switching in power electronics?
In power electronics field switching is common and frequently occurring operation carried out with the help of different simple,complex,old and new devices having different characteristics and operations.Diode,transistors etc are common switching devices used for different purposes when needed.These switching modes are of different interests and purposes.As one switching mode may be conversion of ac to dc where as second is dc to ac,similarly conversion may be from ac to ac or dc to dc depending on the required scenario.
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simple model of switching circuit in power electronics
What is chopper?
A chopper is basically a switching circuitry capable of converting a fixed dc input provided into variable dc output.It is a converter among four normal converters and its conversion functionality is dc to dc.This is useful in many different fields.
Classifications of choppers
A chopper is classified into four following categories.
1. buck converter or we can say this is step down means it downs the parameters of circuitry designed for conversion
2 boost converter this is reverse of buck converter means it is step up converter this property is obvious from name
3.cuk converter is a special sort of converter having feature of continuous current and both step up and down qualities
Operations of chopper?
a) constant frequecny mode
b) variable frequncy or frequency modulation
Applications of chopper in different fields
1. in industrial applications it converts fixed dc to variable dc
2. a chopper is just like a transformer that steps up or down a dc voltage
3. choppers are used for the traction motor control in
- electric automobiles
- trolley cars
- marine hosts
- forklift trucks
- smooth acceleration control
- smooth acceleration control
- high efficiency
- fast dynamic response
5. choppers are used in regenerative braking of dc motors
6. they are used in dc voltage regulators
7. are used in conjunction with an inductor to generate a dc current source.
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,,,,
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