A new family of compact power modules for high-efficiency solar inverters

1. Introduction

In most cases, the DC-AC inverter topology is based on a full-bridge circuit topology fed directly by solar cells , as shown in Figure 1.

Figure 1. Solar inverter with full-bridge circuit

However, when the solar cell's voltage is low or varies greatly, inserting a boost converter after the solar panel can provide a constant DC voltage to the full-bridge circuit, as shown in Figure 2.

Figure 2. Solar inverter with boost chopper and full-bridge circuit.

The full bridge circuit can also be simplified to a phase arm and a capacitor voltage divider. However, compared with the full bridge circuit, this phase arm must withstand twice the current at the same output power. Because the power module has a very symmetrical design, it is easy to form a phase arm with twice the current capacity by simply connecting the two phase arms of the bridge circuit in parallel.

In this article, only the full-bridge circuit topology is considered. System size, performance, reliability and cost are important, but the efficiency of the inverter is the most critical parameter. Achieving the highest possible efficiency of a solar inverter is not only a basic requirement for saving precious energy, but is also crucial for reducing the cost of electricity production. To achieve this goal, the full-bridge circuit needs to adopt a single-pole switch DC-AC inverter topology. In order to minimize the output filter, the lower arm switch of the full-bridge operates at a high frequency, while the upper arm switch operates at the transmission line frequency. With this operating mode, the inverter has the advantages of high frequency operation, while only two switches in the full-bridge circuit have switching losses, and the other two switches have only negligible conduction losses.

These power devices are integrated into a flat and compact package using the most advanced technology to provide solutions for high power density and high reliability inverters with a maximum output power of 10 kW.

2. Power module series for full-bridge circuit solutions

The two power module series are dedicated to providing 600V and 1200V modules for the two main AC grids and can meet the wide range of voltages of solar cells. In order to provide the smallest and most compact solution for the operating frequency range of 15 kHz to 50 kHz at a competitive price, IGBT technology is preferred. Fast NPT IGBT devices are used for the lower arm switches operating at high frequencies. Trench and field stop IGBT devices with the lowest saturation voltage drop are used for the upper arm switches operating at power line frequencies. It is possible to achieve fast switching of the lower arm devices and low conduction loss devices of the upper arm in the full bridge circuit, but usually the phase reversal is achieved by providing drive to the fast devices and avoiding floating position when placed in the lower arm of the full bridge.

At the same time, to improve the efficiency of the inverter, the diodes in this new type of module are matched to the power transistors. The high-speed, soft-recovery Microsemi DQ series diodes can be connected in parallel with the IGBTs in the upper arm and combined with the fast IGBTs in the lower arm to reduce recovery losses. The diodes with low forward voltage drop can protect the IGBTs in the lower arm when the output zero crosses. These latest diodes have much less stress than other diodes and have been used for high-frequency reverse recovery, which can reduce the current rating and help reduce size and cost. 600V, 30A ~ 100A and 1200V, 15A ~ 50A diodes are recommended in the compact, space-saving SP1 and SP3 packages, as shown in Table 1.

Currently, we are able to provide 600V products using CoolmosTM devices, which can operate at a higher switching frequency and minimize switching losses and conduction losses.

Table 1: List of full-bridge modules using SP1 and SP3 packages

3. Inverter circuits for complete boost and full-bridge

3.1 Single module for boost and full-bridge inverters

When the voltage of the solar cell varies greatly, it makes sense to use a boost converter to provide a regulated DC voltage (400V~800V) to the full bridge circuit. In order to give the best adaptability to the overall inverter solution, a kit consisting of 2 modules is preferred, one module is used as the boost switch and the other module is used as the full bridge switch.
Because a complete inverter uses a boost circuit stage and a full bridge circuit, there may be 2 separate packages. It is of course very important to optimize the size and performance of each package first, but these optimizations must also be suitable for the assembly of 2 modules.
To achieve this goal, a combination of a 3KVA module in a SOT227 package as the boost switch and a SP1 package module as the full bridge is preferred, as shown in Figure 3.
The base of the SOT 227 is 25.4mm x 38.1mm, while the base of the SP1 package is 40.8mm x 51.6mm. Both packages have a height of 12mm, so they can be mounted side by side on the same heat sink and wired to the same printed circuit board. The SOT227 provides screw terminals, while the SP1 is connected by soldering pin leads.

Figure 3a Boost stage using SOT227 package (P<3KW)

Fig.3b Full-bridge circuit using SP1 package (P>3KW)

When the output power is greater than 3KVA, the best solution is to use a combination of SP1 packaged modules as boost switches and SP3 packaged modules as full-bridge switches, as shown in Figure 4.

