AMSAT-MPPTDetailed Design Review
Dan CorrieroIan MacKenzieBrent SalmiBryce Salmi
Customer Needs
Customer Need # Description Comment/Status
CN1 Maximize energy transfer between solar panel and load
Efficiency
CN2 Meet environmental requirements Temperature, Radiation
CN3 Provide limited output voltage Customer supplied upper limit
CN4 Meet mechanical constraints Component size and form factor
CN5 Communicate status information with satellite IHU Health and status information
CN6 Recover from soft errors in software Watchdog, ECC
Engineering Specifications
Eng. Spec # Description Marginal
ValueIdeal Value Measure
S1 Maximum Power Output >7.24 7.24 Watts
S2 Input Voltage Max >28 28 Voltage
S3 Limited Output Voltage >3.3 4.1 Voltage
S4 MPPT Efficiency* >90% 90.00% Percentage
S5 MPPT Response Time <100 100 Milliseconds
S6 Maximum Operational Temperature >85 85 Celsius
S7 Minimum Operational Temperature <-40 -40 Celsius
S8 TID Radiation Expectation* 30 >30 KiloRad
S9 Component Height Restriction <8.1 5 Millimeter
S10 Layout Area Constraint 144.5 <144.5 Cm^2
S11 IHU Communications NA NA Pass/Fail
MPPT System Overview
Constant Voltage MPPT
Adjusting the solar panel voltage to the effective MPPT voltage over panel temperature eliminates a significant source of MPPT tracking error
* Image(s) reproduced from: IEEE:11904425, "A MPPT approach based on temperature measurements applied in PV systems"
Constant Voltage MPPT
Regulate solar panel voltage Solar cells only provide max power at MPP Voltage Output voltage not "regulated"! Fast "response" and stable
Typically fixed voltage Low tracking efficiency (~80%)
Panel Temp Scaling Increases tracking efficiency drastically! Must know solar cells in use
This is NOT a "Power Supply"
Regulate the input voltage Output voltage unregulated and can "swing"
from 3.3V-4.1V Limit output voltage if needed during low-load
conditions Will move "away" from maximum power point
Will work without Fox-2 battery MPPT operation does not depend on a battery to
track the maximum power point as many Cubesats pursue
System Detail Diagram
MPPT System Layout - Channels
Detailed Design Overview
Panel Voltage Control
System Control
Radiation Effects
Total Dose Ionization Degrades oxides (MOSFETs)
Threshold Voltage shifts Component parameters degrade Some materials/devices more susceptible Single Event Effects (SEE)
Transients Soft Errors Permanent Damage (Latch Up)
Radiation Mitigation
Use commercial components that have good radiation test data
Use flight tested components Avoid "vulnerable" circuits and materials
Used good engineering practices Will never be 100% guaranteed
UC2524A PWM ICFEATURES Complete PWM Power Control Circuitry Uncommitted outputs for Single-Ended or Push-Pull Applications Low Standby Current ... 8 mA Typical Interchangeable with SG1524, SG2524 and SG3524
UC2524 Radiation Data
This radiation data shows a consistent radiation tolerance
Provides reasonable assurance of radiation tolerance
Similar radiation data found for other components if possible
Temperature Sensing - RTD
Requirements Provide a voltage based on
panel temperature -60° C to +60° C MPPT accuracy dependent
on temperature accuracy Repeatable measurements
RTD - Benefits Large temperature ranges Very accurate Linear over specification
range
RTD - Drawbacks Positive temperature
coefficient
Temperature Sensing - RTD
RTD Driver
Constant current increases linearity and reduces self heating
RTD - Amplification and Scaling
Please see "Theory of Operation" document for details
RTD - System Integration
Provides a "scaled" 2.5V reference based on solar panel temperature and is scaled to match the expected Vmpp of the panels at a given temperature.
The DC-DC converter will attempt to "match" this reference with the scaled version of the solar panel voltage, thus achieving maximum power point.
RTD - Circuit Simulation (100 Ω)
*Vrtd scaled to match max power point panel voltage over temperature.
