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How To Ensure Solar Inverter Efficiency
- Apr 16, 2018 -

The general structure of the photovoltaic inverter is shown in Figure 1. Three different inverters are available. Sunlight shines on the solar modules connected in series, and each module contains a series of solar cells in series. The direct current (DC) voltage generated by the solar module is on the order of hundreds of volts, depending on the light conditions of the module array, the temperature of the battery, and the number of series modules.

The primary function of this type of inverter is to convert the input DC voltage to a stable value. This function is achieved through boost converters and requires boost switches and boost diodes.

In the first configuration, the boost stage is followed by an isolated full-bridge converter. The role of the full-bridge transformer is to provide isolation. The second full-bridge converter on the output is used to transform the DC-DC of the first-stage full-bridge converter into an alternating current (AC) voltage. Its output is then filtered before being connected to the AC grid network via an additional double-contact relay switch in order to provide safe isolation in the event of a fault and isolation from the power grid at night.

The second structure is a non-isolated scheme. Among them, the AC AC voltage is directly generated by the DC voltage output from the boost stage.

The third structure utilizes an innovative topology of power switches and power diodes to integrate the functions of the boost and AC AC generation sections in a dedicated topology.

Although the conversion efficiency of solar panels is very low, it is very important that the efficiency of the inverter be as close as possible to 100%. In Germany, a 3kW series module installed on a south-facing roof is expected to generate 2550 kWh per year. If the inverter efficiency increases from 95% to 96%, 25kWh can be generated annually. The use of additional solar modules to generate these 25kWh costs is equivalent to adding an inverter. Since efficiency increases from 95% to 96% will not double the inverter's cost, investing in more efficient inverters is an inevitable choice. For emerging designs, the most cost-effective way to improve inverter efficiency is the key design criterion.

The reliability and cost of the inverter are two other design criteria. Higher efficiency can reduce temperature fluctuations on the load cycle, thereby improving reliability, so these guidelines are actually related. The use of modules also improves reliability.

All topologies shown in Figure 1 require fast-switching power switches. Boost stages and full-bridge converter stages require fast switching diodes. In addition, switches optimized for low frequency (100Hz) conversion are also useful for these topologies. For any particular silicon technology, switches optimized for fast switching have higher conduction losses than switches optimized for low frequency switching applications.

Switches and Diodes for Boost Stages

The boost stage is generally designed as a continuous current mode converter. Depending on the number of solar modules in the array used by the inverter, a 600V or 1200V device is used.

The two options for power switching are MOSFET and IGBT. In general, MOSFETs can operate at higher switching frequencies than IGBTs. In addition, the effect of the body diode must always be taken into account: there is no problem in the boost stage because the body diode does not conduct in the normal operating mode. The MOSFET conduction loss can be calculated from the on-resistance RDS(ON), which is proportional to the effective die area for a given MOSFET family. When the rated voltage is changed from 600V to 1200V, the conduction loss of the MOSFET is greatly increased. Therefore, even if the rated RDS(ON) is equivalent, the 1200V MOSFET is not available or the price is too high.

Super-junction MOSFETs can be used for boost switches rated at 600V. For high frequency switching applications, this technology has the best conduction loss. There are currently available MOSFETs in the TO-220 package, RDS(ON) values below 100 milliohms, and MOSFETs in the TO-247 package with RDS(ON) values below 50 milliohms.

For solar inverters that require 1200V power switching, IGBTs are the right choice. More advanced IGBT technologies, such as NPT Trench and NPT Field Stop, are optimized for reducing conduction losses at the expense of higher switching losses, which makes them less suitable for boost applications at high frequencies.

Fairchild has developed an FGL40N120AND device based on the old NPT planar technology that can improve the boost circuit efficiency at high switching frequencies, with an EOFF of 43uJ/A, compared to 80uJ/A for EOFF using more advanced technology devices. However, it is very difficult to obtain this kind of performance. The disadvantage of the FGL40N120AND device is its higher saturation voltage drop, VCE(SAT) (3.0V versus 2.1V at 125oC), but its low switching losses at high boost switching frequencies are enough to make up for this. The device also integrates anti-parallel diodes. Under normal boost operation, the diode will not turn on. However, during start-up or during transient conditions, the boost circuit may be driven into the operating mode, at which point the anti-parallel diode will conduct. Since the IGBT itself does not have an inherent body diode, this co-packaged diode is needed to ensure reliable operation.

For boost diodes, fast recovery diodes such as StealthTM or carbon silicon diodes are required. Carbon-silicon diodes have a very low forward voltage and loss. However, they are currently very expensive.

When selecting a boost diode, the effect of the reverse recovery current (or the junction capacitance of the carbon-silicon diode) on the boost switch must be taken into account because this leads to additional losses. Here, the new Stealth II diode FFP08S60S offers higher performance. When VDD = 390V, ID = 8A, di/dt = 200A/us, and the case temperature is 100oC, the calculated switching loss is lower than the 205mJ parameter of the FFP08S60S. With the ISL9R860P2 Stealth diode, this value is 225mJ. This also increases the efficiency of the inverter at high switching frequencies.

 Switches and Diodes for Bridge and Specialty Stages

After filtering, the output bridge produces a 50Hz sinusoidal voltage and current signal. A common implementation is to use a standard full-bridge architecture (Figure 2). In the figure, if the upper left and lower right switches are turned on, a positive voltage is applied between the left and right terminals; the upper right and lower left switches are turned on, and a negative voltage is loaded between the left and right terminals.

For this application, only one switch is turned on during a certain period of time. One switch can be switched to PWM high frequency, and the other switch is at 50Hz low frequency. Since the bootstrap circuit relies on the conversion of the low-side device, the low-side device is switched to the PWM high frequency, and the high-side device is switched to the low frequency of 50 Hz.

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This application uses a 600V power switch, so a 600V superjunction MOSFET is well suited for this high speed switching device. Because these switching devices can withstand the full reverse recovery current of other devices when the switch is turned on, fast recovery of super-junction devices such as the 600V FCH47N60F is an ideal choice. Its RDS(ON) is 73 milliohms, which is very low compared to other similar fast recovery devices. When this device is switched at 50Hz, there is no need to use the fast recovery feature. With excellent dv/dt and di/dt characteristics, these devices can improve system reliability compared to standard superjunction MOSFETs.

Another option worth exploring is the use of the FGH30N60LSD device. It is a 30A/600V IGBT with a saturation voltage VCE(SAT) of only 1.1V. Its turn-off loss EOFF is very high, up to 10mJ, so it is only suitable for low-frequency conversion. A 50 milliohm MOSFET has an on-resistance RDS(ON) of 100 milliohms at operating temperature. Therefore, at 11A, it has the same VDS as the VCE (SAT) of the IGBT. Since this type of IGBT is based on older breakdown technologies, VCE(SAT) does not change much with temperature. Therefore, this IGBT can reduce the overall loss in the output bridge, thereby increasing the overall efficiency of the inverter.

It is also useful to switch the FGH30N60LSD IGBT from one power conversion technology to another in each half cycle. The IGBT is used here as a topology switch. For faster switching, regular and fast recovery superjunction devices are used.

For the 1200V dedicated topology and full-bridge architecture, the previously mentioned FGL40N120AND is a very suitable switch for new high-frequency solar inverters. Stealth II, HyperfastTM II diodes, and carbon-silicon diodes are good solutions when dedicated technology requires diodes.