**2. Review of voltage regulators**

### **2.1. Switching-mode power supply (SMPS)**

A switching-mode power supply (SMPS) is switched at very high frequency. Conversion of both step down and step up of voltage is possible using SMPS. Uses of SMPSs are now universal in space power applications, computers, TV and industrial units. SMPSs are used in DC-DC, AC-AC, AC-DC, DC-AC conversion for their light weight, high efficiency and isolated multiple outputs with voltage regulation. Main parts of a Switching-mode power supply are:

(a) Power circuit, (b) Control circuit.

Figure 1 shows the block diagram of a SMPS. The power circuit is mainly the input, output side with the switching device. The switching device is continuously switched at high frequency by the gate signal from the control circuit to transfer power from input to the output. The control circuit of a SMPS basically generates high frequency gating pulses for the switching devices to control the output voltage. Switching is performed in multiple pulse width modulation (PWM) fashion according to feedback error signal from the load. High frequency switching reduces filter requirements at the input and output sides of the converter. Simplest PWM control uses multiple pulse modulations generated by comparing a DC with a high frequency carrier triangular wave.

The PWM control circuit is commonly available as integrated form. The designer can select the switching frequency by choosing the value of RC to set oscillator frequency. As a rule of thumb to maximize the efficiency, the oscillation period should be about 100 times longer than the switching time of the switching device such as Transistor, Metal oxide semiconductor field-effect transistor (MOSFET), Insulated gate bipolar transistor (IGBT). For example, if a switch has a switching time of 0.5 us, the oscillator period would be 50 us, which gives the maximum oscillation frequency of 20 KHz. This limitation is due to the switching loss in the switching devices. The switching loss of switching devices increases with the switching frequency. In addition, the core loss of inductor limits the high frequency operation.

**Figure 1.** Block diagram of Switching-mode power supply (SMPS).

#### **2.2. DC-DC converter**

122 An Update on Power Quality

in low power and high power applications.

**2. Review of voltage regulators** 

(a) Power circuit, (b) Control circuit.

supply are:

**2.1. Switching-mode power supply (SMPS)** 

a DC with a high frequency carrier triangular wave.

chip SG1524B is used, thus circuit is compact and more viable.

Dynamic Voltage Restorer (DVR) is sometimes used to regulate the load side voltage [19- 21]. The DVR requires energy storage device to compensate the voltage sags. Flywheels, batteries, superconducting magnetic energy storage (SMES) and super capacitors are generally used as energy storage devices. The rated power operation of DVR depends on the size and capacity of energy storage device which limits its use in high power applications. Whereas, switching regulator needs no energy storage devices, therefore, can be used both

The objective of this chapter is to describe the operation and design procedure of a switch mode AC voltage regulator. Firstly, some reviews of the regulators are presented then the procedure of design and analysis of a switch mode regulator is described step by step. Simulation software OrCAD version 9.1 [22] is used to analyze the regulator. The proposed regulator consists mainly two parts, power circuit and control circuit. The power circuit consist two bi-directional switches which serve as the freewheeling path for each other. A signal generating control circuit is to be associated with the power circuit for getting pulses of the switches. In the control circuit, a commercially available pulse width modulator IC

A switching-mode power supply (SMPS) is switched at very high frequency. Conversion of both step down and step up of voltage is possible using SMPS. Uses of SMPSs are now universal in space power applications, computers, TV and industrial units. SMPSs are used in DC-DC, AC-AC, AC-DC, DC-AC conversion for their light weight, high efficiency and isolated multiple outputs with voltage regulation. Main parts of a Switching-mode power

Figure 1 shows the block diagram of a SMPS. The power circuit is mainly the input, output side with the switching device. The switching device is continuously switched at high frequency by the gate signal from the control circuit to transfer power from input to the output. The control circuit of a SMPS basically generates high frequency gating pulses for the switching devices to control the output voltage. Switching is performed in multiple pulse width modulation (PWM) fashion according to feedback error signal from the load. High frequency switching reduces filter requirements at the input and output sides of the converter. Simplest PWM control uses multiple pulse modulations generated by comparing

The PWM control circuit is commonly available as integrated form. The designer can select the switching frequency by choosing the value of RC to set oscillator frequency. As a rule of thumb to maximize the efficiency, the oscillation period should be about 100 times longer than the switching time of the switching device such as Transistor, Metal oxide semiconductor field-effect transistor (MOSFET), Insulated gate bipolar transistor (IGBT). For Figure 2 illustrates the circuit of a classical linear power converter. Here power is controlled by a series linear element; either a resister or a transistor is used in the linear mode. The total load current passes through the series linear element. In this circuit greater the difference between the input and the output voltage, more is the power loss in the controlling device (linear element). Linear power conversion is dissipative and hence is inefficient.

