NL2008098C2

NL2008098C2 - Power converter for photovoltaic modules, photovoltaic modules and method.

Info

Publication number: NL2008098C2
Application number: NL2008098A
Authority: NL
Inventors: Bastiaan Joannes Dani L Vermulst
Original assignee: Prodrive B V
Priority date: 2012-01-11
Filing date: 2012-01-11
Publication date: 2013-07-15

Description

Power converter for photovoltaic modules, photovoltaic modules and method

Fossil fuels, such as coal, oil and gas are used all over the world as the main energy sources, while burning these fuels develops greenhouse gas and causes serious 5 environmental pollution. Furthermore, fossil fuels are not a sustainable source of energy, while, concurrently, renewable energy is sustainable, characterized by low greenhouse gas emission and little environmental pollution. Several renewable energy sources exist, using for instance hydro power, tidal power and photovoltaic power.

10 Photovoltaic energy is commonly produced using the photovoltaic effect by converting photons into moving electrons, which is done with a photovoltaic cell, for example a c-Si (crystalline silicon) cell. A typical c-Si cell reaches an open circuit output voltage of 0.6 V and achieves maximum output power at an output voltage of approximately 0.5 V, which varies with, among others, changing irradiance and temperature. The 15 maximum output power voltage of one single cell is too low for efficient conversion to the mains voltage and therefore many photovoltaic cells (typically 60) are connected in series to form a photovoltaic module. Consecutively, a number of these photovoltaic modules are connected in series to form a photovoltaic array which is connected to a central electric power inverter.

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However, this configuration has disadvantages. Even if all elements would operate with ideal 100% efficiency, the series connection of the cells and modules may prohibit modules from operation at maximum output power and therefore no maximum amount of energy is obtained from the system. Another problem is a lack of information 25 regarding the operating conditions of each module.

Concepts proposing a better performance compared to central-inverter systems are known. The publication “Module-level dc/dc conversion for photovoltaic systems,” in 33rd International Telecommunications Energy Conference 2011, October 2011, pp. 1-30 9” shows a method for improving performance of a photovoltaic array under partially shaded conditions by bypassing excess current, allowing all modules to achieve maximum output power conditions. However, while module-level DC/DC converters are cheap and reliable, the continued need for a central inverter with its single point-of-failure and non-modular property is disadvantageous.

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For this reason, systems using de-central inverters (or micro inverters) have been proposed. The publication “Module integrated electronics - an overview,” in In proc., 25th European Photovoltaic Solar Energy Converence, September 2010, pp. 3708 -5 3714 gives an example. Although decentral inverters do not suffer the aforementioned issues of lacking modularity, this concept still has disadvantages. One problem is that the increased amount of electronics in the system requires a very low failure rate per inverter in order to attain a failure rate comparable with the central-inverter system. Moreover, reaching an efficiency comparable to central inverters is problematic because 10 of the much higher voltage gain required (30 V input instead of 100 V-600 V input) and the larger amount of stand-by power needed as there are more electronics involved.

The publication “An Isolated High Step-Up Forward/Flyback Active-Clamp Converter with Output Voltage Lift” by PH. Kuo et al., IEEE, 2010 describes an inverter for 15 obtaining such high voltage gain. However, increasing demands for efficiency and reliability still require a de-central inverter for photovoltaic modules with a higher efficiency and longer lifetime. It is therefore a goal of the present invention to attain a near-optimal topology from efficiency and reliability point of view.

20 The present invention therefore proposes a power converter according to claim 1. The thus obtained converter is advantageous because of its low complexity. While resonant full-bridge converters require complex control in order to allow soft-switching, the converter allows to be duty-cycle controlled. Furthermore, the flyback converter requires one or two switches, while a full-bridge converter requires four switches.

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The equal orientation of the primary and the secondary winding of the transformer delivers multiple advantages over the Isolated High Step-Up Forward/Flyback Active-Clamp Converter with Output Voltage Lift according to the prior art, because it results in different behaviour.

