Transcript
DN0023 Design note Charging a 12V lead acid battery in solar applications with the SPV1020 Designs from our labs describe tested circuit designs from ST labs which provide optimized solutions for specific applications. For more information or support, visit www.st.com
By Edward Friedman and Nicola Siciliano
Main components SPV1020
Interleaved DC-DC Boost Converter With Built-In MPPT Algorithm
MC33171
Low Power Single Bipolar Operational Amplifiers
TSC101A
High Side Current Sense Amplifier
SEA05L
Advanced Constant Voltage And Constant Current Controller With Very Efficient LED Pilot-Lamp Driver
2STR2160
Low Voltage Fast-Switching PNP Power Transistor
STPS3L25S
Low Drop Power Schottky Rectifier
Specification Circuit specification: Photovoltaic panel with Vmp=18V 12V Lead acid Battery Iout=2A
Introduction Off grid solar power applications store the harvested energy in batteries for later use. Controlling the charging of the batteries and harvesting the maximum available power from the solar array is a key requirement for the charging system. For fixed, non-mobile applications, rechargeable lead acid batteries provide a good power-to-weight ratio. They also have high surge current capability and are well suited for driving DC motors for applications such as pumps that usually require high inrush currents. Photovoltaic technology combined with rechargeable lead acid batteries is a good solution for fixed location solar energy systems.
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Photovoltaic basics and the need for MPPT The equivalent circuit of a solar cell is shown in Figure 1. The series and parallel resistors represent losses and shape the voltage-current output characteristic of the cell. Typical solar panels consist of many cells connected in series and parallel. Figure 1. Solar Cell
A typical solar panel’s output current vs. output voltage is shown by the red curve in Fig.2. The blue curve shows the panel power output as a function of output voltage. Figure 2. Typical solar panel’s output current vs. output voltage
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As seen from the blue curve, maximum power is extracted from the panel at a unique output voltage and current, shown as Vm and Im. Maximum power is extracted from the panel when the load resistance Vm/Im is equal to the source resistance of the panel. A maximum power point tracking (MPPT) circuit is needed to operate the panel at its maximum power point (MPP). The SPV1020 used in this application employs a Perturb and Observe algorithm shown in Figure 3 to determine the Vm and Im operating point. Figure 3. Perturb and Observe
Circuit description The solution presented here charges a 12V battery, using an 18V photovoltaic panel. As shown on the block diagram on Figure 4, the main sections are: SPV1020 boost converter with MPPT Differential to single ended voltage feedback (MC33171) Current sensing (TSC101A) Constant current constant voltage (CC/CV) controller (SEA05L) Boost Converter In a conventional boost converter configuration, the output voltage, Vout, is higher than the input voltage, Vin, with both voltages referenced to the system ground (GND). For most photovoltaic systems this is the negative terminal of the solar panel. In this example, an alternate topology is used where the positive terminal of the solar panel connected to the system ground (GND) and the negative terminal of the solar panel becomes a negative input rail with respect to ground. In this way, the boost converter is converting a negative input voltage to a positive output voltage, both referenced to the common ground. Because the panel is a floating source, it can be connected in this way. The advantage of this topology is that it allows the charging of a 12V battery from an 18V photovoltaic panel, without requiring an additional buck converter after the step up converter with MPPT to drop the boosted panel output voltage down to the battery voltage.
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Voltage Feedback In this topology, the conventional method of implementing voltage feedback through a resistor divider cannot be used. A differential sensing method is needed that would keep the voltage between Vin and Vout constant and still provide the feedback to the SPV1020 which is referenced to Vin-. This is accomplished using the MC33171 as shown in the schematic of Figure 5. The following equation shows the relationship between the differential voltage (V_OUT – Vbatt-) and VCTRL. 1.1
(V_OUT – Vbatt-)/R9 = VCTRL/(R10+R11)
R9 is the top resistor in the divider associated with MC33171 differential voltage sensing IC and the PNP transistor Q1. R10 and R11 are the bottom resistors. R11 allows a fine tuning of the differential (V_OUT – Vbatt-).
Current Sensing In a battery charger application, current monitoring is mandatory. In this alternative topology, this function can be implemented by using a high side current sensor such as the TSC101. This device works similarly to the voltage feedback previously mentioned. It differentially senses the drop across the sense resistor R7 and provides a voltage referenced to Vin- according to the following equation: 2.1
TSC101A OUT (pin1) = (IOUTx R7) x 20.
The gain of the TSC101A equals 20. The R12/R13 resistor divider at the output of the TSC101A reduces the input voltage to the SEA05L by a factor of 1/12 (ISNS). Therefore, the relationship between the drop across the sense resistor R16 and the ISNS signal is: 3.1
ISNS=((IOUTxR7)x20)/12
CC/CV The SEA05L manages the transition between constant current and constant voltage modes of operation according to the feedback coming from the voltage sensing and current sensing sections. The CC/CV engine is the appropriate solution in cases where the power available is greater than the power required. In a solar application, the power available changes with the insolation so there are conditions (such as early in the morning, late in the evening, cloudy days etc…) when the power available is less than the power required to charge the battery and a standard CC/CV controller will overload the panel.
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Thanks to the SPV1020 MPPT function, this condition is prevented because the power extracted from the panel will always be maximized in accordance with the input power available. When in this condition, the MPPT function prevails over the SEA05L, limiting the power out to the MPP (maximum power point) of the panel.