The circuit of Fig. 3 illustrates basic principle of a DC-DC switching-mode power converter. The controlling device is a switch. By controlling the duty cycle, (the ratio of the time in on positions to the total time of on and off position of a switch) the power flow to the load can be controlled in a very efficient way. Ideally this method is 100% efficient. In practice, the efficiency is reduced as the switch is non-ideal and losses occur in power circuits. Hence, one of the prime objectives in switch mode power conversion is to realize conversion with the least number of components having better efficiency and reliability. The DC output voltage to the load can be controlled by controlling the duty cycle of the rectangular wave supplied to the base or gate of the switching device. When the switch is on, it has only a small saturation voltage drop across it. In the off condition the current through the switch is zero.

The output of the switch mode power conversion circuit (Fig. 3) is not pure DC. This type of output is applicable in some cases such as oven heating without proper filtration. If constant DC is required, then output of converter has to be smoothed out by the addition of low-pass filter.

Power Quality Improvement Using Switch Mode Regulator 125

BJT R + \_ Vin Vout

L D

BJT

C1

Vg

L1

C

L2

D

C2

R Vout

as input voltage. In Buck-Boost and Cûk regulator, the polarity of output voltage is opposite to that of the input voltage, therefore theses regulators are also called inverting regulators.

(a) Buck regulator (b) Boost regulator

+ \_ Vin Vg

R

<sup>V</sup> R out <sup>C</sup>

**Figure 4.** Circuit diagram of DC-DC regulator, (a) Buck regulator, (b) Boost regulator, (c) Buck-Boost

(c) Buck-Boost regulator (d) Cûk regulator

The AC voltage regulator is an appliance by which the AC output voltage can be set to a desired value and can be maintained constant all the time irrespective of the variations of input voltage and load. This subject is vast and the field of application extends from very large power systems to small electronic apparatus. Naturally, the types of regulators are also numerous. The design of the regulators depends mainly on the power requirements and

The voltage regulations by tap-changing switches are used in many industrial applications where the maintenance of output voltage at a constant value is not very stringent, such as ordinary battery chargers, electroplating rectifiers etc. For smaller installation, off-load tap

regulator, (d) Cûk regulator.

\_ Vin <sup>L</sup> Vg

BJT

BJT

Vg

\_ Vin <sup>D</sup>C Vout

L

D

+

+

**2.3. AC-AC converter** 

degree of stability.

The AC voltage can be regulated by the following ways. a. Solid-state tap changer and steeples control by variac

*2.3.1. Solid-state tap changer and stepless control by variac* 

b. Solid-tap changer using anti-parallel SCRs c. Voltage regulation using servo system

d. Phase controlled AC regulator e. Ferro-resonant AC regulator f. Switch mode AC regulator

**Figure 2.** Linear (dissipative) DC-DC power conversion circuit.

**Figure 3.** Switching-mode (non dissipative) DC-DC power conversion circuit.

### *2.2.1. Types of DC-DC converter*

There are four basic topologies of switching DC-DC regulators:


The Circuit diagram of four basic DC-DC switching regulators is shown in Fig. 4. The expression of output voltage for the four types of DC-DC regulators are as follows:

$$\text{For Buck regulator, } \mathbf{V}\_{\text{out}} = \mathbf{k} \mathbf{V}\_{\text{in}} \text{ , For Bosst regulator, } \mathbf{V}\_{\text{out}} = \frac{\mathbf{V}\_{\text{in}}}{1 - \mathbf{k}}$$

For Buck- Boost regulator and Cûk regulator, in out -kV <sup>V</sup> 1 k

Where k is the duty cycle, the value of k is less than 1. For Buck regulator output voltage is always lower than input voltage, for Boost regulator output voltage is always higher than input voltage. For Buck-Boost regulator and Cûk regulator output voltage is higher than input voltage when the value of k is higher than 0.5, and output voltage is lower than input voltage when the value of k is lower than 0.5. When k is equal to 0.5 output voltage is same as input voltage. In Buck-Boost and Cûk regulator, the polarity of output voltage is opposite to that of the input voltage, therefore theses regulators are also called inverting regulators.

