What is an Uncontrolled Rectifier?

An uncontrolled rectifier is a type of AC to DC converter that provides fixed DC output voltage from a given AC input supply. These rectifiers are used in applications where fixed DC voltage is essential. Uncontrolled rectifier usually consists of an input filter capacitor, power electronics switches such as MOSFET/IGBT/BJT, inductor, diode, a high-frequency transformer(in an isolated configuration only, for isolation), and control circuits to provide regulated DC output. The PWM waveform with a high-frequency range (kHz or MHz) controls the switching period of the power electronics switches. They are ideal for several applications such as medical, industrial, communication, broadcast, IoT, space, low-power ITE, and consumer electronics.

Uncontrolled rectifiers are further classified into two configurations: Isolated and Non-isolated.

• An isolated uncontrolled rectifier uses a high-frequency transformer (flyback transformer or normal transformer) to establish galvanic isolation between the input AC supply and output DC circuit. This isolation provides several benefits such as ensuring the safety of human and sensitive devices from the high and potentially hazardous AC input voltage, breaking ground loops (helpful to isolate the noisy circuits from sensitive circuits), and avoiding floating output. The isolated uncontrolled rectifier employs various switching power supply topologies such as a Flyback converter, Forward converter, push-pull converter, bridge SMPS, isolated cuk, ringing choke converter, and resonant LLC converter.
• A non-isolated uncontrolled rectifier doesn’t need a transformer for operation. The input AC and output DC circuits share the common ground, and current can flow between them. Because of the absence of a transformer, these rectifiers have several advantages such as low cost, small size, high efficiency, and regulation. They can operate at a higher switching frequency which further reduces the size of passive components (inductors and capacitors). The non-isolated uncontrolled rectifier employs various switching power supply topologies such as a step-down/buck converter, step-up/boost converter, buck-boost converter, cuk converter, split-pi converter, zeta converter, and SEPIC converter.

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