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.
March 2013
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1/25
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Contents
AN3319
Contents
1
2
3
4
5
6
Application overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Boost switching application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
SPV1040 description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Application example . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Schematic and bill of material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
External component selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
6.1
Optional Schottky . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
7
Layout . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
Appendix A SPV1040 parallel and series connection . . . . . . . . . . . . . . . . . . . . . 20
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
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AN3319
List of figures
List of figures
Figure 1.
Figure 2.
Figure 3.
Figure 4.
Figure 5.
Figure 6.
Figure 7.
Figure 8.
Figure 9.
Figure 10.
Figure 11.
Figure 12.
Figure 13.
Figure 14.
Figure 15.
Figure 16.
Boost application schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
PV cell curve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Inductor current in continuous mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Inductor current in discontinuous mode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Typical application schematic using the SPV1040 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
SPV1040 equivalent circuit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
MPPT working principle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
SPV1040 internal block diagram . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
STEVAL-ISV006V2 top view . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
STEVAL-ISV006V2 bottom view . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
STEVAL-ISV006V2 schematic . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
STEVAL-ISV006V2 IOUT filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
STEVAL-ISV006V2 PCB top view . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
STEVAL-ISV006V2 PCB bottom view . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19
SPV1040 output parallel connection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
SPV1040 output series connection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
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Application overview
AN3319
1
Application overview
Figure 1
shows the typical architecture of a boost converter based solar battery charger:
Figure 1. Boost application schematic
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
2
, T
amb
=25 °C)
are:
•
•
•
•
V
OC
(open circuit voltage)
V
MP
(voltage at maximum power point)
I
SC
(short-circuit current)
I
MP
(current at maximum power point)
Figure 2
shows the typical characteristics of a PV cell:
Figure 2. PV cell curve
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.
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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
w
) is typically driven by a fixed frequency
square waveform generated by a PWM controller.
When S
w
is closed (t
on
) the inductor stores energy and its current increases with a slope
depending on the voltage across the inductor and its inductance value. During this time the
output voltage is sustained by C
OUT
and the diode does not allow any charge transfer from
the output to input stage.
When S
w
is open (t
off
), the current in the inductor is forced, flowing toward the output until
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
The energy stored in the inductor during t
on
is ideally equal to the energy released during
t
off
, therefore the relation between t
on
and t
off
can be written as follows:
t
on
D
= -------------------------
-
(
t
on
+
t
off
)
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)
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