Texas Instruments AN-2241 User manual

AN-2241 SM72445 Evaluation Board
User's Guide
Literature Number: SNVA649
May 2012

Contents
1 Introduction ........................................................................................................................ 5
2 System Overview ................................................................................................................ 5
3 SM72445 Evaluation module Design Specification .................................................................. 6
4 Schematic .......................................................................................................................... 9
5 Bill of Material ................................................................................................................... 11
2Table of Contents SNVA649–May 2012
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List of Figures
1 Evaluation Board Simplified Diagram .................................................................................... 5
2 Panel Mode Operation Using Converter's FET (“Boost” waveform is taken at drain of Q3, “Buck”
waveform was taken at drain of Q4.)..................................................................................... 6
3 Slew Rate Limit (between the two markers) and Soft OVP (after the second marker)............................ 7
4 Start-Up Pattern With Unloaded Output ................................................................................. 7
5 Load Connect and Stand-By Sequence After Start-Up................................................................ 8
6 Circuit Schematic ........................................................................................................... 9
7 Top Assembly Diagram................................................................................................... 10
8 Bottom Assembly Diagram............................................................................................... 11
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User's Guide
SNVA649–May 2012
AN-2241 SM72445 Evaluation Board
The SolarMagic™ SM72445 Evaluation Board is designed to demonstrate the operation and the
capabilities of the SM72445 MPPT controller. The board consists of a high efficiency, four-switch Buck-
Boost DC/DC converter and a bidirectional FET based switch designed to bypass the DC/DC converter for
cases when DC/DC conversion is not needed and during failure of the DC/DC converter (over-current
protection, over-temperature, and so forth). The board also includes auxiliary circuits such as driver,
current sensor, temperature sensor and additional OVP circuitry.
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DC/DC converter
Panel Mode
Bi-directional switch
SM72445
SM72295
Curr.
Sense amp.
Comparator
Temp. sensor
PV
Panel
LOAD
PWM signal
Gate Drive
Current
sense
Input
Current
sense
Input voltage
sense
Output voltage
sense
Protection trigger
Out current sense
Protection
trigger
www.ti.com Introduction
1 Introduction
The SM72445 is designed to maximize the energy production of a photovoltaic panel through the control
of a four switch buck-boost converter as well as an optional FET bi-directional switch (two FETs) designed
to maximize the efficiency at 1:1 conversion ratios. The maximum voltage and power transferred depend
on the component choice (driver, passive components, FET, and so forth). The SM72445 performs
optimization of the operating point of the solar panel through a high speed Perturb&Observe algorithm.
The input voltage and current are sampled at a high rate and digitally filtered while the algorithm updates
the output PWM waveform at a rate up to 1.2ms/step. The PWM waveforms include dead-time and can be
directly fed to the drivers controlling the DC/DC converter’s switches. The SM72445 is designed to
operate at 110kHz, 137kHz and 220kHz depending on the operating mode set. The SM72445 includes
and I2C slave module with 8 address settings to communicate monitoring information and to allow remote
shut-down and configuration modifications.
The SolarMagic™ SM72445 Evaluation Board is based off of the SM3320-1A1 power optimizer, and
shares all of its best-in-class power optimization features. This includes its ability to mitigate real-world
mismatch, its 99.5% peak efficiency, and its Panel-Mode operation. In addition, the same ultra-low profile
form factor of the original SM3320-1A1 is maintained so that it can be integrated into the same junction
box designs.
This Evaluation module is fully operational and can be used with panels from 15V up to 50V (absolute)
operating voltage and up to 300W of power. Proper heat-sinking of the Power FETs must be achieved for
power levels above 50W at room temperature.
The absolute maximum output voltage is 50V. The maximum operating current is 11A
2 System Overview
Figure 1 depicts how the SM72445 design would be implemented in its intended application. In this
example, the SM72445 senses the input and output voltages and currents, filters them digitally and
calculates power. It then issues four PWM pulses to the gate drivers to control the converter's switches.
