Switching patterns, harmonics of Cascade H Bridge
Article By : David Levett, Tim Frank, Márcio Sari, Uwe Jansen, Klaus Vogel
CHB has become a popular topology and is regularly selected by power electronics design engineers for MVDs, not least because it offers a relatively simple solution from a technical perspective.
« Previously: Why CHB is popular topology for medium voltage drives
The H bridge can be switched to generate three different output voltages as can be seen with reference to Figure 2. For a positive voltage, S1 and S4 are turned on and for a negative voltage S2 and S3 are turned on. For a zero voltage state, either S1 and S2 or S3 and S4 are turned on.
In the simplest mode of operation the output sine wave can be built up by turning each cell on in sequence with Cell E first, then D through to A with each step increasing the output voltage by ≈1000VDC. PWM switching is used at each level to shape the waveform into a sine wave. This level shift modulation has a drawback in that the power semiconductors in each cell do not see equal losses and the transformer windings do not draw equal current, thereby increasing the input harmonic currents.
Alternatively, angle or phase shift modulation does create equal losses across the cells. The pattern (shown in Figure 5) uses a standard common sine wave reference for all cells, but phase shifts the PWM reference by a factor of 180° divided by the number of cells, in this case, 36°. This staggers the PWM patterns and prevents two cells switching at the same time, it also shares the zero voltage states equally between the two possible choices. Figure 5 shows the phase shift between cells A and D at 3 x 36 = 108°. There are numerous nuances for PWM pattern generation, such as space vector modulation, all with targeted technical advantages—as with regular drives.
Figure 5: Angle or phase shift cell switching pattern for a half positive sine wave.
Estimation of power losses
The losses are dependent on the type of switching pattern used. However, a simple approximation for the losses can be made using an online tool such as the Infineon’s Iposim (sign-in is required); estimating the losses assuming single phase operation. A rough “rule of thumb” is that at a 60Hz fundamental output frequency, with a 1kHz switching frequency on an air cooled heat sink the IGBT modules should be capable of an RMS output of 0.6-0.7x the DC current rating of the module. So, for a 300A module the RMS current can be ≈ 180-210 ARMS.
Available Power Silicon modules
For the three phase rectifier standard 20mm, 34mm and 50mm dual diode modules are available in both solder bond and pressure contact technology as shown in Figure 6.
Figure 6: Infineon 20mm, 34mm and 50mm rectifier modules.
For the H bridge, 1700V IGBT devices are often selected as this allows for a higher cell voltage, up to a maximum of ≈1200VDC enabling fewer cells in series. At the same time, 1700V silicon and peripheral components such as rectifiers and bus capacitors are readily available due to their use in 690VAC rated drives. Infineon has increased its portfolio of IGBT modules in industry standard packages to match the requirements of CHB drives as shown in Figure 7. Dual modules in the 62mm or EconoDUAL 3 packages are available up to 600 A and complete H bridges, for a more compact design, are available in the EconoDUAL 3 and Econo 3 packages. The latter is suitable for PCB based designs using press fit technology.
Figure 7: Modules options offered by Infineon tailored for a cascade cell H bridge.
Conclusion
CHB has become a popular topology and is regularly selected by power electronics design engineers for MVDs, not least because it offers a relatively simple solution from a technical perspective. By using topologies close to standard AC drives that can be built with high-volume components, overall system costs are reduced. The inherent modularity offers simplicity and other advantages, including reducing spares requirements.
First published by EDN.
« Previously: Why CHB is popular topology for medium voltage drives