**Figure 4.** Circuit diagram of DC-DC regulator, (a) Buck regulator, (b) Boost regulator, (c) Buck-Boost regulator, (d) Cûk regulator.

## **2.3. AC-AC converter**

124 An Update on Power Quality

+

+

**Figure 2.** Linear (dissipative) DC-DC power conversion circuit.

R \_ Vin

\_ Vin Vout

R1

R2

V

Vin

Vout

V Vin t

t

Vout

**Figure 3.** Switching-mode (non dissipative) DC-DC power conversion circuit.

Vout

The Circuit diagram of four basic DC-DC switching regulators is shown in Fig. 4. The

Where k is the duty cycle, the value of k is less than 1. For Buck regulator output voltage is always lower than input voltage, for Boost regulator output voltage is always higher than input voltage. For Buck-Boost regulator and Cûk regulator output voltage is higher than input voltage when the value of k is higher than 0.5, and output voltage is lower than input voltage when the value of k is lower than 0.5. When k is equal to 0.5 output voltage is same

<sup>V</sup> <sup>V</sup> 1 k


expression of output voltage for the four types of DC-DC regulators are as follows:

There are four basic topologies of switching DC-DC regulators:

For Buck regulator, V kV out in , For Boost regulator, in out

For Buck- Boost regulator and Cûk regulator, in out

*2.2.1. Types of DC-DC converter* 

BJT

c. Buck-Boost regulator and

a. Buck regulator b. Boost regulator

d. Cûk regulator.

The AC voltage regulator is an appliance by which the AC output voltage can be set to a desired value and can be maintained constant all the time irrespective of the variations of input voltage and load. This subject is vast and the field of application extends from very large power systems to small electronic apparatus. Naturally, the types of regulators are also numerous. The design of the regulators depends mainly on the power requirements and degree of stability.

The AC voltage can be regulated by the following ways.


#### *2.3.1. Solid-state tap changer and stepless control by variac*

The voltage regulations by tap-changing switches are used in many industrial applications where the maintenance of output voltage at a constant value is not very stringent, such as ordinary battery chargers, electroplating rectifiers etc. For smaller installation, off-load tap changing switches are used and for large installation on-load tap changing switches are used. The switches are generally incorporated at the secondary of the transformer. For a low voltage high current load, the switches are provided on the primary side of the transformer due to economical reason. For line voltage correction, taps are provided on the primary of the transformer. For three-phase transformers three pole tap changing switches are used.

Power Quality Improvement Using Switch Mode Regulator 127

through three anti-parallel switches. When the SCR1-SCR2 switch is fired, tap 1 is connected to the load. Similarly taps 2 and 3 can be connected to the load through the SCR3-SCR4 and CSR5-SCR6 switches respectively. Thus, any number of taps can be connected to the load with similar SCR switches. When one group of SCRs operates for the whole cycle and other groups are off, the voltage corresponding to the tap of that group appears at the load. Changeover from one loop to the other is done simply by shifting the firing pulses from one

With resistive load, the load current becomes zero and the SCRs stop conduction as soon as the voltage reverses its polarity. Therefore, when one group is fired, the other groups are commutated automatically. With reactive loads, the situation is complicated by the fact that the zero current angle depends on the load power factor. This means that the SCR conducts a finite value of current at the time of reversal of line voltage. This results in either preventing a tap change due to reverse bias on the SCR to be triggered or causing a short

Voltage regulators using servo systems are quite common. Both single and three-phase types are available. The rating of this type of regulator is quite high and is more economical for high power rating. This regulator normally consist a variac driven by a servomotor, a sensing unit and a voltage and power amplifier to drive the motor in a reversible way. Various types of driving motor may be used for regulating the unit, such as direct current, induction and synchronous motors. However, in all cases, the motor must come to rest rapidly to avoid overrun and hunting. The amount of overrun may be reduced by dynamic braking in the case of a DC motor or by disconnecting the motor from the variac by a clutch as soon as the signal from the measuring unit ceases. The main disadvantage of this type of

Voltage regulators using SCRs are quit common. The load voltage is regulated by controlling the firing instants of the SCRs. There are various circuits for single phase and three phase regulators using SCRs. Though the output voltage can be precisely controlled by this method, the harmonic introduced in the load voltage are quit large and this circuit is used for applications where the output voltage waveform need not be strictly sinusoidal. The circuit arrangement for a single phase SCR regulator is shown in Fig. 6 and Fig. 7.