The SM72445 also operates an additional switch (called Panel-Mode or PM switch) to optimize efficiency
when operating close to a 1:1 conversion ratio. Optionally, the switch can be omitted for reduced cost. The
SM72445 then utilizes the converter at a 1:1 conversion ratio and increases efficiency by reducing the
switching frequency of the converter
Figure 6 shows the detailed schematic with component references.
Figure 1. Evaluation Board Simplified Diagram
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Buck
Boost
SM72445 Evaluation module Design Specification www.ti.com
3 SM72445 Evaluation module
Design Specification
Symbol Parameter Min Typ Max
VMPP PV Module MPP Voltage 15 Vdc 40Vdc
PMPP PV Module Power 10 W 350 W
VOC PV Module Open Circuit Voltage 50 Vdc
ISC PV Module Short-Circuit Current 11A
VOUT Output Voltage 0 Vdc 43 Vdc
IOUT Output Current 12.5A
OVP Overvoltage Protection Threshold 45V
OTP Overtemperature Protection Threshold 125oC
MPPEFF MPP Efficiency 98.5%
PMEFF Panel-Mode Efficiency 99.5%
CONFIGURATION MODES
The SM72445 is configured through I2C communication or through resistor settings on pin A0, A2, A4 and
A6. The default configuration setting on the board is:
Pin name Pin Voltage Description
A0 3.0V “Soft” output voltage limit of 40V (AVOUT=3V)
A2 0V 220kHz operation, uses dedicated FET switch for higher efficiency
A4 5.0V Internal current protection not used
A6 1.56V Long slew rate limit, 60s in a 1:1 conversion ratio at start-up before enabling
optimization
Modifications to the configuration can be performed by changing the resistor settings. The change in
configuration will only happen at start-up and during reset of the IC.
“PANEL MODE” OPERATION
The board is configured to use “Panel Mode” FETs to reach the highest possible efficiency at a 1:1
conversion ratio. The SM72445 will disconnect the DC/DC converter and turn on this switch whenever it
detects that the panel is matched to the load. The detail of this operation can be found in the SM72445
datasheet. Alternatively, the SM72445 can be configured to run the DC/DC converter in a Panel-Mode
behavior instead, driving the high-side FETs of the converter in a mostly ONstate (Figure 2) and achieving
the same function as the dedicated Panel-Mode FETs. This option optimizes the component count and the
cost of the solution but slightly reduces the efficiency of the system in the band close to a 1:1 conversion
ratio. To configure the board in this mode, the resistor couple R29 and R39 can be changed to modify the
voltage at pin A2. In addition, T1 should be removed. Check the SM72445 datasheet for additional details
on the different configuration settings.
Figure 2. Panel Mode Operation Using Converter's FET (“Boost” waveform is taken at drain of Q3,
“Buck” waveform was taken at drain of Q4.)
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www.ti.com SM72445 Evaluation module Design Specification
“SOFT” OUTPUT VOLTAGE LIMIT
The soft output voltage limit of “Soft OVP” is designed to prevent the output voltage from reaching
dangerous level when the system is running the converter in boost mode. This mechanism works by
reducing the duty cycle of the converter when the voltage at pin AVOUT is increasing above the voltage at
pin A0.
SLEW RATE LIMIT
The slew rate limit is designed to limit the rate of rise of the output voltage when and only when the
converter is running in boost mode. The rise of the duty cycle will be limited to ensure that the output
voltage will rise in a controlled manner, especially during transients such as when the load is reduced or
gets removed (before the SM72445 returns in stand-by mode). To test the slew rate limit, start-up the
system with a current load at 1.5A and with an input that has a maximum power point above 3A. The
output voltage will increase very rapidly until it reaches the input voltage. The output voltage will then
increase slowly above the input voltage as limited by the slew rate setting programmed.
