DC to DC Power Converters: Function, Types, Operation & Applications

DC to DC power converters also referred to as choppers provide the means to change one DC voltage to another. Normally the conversion is to a lower voltage however, we also have step-up converters.

DC to DC power converters are fed from a DC supply usually comprising an uncontrolled AC to DC converter or alternatively a battery supply; the controlled DC output can then be used to control a DC machine as in the case of the controlled AC to DC converters.

DC drivers utilizing controlled AC to DC converters have a number of shortcomings which are overcome by the DC to DC converters. Some of the limitations of controlled AC to DC converters are described as follows:

• The inability of a thyristor to interrupt current implies that an alternating supply is needed to commutate the converter; this prevents operation from a DC supply. This is a common requirement on battery vehicles and DC fed rail traction.
• Because of the delay inherent in thyristor switching (3.3 ms in a 50 Hz 3-phase converter) the current control loop band width of the converter is limited to approximately 100 Hz, which is too low for many servo drive applications.
• The thyristor controlled AC to DC converters have an inherently poor input power factor at low output voltages. (Near unity power factor can be achieved by employing uncontrolled rectifier feeding a DC to DC converter.
• The DC ripple frequency is determined by the AC and is for a 50 Hz supply frequency, 100 Hz for single-phase and 300 Hz for three-phase fully-controlled bridges. This means that additional smoothing components are often needed when using high speed machines, permanent magnet motors or other special motors with low armature inductance.
• Electronic short-circuit protection is not economically possible with thyristor converters. Protection is usually achieved by fuses.

Nonetheless, DC to DC converters are more complex and rather less efficient than AC to DC converters. DC to DC converters are used in DC servo drives, rail traction drives and in small fractional kW drives utilizing permanent magnet motors.

In developing DC to DC converters, any of the various types of transistors may be used such as BJT, MOSFET and IGBT or gate turn-off thyristors (GTO) may be employed.

Types of DC to DC Converters

Step-down DC to DC Converters/Buck Converters

Single-quadrant DC to DC Converter

This is the most basic DC to DC converter. An illustration of this type of converter is shown below:

The output voltage is changed by pulse-width modulation (PWM) i.e. by varying the time for which the transistor T is turned on. Thus, the voltage applied to the motor is in the form of a square wave of varying periodicity. Since the motor is inductive, the current waveform is smoothed; the flywheel diode D carrying the current while the transistor is turned OFF.

The circuit above is only capable of supplying unidirectional current and voltage to the motor and is therefore not capable of four-quadrant operation, i.e. reversing or regenerating.

The applications for this circuit are normally limited to drives below 5 kW and simple variable speed applications.

Two quadrant DC to DC Converter

In order to achieve full four-quadrant operation, a converter must be capable of supplying reversible voltage and current to the motor. A circuit capable of two-quadrant operation i.e. motoring and braking in one direction only is illustrated below:

The converter is able to reverse the current flow to the motor but unable to reverse the motor terminal voltage and hence the speed. During motoring, the converter operates as a basic chopper with T1 and D2 carrying the current. During the braking or regeneration, T1 is inoperative and T2 controls the current. During its ON periods, motor current builds up negatively, limited by the motor inductance L. when T2 turns OFF, the only path for the current is via D1 back into the supply; thus the circuit is regenerative.

Since this circuit is not capable of motor speed reversal, it is typically employed in unidirectional applications. However, because of its simplicity it is sometimes used in traction applications where reversing is performed by means of a changeover to reverse the armature or field supply.

Four-quadrant DC to DC Converter

The figure below illustrates a basic four-quadrant converter capable of supplying reversible voltage and current, that is, reversing and regeneration.

During motoring, positive output transistors T1 and T2 are switched ON during the ON period, whereas diodes D2 and D4 conduct during the OFF period. When D2 and D4 conduct, the motor supply is reversed and as a result the voltage is reduced to zero at 50% duty cycle. Any reduction of duty cycle below 50% will cause the output voltage to reverse but with current in the same direction; thus, the speed is reversed and the drive is regenerative. With transistors T2 and T3 conducting, the current is reversed and as a result a full four-quadrant operation is achieved.

One shortcoming of this converter is that the amplitude of the output ripple voltage is twice that of the simple converter and the current ripple is therefore worse. This problem can be overcome by a technique termed to as double-edged modulation. With this technique the flywheel current is circulated via a transistor and a diode during the OFF period. For instance, after T1 and T2 have been conducting, T4 is turned OFF and T3 ON, so that the flywheel current circulates via T1 and D3. The net effect is a reduction in ripple voltage and a doubling of ripple frequency.

The four-quadrant DC to DC converters are extensively employed in high performance DC Drives such as servos.

Step-up DC to DC Converters/Boost Converters

Just like step down converters, various alternative configurations exist for step-up converters. The diagram below shows an arrangement of a step-up converter.

When T is turned ON, current builds up in inductor L; When T is turned OFF  the energy stored in L is transferred to the capacitor C via D. When the capacitor voltage, which is the same as the motor armature voltage, reaches the desired level, T is turned ON once more. C cannot discharge via T as diode D is reverse biased. In this way, a stabilized voltage typically twice the input DC voltage can be obtained. This circuit is specifically useful when operating on low voltage supplies and can lead to very cost effective converter design.

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• Key Facts about Thyristor DC Motor Drives Operation
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