In this process, we will look at the main components and component selections in the design of the SMA "Sunny Boy" series of solar inverters, from Vishay's EMI suppression capacitors to Texas Instruments (TI)'s TMS320F2812 DSP, with special emphasis on isolation and protection , Through clever use of opto-isolated MOSFET gate drivers, such as Avago’s HCPL-316J and HCPL-J312.

Note: For more analysis on the often overlooked topic of optical isolation, we provide an in-depth video interview (see below).

Photovoltaic (PV) power systems consist of multiple components, such as photovoltaic solar panels that convert sunlight into electrical energy, mechanical and electrical connections and supports, and solar inverters, which are essential for delivering electricity generated by solar energy to the grid . Figure 1 shows a general but all-encompassing complete photovoltaic system block diagram.

What is a photovoltaic solar inverter?

The main function of the inverter is to convert the variable voltage direct current from the sunlight on the photovoltaic panel or battery storage into a specific alternating current voltage and frequency, which the power supply uses and feeds back to the grid. Of course, the AC output varies from region to region. North America uses 60-Hz 115 VAC, and most of Europe uses 50-Hz 230 VAC.

Entered SMA Solar Technology AG, headquartered in Germany, with the "Sunny Boy" series of solar inverters. The inverter boards we see in Figure 2 are used in Sunny Boy 3000TL, 4000TL and 5000TL transformerless versions, with rated powers of 3kW, 4kW and 4.6kW AC output power systems (@230v, 50 Hz), respectively. 

The inverter card uses multi-string technology with two independent DC converters, making highly complex generator configurations easy to implement. This part of the input is visible in the lower left quadrant of Figure 2. Each of the two DC inputs uses Vishay EMI suppression capacitor #339MKP as part of the filter, which also includes a DC common-mode filter inductor wound on a common core and a 15 uF boost converter smoothing capacitor # MKPC4AE series, as shown in the left quadrant of Figure 2 below.

Also on the DC input side, two relays are used to monitor the insulation resistance in pure IT AC systems according to IEC 61557-8. See Figure 2 upper left quadrant.

What is measured is the insulation resistance between the system circuit and the system ground. When it is lower than the adjustable threshold, the output relay switches to a fault state.

For these relays, the superimposed DC measurement signal is used for measurement. Calculate the insulation resistance value of the system under test from the superimposed DC measurement voltage and its combined current. Note the Hall-effect current measurement sensor in Figure 2.

One of the most impressive features of this SMA inverter card is the use of very high-quality active and passive components, which enhance the reliability and performance of the power inverter design.

The first DC function encountered in the signal chain is the MPP function.

This inverter task can compensate for environmental conditions that affect power output. For example, the output voltage and current of photovoltaic panels are extremely susceptible to temperature and light intensity per unit cell area (called "irradiance"). The battery output voltage is inversely proportional to the battery temperature, and the battery current is directly proportional to the irradiance.

Extensive changes in these and other key parameters result in significant changes in the optimal inverter voltage/current operating point. The inverter solves this problem by using closed-loop control to maintain operation under the so-called MPP, where the product of voltage and current is at the highest value. SMA uses the OptiTrac global peak MPP tracker. The tried-and-tested operation tracker management system OptiTrac can find and use the best operation point. Although the photovoltaic power station with this additional function is partially shaded, it can still provide good output. The TI DSP controller is the brain behind Maximum Power Point Tracking (MPPT).

The most common algorithm for determining MPP is that the controller perturbs the panel's operating voltage and observes the output during each MPPT period. The algorithm continues to oscillate around the MPP over a wide enough range to avoid local but misleading peaks in the power curve caused by, for example, movement in the clouds or some other conditions affecting the curve. The perturbation and observation algorithm is inefficient because it oscillates away from the MPP in each cycle.

Another method is the incremental inductance algorithm, which solves the derivative of the power curve with respect to 0. According to the definition, it is a peak value and then stabilizes at the analytic voltage level. Although this method has no inefficiency caused by oscillation, it has other risks of inefficiency because it may stabilize at a local peak instead of MPP. The combined method maintains the level determined by the incremental inductance algorithm, but scans at intervals over a wider range to avoid selecting local peaks. Although this method is the most effective, it also requires maximum performance from the controller.

Figure 3 shows how the determination of MPP varies with different conditions.

