Chopper (electronics)

Schematic of an inverter using a vibrator as a chopper.

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

Choppers may be classified on several bases.

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

Applications

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

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:

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 to 1 or between 0 to 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/ 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.[1]

Some example chopper and auto-zero amplifiers are LTC2050,[2] MAX4238/MAX4239[3] and OPA333.[4]

Examples and circuits

The Keithley Instruments Model Nanovolt Null Detector and the 147 Leeds & Northrup 9838-1 Guarded Nanovolt Detector both use mechanical choppers. The choppers are like buzzers and you can hear them at an audio frequency.

The Keithley manual is available at their website.[5] And the Leeds & Northrup has a similar circuit.

The Keithley has been used to look for the I-R DC voltage of a superconductor.[6] The system noise was such that a voltage as low as 10 nanovolts could be detected, and none was found.

A Leeds & Northrup was bench tested and found to be capable of detecting 50 nanovolts.[7]

See also

References

  1. ↑ US Patent 7132883 - Chopper chopper-stabilized instrumentation and operational amplifiers
  2. ↑ LTC2050
  3. ↑ MAX4238/MAX4239
  4. ↑ OPA333
  5. ↑ Keithley manual
  6. ↑ University of New Orleans, Physics Department, Graduate student research.
  7. ↑ A private laboratory in Cross Lanes, WV.

Literature

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