Power Distribution and Regulation

Power Distribution Scenarios

Once battery and solar panels are selected, the different scenarios for distribution of power generated from solar panels to battery and different loads should be considered. The different cases that can arise are:

• Solar panel generating power and battery discharged: Power generated can be used to cater to the loads and charge battery by the remaining power.
• Solar panel generating power and battery is charged: Power generated should be used to power the loads and rest of power can be dissipated in some resistor circuit.
• Solar panel not generating and battery is charged: Depending on the level of discharge the number of loads can be determined that are to be turned on.

Monitoring and Regulation

Now the generated power should be taken to the different loads depending on their needs i.e. voltage requirements etc. Voltage and current regulators can be used to provide the required voltage and current by the load i.e. converting the high or low voltage or current to the levels required by the load. A suitable way is to use a voltage regulator and current limiter in series so that both voltage and current are in the controlled ranges. Different mechanisms to do this with high efficiencies are present that can be selected on priority basis:

• Switching Regulators: Switching regulators rapidly switch a series element on and off. They can operate with both synchronous and non-synchronous switches (FETs). These devices store the input energy temporarily and then releasing that energy to the output at a different voltage level. The switch’s duty cycle sets the amount of charge transferred to the load.
• Linear Regulators: Linear regulators use linear, non-switching techniques to regulate the voltage output from the power supply. The regulator’s resistance varies according to the load and results in a constant output voltage.
  

Overall linear regulators are simple to use, low cost and less noisy as compared to switching regulators but switching regulators are more efficient and have a high voltage range of operation.

Next comes the task of monitoring and controlling the loads whether they are on or off as the status desired by the flight code. This can be done in following ways:

• Measuring the current across the load.
• Checking the status of regulator used for the load.
• Checking the status of protection circuit applied for the load.

While checking the load status there should be a mechanism to control the status of load if it is malfunctioning i.e. drawing more power than prescribed etc. The status can be checked by current measuring circuits or through the status of voltage and current limiter ICs used. Some of these ICs are:

• MAX4372 by Maxim electronics is used (in Pratham) to measure the current through a path.
• TPS2030/1 from Texas Instruments and FDC6324L from Fairchild Semiconductors Integrated circuits are used (in Pratham) as the current limiters for the loads.

Malfunctioning can be caused by changes in temperature and radiation in the surrounding of the circuit or component. Different measures can be taken to avoid these:

• Using a temperature measuring IC or resistor temperature detectors (RTDs) or a thermistor on the device susceptible to temperature changes and changing the device’s status according to the measurements given by them.
• Nowadays some Integrated Circuits (LM29xx series from Texas Instruments are voltage regulators with this quality) have a status pin regarding their temperature related range and accordingly give output which can be interpreted and the IC on or off status can be changed.
• Semiconductors are vulnerable to radiation present in space and they can be protected from radiation by:
  • Coating: Different types of coatings are present in the market for different altitudes of orbit.
  • Container made of special material for vulnerable components: Different metals produce brilliant protection from radiation doses that can be used to protect semiconductor devices.

For all the processing of information about currents and voltages from protection circuit and regulators, microcontrollers can be used that should be able to interpret all the data and act accordingly and turn the load ON or OFF.

Power distribution in different modes

For effective power management, different modes are defined for the satellite's operation. Which components will be ON and which will be OFF is dictated by these modes. Here is a brief description of these modes and their impact on the various components:

Detumbling Mode

This mode exists just after launch, when the satellite is till in bounds with other satellites on the space shuttle and spinning at high angular velocities. In this mode, charging of battery through the solar panels is not very significant and hence, several components, like downlink, certain sensors etc. are kept OFF. Only the components required to bring the angular rates within bounds are kept ON.

Nominal Mode

After the satellite has come out of the detumbling mode and is stable, different components can be switched ON as required, since sufficient power is available from the solar panels to charge the battery.

Emergency Mode

This mode is encountered when the battery voltage drops below a threshold just above the minimum required voltage of the battery (6.6V in case of Pratham, where the minimum required voltage of the battery was 6V). It is a state of emergency for the satellite, and all major components are switched OFF, except perhaps the beacon. The satellite could exist this mode only when the battery has sufficient charge (8V battery voltage in case of Pratham)

Safe Mode

This mode is encountered when the battery voltage drops below the minimum required. All components are shut down. The satellite came out of this mode (in case of Pratham), when the battery voltage rose above 7 V.

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