Improvement of SBC Circuit using MPPT Controller

Transcription

1 Improvement of SBC Circuit using MPPT Controller NOR ZAIHAR YAHAYA & AHMAD AFIFI ZAMIR Electrical & Electronic Engineering Department Universiti Teknologi PETRONAS Bandar Seri Iskandar, 3750 Tronoh, Perak MALAYSIA Abstract: - This paper is about designing Maximum Power Point Tracker (MPPT) controller with Synchronous Buck Converter (SBC) circuit where the main purpose is to improve the converter s performance in terms of output voltage and current. The MPPT controller s characteristics, performance, operation modes, advantages, and disadvantages of SBC are analyzed and observed. PSpice software is used in designing and simulating both circuits. The analysis is carried out based on the average output voltage and current, node voltage, output ripple voltage and current, gate-to-source voltage, and body diode conduction loss of SBC circuit. The details are discussed thoroughly that include limitations and advantages of the controller using MHz switching frequency. It is found that by implementing MPPT controller with SBC, the output voltage and output current have increased by approximately % and 3 % in Continuous Conduction Mode (CCM) and Discontinuous Conduction Mode (DCM) conditions respectively. Besides, it reduces output voltage ripple and current by more than 40 % in CCM compared to conventional SBC. However, in DCM, the output peak-to-peak ripples for both voltage and current have increased more than 0 %. Key-Words: - Body Diode Conduction, MPPT Controller, PSpice Simulation, Synchronous Buck Converter Introduction Maximum Power Point Tracker (MPPT) controller is widely used in the photovoltaic (PV) solar panel applications []. It extracts maximum power from PV panels and delivers to the load or battery. The advantages of MPPT controller have triggered the idea for Synchronous Buck Converter (SBC) integration to study the feasibility of the design. The SBC is applied because it is common in industry for lower power conversion, power management and microprocessor voltage-regulator (VRM) []. Here, the significant differences between SBC and the combination of MPPT and SBC (MPPT-SBC) are used in proposing new solutions to solve issues in the converter. In this work, the output voltage and current, body diode conduction losses, and output ripple peak-topeak for voltage and current in Continuous Conduction Mode (CCM) and Discontinuous Conduction Mode (DCM) in SBC are observed. Therefore this work will eventually suggest if the controller is suitable for high frequency converter. MPPT Controller In this application, the output voltage of converter is captured and the controller can quickly charge to the maximum power. It also controls the output current from the load so that the output power converges to maximum based on the linearity between the maximum output power and the optimal current []. The power delivered to the load is the highest when the source internal impedance matches the load impedance [3]. There are two main groups of MPPT: one that uses analog circuitry and classical feedback control and the other, uses microprocessor to maintain control of the operating point. Analog systems have the advantage of having low cost components, but are more problematic to control. It is difficult to develop a stable system which is able to maintain its efficiency. 3 Synchronous Buck Converter The SBC has advantages due to its bidirectional power flow and higher efficiency compared to buck converter. The performance of the converter at light ISBN:

2 load can be affected where it allows inductor current, i L as shown in Fig. from entering the DCM and maintaining operation in CCM. This means that the performance can be achieved in DCM with correct circuit design. Even though the complexity of the converter is vastly increased due to the need for M switch [4], its operation is not so complicated. It consists of an inductor and two switches that control the load behavior. M and M conduct alternately between inductor and source voltage to store and discharge energy to the load. 4 Body Diode Conduction Body diode conduction occurs when negative voltage occurs at node voltage, V node. When M and M are off, parasitic body diode of M is forward biased and eventually generates undershoot of negative voltage. This body diode conduction is proportional to the dead time, T D. If T D is longer, the body diode conduction will also be longer. Allowing the output current to flow through the body diode of M and M degrades the overall efficiency because it contributes to the body diode conduction losses, P BD. These losses are proportional to the body diode conduction time, t bd. The longer the t BD, the more P BD will be. Thus, a shorter t bd is required for low P BD. This is shown in Fig.. t bd V in Body Diode Conduction Fig. : Effect of Body Diode Conduction The equation to calculate the loss contributed by the body diode conduction is given in Eq. (). P = t V I f () BD assuming that t bd(rise) = t bd(fall) where: V f = body diode forward voltage drop I O = output current f sw = switching frequency bd M M f Node Voltage, V node L o I L C R sw I O V in V O 5 Methodology The work starts with the design and simulation of MPPT controller and MPPT-SBC circuit. Then it proceeds with the analysis of the results with respect to suitable component values. Here, the design requirements are: a) Duty ratio, D = 0.5 b) Input voltage, V in = V c) Output voltage, V O = 3 V d) Switching frequency, f sw = MHz The value of inductor, capacitor and resistor for this circuit are calculated based on Eq. () to Eq. (7) [5]. Inductor voltage, V Output Ripple voltage, di = L = V dt V L in o () V ( t) I ( t) T L s L = (3) 8C Inductor ripple current, I t) = 8C f V ( ) (4) L( sw L t Output inductor, Vin Vo L DTs (5) I t) ( L I L Output capacitor, C = (6) 8 f V Output filter cut-off freq., f c = (7) π L C There are two sets of parameters that have been calculated for both CCM and DCM conditions as indicated in Table. Table : Value of electrical components for CCM and DCM of SBC [6] sw CCM o DCM Resistor, R 3.5 Ω 4 Ω Capacitor, C 0.65 µf µf Inductor, L 5 µh µh Fig. shows the SBC circuit with M connected to the output amplifier of MPPT controller. In this circuit, M is turned on by the Pulse Width Modulation (PWM) signals supplied by PWM and for M, is given by the controller. The following ISBN:

