ST AN3319 Application note
AN3319
Application note
STEVAL-ISV006V2: solar battery charger
using the SPV1040
Introduction
The SPV1040 is a high efficiency, low power and low voltage DC-DC converter that provides
a single output voltage up to 5.2 V. Startup is guaranteed at 0.3 V and the device operates
down to 0.45 V when coming out from MPPT mode. It is a 100 kHz fixed frequency PWM
step-up (or boost) converter able to maximize the energy generated by few solar cells
(polycrystalline or amorphous). The duty cycle is controlled by an embedded unit running an
MPPT algorithm with the goal of maximizing the power generated from the panel by
continuously tracking its output voltage and current.
The SPV1040 guarantees the safety of overall application and of converter itself by stopping
the PWM switching in the case of an overcurrent or overtemperature condition.
The IC integrates a 120 mΩ N-channel MOSFET power switch and a 140 mΩ P-channel
MOSFET synchronous rectifier.
November 2011 Doc ID 18265 Rev 7 1/25
www.st.com
Contents AN3319
Contents
1 Application overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2 Boost switching application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3 SPV1040 description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4 Application example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
5 Schematic and bill of material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
6 External component selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
6.1 Optional Schottky . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
7 Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Appendix A SPV1040 parallel and series connection . . . . . . . . . . . . . . . . . . . . . 20
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
2/25 Doc ID 18265 Rev 7
AN3319 List of figures
List of figures
Figure 1. Boost application schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Figure 2. PV cell curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Figure 3. Inductor current in continuous mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Figure 4. Inductor current in discontinuous mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Figure 5. Typical application schematic using the SPV1040 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Figure 6. SPV1040 equivalent circuit. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Figure 7. MPPT working principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Figure 8. SPV1040 internal block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Figure 9. STEVAL-ISV006V2 top view . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Figure 10. STEVAL-ISV006V2 bottom view . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Figure 11. STEVAL-ISV006V2 schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Figure 12. STEVAL-ISV006V2 IOUT filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Figure 13. STEVAL-ISV006V2 PCB top view . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Figure 14. STEVAL-ISV006V2 PCB bottom view . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Figure 15. SPV1040 output parallel connection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
Figure 16. SPV1040 output series connection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Doc ID 18265 Rev 7 3/25
Application overview AN3319
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1 Application overview
Figure 1 shows the typical architecture of a boost converter based solar battery charger:
Figure 1. Boost application schematic
!-V
The SPV1040 adapts the characteristics of load to those of panel. In fact, a PV panel is
made up of a series of PV cells. Each PV cell provides voltage and current which depend on
the PV cell size, on its technology, and on the light irradiation power. The main electrical
parameters of a PV panel (typically provided at light irradiation of 1000 W/m
are:
● V
● V
● I
● I
(open circuit voltage)
OC
(voltage at maximum power point)
MP
(short-circuit current)
SC
(current at maximum power point)
MP
Figure 2 shows the typical characteristics of a PV cell:
2
, T
=25 °C)
amb
Figure 2. PV cell curve
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MPP (maximum power point) is the working point of the PV cell at which the product of the
extracted voltage and current provides the maximum power.
4/25 Doc ID 18265 Rev 7
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!-V
AN3319 Boost switching application
2 Boost switching application
A step-up (or boost) converter is a switching DC-DC converter able to generate an output
voltage higher than (or at least equal to) the input voltage.
Referring to Figure 1, the switching element (S
) is typically driven by a fixed frequency
w
square waveform generated by a PWM controller.
When S
is closed (ton) the inductor stores energy and its current increases with a slope
w
depending on the voltage across the inductor and its inductance value. During this time the
output voltage is sustained by C
and the diode does not allow any charge transfer from
OUT
the output to input stage.
When S
is open (t
w
), the current in the inductor is forced, flowing toward the output until
off
voltage at the input is higher than the output voltage. During this phase the current in the
inductor decreases while the output voltage increases.
Figure 3 shows the behavior of inductor current.
Figure 3. Inductor current in continuous mode
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/[SN
6
).
,
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RQ
(
RQ
6
6
/54
).
