge the batteries with the kill switch active - a requirement for in-launcher charging.
Main Power Domain
The Main Power domain (MPD) is the power bus for the core satellite functions: the microcontroller, receiver, transmitter, and attitude determination/control system. It is connected to the SBD through the program-required kill switch, such that unless the switch is closed all satellite functions are powered down.In series with the kill switch between the SBD and the MPD is a MAX890 logic level power switch. This device is connected to a MAX706P watchdog timer chip, such that if the satellite microcontroller fails to interrogate the watchdog approximately once per second, the MPD is briefly (~1 second) disconnected from the SBD, then reconnected. This power cycle operation serves two purposes: in the case of a single event upset (SEU) in the microcontroller, it ensures a clean power-on reset; in the case of major system latchup, power is cycled and the latchup is cleared. Also, the satellite power may optionally be cycled before the transmission of every telemetry packet - causing a cold satellite boot every few minutes and clearing otherwise undetectable micro-latchups. (This power cycling operation is not expected to interfere with normal satellite function, as the satellite state is preserved in nonvolatile memory during the brief powerdown/reboot period).The Main Power domain also connects to the battery charge umbilical connector on one of the solar panels, such that when the umbilical cable is plugged in, the MPD is connected to the SBD. This permits operation of the processor, transmitter, etc. in the launch tube, but only if the external umbilical is connected.
Transmitter Power Domain
The Transmitter Power domain (TPD) is a very small power domain connected to the Main Power domain through another MAX890 switch; under software control, power to the transmitter may be connected or disconnected. This is primarily to reduce power consumption via duty cycle control of the transmitter (which is by far the largest consumer of power on the satellite).
Payload Power Domain
The Payload Power Domain (PPD) is also connected to the MPD via a software-controlled switch, and current limited such that an automatic turnoff occurs (and a microcontroller exception generated) if the payload section attempts to draw more than 250 mA. This is primarily to ensure that catastrophic payload failures don't kill the core satellite functions required to communicate with (and potentially repair) the satellite.
Power System Enhancements
The current power system is characterized by our engineering team as the 'Stupid Power System'. It's about as simple as it's possible for it to be and still function. It's primary benefit is its simplicity; it's primary drawback is its relatively poor use of incoming solar power.
Solar cells achieve maximum conversion efficiency at a particular load current, as a function of their instantaneous illumination state. We considered implementing a peak power point converter for each face of the satellite; and while the benefits were impressive (approximately twice the average power as the current system) the complexity was such that implementing the converter was tabled until we have the rest of the satellite integrated and operating.The current system offers circa 100 mW average power per panel over all loads and orientations, providing a total power from six panels of circa 600 mW. Anticipating a 33% eclipse/66% illuminated dark/light cycle, we need to allocate at least a third of this (200 mW) to battery charging. Thus we have available approximately 400 mW (110 mA at 3.7 volts) time average for all satellite functions. (The battery permits us to exceed this dissipation for some period of time, but our average must be <400 mW assuming the satellite operates at all times.)
Now, if our attitude determination/control system works we have the option of holding the satellite in a sun-synchronous orientation that maximizes solar power generation. We calculate that this will increase our available power to between 1.5 and 2 watts. Again, taking a third off the top for eclipse periods, our average available power rises to over a watt. So the most effective enhancement to our current power system appears to be a working attitude control system.