3 guidelines are used to calculate the component values for the Type II controller [7]. Fig.: New MPPT-SBC Circuit. Choose a value for R input, usually between kω and 5 kω. In this work, 3 kω is chosen.. Pick a gain (R /R ) that will shift the Open Loop Gain up to give the desired bandwidth. The following equation is used to calculate R for given R. R Rinput C C F D V = ( R (8) ESR BW OSC ) FLC FESR Vin F LC = (9) π L C M R C L M V node C R ensure the correct output from the designed circuit can be determined. If the simulation does not produce the expected results, the circuit will be redesigned. Table shows the simulation parameters for SBC which includes PWM settings, MOSFETs used and input voltage of the SBC. Table : Parameters of SBC for f s = MHz Components Parameter Settings in PSpice simulator and Component used. PWM V=0, V=5V, T d = 0ns T r = 5ns, T f = 5ns, PW = 40ns, PER= 000ns PWM V=0, V=5V, T d = 65ns, T r = 5ns, T f = 5ns, PW = 70ns, PER= 000ns M, M, IRFP50 V V in 6 Results & Discussions The results start with the observation of PWM signal graph in order to ensure correct PWM signals are fed into the switch. Next, V node is observed to ensure that it is equal to the input voltage and hence indicates correct conduction of the switches. Besides that, operating waveform of i L is used to indicate the operation of SBC either in CCM or DCM. Thorough analysis on the performance of the SBC will be explained. 6. Simulation of MPPT-SBC in CCM F ESR = (0) π R C IL 94.3 ma ma D = 0. 3 () BW f sw I O ma 3. Calculate C : I o(avg) = ma C 0 = π R 4. Calculate C: F LC () V O 3.03 V V o(avg) = V C= C ( π R C f sw ) (3) The purpose of this simulation is to determine whether the results satisfy the design requirements. The graphs obtained will be observed. This is to Fig. 3: I L, I O and V O of MPPT-SBC in CCM Fig. 3 shows the output voltage, output current and inductor current for MPPT-SBC in CCM. It is noticed that I o and I L have increased. The output ripple voltage peak-to-peak, V o(p-p) and output ripples current, I o(p-p) are almost equal to each other ISBN:

4 resulting in %. From I L waveform, it proves that the operation of the controller is in CCM having V o(avg) and I o(avg) are V and ma respectively. More importantly, the output voltage almost satisfies the theoretical value of 3 V. 6. Simulation of MPPT-SBC in DCM IL.66 A ma I O ma V node which is 9.8 V, not V as expected I o,avg = ma V O 3.3 V V o(avg) = 3. V Losses in M =.87 V Fig. 4: V node and V ds for MPPT-SBC in CCM Fig. 4 shows that the node voltage, V node is not V. It only reaches 9.8 V. The remaining.87 V is consumed in M. This is shown in the circle where the switching characteristic of the V ds is different and this issue is yet to be solved. However, V node obtained is sufficient to produce.87 V of output voltage since the voltage drop across L is around 6 V as shown in Fig. 5. V ds Fig. 6: I L, I o, and V o, for MPPT-SBC in DCM Fig. 6 illustrates the output voltage, output current and inductor current of MPPT-SBC in DCM. The V o(avg) is 3. V and I o(avg) is ma. Here, the current ripples (.346 %) are higher than in CCM (0.947 %) and due to that reason, the I o(avg) is lowered by.84 %. V node = 0.89 V V L = 6 V Vds Fig.7: V node and V ds for MPPT-SBC in DCM Fig. 5: V L for MPPT-SBC in CCM Besides that, the SBC also experiences that V node is just 0.89 V and not equal to V. This is shown in Fig.7. This is due to the same reason as for CCM. However, the V o waveform for this controller still produces 3 V because the voltage across L reads 7.79 V as shown in Fig. 8 which is better than CCM. ISBN:

