1. Detailed Design Review
AMSAT-MPPT
Detailed Design Review
Dan Corriero
Ian MacKenzie
Brent Salmi
Bryce Salmi

2. Customer Needs Description Comment/Status Customer Need # 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

3. Engineering Specifications
Eng. Spec #
Description
Marginal Value
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 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 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
Pass/Fail

4. MPPT System Overview

5. 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: , "A MPPT approach based on temperature measurements applied in PV systems"

6. 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

7. 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

8. System Detail Diagram

9. MPPT System Layout - Channels

10. Detailed Design Overview

11. Panel Voltage Control

12. System Control

13. 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)

14. 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

15. UC2524A PWM IC FEATURES Complete PWM Power Control Circuitry
Uncommitted outputs for Single-Ended or Push-Pull Applications
Low Standby Current mA Typical
Interchangeable with SG1524, SG2524 and SG3524

16. 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

17. 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

18. Temperature Sensing - RTD

19. RTD Driver
Constant current increases linearity and reduces self heating

20. RTD - Amplification and Scaling
Please see "Theory of Operation" document for details

21. 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.

22. RTD - Circuit Simulation (100 Ω)
*Vrtd scaled to match max power point panel voltage over temperature.

23. 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

24. DC-DC (Buck Converter)

25. 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

26. 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.

27. 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

28. 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
Store enough charge to hold VOUT constant
Input Capacitor Selection
Rated for open circuit voltage from panels

29. MOSFET Driver

30. MOSFET Driver Two Primary Functions: Acts as a level shifter
Drives gate of MOSFET with a higher current for faster transition times.

31. 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

32. MOSFET Driver

33. UC2524A - PWM Soft Start

34. 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

35. Output Voltage Limit

36. "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

37. Output Voltage Clamp

38. 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

39. Output Diode "ORing"

40. 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)

41. Output Current Sense

42. Output Current Sense Fixed gain of 100 V/V 3.3V operation
MSP V 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.

43. ADC Voltage Scaling & Protection
Resistive divider specific to ADC measurement
LC filter is optional
0Ω "jumper"
Diodes protect from over-voltage and transients

44. 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

45. UC2524 Stability

46. 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
Startup
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
Shutdown

47. NASA Derating
Source NASA PD-ED-1201

48. NASA Derating
Source NASA PD-ED-1201

49. NASA Derating - Calculation
40V P-CHANNEL ENHANCEMENT MODE MOSFET
NASA Derating of Buck Converter Switch
Every component must be checked
Tedious but necessary
Ensures components experience minimal stress
Derated Limits
Operating Conditions

50. Efficiency
All units in Watts except for power efficiency which is in fractional percent.

51. 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)

52. 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

53. Modular Software Structure
All major software functions are separated into modules:
I2C Module
ADC10 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

54. 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
Separating out target-specific functions allows for more hardware-independence. Should the MSP430FR5739 not be suitable (e.g. it fails radiation testing miserably), it can be easily ported to another MCU, even one from a different vendor!
Target-Specific Functions
Functions and interrupt service routines that are microcontroller (target) specific in nature

55. Modular Software Structure

56. Communications Message Flow
MPPT
MCU

57. Communications Message Data
Field
Size (Bytes)
Description
MSG_VER 2
Message control version
SW_BUILD
Software build version
BUS_DC_I
Output bus DC current
+X_PANEL_V
+X solar panel voltage raw value
-X_PANEL_V
-X solar panel voltage raw value
+Y_PANEL_V
+Y solar panel voltage raw value
-Y_PANEL_V
-Y solar panel voltage raw value
+X_PANEL_T
+X solar panel temperature raw value
-X_PANEL_T
-X solar panel temperature raw value
+Y_PANEL_T
+Y solar panel temperature raw value
-Y_PANEL_T
-Y solar panel temperature raw value
MPPT_T
MPPT microcontroller temperature raw value
CHECKSUM 1
CRC-8-CCITT Message checksum

58. MSP430 Support Circuitry

59. 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

60. JTAG Programming JTAG MSP430 Programming I2C Communications
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

61. PCB - Overview

62. Test Plan

63. Circuit Bring-up Bring up board in stages Regulators
Op-amp signal conditioning
Output conditioning
UC2524A
MOSFET Driver
MSP430