Changes

=== Power Regulation and Control ===

=== Power Regulation and Control ===

[[File:Vb811_1mainboardpowerschematic.png | right| thumb]]

[[File:V811 1mainboardpowerschematic.png | right| thumb]]

After power handoff, there are two parallel power paths in the Radio Board.  One is from the output of the diode chip to the 3.3V line, and one from the output of the diode chip to the 5V line.  The 3.3V line needs no control and will run automatically.  However, because of the bake-in of the Iridium module, and the battery current sensitivity at low temperatures, an actively controlled power system for the 5V line is needed to ensure batteries do not die from over-current.  This is because ValBal's high current devices run off of the 5V line (i.e. Iridium, motors), and can easily pull 2A at any given short burst of time, which could destroy the batteries at low temperatures (i.e. degrade their voltage so low as to not recover until they warm up again under no electrical load).  Therefore, instead of acquiring high power, short-burst energy from our cells, we will instead store this energy in a super-capacitor and draw the high burst energy from them instead, when we need it.  The parallel 5V line works as follows: the output of the diode chip (at 4.5V nominal assuming flight batteries) is connected directly to a super-capacitor charger.  This super-capacitor charger determines when, if, and how quickly to charge the super-capacitor (given inputs from the micro-controller).  This charger is required because charging the super-capacitor too quickly is just as dangerous as any other high current application, so we must regulate the super-capacitor charge rate and voltage (the voltage is hard set to 5V).  From a code perspective, there are two things needed to control this super-capacitor charger.  One, there is a super-capacitor charger enable pin, which is active HIGH.  The enable pin has a pulldown resistor, so upon startup the super-capacitor charger will remain off.  Second, the code must control the current limit of the super-capacitor.  This is done in the form of controlling a digital potentiometer that operates over I2C.  The mainboard uses the 100kOhm AD5246 digital potentiometer, with 128 different selectable resistances. Each resistance naturally corresponds to a different current limit.  The potentiometer is linked to the super-capacitor charging rate by the following equation: Current into super-capacitor = 10,000/Rpotentiometer, where everything is in standard units (i.e. amps, ohms).  This means that a charging current of 100mA or 0.1A corresponds to a potentiometer value of 100kOhm, or 100,000 ohms. The maximum current limit for the super-capacitor charger is 1500mA, and the minimum current limit is 100mA.  The super-capacitor itself is a 5V rated, 5F capacitor with a total maximum stored energy of 62.5 Joules.  It takes time to charge this super-capacitor, and the voltage across the super-capacitor will always be changing given the demands of our system.  For example, during charging at initialization, the voltage across the super-capacitor will start at 0V, and will increase to 5V slowly as per the super-capacitor charger current limit.  If our initial current limit is 100mA, for example, it will take 125 seconds to charge the super-capacitor to its fully rated 5V.  The voltage across the super-capacitor will increase quickly at first, and then roll off slowly to 5V.  Now, the since the super-capacitor voltage changes a lot, especially if we drain it with energy required for the Iridium modem, for example, it is not suited to produce a well regulated 5V line.  Therefore, this super-capacitor must be connected to a voltage regulator, in the form of a buck-boost switching converter to produce a clean 5V line.  This is the last step of the 5V power system.  The super-capacitor is connected to a TPS63070 buck-boost switching converter designed to regulate its output voltage to 5V.  The 5V line has an enable pin (active HIGH), which controls the TPS63070.  This is because it allows us the ability to shut off all our 5V systems if they are drawing too much energy, and also allows us to charge up the super-capacitor fully before turning on the 5V line that would drain it.  Temperature directly affects the maximum allowable charging current of the super-capacitor, because this current comes from the batteries.  At low temperatures, we cannot charge the super-capacitor as fast as we can charge it at high temperatures.  The buck-boost switching regulators have a minimum input voltage of 2V, meaning that the super-capacitor voltage must always be above this value in order for the 5V line to be stable.  In addition, the super-capacitor charging current must always be above the 5V line load current (i.e. current drawn from Iridium modem and motors) on average in order to assure that the super-capacitor will not be discharged over time.  This means that we cannot constantly charge the super-capacitor at 100mA if our 5V system current is 120mA.  While we do have current sensors on all the 5V line devices, there is an easier way to determine this: by measuring super-capacitor voltage.  If the voltage drops on average over a relatively long period of time, we know we aren’t charging it at the same rate that it is being discharged at.

