HSPICE Simulations

The PSIM simulations mentioned in last section confirmed the effectiveness of the proposed MPPT algorithm. In this section, the HSPICE simulations are introduced to assist the circuit level design.

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Based on the equivalent circuit of PV module shown in Fig. 2.1and referred to the parameters provided during the PSIM simulation, Fig. 5.21 shows the HSPICE simulation of the solar array. The short-circuit current, ISC, and the open-circuit voltage, VOC, are proportionally downgraded to a reasonable range, which are 50μA and 1.5V, respectively. This equivalent circuit, or the solar array simulator, behaves similarly like a real solar array.

The solar array current, I_PV, increases inverse proportionally to the increment of the solar array voltage, VPV. And there exists a maximum power point where the solar array outputs its maximum energy.

ISC=50μA

VOC=1.5V Fig. 5.21 The HPSICE I-V simulation of the solar array.

The environment factors, which are temperature and solar irradiation, basically change the parameters in the solar array to affect its performance, e.g. the maximum power point. Theses parameters include the parasitic series resistor, R_S, the parasitic parallel resistor R_P, the diode saturation current, I_S, and the diode quality factor, n. Fig. 5.22 demonstrates the influences of these

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parameters on the MPP while others are fixed. Fig. 5.22(a) shows that the MPP increases along with the increase of RS, while the MPP increment is small. In Fig. 5.22(b), it shows that the MPP changes proportionally to R_P. The increments, however, are barely noticeable. The influences of IS on the MPP are depicted in Fig. 5.23(c). Obviously, the MPP decreases as I_S increases, which means they are inverse proportional to each other.

Furthermore, Fig. 5.24(d) shows the relationship between the MPP and n. It is apparent that the MPP increases dramatically when n increases.

In summary, all of these four parameters, R_S, R_P, I_S, and n, have their effects of the MPP, either proportionally or inverse proportionally. Among all of these parameters, n has the most significant influence on the MPP. As a result, it is chosen as the varying parameter to emulate the changes in the atmosphere, so to say, the environment temperatures and the solar irradiation levels. TABLE 5.2 lists the simulated results of VOC and VMPP depending of different diode quality factors. With the help of the information given in TABLE 5.2, one can use different n values to simulation different MPPs. The target of the MPPT circuit is to track the MPPs as close as possible to these simulated MPPs.

TABLE 5.2 Diode Factor vs. Maximum Power Point Voltage Diode Factor

(n)

Open-Circuit Voltage (VOC)

Maximum Power Point Voltage (VMPP)

12 4.62V 3.56V

11 4.23V 3.25V

10 3.84V 2.91V

9 3.45V 2.60V

8 3.06V 2.28V

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MPP increases as Rs increases Rs↑ MPP

(a)

MPP increases slightly as Rp increases Rp↑ MPP

(b)

MPP decreases as Is increases

Is↑ MPP

(c)

MPP increases dramatically as n increases n↑

MPP

(d)

Fig. 5.22 The simulated characteristic P-V curves according to different parameters.

(a)Varying R_S. (b) Varying R_P. (c) Varying I_S. (d) Varying n.

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As mentioned in Chapter 4, the 7-bit up/down counter modulates the duty cycle of the boost converter after receiving the slope information from the slope detection circuit. The current flowing out of the solar array is then adjusted accordingly to pull down or push up the operating voltage, in order to approach the MPP. Fig. 5.23 gives the simulation results of the duty cycle modulation according to the slope changes from positive to negative and from negative to positive.

Eslope

VG

(a)

Eslope

VG

(b)

Fig. 5.23 Simulation waveforms of duty cycle modulation. (a) The changes in VG

according to the changes in slope polarity. (b) The zoom-in view of (a).

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Fig. 5.24(a) shows the case when the slope changes from positive to negative or Eslope changes from high to low. Fig. 5.24(b) depicts the initial waveforms before the duty cycle modulation occurs. The slope condition E_slope is detected as high in Fig. 5.24(c), which means the operating point falls on the left-hand side plane on the power-voltage curve. This results in the decrement of the duty cycle to reduce the current of solar array. The operating voltage, as a consequence, is pulled up to approach the MPP. When the slope condition Eslope is found to be low, it means the operating point falls on the right-hand side of the power-voltage plane. Under this condition, the duty cycle is then increased to pull more current from the solar array. The operating voltage is again reduced to pursue the MPP, as shown in Fig. 5.24(d).

