3. POWER MANAGEMENT CIRCUIT
Figure 5-1 Block diagram of energy harvesting system
Power management circuit is an
important building block for the system that shown in figure 5-1. It is the
direct load for the energy transducer and the circuit’s behavior would affect
the power that can be outputted from the transducer. The power management
circuit should thus be operated so that maximum power can be extracted from the
energy transducer. At the same time, the output of the power management circuit
is used to power the load applications and the output voltage should be stable
and satisfy the load requirement.
3.1. Problem Statement
the input voltage drops below a certain limit, the output voltage must remain
constant to prevent discharge due to the direct connection of the
supercapacitor to the circuit. In addition, supercapacitor should be connected
to the circuit with a switch. That’s why, the connection of the supercapacitor
and the circuit can be disconnected in unintended situations.
3.2. Power Management Circuit Design
3.2.1. Photovoltaic Cell
A photovoltaic (PV) cell is a device that can directly
convert the light energy into the electrical energy. PV cell in the sunlight is
made up of streams of particles called photons. When the photons penetrate the
silicon wafer they give loose electrons they encounter extra energy enough to
enable the electrons to move across the electrical field. Electrons begin to
flow through wafer and round circuit. We have a flow of electrons in other
Figure 5-2-1 PV cell working principle
The charge pump is used to boost the
output voltage of the PV cells to provide power directly to the load circuit
and charge the supercapacitor. The charge pump consists of a two stage voltage
doubler and a clock generator circuit which is basically a ring oscillator
connected to two buffer circuits. Buffers are designed to drive the large stage
capacitors in the voltage doubler circuitry.
Figure 5-2-2 Schematic of the charge pump
The comparator is a circuit that
compares two analog signal (analog signal to analog signal or reference signal
to analog signal ) generates binary signal as the output based on the
comparison. The type and architecture of the comparator significantly influence
the performance of the target application.
Figure 5-2-3 Two stage open loop comparator
As shown in Figure 5-2-3, the comparator contains
input stage, output stage, and differential amplifier. This circuit has many
advantages. One of them is the circuit consumes minimum number of transistor
and thus the circuit area is small. Reference voltage (Vref) of the comparator
is generated by the bandgap reference voltage.
voltage generators are used in DRAM’s, flash memories, and analog devices. The
generators are required to be stabilized over process, voltage, and temperature
variations. The bandgap reference (BGR) is one of the most popular reference
voltage generators that successfully achieve the requirements.
Figure 5-2-4 Bandgap reference circuit
voltage has been converted from the sum of two currents; one is proportional to
Vf (voltage of diode) and the other is proportional to VT (thermal voltage).
Supercapacitors have extremely long-life
cycles, and that’s why they have been identified as a promising type of energy
storage element (ESE). In particular, supercapacitors and photovoltaic (PV)
modules make an excellent combination for energy harvesters.
This academic paper has provided information about
three different topics. Firstly, the designed microchip was tested because of
understanding working properly or not. Measurements were made using two
separate sources. Primarily, dc power supply was used, and results were
observed, after that optical power was used and the results were obtained. Measurements
are shown in tables and graphs.
Secondly, the microchip that are designed through
Mentor Graphics, errors are detected and solved using appropriate methods.
Design rule check (DRC) and layout versus schematic (LVS) are used in order to
find these errors. Approximately 250 layout errors have been fixed. A possible
short circuit was prevented thanks to these solutions.
Finally, the power management circuit has begun to be
designed. The effect of the power management circuits on the systems is
explained. They decide how to optimally use the incoming power. These decisions
are very significant for systems because having the proper power management
system in place can help reduce costs, improve operational efficiency, and meet