After power handoff, there are two parallel power paths in the Radio Board.  One is from the output of the diode chip to the 3.3V line, and one from the output of the diode chip to the 5V line.  The 3.3V line needs no control and will run automatically.  However, because of the bake-in of the Iridium module, and the battery current sensitivity at low temperatures, an actively controlled power system for the 5V line is needed to ensure batteries do not die from over-current.  This is because ValBal's high current devices run off of the 5V line (i.e. Iridium, motors), and can easily pull 2A at any given short burst of time, which could destroy the batteries at low temperatures (i.e. degrade their voltage so low as to not recover until they warm up again under no electrical load).  Therefore, instead of acquiring high power, short-burst energy from our cells, we will instead store this energy in a super-capacitor and draw the high burst energy from them instead, when we need it.  The parallel 5V line works as follows: the output of the diode chip (at 4.5V nominal assuming flight batteries) is connected directly to a super-capacitor charger.  This super-capacitor charger determines when, if, and how quickly to charge the super-capacitor (given inputs from the micro-controller).  This charger is required because charging the super-capacitor too quickly is just as dangerous as any other high current application, so we must regulate the super-capacitor charge rate and voltage (the voltage is hard set to 5V).  From a code perspective, there are two things needed to control this super-capacitor charger.  One, there is a super-capacitor charger enable pin, which is active HIGH.  The enable pin has a pulldown resistor, so upon startup the super-capacitor charger will remain off.  Second, the code must control the current limit of the super-capacitor.  This is done in the form of controlling a digital potentiometer that operates over I2C.  The mainboard uses the 100kOhm AD5246 digital potentiometer, with 128 different selectable resistances. Each resistance naturally corresponds to a different current limit.  The potentiometer is linked to the super-capacitor charging rate by the following equation: Current into super-capacitor = 10,000/Rpotentiometer, where everything is in standard units (i.e. amps, ohms).  This means that a charging current of 100mA or 0.1A corresponds to a potentiometer value of 100kOhm, or 100,000 ohms. The maximum current limit for the super-capacitor charger is 1500mA, and the minimum current limit is 100mA.  The super-capacitor itself is a 5V rated, 5F capacitor with a total maximum stored energy of 62.5 Joules.  It takes time to charge this super-capacitor, and the voltage across the super-capacitor will always be changing given the demands of our system.  For example, during charging at initialization, the voltage across the super-capacitor will start at 0V, and will increase to 5V slowly as per the super-capacitor charger current limit.  If our initial current limit is 100mA, for example, it will take 125 seconds to charge the super-capacitor to its fully rated 5V.  The voltage across the super-capacitor will increase quickly at first, and then roll off slowly to 5V.  Now, the since the super-capacitor voltage changes a lot, especially if we drain it with energy required for the Iridium modem, for example, it is not suited to produce a well regulated 5V line.  Therefore, this super-capacitor must be connected to a voltage regulator, in the form of a buck-boost switching converter to produce a clean 5V line.  This is the last step of the 5V power system.  The super-capacitor is connected to a TPS63070 buck-boost switching converter designed to regulate its output voltage to 5V.  The 5V line has an enable pin (active HIGH), which controls the TPS63070.  This is because it allows us the ability to shut off all our 5V systems if they are drawing too much energy, and also allows us to charge up the super-capacitor fully before turning on the 5V line that would drain it.  Temperature directly affects the maximum allowable charging current of the super-capacitor, because this current comes from the batteries.  At low temperatures, we cannot charge the super-capacitor as fast as we can charge it at high temperatures.  The buck-boost switching regulators have a minimum input voltage of 2V, meaning that the super-capacitor voltage must always be above this value in order for the 5V line to be stable.  In addition, the super-capacitor charging current must always be above the 5V line load current (i.e. current drawn from Iridium modem and motors) on average in order to assure that the super-capacitor will not be discharged over time.  This means that we cannot constantly charge the super-capacitor at 100mA if our 5V system current is 120mA.  While we do have current sensors on all the 5V line devices, there is an easier way to determine this: by measuring super-capacitor voltage.  If the voltage drops on average over a relatively long period of time, we know we aren’t charging it at the same rate that it is being discharged at.