As being restricted by factors such as cost, efficiency and size, the development of high-power solar
LED street light controller is faced with plenty of difficulties. In case that a structure of two independent
DC/DC is applied as the main circuit, it has to face problems such as large size and high cost; in case of
applying the bidirectional BUCK/BOOST circuit, it requires change-over switches to control the solar panel
and LED light. As being restricted by withstanding voltage, on-resistance and cost, a PMOS device cannot be
used as the change-over switch of solar panel and LED light. However, when being used as a change-over
switch, an NMOS device must apply the low-side mode under which the negative ends of the mentioned three
parts are cut off. In the condition of applying the low-side mode, a differential circuit must be used to detect the
voltage of the solar panel. Furthermore, in order to make sure batteries can still be regularly charged after
wearing out in daylight, the controller must be supplied with power through a dual power supply circuit that can
obtain power from both the solar panel and the battery. The demander has a requirement on extremely low
standby power consumption of the product, and thus it is necessary to minimize the circuit that is live while
working in standby mode.
Methods:
The bidirectional BUCK/BOOST circuit structure is applied to the main circuit to realize a higher
change-over efficiency while giving considerations to both cost and size. The NMOS device, model
IRFB4410ZPBF, with a price of about three yuan, is used as the switching device, and the low-side mode is
applied, that is the switches inserted in between negative end of the solar panel or LED light and that of the
DC/DC circuit. The low-cost rail-to-rail operational amplifier LM358 is used to form a differential
amplification circuit for detecting the voltage of the solar panel. A XL1509-12E1 chip that only costs 0.88
yuan/pc is selected as the main change-over chip for the power supply, which has realized the highly-efficient
and low-cost change-over of the power supply. A dual power supply circuit and a step-down protective circuit
are designed for the XL1509-12E1 change-over chip. By comparing solar panel voltage with battery voltage,
the solar panel booting circuit is realized. Only when solar panel voltage is higher than battery voltage, does the
system program start to power it up for running, so that the outage of most of the circuits of the system under
standby mode does not consume energy. Furthermore, the solar panel voltage detecting circuit, the solar panel
booting circuit and several return difference functions are corrected during system debugging.
Results:
The circuit board of the entire controller features small size, low cost and high efficiency. It measures
about 100*62*18mm in size, costs about 60 yuan, and the charge/discharge change-over efficiency reaches up
to over 95%. The controller has many functions: it is capable of operating within a large scope, in which, solar
panel voltage is subject to 15~50V, LED light voltage is subject to 15~60V, battery voltage is subject to
10~35V and battery-end charge/discharge current is 10A; it is capable of adapting to monocrystalline
silicon/multicrystalline silicon/thin-film and many other kinds of solar panels, as well as lithium/lead-acid and
many other kinds of batteries; it is capable of detecting the conversion of day and night, automatically
controlling charging and discharging and automatically making adaptive adjustment according to seasonal
variations; the current to be consumed during standby will be maintained below 3mA, and thus the power
consumption is extremely low.
Conclusion:
By selecting the bidirectional BUCK/BOOST circuit structure, applying low-side mode for
switching of solar panel and LED light, using a differential circuit to detect solar panel voltage, using a low-cost
DC/DC chip to realize power supply change-over, designing a dual power supply circuit, introducing solar
panel booting circuit and other hardware design, as well as MPPT algorithm, state recognition and control,
return difference control and other software design, a solar LED street light control product featuring small size,
low cost, high efficiency and multiple functions is successfully developed.
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