The unidirectional property of a diode is utilized in uncontrolled rectifiers.
The SCRs in AC to DC converters allow control over the conduction time or the phase of the conducting voltage.
Phase-controlled AC to DC converter circuits are used in traction systems, electrochemical processes, magnet power supplies, high voltage DC transmission, and DC motor drives.
An uncontrolled AC to DC converter
Rectifiers, or AC to DC converters, are the most common power electronic circuits in use today. In AC to DC converter circuits, the current and power flow in one direction and are called unidirectional converters. AC to DC converters convert utility power at 50 Hz or 60Hz to controlled DC or uncontrolled DC, depending on the type of converter circuit used. Let’s discuss AC to DC converter circuits in this article.
AC to DC Converter Circuits
In most countries, electrical energy is available in the form of alternating voltage. There is a need to convert alternating voltage to DC voltage for use in electronics equipment. When electronics equipment is plugged into the mains power supply, the internal architecture of the equipment includes an AC to DC converter circuit to convert the AC mains supply to DC supply.
The AC to DC converter forms part of the equipment’s power supply. Typically, the converter circuit is preceded by a step up or step down power transformer. The step-up transformer converts the mains AC voltage to high voltage, whereas the step-down transformer produces low voltage. In general, a transformer helps to achieve the desired DC voltage level. Apart from the step-up or step-down action, the isolation from the mains supply provided by the transformer reduces the risk of electric shock.
The AC to DC converter converts the harmonic AC mains into DC voltage. The AC to DC converter circuit can output either controlled DC or uncontrolled DC, depending on its type. Following the converter, a filter circuit is placed to smoothen the pulsating DC. In most applications, voltage regulators are utilized for regulating the DC voltage constant.
Uncontrolled AC to DC Converters
The unidirectional property of a diode is utilized in uncontrolled rectifiers. In uncontrolled rectifiers, the rectified output current and voltage are functions of the applied inputs. The output voltage level cannot be varied when AC to DC converters with diodes are employed for conversion. Diode rectifier circuit operations are based on the fundamental property of the PN junction diode to conduct as a closed switch when forward biased and not to conduct like an open switch when reverse biased. The principle circuit operation remains the same irrespective of the type of uncontrolled rectifiers, such as half-wave rectifiers or full-wave rectifiers. Both half-wave and full-wave rectifier types are applicable for single-phase as well as three-phase applications.
Uncontrolled half-wave AC to DC converters utilize one diode, converting only positive half cycle AC to DC, whereas full-wave AC to DC converters produce DC from both positive and negative half-cycles. The full-wave AC to DC converter types, center-tap rectifiers and bridge rectifiers, use two diodes and four diodes, respectively.
The pulsating DC output from AC to DC converters with diodes should be ripple-free. For ripple-free DC voltage, large capacitor filters are connected to the DC side. The capacitor charges and discharges to smoothen the pulsating DC. The capacitor charging to the peak of the input voltage draws a large current during the peak of the half-cycle and discharges thereafter. The current flow can be zero for a finite time in uncontrolled AC to DC converters, and the current drawn from the mains supply gets highly distorted. The distortions lead to poor power factors, wastage of electrical energy, and high harmonic components. As harmonic standards are stringent these days, the use of simple diode AC to DC converter circuits is limited, even though they are inexpensive.
Phase-Controlled AC to DC Converters
In uncontrolled AC to DC converters, the level of DC voltage is dependent on the peak voltage and frequency of the applied voltage. In controlled AC to DC converter circuits, silicon-controlled rectifiers (SCRs) or thyristors are used in the place of diodes. The SCRs in AC to DC converters allow control over the conduction time or the phase of the conducting voltage.
SCRs are three-terminal devices with pins: anode, cathode, and gate. Phase-controlled AC to DC converters with SCRs act like diode rectifiers when the gate is triggered (or fired) at the 0° phase angle of the applied AC input voltage. The SCR conducts whenever the gate is triggered and anode to cathode voltage must be positive. One can initiate the gate triggering between 0° and 180° phase of the input AC voltage, and thereby the conduction of the SCR can be controlled. According to the firing of the SCR, the average value of the output DC voltage varies and thus controlled DC is obtained.
Phase-controlled AC to DC converter circuits are used in traction systems, electrochemical processes, magnet power supplies, high voltage DC transmission, and DC motor drives. Irrespective of the controllability, AC to DC converter circuits are designed in printed circuit boards. In the case of phase-controlled AC to DC converters, a gate drive circuit (control circuit) is required, with proper isolation from the converter circuit (power circuit). Cadence offers the proper PCB design software to design power circuits as well as control circuits for residential, commercial, and industrial-power electronic applications.
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