active rectifier

An active rectifier is an iteration of a full bridge rectifier that aims to make Maxwell's demon real by substituting the diodes of a traditional rectifier design with transistors - typically MOSFETs because they have very low resistance from drain to source when conducting. The transistors must be controlled by a comparator circuit which switches them at the zero-crossing point of the AC signal.

Fundamentally, it's a more efficient -but also more costly- way to turn an #AC input to a #DC output. Like the passive design, it will probably have issues with power-factor and ripple on the output side unless it's connected to a smoothing capacitor of some description.

circuit diagram

realization without a transformer.

explanation

• The op-amp comparator circuit provides a square wave input $(v_{comp})$ to the gates of the MOSFETs synchronized with the zero-crossing point of the AC signal ($v_{in}$ or $U$ in the below diagrams).
  • The resistors $R_1$ and $R_2$ should be selected for a feedback ratio that effectively debounces the input with hysteresis.
• The comparator's output drives a pair of BJTs which alternately provide $V_{CC}$ or 0 V potential to the gates of the MOSFETs via the pull-up and pull-down resistors.
  

variation with a transformer

Circuit diagram using a center-tap transformer and two MOSFETs

sources and links

• Comparator - Wikipedia
• Active Bridge Rectifier: Working Principle and Efficiency Advantages
• MOSFET CoolMOS™ S7 600V -Design of active bridge line rectification for SMPS
• Zero crossing detection using comparator circuit (Rev. A)
• 555 Oscillator Tutorial - The Astable Multivibrator