RTD - Percent Error
Preliminary Percent error does not include Op-Amp input offset, OP484 exhibits ~165uV MAX
Directly relates to effective tracking accuracy Would prefer any error to be on left side of MPP
DC-DC (Buck Converter)
Buck Converter
Two Inputs Solar Panel MOSFET Driver
PWM
Output to OR-ing Function System output is to
payload CCM Operation
Output voltage limit and voltage clamp provides minimum load
Buck converter chosen due to input voltage and output voltage range
Snubber circuits are DNP until board is populated. Transients are layout specific
18V Zener Diode to Protect Vgs
Buck Converter
Spec Value
VIN,Max 22.62V
VOUT,Max 4.1V
VOUT, Min 3.3V
fSW 300kHz
IL,Max 3A
22.62V is the worst case scenario input in full illumination at -60C
The low output voltage spec comes from the customer. 3.3V is the minimum voltage the payload can operate.
Buck Converter - Components
*at 300 kHz
Component Selection
Components chosen due to de-rating
Inductor current ripple at 300 kHz = 620 mAOutput voltage ripple at 300 kHz = 20 mV
Buck Converter - Component Selection Inductor Selection
Selected in order to stay out of saturation (handle max current) Low ESR Current Ripple Switching frequency (as fsw increases, so does L) Inductor-current ratio (LIR) balance: trade-off between current ripple and
response time. Size requirements
Diode Selection Schottky used to reduce losses Low VF Max forward current
MOSFET Selection Low Gate Charge High VGS rating Low RDS,ON
Output Capacitor Selection Low ESR Store enough charge to hold VOUT constant
Input Capacitor Selection Rated for open circuit voltage from panels Low ESR
MOSFET Driver
MOSFET Driver
Two Primary Functions: Acts as a level shifter Drives gate of MOSFET with a higher current for faster
transition times.
MOSFET Driver
Output from the UC2524A was modeled in simulation Proper inversion was derived from app note 2kΩ Resistance was used to model rise and fall times
of UC2524A Speed-up capacitor and diode if need be on output for
faster driving
MOSFET Driver
UC2524A - PWM Soft Start
UC2524A - PWM Soft Start
Slowly allows UC2524A PWM to "Ramp Up" Reduces EMI and is less "harsh" Limits inrush current
Only active during UC2524A power up and remains "out of circuit" otherwise i.e. Solar panels enter sunlight and UC2524 internal Vref powers on
Output Voltage Limit
"Soft" Output Voltage Limit
Output voltage limited to 4.1V Reduce PWM duty cycle or completely shut off
Vout scaled and compared to reference (Comparator/OP-AMP) MOSFET "pulls down" input voltage divider
Causes the UC2524A PWM controller to reduce PWM duty cycle (0-95%) Slows down this "reaction" to avoid oscillating PWM action and reduce EMI
Similar to UC2524A shutdown function but exhibits more modulation Basic compensation DNP'ed, further analysis and testing needed
Output Voltage Clamp
Output Voltage Clamp
4.3V output from the buck converter causes comparator to turn on the Darlington transistor to dissipate stored charge in the inductor.
Hysteresis may be needed and is DNP'd Compensation DNP components added
Output Diode "ORing"
Diode ORing
Allows current to flow only to the payload Does not allow other MPPTs or payload battery to send power the "wrong way" Ideal Diode drastically reduces power loss Schottky diode DNP for testing and backup (Radiation testing failure, etc)
Output Current Sense
Output Current Sense
Fixed gain of 100 V/V 3.3V operation
MSP430 3.3V power regulator provides power from battery bus Not dependent on a single solar panel When battery failure occurs it is still powered during sunlight
operation.
