AN INFORMATIVE ARTICLE ON CHOPPERS(DC TO DC) CONVERTERS?

Functions of dc to dc converters

The functions of dc–dc converters are:

• to convert a dc input voltage VS into a dc output voltage VO;
• to regulate the dc output voltage against load and line variations;
• to reduce the ac voltage ripple on the dc output voltage below the required level;
• to provide isolation between the input source and the load (isolation is not always required);
• to protect the supplied system and the input source from electromagnetic interference (EMI)
• to satisfy various international and national safety standards.

circuit diagram of choppers

What is step down (BUCK) converters:

The step-down dc–dc converter, commonly known as a buck converter. It consists of dc input voltage source VS , controlled switch S, diode D, filter inductor L, filter capacitor C, and load resistance R.

What is continuous conduction mode(CCM)?

under assumption that the inductor current is always positive. The state of the converter in which the inductor current is never zero for any period of time is called the continuous conduction mode (CCM).

What is discontinuous conduction mode(DCM)?

When the average value of the input current is low (high R) and/or the switching frequency f is low, the converter may enter the discontinuous conduction mode (DCM). In the DCM, the inductor current is zero during a portion of the switching period.

Gain of converter?

Hence, the dc voltage transfer function, defined as the ratio of the output voltage to the input voltage, is

                       Mv = Vo /Vs

What is step up(BOOST) converter?

• A step-up or a PWM boost converter  is comprised of dc input voltage source VS , boost inductor L, controlled switch S, diode D, filter capacitor C, and load resistance R.  
• When the switch S is in the on state, the current in the boost inductor increases linearly. The diode D is off at the time. When the switch S is turned off, the energy stored in the inductor is released through the diode to the input RC circuit.

What is BUCK BOOST converter?

• A non-isolated (transformer less) topology of the buck–boost converter is shown in Fig. 
• The converter consists of dc input voltage source VS , controlled switch S, inductor L, diode D, filter capacitor C, and load resistance R. With the switch on, the inductor current increases while the diode is maintained off. When the switch is turned off, the diode provides a path for the inductor current.

Cuk converter?

• The circuit of the C`uk converter consists of dc input voltage source VS , input inductor L1, controllable switch S, energy transfer capacitor C1, diode D, filter inductor L2, filter capacitor C, and load resistance R.
• An important advantage of this topology is a continuous current at both the input and the output of the converter.
• Disadvantages of the Cuk converter include a high number of reactive components and high current stresses on the switch, the diode, and the capacitor C1. 
• When the switch is on, the diode is off and the capacitor C1 is discharged by the inductor L2 current. 
• With the switch in the off state, the diode conducts currents of the inductors L1 and L2 whereas capacitor C1 is charged by the inductor L1 current.

Chopper (electronics)

In electronics, a chopper circuit is used to refer to numerous types of electronic switching devices and circuits used in power control and signal applications. A chopper is a switching device that converts fixed DC input to a variable DC output voltage directly. Essentially, a chopper is an electronic switch that is used to interrupt one signal under the control of another.

In power electronics applications, since the switching element is either fully on or fully off, its losses are low, and the circuit can provide high efficiency. However, the current supplied to the load is discontinuous and may require smoothing or a high switching frequency to avoid undesirable effects. In signal processing circuits, use of a chopper stabilizes a system against drift of electronic components; the original signal can be recovered after amplification or other processing by a synchronous demodulator that essentially un-does the "chopping" process.

Classification

This section may be confusing or unclear to readers. In particular, the table columns are unclear and the whole comparison is incoherent. (May 2016) (Learn how and when to remove this template message)

Choppers may be classified on several bases.