5 Fig. 8: V L for MPPT-SBC in DCM Table 3: MPPT-SBC in CCM and DCM CCM DCM V o(avg), (V) I o(avg) (ma) I L(max peak) (A) V o(p-p) (%) I o(p-p) (%) P BD (W) none none From Table 3, the output current has higher value in CCM and slightly lower in DCM. Besides, it also has fewer ripples of about %. However, the ripple increases as it enters DCM. Table 4: Comparison between SBC and MPPT-SBC in CCM SBC [6] V L = 7.79 V MPPT-SBC V o(avg), (V) I o(avg) (ma) V o(p-p) (%).0.35 I o(p-p) (%) P BD (mw) Table 5: Improvement of MPPT-SBC in CCM Improvement of MPPT-SBC to SBC (%) V o(avg), (V).6 I o(avg) (ma).69 V o(p-p) (%) 4.77 I o(p-p) (%) 4.36 P BD (mw) 00 Table 4 and Table 5 illustrate the comparisons and improvements of the SBC and MPPT-SBC in CCM respectively. From Table 4, it shows that MPPT-SBC has achieved higher V o(avg) of 3.09 V and I o(avg) of 88.7 ma compared to the SBC [6]. Eventually, this satisfies V o(avg) requirement of 3 V. When the converter is integrated with MPPT controller, the P BD is found to be zero in addition to lower V o(p-p) and I o(p-p) of only 0.95 %. This gives result in an improvement of more than 40 % in ripples. Table 6: Comparison between SBC and MPPT-SBC in DCM SBC [6] MPPT-SBC V o(avg), (V) I o(avg) (ma) V o(p-p) (%) I o(p-p) (%) P BD (mw) Table 7: Improvement of MPPT-SBC in DCM Improvement of MPPT-SBC to SBC (%) V o(avg), (V) 3.8 I o(avg) (ma) 3.07 V o(p-p) (%) I o(p-p) (%) -.48 P BD (mw) 00 Table 6 and Table 7 show the comparisons and improvements of the SBC [6] and MPPT-SBC in DCM mode respectively. When implementing MPPT-SBC, this results in higher V o(avg) and reduction in I o(avg) which are 3. V and ma. Here, the improvement of 3.8 % for V o(avg) and 3.07 % for I o(avg) are achieved. The V o(p-p) and I o(p-p) for MPPT-SBC is considerably low which are both around.35 % but lower ripple can be achieved with SBC [6] circuit which are only.0 % and.06 % respectively. An increase in ripples at the output can decrease the overall performance of the circuit. Nevertheless, the converter shows a reduction in P BD in DCM when it is connected to MPPT controller as shown in Table 6. ISBN:

6 7 Conclusion It is found that by implementing MPPT-SBC, the output voltage and output current have increased by approximately % in both CCM and DCM conditions compared to conventional SBC. The MPPT-SBC in DCM condition has shown better performance in output current, output voltage and body diode conduction loss compared to CCM. However, its output ripple peak-to-peak voltage and current have increased significantly. These are the drawbacks in the design. References: [] N. Mutoh and T. Inoue A Controlling Method for Charging Photovoltaic Generation Power Obtained by a MPPT Control Method to Series Connected Ultra-Electric Double Layer Capacitors, IEEE Trans. Power Electronics, vol. 4, 004, pp.64 7 [] L. Castaner and S. Silvestre, Modeling Photovoltaic Systems, John Wiley & Sons., st edition, 00 [3] F. L. Antunes and J. L. Santos, Maximum Power Point Tracker for PV Systems, World Climate & Energy Event, 003 [4] -, Buck Converter, Wikipedia Org, 0 [5] I. Batarseh, Equal Pulse (Uniform) PWM, Power Electronics Circuits, John Wiley & Sons Inc., Ch 9, 004, pp [6] N. F. Ahmad, The Study and Evaluation of Output Parameters in SRBC, Bachelor Dissertation, Department of Electrical and Electronic Engineering, Universiti Teknologi PETRONAS, Malaysia, 0 [7] D. Mattingly Designing Stable Compensation Networks for Single Phase Voltage Mode Buck Regulators, Intersil Corporation, 003. ISBN: