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As shown in Fig.10, efficiency of the full bridge LLC converter is above 96% within MPPT voltage range from 25 to 39V and peak efficiency is high up to 97.7%. Fig.10 Efficiency of the LLC converter vs. V,,, and P,

Figure 10 As shown in Fig.10, efficiency of the full bridge LLC converter is above 96% within MPPT voltage range from 25 to 39V and peak efficiency is high up to 97.7%. Fig.10 Efficiency of the LLC converter vs. V,,, and P,

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Fig.1 System block diagram of PVMI
Fig.1 System block diagram of PVMI
Zero voltage switching (ZVS) for the primary side switches and zero current switching (ZCS) for the secondary side diodes can be realized via resonance existing in LLC resonant converter. As shown in Fig. 2, vertical and horizontal axis represents the switching frequency f;, and the phase-shift duty cycle D respectively. Power can be converted efficiently under narrow-range input voltage with control I 1.e. traditional PFM, where the voltage gain is only regulated with f, | 71. Great challenges in magnetic components, system control, and system stability occur when applying control I to PV panels because of the wide-range switching frequency resulted from wide-range input voltage of PV panel and wide-range load variation. Efficient power conversion under wide-range input voltage can be realized through applying control II. As depicted in Fig.2, PFM is adopted in a region where f, is smaller than a specific value, while higher f; for regulation is needed, PS-PWM is used , and so f; range is limited. The efficiency, however, will decrease under PS-PWM on occasions of high input voltage or light load due to the large D which tending to result in the increases of internal circumfluent current loss. Fig.2 Control scheme for full-bridge LLC resonant converter
Zero voltage switching (ZVS) for the primary side switches and zero current switching (ZCS) for the secondary side diodes can be realized via resonance existing in LLC resonant converter. As shown in Fig. 2, vertical and horizontal axis represents the switching frequency f;, and the phase-shift duty cycle D respectively. Power can be converted efficiently under narrow-range input voltage with control I 1.e. traditional PFM, where the voltage gain is only regulated with f, | 71. Great challenges in magnetic components, system control, and system stability occur when applying control I to PV panels because of the wide-range switching frequency resulted from wide-range input voltage of PV panel and wide-range load variation. Efficient power conversion under wide-range input voltage can be realized through applying control II. As depicted in Fig.2, PFM is adopted in a region where f, is smaller than a specific value, while higher f; for regulation is needed, PS-PWM is used , and so f; range is limited. The efficiency, however, will decrease under PS-PWM on occasions of high input voltage or light load due to the large D which tending to result in the increases of internal circumfluent current loss. Fig.2 Control scheme for full-bridge LLC resonant converter
2. The effect of PS-PWM on limiting switching frequency will be weakened with the increases of Q. It means PFM control performs better than PS-PWM control under heavy loads. Fig.3. fvs. Q and D @ k=3.7 under PFM+PS-PWM mode
2. The effect of PS-PWM on limiting switching frequency will be weakened with the increases of Q. It means PFM control performs better than PS-PWM control under heavy loads. Fig.3. fvs. Q and D @ k=3.7 under PFM+PS-PWM mode
Fig.4 Program flowchart of LLC converter with hybrid control The program flowchart of the hybrid control is shown in Fig.4, which is described step-by-step as following. The output voltage Vi; of the LLC resonant converter is compared with the reference voltage in analog to digital (AD) interrupt; thus obtain error Voe70,. The converter will be controlled with PFM if the new calculated switching frequency f; through PI controller is smaller than resonant frequency /;. If not, the PS-PWM+PFM control mode will be activated. Then a modification of phase-shift duty cycle 4D can be calculated according to frequency error which equaling to the differences between f,and f;. Finally, f; and D will be updated by adding AD to a reference phase-shift duty cycle Dpase which had been pre-stored in the register.
Fig.4 Program flowchart of LLC converter with hybrid control The program flowchart of the hybrid control is shown in Fig.4, which is described step-by-step as following. The output voltage Vi; of the LLC resonant converter is compared with the reference voltage in analog to digital (AD) interrupt; thus obtain error Voe70,. The converter will be controlled with PFM if the new calculated switching frequency f; through PI controller is smaller than resonant frequency /;. If not, the PS-PWM+PFM control mode will be activated. Then a modification of phase-shift duty cycle 4D can be calculated according to frequency error which equaling to the differences between f,and f;. Finally, f; and D will be updated by adding AD to a reference phase-shift duty cycle Dpase which had been pre-stored in the register.
Fig.6 M vs. k and Q under PFM+ PS-PWM mode As shown in Fig.5, Jo rms’, io of VS. k and Q in the LLC resonant converter operating PFM mode with f=0.8 are plotted, respectively. Here Jg ms and ig oare normalized as Jo jms" and ig of - It is shown that large k and Q will achieve small currents, then resulting in the low switches loss. And M will be decreased with increasing of & and Q, resulting in wider switching frequency range. Under PFM+PS-PWM mode, the currents behavior is the same like that of PFM mode. In contrast, its M will be increased with increasing of k, as shown in Fig.6. That is to say, large & will not result in wider switching frequency range. Therefore, in the design k and Q of resonant tank, only effect of A and Q on M under PFM control mode is considered.