Figure 4a Boost stage using SP1 package (P>3KW)

Figure 4b Full-bridge circuit using SP3 package (P>3KW)

When the highest output power is reached, both the input boost stage and the output full-bridge module may need to adopt the SP3 package.

Combined with the 40.8mm x 51.6mm footprint of the SP1 package , the SP3 occupies an area of ​​40.8mm x 73.4mm. Both the SP1 and SP3 modules are 12mm high and can be wave soldered to the same PCB .

To minimize the magnetic components, especially the boost inductor, the boost converter must operate at high frequency, typically 100 kHz. For 600V applications, CoolmosTM devices give the best performance at high frequencies. Currently available are 45 mΩ modules in SOT227 and 24 mΩ modules in SP1. The matching diodes in all these products are the latest DQ soft fast recovery diodes. For high voltage applications, if the boost stage operates at high frequency, then MOSFET is the best choice. Because the device's on-resistance Ron increases significantly with increasing MOSFET voltage , a MOSFET device with a large chip area is required when the output power increases. The boost chopper products currently available are 180mΩ and 300mΩ modules in SP1 packages with breakdown voltages of 1000V and 1200V respectively. To minimize the cost of the boost function, a fast IGBT can be used to replace the MOSFET device when the frequency can be reduced to 25 kHz . This purpose can be achieved by using a 1200V, 50A~100A fast IGBT boost stage in the SP1 package.

Table 2 gives a summary of the boost stage module.

Table 2a. MOSFET and CoolmosTM boost stage module

Table 2b. IGBT boost stage module

These boost stage modules can be combined with the full-bridge modules in SP1 and SP3 packages described in the previous full-bridge module section. The voltage, current ratings and process technology of the devices should be selected according to the inverter output power and the selected switching frequency.

3.2 Integrated modules for boost and full-bridge inverters

Combining the boost stage with the full-bridge circuit in the same package allows the size of the inverter to be further reduced. Two products are available with voltages of 600V and 1200V. For each voltage rating, the two smallest power devices are modules in the SP4 package with a base of 40.4mm x 93mm (see Figure 5). The two largest power devices are integrated in the flat SP6-P package (base is 62mm x 108mm see – Figure 6). The boost stage in the 600V product is made of CoolmosTM transistors, while the 1200V product is made of fast IGBTs to save space and cost. Table 3 summarizes the existing integrated modules for boost and full-bridge circuits.

Table 3: Solar modules with integrated boost and full-bridge circuits

Figure 5 3D image of SP4 package

Figure 6 3D image of SP6-P package

4. Performance comparison

Various technology combinations are compared and the efficiency is studied as a function of output power. The performance at different operating frequencies is also studied in order to better determine the impact of switching losses at different switching speeds.

In order to make a reasonable comparison of different solutions, the efficiency corresponding to the normalized output power P/Pnom is given.

To avoid any audible audio noise and to minimize the magnetic components, fast switching is typically performed at an operating frequency of 20 kHz.

Figure 7 shows the efficiency function relationship corresponding to the normalized output power P/Pnom

- All four switches of the full bridge use only trench and field stop IGBTs.

- Only fast NPT IGBTs are used,

- The lower arm adopts fast NPT IGBT, and the upper arm adopts an optimized combination of low conduction loss IGBT devices trench (Trench) and field stop (Field stop) IGBT.

Figure 7 Efficiency curves of Trench and Field stop, NPT and Hybrid Trench/NPT at 20 kHz

Trench and field stop IGBTs are devices designed to operate at frequencies up to 20 kHz. The low saturation voltage VCEsat combined with reasonable switching times allows efficiencies of between 96% and 97%. Despite the higher conduction losses of fast NPT IGBTs, the efficiency can be further improved due to reduced switching losses. The combination of low switching loss fast IGBTs and low conduction loss trench and field stop IGBTs performs about 1% better than the previous combination, with an overall efficiency of more than 98% over a wide input power range.

To further improve efficiency, the operating frequency can be reduced to 16 kHz, noting that this reduction is limited by audible noise and cannot affect the size of the magnetic components (see Figure 8).

Figure 8 Efficiency curves of Trench and Field stop, NPT and Hybrid Trench/NPT at 16 kHz

For trench and field stop IGBTs, reducing the frequency from 20 kHz to 16 kHz can achieve efficiencies greater than 97%, very close to the 98% efficiency of fast NPT IGBTs, while hybrid IGBT technology has efficiencies above 98%.

In some cases, further reducing the size of the magnetic components, especially the output filter, requires increasing the operating frequency into the 50 kHz range.