Capacitors are generally used to store energy that must be stored and retrieved by the inverter. This capacitor is usually located on the PV bus and must be large enough to control the voltage ripple on the bus. Otherwise, this ripple will be detrimental to MPPT accuracy.

Electrolytic capacitors are very suitable for controlling ripple because of their low equivalent series resistance (ESR) and high capacitance per unit volume. Groups of smoothing capacitors can be seen along the top edge of the printed circuit board in Figure 2.

Next is the step-up DC-DC converter, which can boost the DC input to the switching MOSFET bridge so that the inverter can effectively generate a 230V, 50Hz AC sine wave to send to the grid. The DC-DC boost converter is included in a separate power module connected to the back of the inverter card along with the H5 switch bridge. The module dissipates heat well to the chassis. Please refer to the upper middle area of ​​the circuit board in Figure 2. The module will be installed in the final assembly.

Figure 4 shows the basic DC/AC conversion circuit or inverter necessary in a typical transformerless configuration, where:

The idea behind transformerless switches existed long before the development of the photovoltaic market. Equipment engineers already know that a pair of field-effect transistors work best when they are fully on or off, when no current flows through them, and they do not dissipate power. Therefore, theoretically, the efficiency of amplifying an ideal square wave is 100%.

If a signal is modulated by a higher frequency square wave, the result is pulse width modulation (PWM), and the corresponding circuit is called class D. In this way, it is possible to convert direct current to direct current, or effectively switch direct current to alternating current. For solar inverters, the technology could not be used in the past due to the high cost of switching MOSFETs and IGBTs. However, these devices are becoming cheaper and faster every year, so this technology is more cost-effective than analog switching to large amounts of copper and iron. The same technology makes electric vehicles feasible.

Transformerless inverters have been on the market in Europe for many years. SMA obtained UL certification in August 2010 and can be distributed in the United States. This certification applies to SMA's transformerless Sunny Boy 8000TL-US, Sunny Boy 9000TL-US and Sunny Boy 10000TL-US inverters, and is certified for compliance with "UL Standard 1741 for Photovoltaic and Battery Powered Inverters", among which For the first time, the requirements for transformerless inverters are included. Transformerless inverters are much lighter than their electrically isolated inverters, and because of their advanced switching circuits, they can provide a wider operating voltage range than traditional inverters.

The disadvantage of no galvanic isolation is that ground faults can damage the inverter and cause electrical fires. For a transformer, if the secondary is short-circuited, all current will flow through the primary, and once the transformer overheats, it will (hopefully) be stopped by a thermal cutout. If not, if there is no protection, or if the protection fails to detect a ground fault and trips, the large MOSFET or IGBT will immediately fail in a rather catastrophic manner. Fortunately, the possibility of such an event is extremely small, and all such inverters must be protected against ground faults in accordance with UL 1741. However, the burden on the installer is still to ensure that the feedback current is taken into account when determining the size of the combiner and disconnecting the fuse without detecting a ground fault.

Therefore, as long as the correct simple calculations are performed, the disadvantages of transformerless inverters are few and the benefits are many. However, photovoltaic inverters also provide many other key functions. Photovoltaic inverters also provide a grid disconnection function to prevent the photovoltaic system from supplying power to a disconnected utility; that is, an inverter that remains online during a grid disconnection or transmission of power through an unreliable connection can cause Photovoltaic systems feed back to local utility transformers, generating thousands of volts on utility poles and endangering utility workers. Safety standard specifications IEEE 1547 and UL 1741 stipulate that when the AC line voltage or frequency is not within the specified limits, all grid-connected inverters must be disconnected, and if the grid no longer exists, they must be shut down. After reconnecting, the inverter can only supply power after detecting the rated mains voltage and frequency within five minutes. This can be seen in the upper right quadrant of Figure 2, using four LF-G grid safety cut-off relays rated at 22A and 250VAC. But again, this is not the end of the inverter's responsibilities. In addition to these tasks, the inverter also supports manual and automatic input/output disconnect service operations, EMI/RFI conduction and radiation suppression, ground fault interruption, PC-compatible communication interface (Bluetooth of the "Sunny Boy" series) and more. The inverter uses a rugged and durable package and is expected to operate outdoors at full power for more than 25 years! A typical single-phase PV inverter like an SMA board uses a digital power controller, DSP, and a pair of high-side/low-side gate drivers to drive a pulse-width modulation (PWM) full-bridge converter. The full H-bridge topology is used for this and many excellent inverter applications because it has the highest power carrying capacity of any switch-mode topology. SMA uses H5 technology, where the fifth power semiconductor between the input capacitor and the H-bridge suppresses the oscillation caused by the charge loss, and again significantly reduces the power loss. H5 is a significant improvement on the classic inverter bridge circuit (H4 topology), with a maximum conversion efficiency of 98%. In order to prevent the potential fluctuation of the photovoltaic generator, this architecture disconnects the DC side from the AC side during the freewheeling period of the inverter. Compared with the normal full H4 bridge seen in Figure 4, the H5 topology shown in Figure 5 only requires one more switch. The switches T1, T2 and T4 operate at a high frequency around 20 kHz, and T1 and T3 operate at the grid frequency, in this case 50 Hz. During the freewheeling period, T5 opens, disconnecting the DC side and the AC side. The freewheeling path is closed by the reverse diodes of T1 and T3 for positive current, and the diodes of T3 and T1 for negative current.

