Design of Power Switching and Distribution Circuits for Solar Power Generation

Yun-Parn Lee and En-Chi Liu

1.Introduction

When solar panels receive sunlight, a device can be used to track the maximum power so as to ensure efficiency and maximum power generation.An automatic switching circuit can be utilized to switch power between the solar panel system and the general municipal power system as needed.When the amount of generated solar power and stored battery power is insufficient for the demand, the power will be automatically switched to the municipal city grid.The load can be utilized for 24 hours.The output of the battery storage system is channeled through a quasi-resonant pulse-width-modulated (PWM) DC to AC inverter[1], an AC conversion transformer, and a power filter to obtain 110 V AC output which can be utilized for general household use.Fig.1 shows the architecture of the solar power system.Fig.2 shows a block diagram of the power switching system.

There are many research papers on photosensitive cells and its applications have been published before.Miranda[2]proposed a photo-sensitive resistor in an overload preventing design.Layer[3]suggested using a photosensitive resistor to design an automatic ellipsometer.In addition, the photosensitive resistor has been used extensively in camera’s shutter circuit for photography[4],[5]and Pacholok[6]proposed a high power factor switching type battery charger.Lee[7]put forward a wall switch and lamp assembly unit to use the photosensitive resistor.However, Lee did not show any detail circuit diagram.Sham[8]suggested a photosensitive switching apparatus for an electric appliance.Delwiche[9]suggested using a cadmium-sulfide (CdS) photosensitive resistor in their electronic circuit design on a broadcast call unit for bird control.Although Lee[10]and Ranjit[11]put forward an excellent power distribution system for future homes,and the detailed circuit design are yet to be done.In this paper, we proposed to use the photosensitive resistor with relays to construct a solar power switching circuit and a power distribution circuit.In Section 2, we show the parallel connection between the municipal grid and solar power system, the detailed circuit design principles and small signal equivalent circuit are presented.In Section 3, we give the experimental data and test results.Finally, conclusions are drawn.

Fig.1.Diagram of the solar power generation system.

Fig.2.Block diagram of the solar power automatic switching system.

2.Parallel Connection between the Municipal Grid and Solar Power System

The power from the solar energy conversion system needs to be converted from the general AC 110 V current to a total of five DC voltages[12], which are used in the quasi-resonant PWM DC to AC converter.This is done with a power switching circuit and a power filtering circuit.One of these voltages is +15V used to supply the sinusoidal generating circuit system[13].A solar power distribution circuit is illustrated in Fig.3.Fig.4 is the circuit for AC to DC converters.

2.1 Booster Circuit and Reverse Voltage Circuit

This section describes the solar energy booster circuit and the reverse voltage circuit[14].The solar energy from the panels flows through the charge and discharge controller to the storage batteries.By connecting a boosting circuit, the inputs are further divided into +20 V and –20 V.The purpose of this circuit is to facilitate the exchange of power between the municipal power grid and the solar power supply.To achieve this purpose, we utilize an integrated circuit (IC) MC34063 which is in line with our needs.It can not only boost the voltage but also convert it to a negative voltage of –20 V.

Fig.5 shows the comparator invert input of the pin 5 connected to the external resistors R1and R2and the feedback circuit for the generation of the output voltage,which can be written as

Fig.4.Diagram of AC to DC converter circuit.

It is also can be seen that the RS flip-flop is driven by an internal oscillator.From a comparison of the voltage at the pin 5 and MC34063’s internal reference voltage of 1.25 V,we note that when the voltage at pin 5 is less than the reference voltage, the comparator output of the jump at the Q-ended is in the high-voltage state, therefore T1and T2are conducted.When the input voltage increases the charge on the output filter capacitor, the voltage at the pin 5 is higher than the reference voltage of 1.25 V, therefore, the RS flip-flop is blocked.This means that the Q-ended is in the low-voltage state, and T1and T2are not conducted.The oscillator input is used to monitor the peak current of the oscillator pulse.The capacitance C0which connects to the pin 2 and pin 5 can monitor the oscillator frequency.In addition, C0can decide the switching time of the switch T1.

