FPGA high efficiency, low noise pulse frequency space vector modulation--Part I

Dr. Giulio Corradi, Xilinx Industrial, Scientific & Medical Group (ISM) Germany -October 04, 2012

Power modulation is crucial in motor control to ensure high efficiency, fast response time, low ripple torque, low harmonic generation and low acoustic noise. Pulse Width Modulation (PWM) is typically used in continuous methods like Space Vector Modulation (SVM) and discontinuous methods such as Flat-Top Modulation (FTM) exhibit harmonics and require complex mechanisms to mitigate such harmonics. The properties of different modulation methods like Pulse Frequency Modulation (PFM) or Pulse Density Modulation (PDM) can be used advantageously for standalone, or in combination with PWM, to push and spread the noise out of the band of interest, properly shaping the power signal to reduce the acoustic noise, harmonics ripple, distortion, switching losses and the overall modulator complexity.

PFM is clearly beneficial at a modulation index higher than 0.4, thus the realization of a versatile power modulator allowing on-the-fly switching between a space vector PWM and a space vector PFM, coupled with a fast current loop, achieves less switching, a finer torque ripple control and satisfies many motor control applications. This article describes the PWM- PFM versatile modulator, along with its advantages and practical results when applied to Brushless DC (BLDC) motors, Permanent Magnet Synchronous (PMSM) motors and Stepper motors as part of a set of functional motor control libraries implemented for the latest generation of 7 Series All Programmable FPGAs.

Power inverters are at the heart of most power control systems or electrical drives, where digital modulation is used to transfer commanded voltages or currents. Controllable magnitude and frequency are used as a series of high frequency pulses to loads or motors. Power semiconductor switches receive low voltage pulses and transform them into power pulses adequate for the load or motor. The load behaves, in general, as an integrator reconstructing the magnitude and frequency of the original commanded signal.

Real power semiconductor devices, such as MOSFETs, IGBTs, GTOs, etc., experience important limiting factors that decrease the efficient power transfer to the load. This often results in unwanted conduction losses, switching losses, unwanted harmonics, thermal runaway or audible noise. In recent years the overall performance of such power semiconductors has improved, which has opened the door to new design approaches.

odulation strategy greatly influences the power switch limiting factors. In particular the harmonics produced by the modulation process interacting with electric or magnetic properties of the load produce forces t h a t interfere with mechanical and structural elements creating vibrations and decreasing the load lifetime. Mechanical mitigation strategies to reduce such effects are valid but add extra cost and do not always produce the required result.

Harmonics created by modulation also produce electromagnetic interferences (EMI) that must be minimized to achieve the desired level of electromagnetic compatibility (EMC) required by industry norms and power supply distribution. Mitigation strategies consist o f putting electrical filters and chokes, to block or attenuate such harmonics, but these additional components also result in extra cost and extra space, thus also reducing the reliability of the power systems.

perating on digital modulation to shape the harmonic contents at the source before being transferred to the load, offers important advantages and avoids or reduces the extra cost required to reduce noise and EMI at the destination.

Power digital modulation has been studied in great detail, but results of such studies are difficult to find. For single-level inverters several pulse-programming methods can be used where the desired voltage waveform may be incorporated into the pulse train by:
  • PWM by varying the width of the individual pulses
  • PDM by varying the distance between the pulses
  • PFM varying the repetition frequency of a pulses train

PWM has found wider applications in power systems as the volt-second balance necessary to recreate intended magnitude at the load can be easily generated.  In terms of spectral efficiency, PWM introduces significant harmonics content to the form 6k±1 (k=1…N) beyond its fundamental frequency. Such harmonics are directly responsible for the mechanical interactions and EMI issues. PDM and PFM possess better spectral efficiency than PWM providing better EMI mitigation and intrinsic harmonics spreading.

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