Hello,
I’m designing a circuit to measure the IV characteristics of an illuminated solar cell. The idea is to charge a capacitor with the solar cell. The capacitor acts as a variable load. A current sensor measures the current flowing to the capacitor and the voltage across the capacitor is measured by a voltage sensor. The values of the current and voltage sensors during the time the capacitor is being charged directly corresponds to the required set of values for IV characteristics. This method is used to characterize large PV modules. I’m hoping to use the same technique to measure the IV characteristics of a single solar cell.
I’ve incorporated a switching arrangement in the design so that the capacitor may be charged and discharged at times controlled by the user. An MCU uses PWM to control two MOS switches. The first switch charges the capacitor from 0 V to the open circuit voltage of the solar cell. The next switch is used to discharge the capacitor through a resistor so that it can be used for another measurement.
I’ve created a circuit based on this idea and simulated it using LTSpice IV. I’m using an inverter between the two switches so that the switching takes place in an alternating fashion. Because 7404 inverter output is between 0 and 1 V, I’ve used a non-inverting amplifier to change this to 0 and 5V, so that MOSFET switching action takes place. The default ltspice NMOS was used.
However, the simulation yields unexpected results. I’ve used the simple equivalent circuit consisting of a current source, diode and series and shunt resistances to model the solar cell. However, the simulation shows that the capacitor charges and discharges between two fixed DC levels. That is, the capacitor does not discharge completely to zero. In fact, the capacitor voltage doesn’t even start from 0V at t = 0s.
After some experiments, I’ve found that the level to which the capacitor voltage drops during discharge is dependent on the diode model I’ve chosen. So I decided to model a diode which would approximate the solar cell diode. As far as I could see, the diode saturation current and ideality factor (or emission coefficient N) are the only parameters that would affect the diode forward characteristics. So I chose the following values:
Assuming an open circuit voltage of 0.8V and a short circuit current of 0.5A, I determined the value of reverse saturation current as
I0 = 7.27fA.
[This is found by the relation Iph = I0*(exp(qVoc/(nkT))-1) and Iph approx. equals Isc; Here T = 300K]
I chose an emission coefficient N =1. I don’t know what the value will be, but N is ideally 1 from what I understand.
With this I used the following model:
.model solar_diode D (IS=7.27f RS=42.0m BV=50.0 IBV=5.00u + CJO=39.8p M=0.333 N=1.45 TT=4.32u)
The other values of the model file were substituted from a 1N4001 model file. I don’t know if they are fully suited for a solar cell.
When I simulated the circuit, the capacitor voltage was varying between 574 mV and 594mV.
For IV characteristic measurement, I need the capacitor to charge between 0V and 0.8V (Voc). The values I get by simulation don’t make much sense to me.
Why isn’t the capacitor fully charging? Why isn’t it fully discharging?
Why doesn’t the capacitor start at 0V when t = 0? (The time at which ltspice starts saving data is 0s).
I tried varying the capacitance, the resistance R1, and the gate voltage of the MOSFET switch. None of this brought the desired results. What I conclude is that I’ve made some errors while modelling the diode.
Can someone help me by pointing out where I went wrong? Is this method not suitable to measure the IV characteristic?
SPICE NETLIST:
The netlist won't work readily because I've appended the abovementioned model file to standard.dio.
I’m designing a circuit to measure the IV characteristics of an illuminated solar cell. The idea is to charge a capacitor with the solar cell. The capacitor acts as a variable load. A current sensor measures the current flowing to the capacitor and the voltage across the capacitor is measured by a voltage sensor. The values of the current and voltage sensors during the time the capacitor is being charged directly corresponds to the required set of values for IV characteristics. This method is used to characterize large PV modules. I’m hoping to use the same technique to measure the IV characteristics of a single solar cell.