Fig.6 M vs. k and Q under PFM+ PS-PWM mode As shown in Fig.5, Jo rms’, io of VS. k and Q in the LLC resonant converter operating PFM mode with f=0.8 are plotted, respectively. Here Jg ms and ig oare normalized as Jo jms" and ig of - It is shown that large k and Q will achieve small currents, then resulting in the low switches loss. And M will be decreased with increasing of & and Q, resulting in wider switching frequency range. Under PFM+PS-PWM mode, the currents behavior is the same like that of PFM mode. In contrast, its M will be increased with increasing of k, as shown in Fig.6. That is to say, large & will not result in wider switching frequency range. Therefore, in the design k and Q of resonant tank, only effect of A and Q on M under PFM control mode is considered.
Fig.7 k, O area meeting the voltage gain and ZVS k and Q, shown as overlapped parts in Fig.7, satisfies both Inequality (2) and (4) . Then finding & and Q to maximize the multiplication of & and Q can achieve optimal design.
Fig.7 k, O area meeting the voltage gain and ZVS k and Q, shown as overlapped parts in Fig.7, satisfies both Inequality (2) and (4) . Then finding & and Q to maximize the multiplication of & and Q can achieve optimal design.
f=83.8kHz @ input voltage 39V (under PFM +PS-PWM mode) Using the control scheme and design parameters, simulations of the PVMI are carried. Some key waveforms are shown in Fig.8. vgs, Vas, ip, Vp, V4B, itr represent drive signal of MOSFET, voltage across drain and source, current and voltage of rectifier diode, voltage across A and B in Fig.1, current of resonant inductor, respectively. It is can be seen that, the LLC converter operates with resonant frequency of 65.5 kHz at nominal input voltage 31V and full-load, while operates with PFM+PS-PWM mode, and f;= 83.8 kHz, D=0.2 at input voltage of 39V and full-load. The switching frequency ranges from 47.3 kHz to 111 kHz. The MOSFETs operate with ZVS under full input voltage and loads. It should be noted that ZCS for the diodes, which is in favor of suppressing voltage spike, is also achieved even when f; is higher than f,. The reason is hat the diodes current lead because of phase shift. This ZCS behavior is great different with that under single PFM control .The simulation has good agreement with the design. Fig.8.Key waveforms of LLC converter vs. input voltage
f=83.8kHz @ input voltage 39V (under PFM +PS-PWM mode) Using the control scheme and design parameters, simulations of the PVMI are carried. Some key waveforms are shown in Fig.8. vgs, Vas, ip, Vp, V4B, itr represent drive signal of MOSFET, voltage across drain and source, current and voltage of rectifier diode, voltage across A and B in Fig.1, current of resonant inductor, respectively. It is can be seen that, the LLC converter operates with resonant frequency of 65.5 kHz at nominal input voltage 31V and full-load, while operates with PFM+PS-PWM mode, and f;= 83.8 kHz, D=0.2 at input voltage of 39V and full-load. The switching frequency ranges from 47.3 kHz to 111 kHz. The MOSFETs operate with ZVS under full input voltage and loads. It should be noted that ZCS for the diodes, which is in favor of suppressing voltage spike, is also achieved even when f; is higher than f,. The reason is hat the diodes current lead because of phase shift. This ZCS behavior is great different with that under single PFM control .The simulation has good agreement with the design. Fig.8.Key waveforms of LLC converter vs. input voltage
f=65.5kHz@ input voltage 31V (under PFM mode)
f=65.5kHz@ input voltage 31V (under PFM mode)
4.2 PVMI prototype
4.2 PVMI prototype
fs = 49.1kHz @ input voltage V,y= 25V, output power P,=250W
fs = 49.1kHz @ input voltage V,y= 25V, output power P,=250W
Jf: =63.6kHz @ input voltage V,,= 31V, output power P,=250W
Jf: =63.6kHz @ input voltage V,,= 31V, output power P,=250W
The MPPT key waveforms are given in Fig.13. Under the test, the Regatron TCP.16.800.400.PV is used as PV simulator. The maximum power point is set to Pyp=250W and Vyp=36V. vpy, ipy , ppv iS output voltage, current and power of the
The MPPT key waveforms are given in Fig.13. Under the test, the Regatron TCP.16.800.400.PV is used as PV simulator. The maximum power point is set to Pyp=250W and Vyp=36V. vpy, ipy , ppv iS output voltage, current and power of the
simulator, and ig,iqis the injected current of the inverter. The test shows that the inverter can search to the maximum power point quickly, and has good MPPT accuracy up to 99.3%. Fig.12 efficiency of the PVMI vs. V,@ Vgrid=220Vac
simulator, and ig,iqis the injected current of the inverter. The test shows that the inverter can search to the maximum power point quickly, and has good MPPT accuracy up to 99.3%. Fig.12 efficiency of the PVMI vs. V,@ Vgrid=220Vac
Fig. 13 MPPT key waveform @ Pyp=250W, Viyp=36V V. Conclusion The PVMI will meet more challenge of achieving high efficiency because of wide-range input voltage, A PVMI with two high-frequency power stages is proposed, and the full bridge LLC resonant converter with hybrid control is used for the first power stage to meet PV wide range input voltage. The LLC resonant converter operates PFM mode when switching frequency is lower than the resonant frequency; otherwise, it
Fig. 13 MPPT key waveform @ Pyp=250W, Viyp=36V V. Conclusion The PVMI will meet more challenge of achieving high efficiency because of wide-range input voltage, A PVMI with two high-frequency power stages is proposed, and the full bridge LLC resonant converter with hybrid control is used for the first power stage to meet PV wide range input voltage. The LLC resonant converter operates PFM mode when switching frequency is lower than the resonant frequency; otherwise, it
Related topics:
EngineeringComputer SciencePulse Width ModulationMaximum Power Point TrackingPhotovoltaic systemLLC resonant converter
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