ADC Voltage Scaling & Protection
Resistive divider specific to ADC measurement LC filter is optional
0Ω "jumper" Diodes protect from over-voltage and transients
Powering the MPPT
Each MPPT regulates its own 5V supply for op-amps and other circuitry
Linear regulators are simple, low noise Low Drop Out - Turns on before UC2524A
UC2524 Stability
Star
tup
MPPT Startup and Shutdown
Voltage Device(s) Turning ON
0V OFF (MSP430 always on if BATTERY is good)
6.75V 5V Regulator (RTD Temp, PWM Limit, Voltage Clamp)
8.5V UC2524A
Vout Turn ON MSP430 if BATTERY Failure
Voltage Device(s) Turning OFF
8V UC2524A
Vout Turn OFF MSP430 if BATTERY Failure
6.75V 5V Regulator (RTD Temp, PWM Limit, Voltage Clamp)
0V N/A
Shut
dow
n
NASA Derating
Source NASA PD-ED-1201
NASA Derating
Source NASA PD-ED-1201
NASA Derating - Calculation40V P-CHANNEL ENHANCEMENT MODE MOSFET
NASA Derating of Buck Converter Switch
Derated Limits Operating Conditions
Every component must be checked Tedious but necessary Ensures components experience minimal
stress
Efficiency
All units in Watts except for power efficiency which is in fractional percent.
Selected Microcontroller
Texas Instruments MSP430FR5739
Features: 16-Bit RISC Architecture 16KB FRAM Nonvolatile Memory Built in Error Coding and Correction (ECC) 81.4 µA/MHz active current consumption 14-Channel 10-Bit Analog-to-Digital Converter (ADC) with
Internal Reference eUSCI with support for UART, I2C, and SPI 16-Bit Hardware Cyclic Redundancy Checker (CRC) 32-Bit Hardware Multiplier (MPY) Memory Protection Unit (MPU)
MPPT Health and Status Firmware
Primary functions: Measure analog inputs using an ADC Report measurements to IHU upon request
Secondary function: Take measures to protect firmware integrity from
random memory errors due to radiation events
Modular Software Structure
All major software functions are separated into modules:
Idea is that each module is relatively generic and independent of every other module, and are all
connected at a single point
I2C Module
ADC10 Module
CRC8 Module
SEU Protection
Module
Modular Software Structure
Each module consists of three parts:
Target-Specific Functions
Library Functions
InterfaceInterface between module and any code outside of module (e.g. Main)
Functions necessary to carry out module operations, but contain no hardware (target) dependence
Functions and interrupt service routines that are microcontroller (target) specific in nature
Modular Software Structure
Communications Message Flow
MPPTMCU
Communications Message DataField Size
(Bytes) Description
MSG_VER 2 Message control version
SW_BUILD 2 Software build version
BUS_DC_I 2 Output bus DC current
+X_PANEL_V 2 +X solar panel voltage raw value
-X_PANEL_V 2 -X solar panel voltage raw value
+Y_PANEL_V 2 +Y solar panel voltage raw value
-Y_PANEL_V 2 -Y solar panel voltage raw value
+X_PANEL_T 2 +X solar panel temperature raw value
-X_PANEL_T 2 -X solar panel temperature raw value
+Y_PANEL_T 2 +Y solar panel temperature raw value
-Y_PANEL_T 2 -Y solar panel temperature raw value
MPPT_T 2 MPPT microcontroller temperature raw value
CHECKSUM 1 CRC-8-CCITT Message checksum
MSP430 Support Circuitry
MSP430 Power - Latchup Protection
Dedicated 3.3V regulator Always powered when battery bus is powered (Battery/MPPT output) Low drop out
Latchup protection for Single Event Upset (SEE) radiation events 50mA current trip setting (2mA MAX expected) In use by AMSAT
JTAG Programming
JTAG MSP430 Programming Allows for programming and the
MSP430 on-board Connector and height restrictions
not part of PCB requirements Located "outside MPPT area" Not restricted to height
limitations since this is an engineering test component
I2C Communications I2C transmitted to AMSAT
connector Engineering test connector also on-
board for easy access
PCB - Overview
Test Plan
Circuit Bring-up
Bring up board in stages Regulators Op-amp signal conditioning Output conditioning UC2524A MOSFET Driver MSP430