The concept of the stabilization of AC voltage using a saturated transformer is rather old. The basic circuit arrangement consists of a linear reactor or transformer T1 and a nonlinear saturated reactor or transformer T2 connected in series as shown in Fig. 8. Since the two elements T1 and T2 are in series, the current through them is the same. Transformer T2 is operated under saturated. The voltage division between the two is according to their

group of SCRs to the other.

circuit between the taps through two SCRs.

*2.3.3. Voltage regulation using servo system* 

regulator is the low life of the contact points of the relays.

*2.3.4. Phase controlled AC voltage regulator* 

*2.3.5. Ferro-resonant AC voltage regulator* 

In off-load tap changer, the output is momentarily cut-off from the supply. It is therefore used for low capacity equipment and where the momentary cut-off of the supply is not objectionable for the load. The major limitation of the off-load tap changing switches is the occurrences of arcs at the contact points during the change-over operation. This shortens the life of off-load rotary switches, particularly of high current ratings. In Fig. 5 (a), three fourposition switches of an off load tap changer are shown, such that the minimum of X volts per step are available at the output.

The voltage is corrected by tap-charging switches in steps. Where stepless control is required, variable autotransformers or variacs are used. The normal variac consists of a toroidal coil wound on a laminated iron ring. The insulation of the wire is removed from one of the end faces and the wire is grounded to ensure a smooth path for the carbon brush. Carbon brush is used to limit the circulating current, which flows between the short-circuited turns.

A Buck-Boost transformer is sometimes used for AC voltage regulation when the output voltage is approximately the same as the mean input voltage as shown in Fig. 5(b). In this case if the output voltage is less than or greater than the desired value, it can be increased or decreased to the desired value by adding a suitable forward or reverse voltage with the input through the Buck-Boost transformer.

**Figure 5.** Circuit diagram of AC voltage controller using (a) Off load tap changer and (b) Buck-Boost transformer and variac.

#### *2.3.2. Solid tap changer using anti-parallel SCRs*

Anti-parallel SCRs combinations can replace the voltage sensitive relay in the tap-changing regulator. Figure 6 shows a tap changer with three taps which can be connected to the load through three anti-parallel switches. When the SCR1-SCR2 switch is fired, tap 1 is connected to the load. Similarly taps 2 and 3 can be connected to the load through the SCR3-SCR4 and CSR5-SCR6 switches respectively. Thus, any number of taps can be connected to the load with similar SCR switches. When one group of SCRs operates for the whole cycle and other groups are off, the voltage corresponding to the tap of that group appears at the load. Changeover from one loop to the other is done simply by shifting the firing pulses from one group of SCRs to the other.

With resistive load, the load current becomes zero and the SCRs stop conduction as soon as the voltage reverses its polarity. Therefore, when one group is fired, the other groups are commutated automatically. With reactive loads, the situation is complicated by the fact that the zero current angle depends on the load power factor. This means that the SCR conducts a finite value of current at the time of reversal of line voltage. This results in either preventing a tap change due to reverse bias on the SCR to be triggered or causing a short circuit between the taps through two SCRs.

#### *2.3.3. Voltage regulation using servo system*

126 An Update on Power Quality

per step are available at the output.

input through the Buck-Boost transformer.

16X 16X 16X

Coarse

Fine

4X 4X 4X

*2.3.2. Solid tap changer using anti-parallel SCRs* 

(a) Off load tap changing switch arrangement.

X X X

transformer and variac.

Input

changing switches are used and for large installation on-load tap changing switches are used. The switches are generally incorporated at the secondary of the transformer. For a low voltage high current load, the switches are provided on the primary side of the transformer due to economical reason. For line voltage correction, taps are provided on the primary of the transformer. For three-phase transformers three pole tap changing switches are used.

