Design of control signals for Uniform PWM Full-Bridge DC-DC Converter
Abstract- DC drive systems controlled by pulse-width modulation (PWM) techniques are expected to replace the conventional phase controlled systems. This is due to their simple control, high reliability, low cost and fast response. Two different uniform PWM control techniques are considered; PWM with bipolar voltage switching and PWM with uni-polar voltage switching. This project deals with the design of control circuit for both the bipolar and uni-polar voltage switching. I. INTRODUCTION Up to now, single-phase and three phase systems have been dominated by thyristor line frequency phase controlled rectifiers especially in large power ratings. Such converters have the drawbacks of their poor p oor input power factor especially when the firing angle of the converter increases and the high harmonic content in the input ac line current due to its discontinuous nature . Also, the current through phase controlled converters is Uni directional, while the output voltage can reverse polarity. The two quadrant operation with the reversible voltage is not suited for dc motor b raking, which requires the voltage To be unidirectional but the current to be reversible. Therefore, if regenerative braking is required, two back-to-back connected thyristor converters can be used. This, in fact, gives a capability to operate in all the four quadrants. For the same armature inductance, the ripple of the load current and the zone of discontinuous conduction operation are reduced when compared to conventional converters. Different PWM control techniques were later investigated in order to reduce the amplitudes of the lower-order harmonics of the line current. PWM control techniques also allowed the switching frequency to be raised due to the simplicity of the control strategy and the availability of fast switching devices with reasonably power ratings. However, this will be on the expense of the increased switching losses.
II. THE SINGLE-PHASE DC DRIVE SYSTEM CONFIGURATION Figure 1 shows the circuit diagram the single-phase dc drive system with a full-bridge dcdc converter. Since the input bridge diode rectifier is uncontrolled, its dc output voltage is of constant amplitude. The motor voltage can be controlled in magnitude as well as polarity. Similarly, the magnitude and the direction of the motor current can be
controlled. Therefore, this drive system allows the operation to be in the four quadrants of V-I plane, and the power flow can be either from the supply to the motor or from the motor to the supply. Since the dc link current changes direction instantaneously, it is important that the input to the converter be a dc voltage source with a low internal impedance. This is achieved by inserting a capacitor filter as shown in Fig. 1 to provide this low impedance path to the dc input current.
The full-bridge converter consists of two legs; each leg consists of two MOSFET switches (MI, M2 and M3 M3,, &M4) and their respective anti-parallel diodes D1, D2 and D3, D3, D4). The switches in the same leg cannot be switched-on simultaneously to avoid short-circuiting the ac supply. In single quadrant dc drive systems, the polarity of the motor voltage is unidirectional, hence the switching device is pulse-width modulated by comparing a switching frequency saw-tooth with the control voltage. In contrast, the motor voltage supplied by bridge converters is reversible in polarity and and,, therefore a switching frequency triangular waveform is used for PWM of the converter switches. In the presented study, two uniform u niform PWM control techniques are considered:
1. PWM with bipolar voltage switching, where switches M1, M4 and M2, M3 are treated as two switch pairs where each pair of switches is turned on and off simultaneously. 2. PWM with uni-polar voltage switching, where the switches in each leg are controlled independently of the other leg.
Block diagram of the control circuit of the PWM technique using UPVS & BPVS
The above control circuit is realized by using 741 op-amps, because op-amp has high input impedence ,less loading effect ,and desired gain can be obtained in both inverting and no inverting mode of operations. The signal conditioning can be done using operational amplifiers with fewer efforts.
To get desired control signals, first a square wave is generated with variable frequency and variable pulse width i.e. duty cycle. Then it is fed to an integrator circuit which converts this square in to a triangular wave. In the next stage this triangular wave is compared with reference voltage which is the cause for the variation of pulse width. And the frequency of the generated control signals is same as that of the input frequency of the square wave. In unipolar voltage switching comparator 2 comes in to role in which the same input is given as in case of comp1.by using inverters here we obtain the control signals for unipolar voltage switching.
III. UNIFORM PWM DC DRIVE SYSTEM WITH BIPOLAR VOLTAGE SWITCHlNG. In this type of PWM control, the switch pairs Ml, M4 and M2, M3 are turned on and off simultaneously, where one of the two switch pairs is always one. The uniform PWM with Bipolar voltage switching control strategy is illustrated in Fig.2 for a switching frequency of 1 kHz. The switching patterns are generated by comparing a triangular carrier Waveform (Vd with a dc control voltage VJ). VJ). For closed loop control, this control voltage represents the speed feedback signal. The switching patterns are generated in such a way that when V2V, switches M1 and M4 are turned on otherwise, switches M2 and an d M3 are Turned on
Studying Fig.2 shows that the duty ratios of the switch pairs are given by:
This shows that the average motor voltage varies linearly with the input control signal. Also, the motor voltage jumps between positive and negative so that this switching strategy is referred to as the bipolar voltage switching PWM. It has to be noted that the duty ratio can be varied between 0 and 1, depending on the magnitude and polarity of the control voltage. Therefore, V can be continuously varied in a range from vd to +vd, +vd, while the motor current
can
be either positive or negative such that the dc
motor can be operated in the four quadrants of the V-I plane. The single-phase dc drive system of Fig.1 is simulated using PSpice with an exact model of the dc motor. An Expanded waveform of the switch currents are shown in Fig.3, where the negative values of currents correspond to the current carried out by the anti-parallel diode.
The following modes of operation can be identified:
1. Powering mode: mode: The motor current is supplied through M1 and M4 (for positive motor voltage) or through M2 and M3 (for negative motor voltage). 2. Regenerating mode: When the switches are made off and off and the energy is returned back to the supply through diodes D1 and D4 or through D2 and D3.
IV. UNIFORM PWM DC DRIVE SYSTEM WITH UNI-POLAR VOLTAGE SWITCHING: In the uniform PWM control technique with unipolar voltage switching, a triangular waveform is compared with the control voltages V, and -V, to obtain the switching patterns for the converter switches M1, M2 and M3, W respectively. The switching patterns are such that when V>V, MI is on, and when -Vc>Vh M3 is on where switches in the same leg have complement switching patterns. The switching patterns of the uniform PWM control strategy for unipolar voltage switching are illustrated in Fig.7.Examining Fig.7 shows that the duty ratios of the switches a1 and a2, and therefore the output voltage V, are the same as those of the PWM with bipolar voltage switching where the output voltage varies linearly with the control voltage.
The different switch currents are shown in Fig.8. Therefore, the following modes of operation can be defined: 1. Powering mode: The motor current is carried out by the switch pair MI and W back to the supply. 2. Free-wheeling mode: The motor current free-wheels through a power switch and a diode such as M1, D3 or M3, M3, D1 or M2, D1 or W, W, D2 where no power is drawn
from the ac supply. 3. Regenerating mode: This mode occurs during light load or breaking conditions where the energy is returned back to the supply through diodes D1 and D4. D4.
V. CONCLUSIONS The performance characteristics of a single-phase dc drive system controlled by uniform PWM full-bridge dc-dc converter has been presented. Two uniform PWM control techniques have been investigated. The first one with bipolar voltage switching, while the second one with unipolar voltage switching. It is also observed that it is advantageous to use unipolar voltage switching because more modes mod es of operation is possible in this mode.
