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AMSAT-MPPT
Detailed Design Review
Dan Corriero
Ian MacKenzie
Brent Salmi
Bryce 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
#
Marginal
Value
Description
Ideal
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
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Regulate solar panel voltage
o
Solar cells only provide max power at MPP Voltage
o
Output voltage not "regulated"!
o
Fast "response" and stable
Typically fixed voltage
o
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Low tracking efficiency (~80%)
Panel Temp Scaling
o
Increases tracking efficiency drastically!
This is NOT a "Power Supply"
•
Regulate the input voltage
o Output voltage unregulated and can "swing"
from 3.3V-4.1V
o Limit output voltage if needed during low-load
conditions
 Will move "away" from maximum power point
•
Will work without Fox-2 battery
o 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
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Total Dose Ionization
Degrades oxides (MOSFETs)
o
Threshold Voltage shifts
Component parameters degrade
Some materials/devices more susceptible
Single Event Effects (SEE)
o
o
o
Transients
Soft Errors
Permanent Damage (Latch Up)
Radiation Mitigation
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Use commercial components that have
good radiation test data
Use flight tested components
Avoid "vulnerable" circuits and materials
o
Used good engineering practices
Will never be 100% guaranteed
UC2524A PWM IC
FEATURES
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
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UC2524 Radiation Data
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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
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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
•
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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
•
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Preliminary Percent error does not include Op-Amp input
offset, OP484 exhibits ~165uV MAX
Directly relates to effective tracking accuracy
o Would prefer any error to be on left side of MPP
DC-DC (Buck Converter)
Buck Converter
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•
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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
Two Inputs
o Solar Panel
o MOSFET Driver
PWM
Output to OR-ing
Function
o System output is to
payload
CCM Operation
o Output voltage limit
and voltage clamp
provides minimum
load
Buck Converter
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•
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
Component Selection
*at 300 kHz
Components chosen due to de-rating
Inductor current ripple at 300 kHz = 620 mA
Output voltage ripple at 300 kHz = 20 mV
Buck Converter - Component Selection
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Inductor Selection
o Selected in order to stay out of saturation (handle max current)
o Low ESR
o Current Ripple
o Switching frequency (as fsw increases, so does L)
o Inductor-current ratio (LIR) balance: trade-off between current ripple and
response time.
o Size requirements
Diode Selection
o Schottky used to reduce losses
o Low VF
o Max forward current
MOSFET Selection
o Low Gate Charge
o High VGS rating
o Low RDS,ON
Output Capacitor Selection
o Low ESR
o Store enough charge to hold VOUT constant
Input Capacitor Selection
o Rated for open circuit voltage from panels
o Low ESR
MOSFET Driver
MOSFET Driver
•
Two Primary Functions:
o Acts as a level shifter
o Drives gate of MOSFET with a higher current for faster
transition times.
MOSFET Driver
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Output from the UC2524A was modeled in
simulation
o Proper inversion was derived from app note
o 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
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Slowly allows UC2524A PWM to "Ramp Up"
o Reduces EMI and is less "harsh"
o Limits inrush current
Only active during UC2524A power up and remains "out of circuit" otherwise
o i.e. Solar panels enter sunlight and UC2524 internal Vref powers on
Output Voltage Limit
"Soft" Output Voltage Limit
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Output voltage limited to 4.1V
o Reduce PWM duty cycle or completely shut off
Vout scaled and compared to reference (Comparator/OP-AMP)
MOSFET "pulls down" input voltage divider
o Causes the UC2524A PWM controller to reduce PWM duty cycle (0-95%)
o 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
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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
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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
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Fixed gain of 100 V/V
3.3V operation
o MSP430 3.3V power regulator provides power from battery bus
o Not dependent on a single solar panel
o When battery failure occurs it is still powered during sunlight
operation.
ADC Voltage Scaling & Protection
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Resistive divider specific to ADC measurement
LC filter is optional
o 0Ω "jumper"
Diodes protect from over-voltage and transients
Powering the MPPT
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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
MPPT Startup and Shutdown
Startup
Voltage
0V
Device(s) Turning ON
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
Shutdown
Voltage
8V
Vout Turn OFF
6.75V
0V
Device(s) Turning OFF
UC2524A
MSP430 if BATTERY Failure
5V Regulator (RTD Temp, PWM Limit, Voltage Clamp)
N/A
NASA Derating
Source NASA PD-ED-1201
NASA Derating
Source NASA PD-ED-1201
NASA Derating - Calculation
40V P-CHANNEL ENHANCEMENT MODE MOSFET
NASA Derating of Buck Converter Switch
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Every component must be checked
Tedious but necessary
Ensures components experience minimal
stress
Derated Limits
Operating Conditions
Efficiency
All units in Watts except for power
efficiency which is in fractional
percent.
Selected Microcontroller
Texas Instruments
MSP430FR5739
Features:
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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:
ADC10
Module
I2C
Module
CRC8
Module
SEU
Protection
Module
Idea is that each module is relatively generic and independent of every other module, and are all connected at
a single point
Modular Software Structure
Each module consists of three parts:
Interface
Interface between module and
any code outside of module (e.g.
Main)
Library
Functions
Functions necessary to carry out
module operations, but contain no
hardware (target) dependence
Target-Specific
Functions
Functions and interrupt service
routines that are microcontroller
(target) specific in nature
Modular Software Structure
Communications Message Flow
MPPT
MCU
Communications Message Data
Field
Size
(Bytes)
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
Description
MSP430 Support Circuitry
MSP430 Power - Latchup
Protection
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Dedicated 3.3V regulator
o Always powered when battery bus is powered (Battery/MPPT output)
o Low drop out
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Latchup protection for Single Event Upset (SEE) radiation events
o 50mA current trip setting (2mA MAX expected)
o In use by AMSAT
JTAG Programming
JTAG MSP430 Programming
• Allows for programming and the
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MSP430 on-board
Connector and height restrictions
not part of PCB requirements
o Located "outside MPPT area"
o Not restricted to height
limitations since this is an
engineering test component
I2C Communications
• I2C transmitted to AMSAT
•
connector
Engineering test connector also onboard for easy access
PCB - Overview
Test Plan
Circuit Bring-up
•
Bring up board in stages
o Regulators
o Op-amp signal conditioning
o Output conditioning
o UC2524A
o MOSFET Driver
o MSP430