Introduction

In power electronics, several Pulse Width Modulation (PWM) schemes have been successfully employed depending on the particular application. Most of the conventional PWM schemes, being deterministic in nature, produce a predetermined harmonic content. This can create a number of issues in real-world applications like the production of acoustic noise, radio interference, and mechanical vibration. In applications where interference with the environment and other equipment need to be mitigated, for example in industrial motor drives, traction drives, electric vehicles, the conventional PWM schemes inherently do not perform efficiently and additional equipment like electromagnetic interference (EMI) filters need to be added. One available option to cope with issues resulting in these applications is to increase the switching frequency of the conventional PWM schemes i.e.>18kHz [Capitaneanu, Stefan Laurentiu, et al. "On the acoustic noise radiated by PWM AC motor drives." Automatika 44.3-4 (2003): 137-145]. However, this causes the switching losses to increase significantly. In such applications random pulse width modulation (RPWM) has been found to be effective to mitigate the cited issues without the need to considerably increase the switching frequency.

In RPWM the width of each switching pulse varies stochastically. This causes the harmonics cluster to spread over a large range thus reducing the size of separate filters or entirely avoiding the use of filters in certain applications. RPWM technique has successfully been utilized in many power electronics applications e.g. in industrial motor control drives where the acoustic noise needs to be checked.

Usually, high-frequency PWM and RPWM signals for commercial sophisticated systems are implemented using Digital Signal Processors (DSP) and Field Programmable Gate Arrays (FPGA). However, these devices are more generic, powerful, and flexible which makes them quite expensive. Similar precision and high-frequency timing requirements needed for RPWM generation can be met with a low-cost GreenPAK IC. Many suitable RPWM schemes, especially for open-loop applications, can be implemented using the GreenPAK ICs. Thus, the explicit programming or coding of embedded DSPs, MCUs, or FPGAs is replaced by a simple interface provided in the GreenPAK Designer software. In addition, the size of the overall control circuit is considerably reduced.

There are several ways of producing the RPWM for three-phase inverter applications. In this project, a suitable RPWM technique is presented that can be implemented using the available GreenPAK GreenPAK IC's resources. The RPWM technique is implemented using the dual matrix IC SLG46620. Appropriate theoretical proposals and experimental results are also presented including the output voltage waveforms and their harmonic content that would justify the proposed strategy.

The complete circuit design file can be found here. It was created in the GreenPAK Designer software, a part of the Go Configure Software Hub (available for free, GUI-based).

1. Proposed RPWM Scheme

The block diagram of the RPWM scheme driving a three-phase inverter is shown in Figure 1.


Figure 1: Block Diagram of the Proposed Scheme

Two saw-tooth signals, labeled as p2 and p3 (with values ranging: 0-1), 180 ˚C phase apart, are compared with a constant value p1 (with the value range: 0-1) to give a different type of pulses labeled as p5 and p6. The waveforms of these pulses (p5 and p6) are shown in Figure 2 and Figure 3. A binary pseudo-random number generator (labeled as p4) with the waveform shown in Figure 4 is employed to randomly select a pulse out of the signals p5 and p6 using the logical operators as shown in the above block diagram. This generates a train of pulses p10, which is shown in Figure 5. The signal p10 is passed through AND gates along with 10ms long pulses generated...

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