Fig.3 Block diagram of solar power distribution circuits.

Fig.5.MC34063 and booster circuit.

Fig.6.Reverse-voltage circuit.

Fig.5 is an illustration of the booster circuit formed by MC64063.When the switch T1turns on, the power supply goes through the resistance RSCand the inductor L.The inductor begins to store energy,but C0 is responsible for providing energy Voutto the load.When the switch T1turns off, the inductor releases the energy to C0and the load.During the time when the inductor releases energy, since the polarity is the same, the load voltage can be higher than the supplied voltage.However, due to back and forth oscillation of the switch, if we adjust the time constant of the load to be high enough, a continuous DC voltage can be supplied to the load.

As shown in Fig.6, we can see that when the power turns on, the current flows through pin 1 and pin 2 of MC34063 and the inductor L.The inductor L stores energy.When T1is not conducted, the current flowing through the inductor cannot increase simultaneously, therefore, the diode is conducted.The flow of inductor energy through the diode provides a negative voltage output to the load.

2.2 Switching Circuits and Darlington Amplifier

The solar energy input is connected in parallel with a light bulb.As shown in Fig.7, when the solar power is sufficient, the light bulb indicates the output; on the other hand, when the solar power is insufficient, the light-sensitive resistors do not receive light.This indicates when the system is supplied by the city grid.Light-sensitive resistors are used as a detection device for solar power switching.The photosensitive resistors are commonly made of cadmium sulfide (CdS).Thus, when the sunlight is strong, the resistance becomes smaller and vice versa[15].Fig.8 shows the relation between resistance of the photosensitive resistor and the photon flux density.In addition, the relationship can be expressed by

Fig.7.Solar power switching circuit in parallel connection with the city grid.

where η stands for the quantum efficiency, Φ is the electronic charge, and A is the area of the photosensitive resistor.Because of the different material quality, receiving light area, and other factors, we have different ranges of resistance changes.

We want to use a Darlington amplifier[15]with relays and photosensitive resistors for switching purposes.Two cascaded transistors are used to build the Darlington amplifier for a large current gain.Combining the relay with a Darlington amplifier is an easy and inexpensive method to build a power switching circuit.Fig.9 shows a small signal equivalent circuit of the power switching circuit.In this diagram, RCDSstands for the photo-sensitive resistor, IRis the current flow through the relay coil, β1and β2are the hFE, where hFEis the small-signal current gain of the two transistors, and gm1and gm2are the conductance of the two transistors in the Darlington pair configuration.Therefore,we have

where ISis the short circuit current between base and emitter, rπ1is internal resistance between base and emitter,and Vπ1is the voltage across base and emitter.Also,

The current driving the relay is equivalent to

So, the current gain of this configuration is equivalent to

The finished implementations of the power switching circuits are shown in Fig.8 and Fig.10.

3.Experimental Results and Discussion

Energy conversion efficiency is between 0 and 100%,which is a quantitative measurement of the output power versus the input power.If we express that the power delivered by the solar panel is PSolar, the power delivered by the AC source is PAC, and the power output used by the device is Pout, the energy conversion efficiency can be expressed as (7), when the power circuit is connected to the AC source:

Or, the energy conversion efficiency can be expressed as(8), when the power circuit is connected through the solar panel:

In our experiments, we tested and actually measured two different devices and plotted the energy efficiency diagram in Fig.11.

Fig.8.Resistance versus the irradiance of CdS photosensitive resistor.

Fig.9.Small equivalent circuit of photo-sensitive resistor and Darling amplifier configuration.

Fig.10.Power switching circuit test set up.

Fig.11.Actual data of power efficiency vs.input power of solar power and AC power.

4.Conclusions

Most of the power switching circuits are currently using mechanical devices which are bulky and easily damaged.In this study, we design and analyze a photovoltaic switching circuit using the photosensitive resistor, Darlington amplifier, and relays, which can quickly switch power input between the city grid and solar energy system.Under an independent mode of operation, this system can directly convert solar power into 60 Hz, 110 V AC in order to provide a power source for small household loads.Our experimental data confirm the feasibility of the system.

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