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Firstly, the converter according to the prior art is only able to transfer energy to the output when the main switch is off. As a result, the bandwidth that can be achieved is a factor 2 lower compared to the solution according to the present invention. The bandwith here is a ratio between the response time of the converter and the switching 3 frequency. In the topology according to the art, the main switch needs to be switched on for charging energy, which is released only when the main switch is switched off (flyback mode). The topology according to the present invention leads to direct energy output when the main switch is switched on, which enables for instance to respond 5 quickly to current peaks at the output side. Another advantage is that this direct control takes away the disadvantage of instabilities caused by the slow response times according to the prior art.

A further advantage with respect to the state of the art is that the output current ripple 10 and the transformer winding ratio are inversely proportional, while for the proposed solution both are straightforward proportional. This allows smaller output current ripple with smaller transformer winding ratio.

In an embodiment, the power converter has an isolated topology, which reduces 15 common-mode (leakage) current and reduces requirements on isolation of the Photovoltaic module, both adding to the flexibility and usability of the system.

The choice for an isolating transformer, which typically has a lower efficiency compared to inductor-based step-up converters, is still proposed in this embodiment 20 despite of prejudice according to the art.

In a practical realisation, active-clamping is realised with a resonance-capacitor (Cr), connected in parallel to the switch, and a switchable clamping-capacitor (Cl) with a serial switch (SW2), connected in parallel to the primary winding.

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The voltage-lifting cell may be composed of a voltage lifting capacitor, connected to the secondary winding of the converter, a first voltage-lifting diode, coupled between the secondary winding of the converter and the voltage-lifting capacitor, and a second voltage-lifting diode, coupled between a cathode of the first voltage-lifting diode and a 30 positive power-output clamp.

The converter according to the invention is distinguished from a fullbridge converter by the current waveshapes, because both the input and output of the converter have a large current ripple. However, because the converter in this case is a step-up converter with relatively low output currents, the output current ripple does not have to be an issue.

4 A single converter without interleaving may be used when a high reliability and/or low 5 costs are required. A reduced number of switches is also beneficial for designing the reinforced isolation barrier between a safety-extra-low voltage input and the output, for instance a mains connection, because less gatedrive signals have to cross the isolation barrier. A relatively large input filter may be required to reduce the converter input current ripple.

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The large input current ripple can also be reduced by using two or more interleaved converters. This maintains control simplicity and, more importantly, allows more flexibility on transformer load distribution compared to a single full-bridge converter. For an interleaved converter it is possible to implement active-clamping with zero-15 voltage, zero-current switching while folding both stages into each other.

The converter input power (i.e. input voltage and current) must be ripple-free to ensure the maximum-power-point tracking operates accurately. However, the inverter output power has a large ripple because of the single-phase alternating output current. This 20 energy may be stored by using a capacitor, preferably at the output of the converter at a high operating voltage.

The topology can be further compacted with a buck converter generating rectified sine waves and a rectifier stage, while the buck converter can also be used as one leg of the 25 half-bridge. This step reduces cost by reducing the number of high-voltage switches by one third, increases efficiency as the total series resistance in the current path is now lower (less switches in series) and also increases reliability because less components are used while keeping equal component stress. A downside of integrating the buck converter and the inverter is the voltage step which must be made by the buck 30 converter. The transient could cause electromagnetic interference issues if it is not handled properly.

The invention will now be elucidated with reference to the following figures, wherein: - Figure 1 shows a topology of a power converter according to the invention; 5 - Figure 2 shows a transformer-less model of the converter from figure 1; - Figure 3 shows voltage-lift-cell modes in de converter from figures 1 and 2; - Figure 4 shows an overview of wave shapes when the converter is operated; - Figures 5a and 5b show the converter with active clamp using hard-switching at turn-5 off and the current through the transformer at main-switch turnoff; and - Figure 6a and 6b show waveforms in case of hard-switching and soft-switching.

