PULSE WIDTH MODULATION TECHNIQUES.docx

7/27/2019 PULSE WIDTH MODULATION TECHNIQUES.docx

http://slidepdf.com/reader/full/pulse-width-modulation-techniquesdocx 1/4

PULSE WIDTH MODULATION TECHNIQUES

The three phase, six-step inverter control is simple and the switching loss is low because there

are only six switchings per cycle of fundamental frequency. Unfortunately, the lower order harmonics of

the six-step voltage wave will cause large distortions of the current wave unless filtered by bulky

uneconomical low-pass filters. Besides, the voltage control by the line-side rectifier has the usualdisadvantages.

PWM Principle

Because an inverter contains electronic switches, it is possible to control the output voltage as

well as optimize the harmonics by performing multiple switching within the inverter with the constant

dc input voltageVd. The PWM principle to control the output voltage is explained as shown in the fig. The

fundamental Vl has the maximum amplitude (4Vd/π) at square wave, but by creating two notches as

shown, the magnitude can be reduced. If the notch widths are increased, the fundamental voltage will

be reduced.

Fig. PWM principle to control output voltage

PWM Classification:

There are many possible PWM techniques proposed in the literature. The classification of PWM

techniques can be given as follows:

Sinusoidal PWM (SPWM)

Selected harmonic elimination (SHE) PWM

Minimum ripple current PWM

Space Vector PWM (SVM)

Random PWM

Hysteresis band current control PWM



Sinusoidal PWM with instantaneous current control

Delta Modulation

Sigma-delta modulation

Hysteresis-Band Current Control PWM

Principle of hysteresis-band current control

Hysteresis-band PWM is basically an instantaneous feedback current control method of PWM

where the actual current continually tracks the command current within a hysteresis band. The figure

explains the operation principle of hysteresis-band PWM for a half-bridge inverter. The control circuit

generates the sine reference current wave of desired magnitude and frequency, and it is compared with

the actual phase current wave. As the current exceeds a prescribed hysteresis ban, the upper switch in

the half bridge is turned off and the lower switch is turned on. As a result, the output voltage transitions

from +0.5Vd to -0.5Vd and the current starts to decay. As the current crosses the lower band limit, the

lower switch is turned off and the upper switch is turned on. A lock-out time (td) is provided at each



transition to prevent a shoot through fault. The actual current wave is thus forced to track the sine

reference wave within the hysteresis band by back-and-forth switching of the upper and lower switches.

The inverter then essentially becomes a current source with peak-to-peak current ripple, which is

controlled within the hysteresis band irrespective of Vd fluctuation. When the upper switch is closed, the

positive slope of the current is given as

Where 0.5Vd is the applied voltage, Vcm sine Ѡet = instantaneous voltage of the opposing load CEMF, and

L=effective load inductance. The corresponding equation when the lower switch is closed is given as

The peak-to-peak current ripple and switching frequency are related to the width of the

hysteresis band. For example, a smaller band will increase switching frequency and lower the ripple. An

optimum band that maintains a balance between the harmonic ripple and inverter switching loss is

desirable. The hysteresis-band PWM can be smoothly transitioned to square wave voltage mode

through the quasi PWM region. In the low speed region of the machine, when the CEMF is low, there is

no difficulty in the current controller tracking. However, at higher speeds, the current controller will

saturate in part of the cycle due to a higher CEMF and fundamental frequency-related harmonics will

appear. At this condition, the fundamental current will be less and its phase will lag with respect to the

command current.

Control block diagram of hysteresis band PWM

Fig shows the simple control block diagram for a hysteresis-band PWM implementation. The error in the

current control loop is impressed at the input of a comparator with a hysteresis band, as shown. The

bandwidth of HB is given as



HB=V

Where V=comparator supply voltage. The condition for switcjing the devices are:

Upper switch on:

(i*-i)>HB

Lower switch on:

(i*-i)<-HB

For a three-phase inverter, a similar control circuit is used in all phases.

The hysteresis-band PWM has been very popular because of its simple implementation, fast transient

response, direct limiting of device peak current, and practical insensitivity of dc link voltage ripple thatpermits a lower filter capacitor. However, there are a few drawbacks of this method. It can be shown

that the PWM frequency is not constant (varies within a band) and, as a result, non-optimum harmonic

ripple is generated n the machine current. An adaptive hysteresis band can alleviate this problem. It can

be shown that the fundamental current suffers a phase lag that increases at higher frequency. This

phase deviation causes problems in high-performance machine control. Of course, isolated neutral load

(which is not discussed here) creates additional distortion of the current wave.

PULSE WIDTH MODULATION TECHNIQUES.docx

Documents