Today's programmable digital signal processors (DSPs) provide capabilities that make generating advanced pulse-width-modulation (PWM) signals easier than previous techniques. Using a DSP makes it easy to change carrier frequency and PWM scheme simply through reprogramming. In addition, they allow generation of three pairs of complementary PWM waveforms with programmable dead bands for three-phase voltage-source inverters. The combination of these features helps reduce the number of chips needed to implement the entire circuit, increasing reliability and lowering the overall cost of the system.
As a result, advances in solid-state power devices and high-performance processors have substantially increased the use of switching power converters in more modern motor drives to convert and deliver the required energy to each motor. There also are many advantages of PWM-based switching power converters over linear power amplifiers. Some key benefits include easy implementation and control, no temperature variation and aging-caused drifting or degradation in linearity, compatibility with today's digital microprocessors, and lower power dissipation. These advances, in turn, are helping manufacturers to shorten the time-to-market.
Popular PWM Techniques Three commonly used PWM techniques include sinusoidal, hysteresis (bang-bang), and space-vector (symmetrical or asymmetrical) implementations. Widely used in industrial applications, sinusoidal PWM (SPWM) is the generation of PWM outputs with sine waves as the modulating signals (Fig. 1a). The on and off instances of the PWM signal can be determined by comparing a sine wave (the modulating wave) with a high-frequency triangular wave (the carrier wave). In SPWM, the frequency of the modulating wave determines the frequency of the output voltage. The peak amplitude of the modulating wave determines the modulation index, and in turn controls the rms value of output voltage. Changing the modulation index can vary the rms value of the output voltage and significantly improves the distortion factors, as compared to other multiphase modulation techniques.
To implement SPWM using analog circuits, the following building blocks must be used:
* a high-frequency triangular wave generator;
* a sine wave generator;
* a comparator; and
* an inverter circuit with dead-band generators to produce complimentary driving signals with required dead bands.
All of these building blocks can be implemented using single or multiple chips, however, analog implementation of these circuits does present challenges commonly associated with analog circuits.
Hysteresis PWM refers to the technique where the output is allowed to oscillate within a predefined error band called a "hysteresis band." The switching instants are generated from the vertices of the triangular wave (Fig.1b). Hysteresis PWM techniques do not require any information about the inverter load characteristics. As long as the reference signal is known and the inverter output voltage is not saturated, the inverter output will always follow the reference.
Hysteresis PWM can be implemented with both analog circuits and digital circuits, however, digital implementation utilizing DSPs is becoming more popular due to the processor's programmable flexibility and overall reliability. Any available DSP controller can be utilized to implement this PWM technique.
The Space-Vector PWM technique refers to a special switching sequence of the three-phase voltage source inverters using basic space-vectors to generate the output voltages to the motor. The space-vector PWM technique has been shown to generate less harmonic distortion in output voltages and/or currents applied to the phases of an ac motor. In addition, it provides a more efficient use of the supply voltage in comparison with direct sinusoidal modulation technique.
The objective of space-vector PWM is to approximate the output voltage vector Uout by a combination of the eight switching patterns. With today's DSPs, space-vector PWM can easily be implemented. The on-chip timer and compare unit features available in DSP processors like the TMS320C24x play a key role in the implementation of PWM signal generation.
The event manager module in such processors also has built-in hardware to simplify the generation of symmetric space-vector PWM waveforms (Fig.1c). This on-chip hardware eliminates the need to determine the channel toggle frequency, as it also simplifies the compare register loading requirements so that the user does not have to worry about matching the values with the compare registers.
Symmetric PWM The energy that a switching power converter delivers to a motor is controlled by PWM signals that are applied to the gates or the bases of the power transistors. PWM signals are described as pulse trains with variable pulse width, fixed frequency and magnitude--with one pulse of fixed magnitude in every PWM period. However, the width of the pulses changes from pulse to pulse according to a modulating signal.
When a PWM signal is applied to the gate/base of a power transistor, it causes the turn-on and turn-off intervals of the transistor to change from one PWM period to another PWM period according to the same modulating signal. The frequency of a PWM signal must be much higher than the modulating signal, the fundamental frequency, such that the energy delivered to the motor and its load depends mostly on the modulating signal.
The pulses of a symmetric PWM signal are always symmetric with respect to the center of each PWM period. The pulses of an asymmetric PWM signal always have the same side aligned with one end of each PWM period. Asymmetric PWM can be used for stepper motors and other variable-reluctance motors.
Symmetric PWM methods are often used for three-phase ac induction and brushless dc motors, due to the lower harmonic distortion that is generated on phase currents in comparison to asymmetric PWM methods.
Generating Waveforms To generate the proper signals, engineers require a high-frequency PWM, the flexibility to change frequency in real-time and dead band to secure safe operation of the power converter. DSP controllers have made it more practical for designers to apply space-vector PWM waveform generation. This has allowed high-speed processing to meet high-frequency requirements and programmable frequency changes either from application to application, or in real-time in a given application. High-frequency PWM signals are desirable for better control of the motor phase currents and smoother performance of the motor and load. An additional benefit is optimizing the dead band and eliminating its undesired impacts on the motor phase currents.
Symmetric space-vector PWM has been widely used in three-phase switching power converters, including those used for three-phase ac induction motors. Using DSPs, specifically the TMS320C24x with built-in, on-chip hardware, for such applications, facilitates the implementation of a symmetric space-vector PWM waveform. In addition, the speed of DSPs allows implementation of this kind of PWM at high frequencies. This leaves ample time for the CPU to do other motor control functions. Also, dedicated features on these controllers eliminate the need for external off-chip components.
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