Slope>0 Slope<0

Eslope =0 Eslope =1

(a)

E_slope=1

V_G Duty

(b)

73 Eslope=1

VG Duty↓

(c)

Eslope=0

VG Duty↑

(d)

Fig. 5.24 Duty cycle changes when the slope changes from positive to negative. (a) The waveforms of Eslope vs. VG. (b) Initial duty cycle. (c) When Eslope is detected as high. (d) When Eslope is detected as low.

On the other hand, as the slope changes from negative to positive, the duty cycle changes as shown in Fig. 5.25(a). Fig. 5.25(b) shows the initial waveforms before the slope changes. When the slope is detected to be negative, the duty cycle increases to increase the solar array current and therefore decrease the voltage to track the MPP as can be seen in Fig. 5.25(c).

On the other hand, the slope is positive after the slope transition, which means the operating point falls on the left-hand side of the power-voltage plane. The duty cycle, consequently, needs to decrease in order to raise the solar array voltage as shown in Fig. 5.25(d).

74 Eslope =0

Slope<0 Slope>0

Eslope =1

(a)

Eslope=0

VG Duty

(b)

V_G Duty↑

Eslope=0

(c)

VG Duty↓

E_slope=1

(d)

Fig. 5.25 Duty cycle changes when the slope changes from negative to positive. (a) The waveforms of E_slope vs. V_G. (b) Initial duty cycle. (c) When E_slope is detected as low. (d) When E_slope is detected as high.

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Fig. 5.26 shows the simulation results, both the solar array voltage V_PV and current IPV, of the proposed MPPT algorithm and circuit. Comparing the simulation results (V_MPP) with respect to different diode quality factors n given in TABLE 5.2, it concludes that the proposed MPPT technique can track the maximum power point of solar array and achieve a high tracking efficiency. The simulation results show that the tracking begins with the proposed OCT technique, which detects the open-circuit voltage of the solar array and approximates the operating voltage to around 70% of the open-circuit voltage. The proposed SDT technique then takes place the tracking procedure to accurately track the MPP. Owing to the oscillation around the MPP, both the voltage and current waveforms present certain oscillations during the steady-state, as can be seen in the figure.

n=12 n=11 n=10 n=9 n=8 VPV

IPV

Fig. 5.26 Simulation results of the proposed MPPT technique.

As mentioned before, the proposed OCT technique is aimed to improve the tracking speed during the system power-on period. To show the

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effectiveness of the OCT technique, OCT is turned off deliberately in Fig. 5.27 to compare the tracking speed to the one when it is enabled. It is apparent in the figure that the OCT technique greatly improves the tracking speed during the power-on period. In both cases, whether the OCT technique is enabled or not, the steady-state voltage reaches at 2.91V when the diode quality factor is set to 10 according to TABLE 5.2.

OCT disabled VPV

IPV

OCT enabled

VMPP=2.91V

Fig. 5.27 Simulation results showing the effectiveness of the OCT technique.

To investigate the effectiveness of the proposed MPPT technique and circuit undergoing environmental condition changes, the diode quality factor n is changed from a high value to a low value and from a low value to a high value in Fig. 5.28(a) and Fig. 5.28(b), respectively. The diode quality factor is changed from 10 to 9 in Fig. 5.28 (a) to emulate either the increasing of irradiation level or the decreasing of ambient temperature. The simulation results, VMPP transiting from high to low, confirm the speculations stated in Chapter 2. Furthermore, the tracking efficiency is high as the steady-state voltage almost coordinates to the information given in TABLE 5.2. In Fig.

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5.28(b), the effectiveness of the proposed MPPT technique is examined from another perspective. The diode quality factor is changed from 9 to 10 to simulate either the decreasing of irradiation level or the increasing of ambient temperature. The simulation results show that the tracking efficiency is again guaranteed.

OCT disabled

OCT enabled VPV

IPV

VMPP=2.91V

VMPP=2.60V

n=10 n=9

(a) VPV

IPV

OCT disabled

OCT enabled

VMPP=2.91V VMPP=2.60V

n=9 n=10

(b)

Fig. 5.28 Simulation results of the proposed MPPT technique undergoing environmental condition changes. (a)When the irradiation decreases or the temperature increases. (b)When the irradiation increases or the temperature decreases.

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In conclusion, the HSPICE simulation results confirm the effectiveness of the proposed MPPT algorithm and circuit. The tracking speed during the power-on period is dramatically improved, as speculated in Chapter 3, when the proposed OCT technique is activated. High tracking accuracy, in addition, is also guaranteed since the proposed SDT technique is used.