,
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RII
(
RII
W
RQ
W
RII
The energy stored in the inductor during t
t
, therefore the relation between ton and t
off
D
is ideally equal to the energy released during
on
can be written as follows:
off
t
on
--------------------------=
tont
+()
off
WLPH
!-V
where “D” is the duty cycle of the square waveform driving the switching element.
Boost applications can work in two different modes depending on the minimum inductor
current within the switching period, that is if it is not null or null respectively:
● Continuous mode (CM)
● Discontinuous mode (DCM)
Doc ID 18265 Rev 7 5/25
Boost switching application AN3319
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Figure 4. Inductor current in discontinuous mode
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6
).
,
%
ON
%
OFF
6
/54
6
).
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T
ON
T
OFF
T
IDLE
Obviously the efficiency is normally higher in CM.
Inductance and switching frequency (F
) impact the working mode. In fact, in order to have
sw
the system working in CM, the rule below should be followed:
L
V
OUT
--------------
P
IN
2
D1D–()⋅()
---------------------------------- -
⋅>
2FSW⋅
2
According to the above, L is minimum for D = 50 %.
TIME
!-V
6/25 Doc ID 18265 Rev 7
AN3319 SPV1040 description
MPP
SET
,
,
5
9
9
3 SPV1040 description
The following is a quick overview of SPV1040 functions, features, and operating modes.
Figure 5. Typical application schematic using the SPV1040
L
Lx
V
PV
R
C
IN
XSHUT
3
GND
-
C
INsns
The SPV1040 acts as an impedance adapter between the input source and output load
which is:
Figure 6. SPV1040 equivalent circuit
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3DQHO
V
OUT
I
CTRL_PLUS
I
CTRL_MINUS
V
CTRL
C
OUTsns
639
JP9
'&
=
*/
R
S
R
F1
R
C
F2
F
065
&
065
R
R
065
1
D
OUT
2
V
C
OUT
AM06700v1
065
BATT
!-V
Through the MPPT algorithm, it sets up the DC working point properly by guaranteeing
Z
= Zm (assuming Zm is the impedance of the supply source). In this way, the power
IN
extracted from the supply source (P
= VIN * IIN) is maximum (PM = VM * IM).
IN
The voltage-current curve shows all the available working points of the PV panel at a given
solar irradiation. The voltage-power curve is derived from the voltage-current curve by
plotting the product V*I for each voltage generated.
Doc ID 18265 Rev 7 7/25
SPV1040 description AN3319
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R
START SIGNAL
ZERO CROSSING
g
-
g
Figure 7. MPPT working principle
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Figure 7 shows the logical sequence followed by the device which proceeds for successive
approximations in the search for the MPP. This method is called “Perturb and Observe”. The
diagram shows that a comparison is made between the digital value of the power Pn
generated by the solar cells and sampled at instant n, and the value acquired at the previous
sampling period Pn-1. This allows the MPPT algorithm to determine the sign of duty cycle
and to increment or decrement it by a predefined amount. In particular, the direction of
adjustment (increment or decrement of duty cycle) remains unchanged until condition
Pn≥Pn-1 occurs, that is, for as long as it registers an increase of the instantaneous power
extracted from the cells string. On the contrary, when it registers a decrease of the power
Pn<Pn-1, the sign of duty cycle adjustment is inverted.
In the meantime, SPV1040 sets its own duty cycle according to the MPPT algorithm, other
controls are simultaneously executed in order to guarantee complete application safety.
These controls are mainly implemented by integrated voltage comparators whose
thresholds are properly set.
Figure 8. SPV1040 internal block diagram
Lx
DETECTOR
PWM
CONTROL
DRIVERS
IoutReg
Vin Reg
VoutReg
V
MPP-REF
CLOCK
DAC CODE
Vbg
OVER CURRENT
OVER TEMPERATURE
REVERSE POLARITY
DIGITAL
CORE
XSHUT
MPP-SET
GND
ANALOG BLOCK
Burst Ref
CLOCK
MPP BLOCK
BURST MODE
8/25 Doc ID 18265 Rev 7
V
OUT
-
+
I
CTRL_PLUS
+
-
I
CTRL_MINUS
V
MPP-REF
+
MPP-SET
-
V
+
Vb
CTRL
AM06703v1
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