Figure 1 shows a topology of a power converter for photovoltaic modules according to the invention. The power converter, comprises a transformer, having a primary winding 10 and a secondary winding, a main switch SW1, for switching the primary winding to a power source, an active-clamping module, connected to the primary winding, the active-clamping module comprising a resonance-capacitor (Cr), connected in parallel to the switch, and a switchable clamping-capacitor (Cl) with a serial switch (SW2), connected in parallel to the primary winding. The converter further comprises a voltage-15 lifting module, connected to the secondary winding, and comprising a voltage-lifting capacitor, connected to the secondary winding of the converter, a first voltage-lifting diode, coupled between the secondary winding of the converter and the voltage lifting capacitor; and a second voltage-lifting diode, coupled between a cathode of the first voltage-lifting diode and a positive power output clamp, wherein the orientation of the 20 primary and the secondary winding of the transformer is equal. As can be seen in figure 1, the transformer is an isolating transformer.

Because the transformer is non-ideal in practice it will feature, among others, some leakage and magnetizing inductance. In a normal flyback converter the magnetizing 25 inductance Lm is mostly used in order to transfer energy, whilst the energy stored in the leakage inductance Llk typically is unused and must be dissipated by an RCD-snubber network to prevent over-voltage on the primary-side switch. Various methods exist in order to capture the energy in the leakage inductance, increasing practical efficiency from 85% to 92%. One common method to capture this energy is to apply active-30 clamping (AC). By applying AC not only efficiency improves, but also the voltage wave-shape on the main switch becomes much cleaner and voltage stress is thereby reduced. Furthermore, it can be seen that the current through the primary inductor becomes negative as a result of the resonance between the clamp capacitor and the leakage (and partially magnetizing) inductance.

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The practical efficiency of 92% for a flyback converter using active clamping is still low compared to the 97% efficiency of central inverters. The lower efficiency is caused by high current ripple at the input and therefore a large RMS current, which results in 5 high resistive losses.

In order to reduce magnetic core losses it is preferred that the transformer is used in multiple quadrants with reduced peak excitation. Because a flyback and a forward converter require transformer magnetization in opposite directions, combining them is 10 beneficial for reduced transformer core losses and current ripple. Furthermore, if the output voltages of both converters are summed this reduces the required duty cycle and/or transformer winding ratio.

Because the resonance of the active-clamp circuit already achieves negative coil 15 currents on the primary side, the only restriction for using multiple quadrants is the output rectifier. This is resolved when storing the energy of the flyback phase in a capacitor and connecting this capacitor in series for the forward phase by using a voltage-lift cell (VLC).

20 Figure 2 shows a transformer-less model of the converter from figure 1.

Figure 3 shows voltage-lift-cell modes in de converter from figures 1 and 2. As can be seen from the voltage-lift-cell modes, in the flyback phase the cell node voltage Vn is clamped to ground through diode D1 while the voltage Vs is negative, thus discharging 25 the lift-capacitor. Now, when entering the forward mode the voltage Vs becomes positive and the lift-cell capacitor voltage VI is subtracted from Vs by superposition, boosting the output voltage. This process is illustrated in more detail in Fig. 4.

Figure 4 shows an overview of wave shapes when the converter is operated. Because no 30 silicon Schottky diodes are available at the high output voltage of the converter, having hard-switched diodes will cause large reverse-recovery losses. With this topology, current through the diodes reaches zero before the voltage is reversed. This means the diodes are soft-switched, hence have very low switching losses.

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Figure 5a shows the converter with active clamp using hard-switching at turn-off. However, reduced-voltage switching can be attained by adding a resonance capacitor Cr to the switch node.

5 Figure 5b shows the current through the resonance capacitor Cr at main-switch turn-off will then limit the dV/dt and allow softer turn-off as the voltage will increase at a more limited rate. The main-switch average power dissipation in this example can hereby be reduced from 1000 mW for the hard turn-off to 400 mW for soft turn-off using state-of-the-art switches.

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The maximum capacitor value must be selected in such way that the switch turn-on remains soft, in other words, the energy in the leakage inductance Llk must be larger compared to the energy in the resonance capacitor Cr to ensure Ilk will discharge Cr before the main switch turns on.