Therefore, as long as the correct simple calculations are performed, the disadvantages of transformerless inverters are few and the benefits are many.

However, photovoltaic inverters also provide many other key functions.

Photovoltaic inverters also provide a grid disconnection function to prevent the photovoltaic system from supplying power to a disconnected utility; that is, an inverter that remains online during a grid disconnection or transmission of power through an unreliable connection can cause Photovoltaic systems feed back to local utility transformers, generating thousands of volts on utility poles and endangering utility workers. Safety standard specifications IEEE 1547 and UL 1741 stipulate that when the AC line voltage or frequency is not within the specified limits, all grid-connected inverters must be disconnected, and if the grid no longer exists, they must be shut down. After reconnecting, the inverter can only supply power after detecting the rated mains voltage and frequency within five minutes. This can be seen in the upper right quadrant of Figure 2, using four LF-G grid safety cut-off relays rated at 22A and 250VAC.

But again, this is not the end of the inverter's responsibilities. In addition to these tasks, the inverter also supports manual and automatic input/output disconnect service operations, EMI/RFI conduction and radiation suppression, ground fault interruption, PC-compatible communication interface (Bluetooth of the "Sunny Boy" series) and more. The inverter uses a rugged and durable package and is expected to operate outdoors at full power for more than 25 years!

A typical single-phase PV inverter like an SMA board uses a digital power controller, DSP, and a pair of high-side/low-side gate drivers to drive a pulse-width modulation (PWM) full-bridge converter. The full H-bridge topology is used for this and many excellent inverter applications because it has the highest power carrying capacity of any switch-mode topology. SMA uses H5 technology, where the fifth power semiconductor between the input capacitor and the H-bridge suppresses the oscillation caused by the charge loss, and again significantly reduces the power loss. H5 is a significant improvement on the classic inverter bridge circuit (H4 topology), with a maximum conversion efficiency of 98%. In order to prevent the potential fluctuation of the photovoltaic generator, this architecture disconnects the DC side from the AC side during the freewheeling period of the inverter.

Compared with the normal full H4 bridge seen in Figure 4, the H5 topology shown in Figure 5 only requires one more switch. The switches T1, T2 and T4 operate at a high frequency around 20 kHz, and T1 and T3 operate at the grid frequency, in this case 50 Hz. During the freewheeling period, T5 opens, disconnecting the DC side and the AC side. The freewheeling path is closed by the reverse diodes of T1 and T3 for positive current, and the diodes of T3 and T1 for negative current.

The PWM voltage switching action synthesizes a discrete but noisy 50 Hz current waveform at the output of the full bridge. The high-frequency noise component is filtered by an inductance to produce a 50 Hz sine wave with a moderate amplitude. The H-bridge works through asymmetric unipolar modulation. The high end of the asymmetric H-bridge should be driven by a 50Hz half-wave, depending on the polarity of the power supply, while the opposite low end is PWM modulated to form a sinusoidal shape of the power supply. You will notice that the AC output filter part in Figure 2 has an EMI suppression capacitor on the right side of the inverter card. The output sine filter containing large inductance will also be bolted to this card in this area to complete the AC filter.