On basis of input and output voltage levels:

1. Step-down chopper
2. class A
3. class B
4. class C(combination of A&B)
5. class D
6. class E
7. Step-up chopper
8. class B *explanation of class A, C,D,E chopper

Comparison between step up and step down chopper:

Sr no parameters Step down chopper Step up chopper

1 Range of output voltage 0 to V volts V to +∞ volts

2 Position of chopper switch In series with load In parallel with load

3 Expression for output voltage VL dc = D x V volts Vo = V/ ( 1 – D ) volts

4 External inductance Not required Required for boosting the output voltage

5 Use For motoring operation, for motor load For regenerative braking for motor load.

6 Type of chopper Single quadrant Single quadrant

7 Quadrant of operation 1st quadrant 1st quadrant

8 Applications Motor speed control Battery charging/voltage boosters

On basis of circuit operation:

First quadrant

Two quadrant

Four quadrant

On basis of commutation method:

Voltage commutated

Current commutated

Load commutated

Impulse commutated

Applications[edit]

Most modern uses also use alternative nomenclature which helps to clarify which particular type of circuit is being discussed. These include:

switched mode power supplies, including DC to DC converters.

Speed controllers for DC motors

Class D Electronic amplifiers

Switched capacitor filters

Variable-frequency drives

D.C. motor speed control

D.C. voltage boosting

Battery-operated electric cars

Battery-operated appliances

Battery chargers

Control strategies

For all the chopper configurations operating from a fixed DC input voltage, the average value of the output voltage is controlled by periodic opening and closing of the switches used in the chopper circuit. The average output voltage can be controlled by different techniques namely:

Pulse-width modulation

Frequency modulation

Variable frequency, variable pulse width

CLC control

In pulse-width modulation the switches are turned on at a constant chopping frequency. The total time period of one cycle of output waveform is constant. The average output voltage is directly proportional to the ON time of chopper. The ratio of ON time to total time is defined as duty cycle. It can be varied between 0 and 1 or between 0 and 100%. Pulse-width modulation (PWM), or pulse-duration modulation (PDM), is a technique used to encode a message into a pulsing signal. Although this modulation technique can be used to encode information for transmission, its main use is to allow the control of the power supplied to electrical devices, especially to inertial loads such as motors. The average value of voltage (and current) fed to the load is controlled by turning the switch between supply and load on and off at a fast rate. The longer the switch is on compared to the off periods, the higher the total power supplied to the load. The PWM switching frequency has to be much higher than what would affect the load (the device that uses the power), which is to say that the resultant waveform perceived by the load must be as smooth as possible. Typically switching has to be done several times a minute in an electric stove, 120 Hz in a lamp dimmer, from few kilohertz (kHz) to tens of kHz for a motor drive and well into the tens or hundreds of kHz in audio amplifiers and computer power supplies.

In frequency modulation, pulses of a fixed amplitude and duration are generated and the average value of output is adjusted by changing how often the pulses are generated.

Variable pulse width and frequency combines both changes in the pulse width and repetition rate.

Chopper amplifiers

One classic use for a chopper circuit and where the term is still in use is in chopper amplifiers. These are DC amplifiers. Some types of signals that need amplifying can be so small that an incredibly high gain is required, but very high gain DC amplifiers are much harder to build with low offset and 1/ {\displaystyle f} f noise, and reasonable stability and bandwidth. It's much easier to build an AC amplifier instead. A chopper circuit is used to break up the input signal so that it can be processed as if it were an AC signal, then integrated back to a DC signal at the output. In this way, extremely small DC signals can be amplified. This approach is often used in electronic instrumentation where stability and accuracy are essential; for example, it is possible using these techniques to construct pico-voltmeters and Hall sensors.

The input offset voltage of amplifiers becomes important when trying to amplify small signals with very high gain. Because this technique creates a very low input offset voltage amplifier, and because this input offset voltage does not change much with time and temperature, these techniques are also called "zero-drift" amplifiers (because there is no drift in input offset voltage with time and temperature). Related techniques that also give these zero-drift advantages are auto-zero and chopper-stabilized amplifiers.

Auto-zero amplifiers use a secondary auxiliary amplifier to correct the input offset voltage of a main amplifier. Chopper-stabilized amplifiers use a combination of auto-zero and chopper techniques to give some excellent DC precision specifications.Courtesy of wikipedia...