I’ve incorporated a switching arrangement in the design so that the capacitor may be charged and discharged at times controlled by the user. An MCU uses PWM to control two MOS switches. The first switch charges the capacitor from 0 V to the open circuit voltage of the solar cell. The next switch is used to discharge the capacitor through a resistor so that it can be used for another measurement.
I’ve created a circuit based on this idea and simulated it using LTSpice IV. I’m using an inverter between the two switches so that the switching takes place in an alternating fashion. Because 7404 inverter output is between 0 and 1 V, I’ve used a non-inverting amplifier to change this to 0 and 5V, so that MOSFET switching action takes place. The default ltspice NMOS was used.
However, the simulation yields unexpected results. I’ve used the simple equivalent circuit consisting of a current source, diode and series and shunt resistances to model the solar cell. However, the simulation shows that the capacitor charges and discharges between two fixed DC levels. That is, the capacitor does not discharge completely to zero. In fact, the capacitor voltage doesn’t even start from 0V at t = 0s.
After some experiments, I’ve found that the level to which the capacitor voltage drops during discharge is dependent on the diode model I’ve chosen. So I decided to model a diode which would approximate the solar cell diode. As far as I could see, the diode saturation current and ideality factor (or emission coefficient N) are the only parameters that would affect the diode forward characteristics. So I chose the following values:
Assuming an open circuit voltage of 0.8V and a short circuit current of 0.5A, I determined the value of reverse saturation current as
I0 = 7.27fA.
[This is found by the relation Iph = I0*(exp(qVoc/(nkT))-1) and Iph approx. equals Isc; Here T = 300K]
I chose an emission coefficient N =1. I don’t know what the value will be, but N is ideally 1 from what I understand.
With this I used the following model:
.model solar_diode D (IS=7.27f RS=42.0m BV=50.0 IBV=5.00u + CJO=39.8p M=0.333 N=1.45 TT=4.32u)
The other values of the model file were substituted from a 1N4001 model file. I don’t know if they are fully suited for a solar cell.
When I simulated the circuit, the capacitor voltage was varying between 574 mV and 594mV.
For IV characteristic measurement, I need the capacitor to charge between 0V and 0.8V (Voc). The values I get by simulation don’t make much sense to me.
Why isn’t the capacitor fully charging? Why isn’t it fully discharging?
Why doesn’t the capacitor start at 0V when t = 0? (The time at which ltspice starts saving data is 0s).
I tried varying the capacitance, the resistance R1, and the gate voltage of the MOSFET switch. None of this brought the desired results. What I conclude is that I’ve made some errors while modelling the diode.
Can someone help me by pointing out where I went wrong? Is this method not suitable to measure the IV characteristic?
SPICE NETLIST:
Rich (BB code):
* F:\Engg\Sun\IVtrace Simulation\ivckt.asc
C1 N008 0 3µF
M1 N008 N004 N007 N007 NMOS
M2 N009 N001 N008 N008 NMOS
R1 N009 0 10
A1 N004 0 0 0 0 N003 0 0 BUF
V1 N004 0 PULSE(0 5 0s 50us 50us 5ms 10ms 100) Rser=0
XU1 N003 N002 +15 -15 N001 LT1001
V3 +15 0 15V
R2 0 N002 1k
R3 N002 N001 4k
Isrc1 0 N005 0.2A
Rp1 N005 0 100k
Rs1 N006 N005 1
V2 0 -15 15V
R§Sense_Resistor N006 N007 0.1
D1 N005 0 solar_diode
.model D D
.lib C:\Program Files (x86)\LTC\LTspiceIV\lib\cmp\standard.dio
.model NMOS NMOS
.model PMOS PMOS
.lib C:\Program Files (x86)\LTC\LTspiceIV\lib\cmp\standard.mos
.tran 0 0.1s 0s 0.001s
* Solar Cell
* Opamp power supply
* Non-Inverting Amplifier
* Gain = 5
* <100 mV
.lib LTC.lib
.backanno
.end