Photovoltaic inverter design requires many design compromises, and if you make the wrong trade-offs, it may cause the designer's heartache. For example, photovoltaic systems are expected to operate reliably at full rated output for at least 25 years, but they require competitive prices, forcing designers to make difficult trade-offs between cost/reliability. Photovoltaic systems require high-efficiency inverters because, compared with less efficient inverters, more efficient inverters run at lower temperatures and have a longer service life, and they save money for photovoltaic system manufacturers and users. SMA has done a very good job in this regard. The "brain" behind the control architecture inverter is its controller, which in this case is usually a digital power controller (DPC) or a digital signal processor (DSP). Digital signal processor (DSP)-based controllers, such as the Texas Instruments TMS320F2812 in this design, provide the high level of computing performance and programming flexibility required for real-time signal processing in solar inverters. Highly integrated digital signal controllers help inverter manufacturers create more efficient and cost-effective products to support the growing demand for solar energy in the coming years. The inverter's control processor must meet many real-time processing challenges to effectively execute the precise algorithms required for efficient DC/AC conversion and circuit protection. Although MPPT and battery charge control only require near real-time response, they do involve algorithms with advanced processing capabilities. The digital signal controller combined with high-performance DSP and integrated control peripherals provides an excellent solution for the real-time control of DC/AC converter bridges, MPPT and protection circuits in solar inverters. The DSP controller essentially supports high-speed mathematical calculations for real-time control algorithms. Integrated peripherals such as analog-to-digital converter (ADC) and pulse width modulation output (PWM) can directly detect the input and control the power IGBT or MOSFET, thereby saving system space and cost. On-chip flash memory facilitates programming and data collection, and the communication port simplifies the design of networking with meters and other inverters. The higher efficiency of the DSP controller in the solar inverter has been proved by the design report, the conversion efficiency loss has been reduced by more than 50%, and a significant cost reduction has been achieved. Generally, the controller's firmware is implemented in a state machine format to achieve the most efficient execution using non-blocking (failure) code, thereby preventing execution from inadvertently entering an infinite loop. Firmware execution is hierarchical, usually serving the highest priority functions more frequently than lower-level functions. In the case of photovoltaic inverters, isolated feedback loop compensation and power switch modulation are usually the highest priority, followed by key protection functions supporting safety standards, and finally efficiency control or maximum power point (MPP). The remaining firmware tasks mainly involve optimizing the operation of the current operating point, monitoring system operation, and supporting system communication. Integrated functions can maintain cost-effectiveness while the system is running. TI's TMS320F2812 controller has an ultra-fast 12-bit ADC that can provide up to 16 input channels to perform the current and voltage detection required to achieve a regular sinusoidal waveform. For safety reasons, the ADC can also provide current sensing in a residual current protection device (RCD). Twelve independently controlled enhanced PWM (EPWM) channels provide variable duty cycles for high-speed switching in the converter bridge and battery charging circuit. Each EPWM has its own timer and phase register, allowing the phase delay to be programmed, and all EPWMs can be synchronized to drive multiple stages at the same frequency. Multiple timers can access multiple frequencies, and fast interrupt management can be used to support additional control tasks. Multiple standard communication ports including CAN bus provide simple interfaces for other components and systems. isolation

Photovoltaic inverter design requires many design compromises, and if you make the wrong trade-offs, it may cause the designer's heartache. For example, photovoltaic systems are expected to operate reliably at full rated output for at least 25 years, but they require competitive prices, forcing designers to make difficult trade-offs between cost/reliability. Photovoltaic systems require high-efficiency inverters because, compared with less efficient inverters, more efficient inverters run at lower temperatures and have a longer service life, and they save money for photovoltaic system manufacturers and users. SMA has done a very good job in this regard.

The "brain" behind the inverter is its controller, which in this case is usually a digital power controller (DPC) or a digital signal processor (DSP). Digital signal processor (DSP)-based controllers, such as the Texas Instruments TMS320F2812 in this design, provide the high level of computing performance and programming flexibility required for real-time signal processing in solar inverters. Highly integrated digital signal controllers help inverter manufacturers create more efficient and cost-effective products to support the growing demand for solar energy in the coming years.