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The inverter actually consists of two stages, namely the buck and the unfolding stage. While the efficiency of the unfolding stage is solely determined by the MOSFETs conduction losses because of the low 50 Hz switching frequency, the efficiency of the buck stage is determined by more factors. Therefore, most effort has been put into 20 improving the buck-converter efficiency.

Normally the buck converter switches are hard-switched resulting in high switching losses, especially at high operating voltages. Some methods are available for achieving soft switching, even though some of them require additional components or have high 25 switch voltage stress. By understanding the issue of hard switching, a simple solution is proposed in order to obtain soft-switching solely by smart control algorithms.

Most of the switching losses at high voltage are caused by the reverse-recovery effect, which becomes noticeable when diodes are forced to block the current they are 30 conducting because of a reversed voltage.

Figure 6a shows the switch currents for a normal buck converter, and as can be seen the diode D2 turn-off is hard switched, caused by SW1 turning on. This can be prevented by simply allowing the coil current to change direction. This causes natural 8 commutation of the diodes and therefore removes the losses caused by the reverse-recovery effect. The trade-off here is the higher current ripple at the output and through the switches, requiring larger output filters and increasing the resistive losses.

5 When the inductor current ripple increases, the switching frequency decreases and the switching losses reduce, while the resistive losses increase because of the higher RMS current through the components. Therefore, there is a value for the ripple current which results in minimum total losses, where these losses consist of the sum of gate-charge losses, switching losses and resistive losses. Because switching energy (J) is constant, 10 the switching losses are linear with the switching frequency (Hz).

While the turn-on of the buck converter switches is zero-voltage, the turn-off is done at non-zero current and nonzero voltage, hence the switch will dissipate power. Reduced current and voltage turn-off, however, can be achieved by adding a resonance capacitor 15 to the switch node, in the same manner as applied to the DC/DC converter. In order to preserve soft turn-on the voltage at the switch node must reach the bus voltage, for which a minimum amount of negative current through the inductor, Ir min, is required to charge/discharge the switch node capacitance.

20 While the inverter is active there is current circulating through the buck-stage output inductor in order to maintain soft switching. This results in the inverter always dissipating power, even at zero output power.

The instantaneous output power increases when the instantaneous absolute mains 25 voltage increases. Because relative switching losses decrease for higher instantaneous output power, theoretically it is most efficient to enable the inverter only when the mains voltage reaches its maximum value.

Figure 6b shows the currents when soft-switching according to the present invention.

30 The proposed DC/DC converter is especially useful in high voltage-gain applications, because the voltage-lift cell allows a reduced transformer winding ratio, reducing the amount of wire required and therefore reducing resistive losses in the transformer. Furthermore, the voltage-lift cell introduces output diode soft-switching, allowing the use of cheap silicon diodes instead of silicon-carbide. Comparison of the proposed 9 DC/DC topology with the state of the art shows large improvement in efficiency over the whole power range. The proposed inverter is soft-switched (turn-on only) and does not suffer from reverse-recovery losses because of the inverter’s smart current-ripple scaling.

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Claims

1. Vermogenomvormer voor fotovoltaïsche modules, omvattende - Een transformator, met een eerste wikkeling en een tweede wikkeling;A power converter for photovoltaic modules, comprising - A transformer, with a first winding and a second winding;

2. Vermogenomvormer volgens conclusie 1, waarbij de transformator een isolerende transformator is.The power converter according to claim 1, wherein the transformer is an insulating transformer.

3. Vermogenomvormer volgens conclusie 1 of 2, waarbij de actieve begrenzingsmodule omvat: - Een resonantiecapaciteit (Cr), verbonden parallel aan de hoofdschakelaar; en - Een wisselbare begrenzingscapaciteit (Cl) met een serieschakelaar 20 (SW2), verbonden parallel aan de eerste wikkeling.A power converter according to claim 1 or 2, wherein the active limiting module comprises: - A resonance capacitance (Cr) connected in parallel to the main switch; and - An exchangeable limiting capacity (C1) with a series switch 20 (SW2) connected in parallel with the first winding.

