Electronic Design

Generate Advanced PWM Signals Using DSP

On-chip Timers And Registers Enable DSP Controllers To Generate Symmetric Space-Vector PWM Signals For Three-Phase Motors.

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.

For instance, the TMS320C24x has 3 general-purpose timers, 3 full Compare Units, and 3 simple compare units, Programmable dead-band units and a dedicated space-vector PWM module that can be used for generating PWM outputs. It can generate up to 12 PWM outputs, of which half are complementary with programmable dead-band time.

The general-purpose (GP) timers can be configured to run in up and up/down count modes for generating asymmetrical and symmetrical PWM outputs. The period registers of the GP timers are shadowed to allow on-line change of PWM frequency, which in turn allows wabbling of PWM frequency, which can help spread out the spectrum of the PWM outputs.

The pulse widths of the PWM outputs are determined by values in the compare registers. The compare registers are shadowed, allowing the CPU to write to these registers at any time during the current PWM period. The new compare values can be programmed to become active immediately on underflow or on period match.

The polarities of the PWM outputs can be independently controlled by the action control register and the simple action control register. The polarity of a PWM output can be active high, active low, forced high, and forced low, allowing control of different types of power devices, such as IGBTs, power Mosfets, and bipolars.. The action control registers are also shadowed so users can write to these registers to change the polarities of PWM outputs at any time during a PWM period.

The space-vector PWM module automatically generates space-vector PWM patterns once a starting basic vector and a direction is given in the action control register.

The generation of such PWM outputs is entirely register-based. All the registers are data memory mapped so the CPU can access them as data memory locations. To generate a certain kind of PWM output, the CPU:

* Writes to the pin configuration registers to configure the pins as PWM outputs

* Writes to the GP timer control, compare control registers, the action control registers, and the dead-band control registers to configure PWM frequency, type of PWM waveform to be generated, polarities of the PWM outputs and the dead band

* Continuously updates the compare registers based on newly determined pulse widths.

Any of the mentioned PWM techniques can be used to determine the pulse widths.

One significant advantage of using symmetric space-vector PWM is that the technique applies about 14% more voltage on motor windings in comparison to sinusoidal PWM with the same dc bus voltage. This translates into a more efficient utilization of bus voltage, as well as a motor that can be rated at a higher voltage and lower current to achieve the same horsepower rating.

Using symmetric space-vector PWM results in 10% less phase current, with a reduced power dissipation and heat generation in the power converter and motor. Finally, symmetric space-vector PWM technique generates less harmonics in phase current for less power dissipation and less noise. It has been observed that symmetric space-vector PWM technique generates less audible noise, especially when the dc bus voltage goes above 100 V.

Depicted is the structure of a typical three-phase voltage-source power inverter--Va, Vb, and Vc are the output voltages applied to the windings of a motor (Fig. 2). Q1 through Q6 are the six power transistors that shape the output, which are controlled by a, a', b, b', c, and c'. When an upper transistor is switched on (a, b, or c is 1), the corresponding lower transistor is switched off (a', b', or c' is 0). The on and off states of the upper transistors Q1, Q3, and Q5, or equivalenty the state of a, b, and c, are sufficient to evaluate the output voltage.

The relationship between the switching variable vector \\[a, b, c\\]t and the line-to-line voltage vector \\[Vab Vbc Vca\\]t is given by Equation 1, from which one can easily arrive at Equation 2 that determines the phase voltage vector \\[Vab Vbc Vca\\]t.

As shown in Figure 3 , there are eight possible combinations of on and off patterns for the three power transistors that feed the three-phase power inverter. Notice that the on and off states of the lower power transistors are opposite to the upper ones, so they are completely determined once the states of the upper power transistors are known. The eight combinations and the derived output line-to-line and phase voltages in terms of dc supply voltage Vdc, according to Equation 1 and 2, are shown in Table 1.

Experimental Results
Experimental data results of using the TMS320 DSP, as well as the sinusoidal and space-vector PWM techniques are shown (Fig. 5 and Table 2). A three-phase ac induction motor, rated at 147V, 60 Hz and 1/2 hp is controlled, in this case, based on constant V/Hz principle with a PWM frequency of 25 kHz, sampling frequency of 12.5 kHz, and dc bus voltage of 180V. It can be seen that the space-vector PWM technique generates 14 to 25% more output voltage and an obvious reduction in harmonics in output currents.

TAGS: Digital ICs
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