Chopper circuits

NoU It is possible that at some loealioiis there is no a.e. source available, such as (or battery-operated lifts iirul motor vehicles,. Such applications may also call for a variable d.e. source. When it is so. it can be achieved with the use of a chopper circuit which uses the conventional semiconductor devices. The devices are switched at high repetitive frequencies to obtain the required variation in the output voltage as with the use of a phase-controlled lliyristor rectifier, A typical chopper circuit is shown in Ingure 6.2, i. using diodes and a controlled unidirectional semieonduetor switch, which can be a thyristor or tin IGBT. 

Either a linear servo-amplifier or a chopper can be used. Linear servo amplifiers are not generally used with power wheelchairs primarily because of their lower efficiency than chopper circuits. A motor can be thought of as a filter to a chopper circuit, in this case, the switching unit can be used as part of a speed and current control loop. The torque ripple and noise associated with phase control drives can be avoided by... 

Connection to the supply/line can be by a Diode Front End (DFE) arrangement for two quadrant operations. Addition of a chopper circuit with dumping resistor extends the operation of a DFE drive for accommodation of short term regenerative loads. More commonly for the four quadrant control required by a hoist a fully controlled bridge is used. This is referred to as an Active Front End (AFE). 

A potential safety hazard is the fact that with armature current feedback, the armature current is connected to the operator control and potentiometers maybe operated at high potentials (500 V). This problem can be eliminated using isolation transformers or DC to DC chopper circuits. 

It is also clear that the ripple depends on the frequency - at higher frequency, the ripple is less. However, each tum-on and turn-off involves the loss of some energy, so the frequency should not be too high. A control circuit is needed to adjust foN to achieve the desired output voltage - such circuits are readily available from many manufacturers. The main energy losses in the step-down chopper circuit are... 

In practice all these can be made very low. The efficiency of such a step-down chopper circuit should be over 90%. In higher voltage systems, of about 100 V or more, efficiencies as high as 98% are possible. 

Figure 10.7 Graph of voltage against current for a fuel cell with a step-up chopper circuit that regulates to a voltage a little less than the maximum stack voltage.

The primary reference method used for measuring carbon monoxide in the United States is based on nondispersive infrared (NDIR) photometry (1, 2). The principle involved is the preferential absorption of infrared radiation by carbon monoxide. Figure 14-1 is a schematic representation of an NDIR analyzer. The analyzer has a hot filament source of infrared radiation, a chopper, a sample cell, reference cell, and a detector. The reference cell is filled with a non-infrared-absorbing gas, and the sample cell is continuously flushed with ambient air containing an unknown amount of CO. The detector cell is divided into two compartments by a flexible membrane, with each compartment filled with CO. Movement of the membrane causes a change in electrical capacitance in a control circuit whose signal is processed and fed to a recorder. 

The reduction of cost of electronic speed control for fan motors has led to a much wider use of this method. The general circuit is to invert the supply by first rectifying it to direct current and then pass this through a chopper to produce a new alternating current with the frequency for the new motor speed. 

Figure 9.12. A correlation radiometer where the chopper is replaced by a hybrid circuit. Separate amplifier chains amplify the sum and difference signals which are then multiplied.

CDS is somewhat simpler since no demodulator and filter are needed. However, since the signal is sampled, CDS suffers from noise aliasing. Unless eliminated with appropriate circuit techniques [22, 23], this will result in an elevated noise floor. Chopper stabilization is preferred in printed circuit board implementations since all necessary components are readily available. 

Many other integrated solutions are documented in the literature. These include single-ended and differential solutions employing chopper stabilization [1, 20, 24, 31], circuits using correlated double sampling [2, 3, 32] and applications for capacitive readout circuits and corresponding implementations [15]. 

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