4. Vermogenomvormer volgens één van de voorgaande conclusies, waarbij de spanningsverhoogmodule omvat: - Een spanningsverhoogcapaciteit, verbonden aan de tweede wikkeling 25 van de omvormer; - Een eerste spanningsverhoogdiode, gekoppeld tussen de tweede wikkeling van de omvormer en de spanningsverhoogcapaciteit; en - Een tweede spanningsverhoogdiode, gekoppeld tussen een kathode van de eerste spanningsverhoogdiode en een positieve 30 vermogenuitgangsbegrenzings.4. Power converter according to one of the preceding claims, wherein the voltage increase module comprises: - A voltage increase capacity, connected to the second winding of the converter; - A first voltage increase diode, coupled between the second winding of the inverter and the voltage increase capacity; and - A second voltage increase diode coupled between a cathode of the first voltage increase diode and a positive power output limit.

5. Vermogenomvormer volgens één van de voorgaande conclusies, waarbij een volledige brugomzetter wordt gekoppeld aan de spanningsverhoogmodule.5. Power converter according to one of the preceding claims, wherein a complete bridge converter is coupled to the voltage increase module.

5. Een hoofdschakelaar, voor het schakelen van de eerste wikkeling aan een vermogensbron; - Een actieve begrenzingsmodule, verbonden aan de eerste wikkeling; - Een spanningsverhoogmodule verbonden aan de tweede wikkeling; Met het kenmerk dat de oriëntatie van de eerste en de tweede wikkeling van de 10 transformator gelijk is5. A main switch, for switching the first winding to a power source; - An active limiting module connected to the first winding; - A voltage increase module connected to the second winding; With the characteristic that the orientation of the first and the second winding of the transformer is the same

6. Vermogenomvormer volgens één van de voorgaande conclusies en conclusie 3, omvattende een besturing voor het besturen van ten minste de hoofdschakelaar, waarbij de besturing is ingericht voor het tegenovergesteld aanschakelen van de hoofdschakelaar en de serieschakelaar. 56. Power converter as claimed in any of the foregoing claims and claim 3, comprising a control for controlling at least the main switch, wherein the control is adapted to switch on the main switch and the series switch in opposite direction. 5

7. Vermogenomvormer volgen conclusies 3-6, waarbij de besturing is ingericht voor het door software schakelen van de eerste en tweede spanningsverhoogdiode, door de hoofdschakelaar en de serieschakelaar beide uit te schakelen tussen het tegenovergestelde aanschakelen van de hoofdschakelaar 10 en de serieschakelaar.The power converter according to claims 3-6, wherein the control is adapted to switch the first and second voltage increase diodes by software, by switching off the main switch and the series switch both between the opposite switching on of the main switch 10 and the series switch.

8. Fotovoltaïsche module, gekoppeld aan een vermogenomvormer volgens één van de voorgaande conclusies.A photovoltaic module coupled to a power inverter according to any one of the preceding claims.

9. Fotovoltaïsche module volgens conclusie 8, gekoppeld aan twee of meer geluste omvormers volgens één van de voorgaande conclusies.Photovoltaic module according to claim 8, coupled to two or more looped inverters according to one of the preceding claims.

10. Werkwijze voor het bedienden van een vermogenomvormer volgens één van de voorgaande conclusies 1-5, omvattende het tegenovergesteld aanschakelen van 20 de hoofdschakelaar en de serieschakelaar.10. Method for operating a power converter according to one of the preceding claims 1-5, comprising the opposite switching on of the main switch and the series switch.

11. Werkwijze volgens conclusie 10, omvattende het zacht schakelen van de eerste en tweede spanningsverhoogdiode, door de hoofdschakelaar en de serieschakelaar beide uit de schakelen tussen het tegenovergesteld aanschakelen 25 van de hoofdschakelaar en de serieschakelaar.11. Method as claimed in claim 10, comprising soft switching of the first and second voltage-increasing diode by the main switch and the series switch both from switching between the opposite switching on of the main switch and the series switch.