The 600 V fast NPT IGBT has low turn-off losses and is fully capable of operating at frequencies up to 100 kHz, so acceptable efficiency is certain to be achieved in the 50 kHz range. MOSFET devices have faster switching times and lower switching losses than the fastest NPT devices. So as long as MOSFET devices have low conduction losses, their total losses will naturally be low. The on-resistance RDson of the 600V Cool MOS TM transistor is very small, thus minimizing conduction losses, and has fast switching times.

Figure 9 Efficiency curves of fast NPT/Trench IGBT and CoolMOSTM / Trench switch combination at 50 kHz

The combination of fast NPT and trench IGBTs allows an efficiency of more than 97% at 50 kHz. The combination of CoolMOSTM transistors and trench IGBTs is even more efficient than the previous combination.

If it is not necessary to run at high frequency to reduce the size of the inverter, the highest efficiency can be achieved by operating at 16 kHz using a combination of CoolMOSTM devices and trench IGBTs. Although trench IGBTs and field stop IGBTs operate at low 50Hz power line frequency, it is recommended to use FREDFET devices or CoolMOSTM transistors with faster intrinsic diodes to minimize EMI interference in the system.

Another important characteristic of solar inverters is lifespan and reliability. The EMI/RFI generated by the inverter is also critical.

The key characteristics of SiC diodes are their zero forward voltage drop and zero reverse recovery losses, which provide significant advantages in reducing switching noise and improving performance compared to standard fast silicon diodes.

Under hard switching conditions, the reverse recovery current of the diode has a significant impact on the turn-on energy inside the power switch. Thus, as the switching frequency increases, a considerable amount of turn-on losses are generated in both the power switch and the diode. It must be noted that at the end of the reverse recovery period, some oscillations may occur, resulting in a large amount of noise in the system, which is difficult to eliminate even with expensive and bulky input filters.

Faster recovery characteristics enable much lower switching losses in both the power switch and the diode. The small peak current observed when the SiC diode is turned off is due to the junction capacitance of the Schottky barrier device and not the reverse recovery characteristics. Unlike the configuration using the usual FRED diode, no transient disturbances or ringing were measured. This noiseless switching operation is the key to reducing the size and simplicity of the input filter and plays a major role in meeting strict EMI/RFI regulations.

SiC devices not only have excellent recovery characteristics at room temperature, but also remain unchanged over a wide temperature range. Figure 10 shows a comparison of the reverse recovery characteristics of a 10A/600V Cree SiC diode and a silicon diode with the same current and voltage ratings.

Figure 10 Reverse recovery characteristics of SiC diodes and Si diodes at different junction temperatures

Therefore, the use of SiC diodes can significantly reduce the overall losses of a solar inverter, enabling it to achieve record efficiency. Because lower losses also mean lower operating junction temperatures, this will significantly extend the inverter's operating life, which is critical for solar applications.

Based on this, the most efficient performance can be achieved by using an optimized power device mix technology; low conduction loss IGBTs operating at 50Hz, fast switching devices operating at high frequencies, and SiC diodes combined with fast transistors.

Selecting the switching frequency to the lowest possible 16 kHz will yield the highest possible efficiency, as shown in Figure 13.

Figure 13 Efficiency curves of fast NPT/Trench IGBT and CoolMOSTM / Trench switch combined with SiC diode at 16 kHz

In this article, different configuration combinations are compared at a heat sink temperature of 75°C. When the inverter is operated at the maximum ambient temperature, its efficiency can drop by as much as 1%. SiC diodes with excellent temperature characteristics can increase the efficiency gap under these extreme conditions compared to conventional silicon devices. Using aluminum nitride can further improve the thermal characteristics.

Standard modules use a substrate with better thermal conductivity than the existing aluminum substrate. Because the power device has a better junction-to-case thermal resistance, the operating junction temperature is reduced. For silicon devices, higher junction temperature means higher conduction losses and switching losses, while for SiC devices, it only leads to higher conduction losses. Therefore, the use of aluminum nitride (AlN) substrates can further increase the efficiency of solar inverters and extend their operating life.

“COOLMOS™ is a new transistor family developed by Infineon Technologies AG. “COOLMOS” is a registered trademark of Infineon Technologies AG.

5. Conclusion

This article explains that in order for modern solar inverters to achieve high efficiency goals, combining low conduction losses and fast power device technology in an advanced full-bridge configuration is key.

Microsemi Power Products offers a wide range of application specific power modules that utilize all of the various power device technologies described in this article to ...

SiC switching devices, MOSFETs or IGBTs, will be available in the near future, making it possible to achieve efficiencies better than 99%, the maximum technically achievable.