Figure 4. Block diagram Current sensing
TSC101
Reverse current blocking diode
Vbatt
OFF time
MC33171
Differential voltage sensing
+ Vout
Photovoltaic panel Vmp=18V
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Vin
ON time
-
SPV1020
SPV1020 Boost converter output Vout = Vin + Vbatt
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SEA05L
CC/CV controller
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Measurement results For test purposes, the solution was assembled using an SPV1020 evaluation board (STEVAL-ISV005V2) with some board rework, plus an external circuitry on a breadboard to take care of the dedicated voltage and current sensing. Figure 5 shows the equivalent schematic. The solution was set to provide a differential Voltage (V_OUT – Vbatt-) of 13.8V (VCTRL = 2.5V) and a current limit of 2A (ISNS = 50mV). For the voltage sensing, according to equation 1.1, we have: 1.1
(V_OUT – Vbatt-)/R9 = CTRL/(R10+R11)
13.8V/R9 = 2.5V/(R10+R11), R9/(R10+R11) = 5.52 R9=6K, (R10+R11)=1.087K, R10=1K, R11=250Ω (potentiometer) For the current limit, according to the equation 3.1, we have: 3.1
ISNS=((IOUTxR7)x20)/12
R7=15mΩ The solution’s behavior was consistent for a Vin range from 10V to 20V.
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1 2
PV+
1 2 3 4
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R2 110K
D1
C9 220pF
VIN-
L2
C18
VIN_SNS_M
C11 1uF
XCS SDI SCLK
19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36
L4
L3
L2
L1
VOUT VOUT
L4 L4
PGND PGND
L3 L3
VOUT VOUT
VIN_SNS_M
VCC VIN_SNS
VOUT VOUT
L2 L2
PGND PGND
L1 L1
VOUT VOUT
U1
VIN
VIN
VIN
VIN
18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1
Iin+ path Iin- path Iload+ path Iload- path
CB4 V_OUT
VIN-
VIN-
22nF
C8
1k
R8
VIN-
VIN-
C20
VIN-
VIN
2
5
220pF
C10
470nF
C7
VIN-
28.6K
R4
1M
R3
V_OUT
PZ_OUT
ISNS = 50mV @ Iout = 2A
4 Vm 3 Vp 1 Out
U2 TSC101A
STPS3L25S
D2
Current sensing
VIN-
100nF
VOUT_SNS
dnm
R5
0
R6
Vout_sns = 1V @Vout = 36V
Vreg
4.7uF
C6
4.7uF
C17
L4
CB4
L3
CB3
L2
CB2
L1
CB1
OSC_IN VOUT_SNS PZ_OUT
SDO
L4
L3
CB3
V_OUT
100nF
100nF
100nF
100nF
AGND
47uH
C4
47uH
C3
47uH
C2
47uH
C1
SPV1020 (PSSO36 package)
L1
4.7uF
CB2 4.7uF V_OUT
CB1
C16
V_OUT
4u7F
C5
VIN
High Current Path Legend
VIN-
D3
VIN_SNS
R1 2.2M
VIN-
XCS SDI SCLK SDO
STPS160U
VIN
VIN-
4 HEADER
J4
PV-
CONN FLEX 2
J1
SMB36CA
VIN-
R13 10R
ISNS
R12 110R
R7 15m Ohm
FASTON
7 1
4 5
U3
SEA05L C15 22nF
1 6 3
4.7nF
C14
Date:
Size A4
Title
Thursday , January 03, 2013
Document Number
SPV1020 12V battery charger
V_OUT
C19 100nF
D5
4.7uF
4.7uF
VIN-
C12
C13
Sheet
1
of
1
Rev 1
Vctrl = 2.5V @Vbatt = 13.8V Vbatt=(Vbatt+) - (Vbatt-)
VCTRL
VIN-
R11
VIN
1K
R10
Q1
VIN-
250R
2STR2160
6K
CC/CV controller
R15 220K
VIN-
5 4 2
U4
6
100nF
C21
VIN-
R14 22K
VIN-
MC33171
3
2
V_OUT
J3
R9
FASTON
1 2 3 4
Differential voltage sensing
J2
Vbatt+
SMB36CA
Vbatt1 2 3 4 -
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V R-sense = 30mV @ IOUT = 2A
Figure 5. Application schematic
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Support material
Related design support material STEVAL-ISV009V1: 300 W photovoltaic converter demonstration board based on the SPV1020 STEVAL-ISV005V2 : 240 W photovoltaic battery charger evaluation board based on the SPV1020 Documentation SPV1020: Interleaved DC-DC Boost Converter With Built-In MPPT Algorithm - datasheet MC33171 : Low Power Single Bipolar Operational Amplifiers - datasheet TSC101A: High side current sense amplifier - datasheet SEA05L: Advanced constant voltage and constant current controller with LED driver - datasheet 2STR2160: Low voltage fast-switching PNP power transistor - datasheet STPS3L25S: Low Drop Power Schottky Rectifier - datasheet AN3392: Designing with the SPV1020, an interleaved boost converter with MPPT algorithm AN3971: STEVAL-ISV005V2: solar battery charger for lead acid batteries based on the SPV1020 and SEA05
Revision history Date 18-Dec-2013
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Version 1
Changes Initial release
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