Figure 3. Slew Rate Limit (between the two markers) and Soft OVP (after the second marker)
STARTUP
When starting up at no load, the SM72445 will increase the duty cycle until a 1:1 conversion ratio is
reached. The system will then enter a stand-by mode and wait for the detection of power by sensing the
output current. When the voltage on AIOUT rises above the internal threshold, the system will wait for a
typical period of 64 seconds and exit the stand-by mode. This is to allow the proper start-up of a potential
inverter connected to the string of panels in which the SM72445 based optimizer would be inserted. Upon
exiting stand-by mode, the system will enter “PM mode” (see SM72445 datasheet for further explanation).
MPPT will be enabled if power variation is detected. If no power variation is detected such as when the
system is connected to a DC Electronic load for example, MPPT will be enabled only after the “PM mode”
timer expires after an additional period of 64 seconds.
Figure 4. Start-Up Pattern With Unloaded Output
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Figure 5. Load Connect and Stand-By Sequence After Start-Up
When a load is already present on the output of the board, the SM72445 will increase the duty cycle and
immediately enable MPPT operation when the voltages on AIOUT and AIIN rise above the internal
threshold.
INTERNAL CURRENT PROTECTION
If the voltage at AIOUT rises above the value set in A4, the system will reset. If AIOUT is set at or above
VDDA, this feature will be de-activated. The default setting for this board has this feature de-activated.
Current protection is performed using an external comparator
BOARD CURRENT PROTECTION
The board features a current protection circuit with U11A activating the PM pin of the SM72445 and
forcing the PM mode bypass switch to turn on (or the bridge to switch to 1:1 operation if the board is
reconfigured) when the output current reaches 13.5A. The output current will then be limited by the input
(solar panel) current. It is important to verify that the maximum solar panel current does not exceed the
rating of the components.
TEMPERATURE PROTECTION
U2 and U3 are strategically placed near the highest source of heat in the system (Q1 and Q4). When the
temperature reaches the tripping point (120 C), the ICs will activate the PM mode of the SM72445 by
forcing the PM pin low.
OVER-VOLTAGE PROTECTION
U11B forces the SM72445 to reset if the output voltage rises above 44V.
PANEL MODE SWITCH
Q5 and Q6 form a bi-directional switch controlled by the SM72445 and driven by U13 through T1. The
SM72445 supplies a 440kHz square waveform to turn on the switch.
GATE DRIVER
U7 is a quad 3A gate driver (two high and two low). It contains high-side operational amplifiers and buffers
used to sense the input current for MPPT purposes.
HEATSINKING
SM72445 evaluation board does not come with a heatsink. Therefore, in order to run the evaluation board
at elevated power ratings, an appropriate heatsink should be added on Q1, Q2, Q3 and Q4 as well as
diode D1. Care must be taken to prevent electrical contact between the drains of the MOSFETs in the
process of proper heatsinking. At elevated power operation please note the increase in temperature
across these semiconductor devices.
TEST SETUP
To perform an evaluation on the SM72445 evaluation board, it is suggested that the user connect the
input to a SAS (Solar Array Simulator) and the output to a load bank, preferably in Current mode (Voltage
mode sometimes causes regulation issues with some electronic load when used with the evaluation
board). The following sequence can be used to verify the operation of the board:
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•Connect S.A.S. to input.
•Connect Electronic load to output.
•Turn on SAS Verify proper startup pattern (see Figure 4).
•Set Electronic load to 2A and turn on Electronic load.
•Wait two minutes and verify that SAS is operating at Maximum Power Point. If output voltage is
close to 40V, increase Electronic load current.
•Vary Electronic load current and verify that the SAS is operating at Maximum Power Point in
step-up (lower load current) and step-down (higher load current) operation.
4 Schematic
Figure 6. Circuit Schematic
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Schematic www.ti.com
Figure 7. Top Assembly Diagram
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