The inverter's control processor must meet many real-time processing challenges to effectively execute the precise algorithms required for efficient DC/AC conversion and circuit protection. Although MPPT and battery charge control only require near real-time response, they do involve algorithms with advanced processing capabilities. The digital signal controller combined with high-performance DSP and integrated control peripherals provides an excellent solution for the real-time control of DC/AC converter bridges, MPPT and protection circuits in solar inverters. The DSP controller essentially supports high-speed mathematical calculations for real-time control algorithms.

Integrated peripherals such as analog-to-digital converter (ADC) and pulse width modulation output (PWM) can directly detect the input and control the power IGBT or MOSFET, thereby saving system space and cost. On-chip flash memory facilitates programming and data collection, and the communication port simplifies the design of networking with meters and other inverters. The higher efficiency of the DSP controller in the solar inverter has been proved by the design report, the conversion efficiency loss has been reduced by more than 50%, and a significant cost reduction has been achieved.

Generally, the controller's firmware is implemented in a state machine format to achieve the most efficient execution using non-blocking (failure) code, thereby preventing execution from inadvertently entering an infinite loop. Firmware execution is hierarchical, usually serving the highest priority functions more frequently than lower-level functions. In the case of photovoltaic inverters, isolated feedback loop compensation and power switch modulation are usually the highest priority, followed by key protection functions supporting safety standards, and finally efficiency control or maximum power point (MPP). The remaining firmware tasks mainly involve optimizing the operation of the current operating point, monitoring system operation, and supporting system communication.

Integrated functions can maintain cost-effectiveness while the system is running. TI's TMS320F2812 controller has an ultra-fast 12-bit ADC that can provide up to 16 input channels to perform the current and voltage detection required to achieve a regular sinusoidal waveform. For safety reasons, the ADC can also provide current sensing in a residual current protection device (RCD).

Twelve independently controlled enhanced PWM (EPWM) channels provide variable duty cycles for high-speed switching in the converter bridge and battery charging circuit. Each EPWM has its own timer and phase register, allowing the phase delay to be programmed, and all EPWMs can be synchronized to drive multiple stages at the same frequency. Multiple timers can access multiple frequencies, and fast interrupt management can be used to support additional control tasks. Multiple standard communication ports including CAN bus provide simple interfaces for other components and systems.

In the center of the SMA inverter card, we will find five Avago isolated gate drivers. See Figure 2.

The two isolated MOSFET drivers that control the T1 and T3 switches at a grid frequency of 50 Hz are Avago HCPL-316J, a 2.5A gate drive optocoupler with integrated (VCE) desaturation detection and fault state feedback. The other three isolated MOSFET drivers that control the T2, T4, and T5 switches at higher frequencies are the Avago HCPL-J312, 2.5A output current MOSFET gate drive optocoupler. See Figure 5 for H5 configuration. Especially in the design of transformerless inverters, optocouplers provide reinforced insulation and provide fail-safe protection in the event of a fault. Why is reactive power control important in photovoltaic inverters1? "Sunny Boy" model 3000TL/4000TL/5000TL can provide reactive power control. Reactive power usually occurs at any time when energy is transferred through alternating current. Its importance to solar engineers and photovoltaic system operators is increasing, both for large and small systems. The most important realization: There is no problem with reactive power at all. It is actually a solution to some problems. On July 1, 2010, the photovoltaic system fed into the grid at a medium voltage level in Germany must be able to provide reactive power to the grid. This is explained in the 2008 edition of the Medium Voltage Guidelines of the German Federal Energy and Water Industry Association. For low-voltage power grids, more stringent requirements are being discussed. How does reactive power develop? For DC, the equation is very simple: electrical power is the product of voltage and current. However, with alternating current, things are slightly more complicated, because the intensity and direction of current and voltage change regularly here. See Figure 7.

The two isolated MOSFET drivers that control the T1 and T3 switches at a grid frequency of 50 Hz are Avago HCPL-316J, a 2.5A gate drive optocoupler with integrated (VCE) desaturation detection and fault state feedback. The other three isolated MOSFET drivers that control the T2, T4, and T5 switches at higher frequencies are the Avago HCPL-J312, 2.5A output current MOSFET gate drive optocoupler. See Figure 5 for H5 configuration.

Especially in the design of transformerless inverters, optocouplers provide reinforced insulation and provide fail-safe protection in the event of a fault.

