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

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

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

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 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

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 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 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

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

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

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 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: 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: 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

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

Modular Software Structure

Communications Message Flow MPPT MCU

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

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 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

PCB - Overview

Test Plan

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