In off-load tap changer, the output is momentarily cut-off from the supply. It is therefore used for low capacity equipment and where the momentary cut-off of the supply is not objectionable for the load. The major limitation of the off-load tap changing switches is the occurrences of arcs at the contact points during the change-over operation. This shortens the life of off-load rotary switches, particularly of high current ratings. In Fig. 5 (a), three fourposition switches of an off load tap changer are shown, such that the minimum of X volts

The voltage is corrected by tap-charging switches in steps. Where stepless control is required, variable autotransformers or variacs are used. The normal variac consists of a toroidal coil wound on a laminated iron ring. The insulation of the wire is removed from one of the end faces and the wire is grounded to ensure a smooth path for the carbon brush. Carbon brush is

A Buck-Boost transformer is sometimes used for AC voltage regulation when the output voltage is approximately the same as the mean input voltage as shown in Fig. 5(b). In this case if the output voltage is less than or greater than the desired value, it can be increased or decreased to the desired value by adding a suitable forward or reverse voltage with the

**Figure 5.** Circuit diagram of AC voltage controller using (a) Off load tap changer and (b) Buck-Boost

Output

Medium Output

Ei/2

Input = Ei

> (b) Voltage control by combination of a Buck–Boost transformer and a variac.

Buck – Boost Transformer

Np

Ns

= Ei Ei/2 (Ns/Np)

Anti-parallel SCRs combinations can replace the voltage sensitive relay in the tap-changing regulator. Figure 6 shows a tap changer with three taps which can be connected to the load

used to limit the circulating current, which flows between the short-circuited turns.

Voltage regulators using servo systems are quite common. Both single and three-phase types are available. The rating of this type of regulator is quite high and is more economical for high power rating. This regulator normally consist a variac driven by a servomotor, a sensing unit and a voltage and power amplifier to drive the motor in a reversible way. Various types of driving motor may be used for regulating the unit, such as direct current, induction and synchronous motors. However, in all cases, the motor must come to rest rapidly to avoid overrun and hunting. The amount of overrun may be reduced by dynamic braking in the case of a DC motor or by disconnecting the motor from the variac by a clutch as soon as the signal from the measuring unit ceases. The main disadvantage of this type of regulator is the low life of the contact points of the relays.

#### *2.3.4. Phase controlled AC voltage regulator*

Voltage regulators using SCRs are quit common. The load voltage is regulated by controlling the firing instants of the SCRs. There are various circuits for single phase and three phase regulators using SCRs. Though the output voltage can be precisely controlled by this method, the harmonic introduced in the load voltage are quit large and this circuit is used for applications where the output voltage waveform need not be strictly sinusoidal. The circuit arrangement for a single phase SCR regulator is shown in Fig. 6 and Fig. 7.

#### *2.3.5. Ferro-resonant AC voltage regulator*

The concept of the stabilization of AC voltage using a saturated transformer is rather old. The basic circuit arrangement consists of a linear reactor or transformer T1 and a nonlinear saturated reactor or transformer T2 connected in series as shown in Fig. 8. Since the two elements T1 and T2 are in series, the current through them is the same. Transformer T2 is operated under saturated. The voltage division between the two is according to their impedances. Due to nonlinear characteristics of T2 the percentage change of voltage across it is much smaller compared to the percentage change of input voltage. If a suitable voltage proportional to the current is subtracted from the voltage across T2 a practically constant output voltage can be obtained. The circuit arrangement shown is Fig. 8(a) has some drawbacks such as, no load input current is high, and good output voltage stability cab be achieved only at a particular load. Hence some modifications are necessary to improve its performance. The major modification is to place a capacitor across the saturated transformerT2 that is shown in Fig. 8(b).

Power Quality Improvement Using Switch Mode Regulator 129

causes the circuit to go out of resonance consequently a large change in input current and power factor. For a change in the input voltage, the change in voltage across the resonant circuit is small but the change in voltage at T1 is large, and by suitable proportioning of the voltage, a good degree of stabilization is achieved for the variation of input voltage as well as load current. The simple Ferro-resonant regulator has the following disadvantages:

b. Since the core operates in saturation and output is derived from the tank circuit, the core volume is large, the core losses are high and external magnetic field is also high.