Why is reactive power control important in photovoltaic inverters1?

"Sunny Boy" model 3000TL/4000TL/5000TL can provide reactive power control.

Reactive power usually occurs at any time when energy is transferred through alternating current. Its importance to solar engineers and photovoltaic system operators is increasing, both for large and small systems. The most important realization: There is no problem with reactive power at all. It is actually a solution to some problems.

On July 1, 2010, the photovoltaic system fed into the grid at a medium voltage level in Germany must be able to provide reactive power to the grid. This is explained in the 2008 edition of the Medium Voltage Guidelines of the German Federal Energy and Water Industry Association. For low-voltage power grids, more stringent requirements are being discussed.

How does reactive power develop?

For DC, the equation is very simple: electrical power is the product of voltage and current. However, with alternating current, things are slightly more complicated, because the intensity and direction of current and voltage change regularly here. See Figure 7.

In the public grid, both have sinusoidal trajectories with a frequency of 50 or 60 Hz. As long as the current and voltage are "in phase", that is, moving at the same rhythm, the product of these two oscillation factors will also be an oscillating output with a positive average value-pure active power (Figure 8a).

However, once the sinusoidal trajectories of current and voltage are shifted from each other, their product will be an output with alternating positive and negative signs. In extreme cases, the current and voltage will undergo a quarter-cycle phase shift: when the voltage is zero, the current always reaches its maximum intensity-and vice versa. Result: pure reactive power, the sign completely cancels each other out (Figure 8b).

This phase shift will naturally occur in two directions. It occurs with coils and capacitors in AC circuits-this is usually the case: all motors or transformers have coils (for inductive displacement); capacitors (for capacitance displacement) are also common.

Multi-core cables also function as capacitors, while high-voltage overhead lines can be regarded as extremely long coils. Therefore, a certain degree of phase shift, that is, reactive power, cannot be avoided in the AC power grid. The measurement parameter of phase shift is the shift factor cos(φ), and its value can be between 0 and 1. It can be used to easily convert the output value. The unit of measurement of reactive power is called volt-ampere reactive power (VAR), not watts (see formula 1)

Multi-core cables also function as capacitors, while high-voltage overhead lines can be regarded as extremely long coils. Therefore, a certain degree of phase shift, that is, reactive power, cannot be avoided in the AC power grid. The measurement parameter of phase shift is the shift factor cos(φ), and its value can be between 0 and 1. It can be used to easily convert the output value. The unit of measurement of reactive power is called volt-ampere reactive power (VAR), not watts (see formula 1)

What impact does reactive power have on the grid?

Only active power is the real usable power. It can be used to power machines, make lights glow, or operate electric heaters. Reactive power is different: it cannot be consumed and therefore cannot power any electronic equipment. It just moves back and forth in the grid, thereby acting as an additional load. All cables, switches, transformers and other components require additional consideration of reactive power. This means that they need to be designed for apparent power (the geometric sum of active and reactive power). The ohmic loss during energy conduction is based on apparent power; therefore, additional reactive power will result in greater conduction loss.

Only active power is the real usable power. It can be used to power machines, make lights glow, or operate electric heaters. Reactive power is different: it cannot be consumed and therefore cannot power any electronic equipment. It just moves back and forth in the grid, thereby acting as an additional load. All cables, switches, transformers and other components require additional consideration of reactive power.

This means that they need to be designed for apparent power (the geometric sum of active and reactive power). The ohmic loss during energy conduction is based on apparent power; therefore, additional reactive power will result in greater conduction loss.

Go forward

Photovoltaic systems are a newcomer in the field of energy production. Like other emerging technologies, as the technology matures, photovoltaic systems will undergo rapid changes. Therefore, photovoltaic systems will undoubtedly continue to develop to meet the market's demand for higher capacity, lower cost and higher reliability. As this happens, the functionality of photovoltaic inverters will expand, and designers will need more integrated, application-specific, component-level equipment. As these events unfold, photovoltaic power generation systems will become more common and eventually become a viable part of the mainstream utility, thereby significantly reducing our dependence on fossil fuels.

Thank you for providing detailed information on such related topics. I am curious about the application of dumping back to the grid. I did not see in the block diagram or disassembly how the inverter phase matches the phase part of the grid to which it is connected

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