In switch mode AC voltage regulator, the switching devices are continuously switching on and off with high frequency in order to transfer energy from input to output. The high operating frequency results in the smaller size of the switch mode power supplies since the size of power transformer inductors and filter capacitors is inversely proportional to the frequency. The SMPS are more complicated and more expensive, their switching current can

Four common types of switch mode converters are used in DC-DC conversion. They are Buck, Boost, Buck-Boost and C^UK converters. Researches are trying to modify these DC regulators to regulate AC voltages. Buck- Boost and Cûk converter configuration has been investigated for voltage regulation [17-18]. But in every case it is found that the input power

Voltage sag is an important power quality problem, which may affect domestic, industrial and commercial customers. Voltage sags may either decrease or increase in the magnitude of system voltage due to faults or change in loads. Momentary and sustained over voltage and under voltage may cause the equipment to trip out, which is highly undesirable in certain application. In order to maintain the load voltage constant in case of any fluctuation

In this chapter the principle of operation of high frequency switching AC voltage regulator, design of its filter circuit and snubber circuit are described. Performance of the regulator is also analyzed using simulation software OrCAD version 9.1. Switch-mode power supplies (SMPS) incorporate power handling electronic components which are continuously switching on and off with high frequency in order to provide the transfer of electric energy from input to output. The design of AC voltage regulator depends on power requirement, degree of stability and efficiency. Solid state AC regulator using phase control technique are not new and are widely used in many application such as heating and lighting control etc.

cause noise problems, and simple designs can have a poor power factor.

**3. Design and analysis of switching-mode AC voltage regulator** 

**3.1. Operation principle of switching-mode AC voltage regulator** 

of input voltage or variation of load some regulating device is necessary.

a. The output voltage changes with frequency.

*2.3.6. Switch mode AC voltage regulator* 

factor is very low and the efficiency is poor.

*3.1.1. Operation of power circuit* 

**Figure 6.** Circuit diagram of solid tap changer using anti-parallel SCRs.

**Figure 7.** Phase controlled AC voltage regulator, (a) Using back to back SCR and diode and (b) using inverse parallel SCR (c) Using diode-bridge and single SCR

**Figure 8.** Fero-resonant AC voltage regulator.

The value of the capacitor is such that it resonant with the saturated inductance of T2 at some point. The characteristics of the circuit is such that a small change in voltage across T2 causes the circuit to go out of resonance consequently a large change in input current and power factor. For a change in the input voltage, the change in voltage across the resonant circuit is small but the change in voltage at T1 is large, and by suitable proportioning of the voltage, a good degree of stabilization is achieved for the variation of input voltage as well as load current. The simple Ferro-resonant regulator has the following disadvantages:


#### *2.3.6. Switch mode AC voltage regulator*

128 An Update on Power Quality

transformerT2 that is shown in Fig. 8(b).

Input

SCR1

**Figure 6.** Circuit diagram of solid tap changer using anti-parallel SCRs.

V3

V2

V1

inverse parallel SCR (c) Using diode-bridge and single SCR

SCR2

Input Output

**Figure 8.** Fero-resonant AC voltage regulator.

Output Input

T 2

ET T 1

**Figure 7.** Phase controlled AC voltage regulator, (a) Using back to back SCR and diode and (b) using

Input

(a) (b) (c)

The value of the capacitor is such that it resonant with the saturated inductance of T2 at some point. The characteristics of the circuit is such that a small change in voltage across T2

(a) (b)

ET C ET2

impedances. Due to nonlinear characteristics of T2 the percentage change of voltage across it is much smaller compared to the percentage change of input voltage. If a suitable voltage proportional to the current is subtracted from the voltage across T2 a practically constant output voltage can be obtained. The circuit arrangement shown is Fig. 8(a) has some drawbacks such as, no load input current is high, and good output voltage stability cab be achieved only at a particular load. Hence some modifications are necessary to improve its performance. The major modification is to place a capacitor across the saturated

SCR 2

SCR 4

GR 2

GR 1

GR 3

SCR 6

SCR2

SCR1

Output

Output

SCR1

ET1(P) ET1(S)

Input Output

Output

Input

Lo ad

SCR 5

SCR 3

SCR 1

In switch mode AC voltage regulator, the switching devices are continuously switching on and off with high frequency in order to transfer energy from input to output. The high operating frequency results in the smaller size of the switch mode power supplies since the size of power transformer inductors and filter capacitors is inversely proportional to the frequency. The SMPS are more complicated and more expensive, their switching current can cause noise problems, and simple designs can have a poor power factor.

Four common types of switch mode converters are used in DC-DC conversion. They are Buck, Boost, Buck-Boost and C^UK converters. Researches are trying to modify these DC regulators to regulate AC voltages. Buck- Boost and Cûk converter configuration has been investigated for voltage regulation [17-18]. But in every case it is found that the input power factor is very low and the efficiency is poor.
