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Do we use an active or passive rectifier to convert AC current to DC current, and why?

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    \$\begingroup\$ Please get a bit more specific on what you want to know. Maybe add an example? \$\endgroup\$
    – jippie
    Apr 19, 2012 at 20:56
  • \$\begingroup\$ Ummm I just took an exam and that was literally all it said. It's a basic circuits class so I just need to know if an active or passive rectifier is used to convert AC to DC current. I don't need a very in depth explanation \$\endgroup\$ Apr 19, 2012 at 20:59
  • \$\begingroup\$ I dont know if this goes to deep, but this site gives some explanations about the various pieces of a power supply. Maybe you will find it helpful. \$\endgroup\$
    – PetPaulsen
    Apr 19, 2012 at 21:04
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    \$\begingroup\$ Um, both? Things like wall-wart adapters are passive and things like switching power supplies with PFC are active ... seems like an absurd exam question to me. \$\endgroup\$ Apr 19, 2012 at 21:05

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It really varies. The electricity in your outlet is AC, so the rectification is done in the product. You can either rectify passively using just diodes, or actively using thyristors* or MOSFETs. Diodes are nearly always used since they are the cheapest and simplest solution. Once you reach too large currents, then the forward voltage of the diode will generate too much waste heat, suggesting the use of active rectification using MOSFETs. You generally see active rectification in very high current environments, or other exotic applications where control is desired. MOSFETs have less waste heat at high currents due to their low RDS(on), or basically their resistance when conducting, and they have no additional forward voltage otherwise, compared to a diode which has very low (incremental) resistance but a sizable voltage drop.

Current squared times resistance is for MOSFET power dissipation when just concerned about on resistance.

Current times voltage drop is power dissipation for a diode when just concerned with forward voltage.

Power factor correction (PFC), is also one of the normal uses of active rectification.

*Thyristors, such as SCRs (silicon controlled rectifiers), work essentially like a diode, including having the (roughly) fixed voltage drop, but turn-on can be triggered (after which it remains on until reverse-biased). This is often used in industrial equipment, when a variable output voltage is needed, and when phase control is acceptable (that is, cutting off a portion of the input waveform to reduce output voltage). In fact this even works in reverse, allowing power to be fed back into the mains, or new AC output to be generated (SCR inverter / motor drive). TRIACs are the other type of thyristor, which would not normally be used in rectifiers due to their bidirectional operation, but can be used in related circuits, for example to switch power to a (passive) rectifier, or to bypass an inrush current limiting resistor.

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  • \$\begingroup\$ Sorry for the bad spelling, I answered this on the tablet and editing it is a huge pain in the butt, I will edit this later in a few hours when i get to a computer. \$\endgroup\$
    – hak8or
    Apr 19, 2012 at 22:55
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    \$\begingroup\$ Or I'll do it for you 11 years later. \$\endgroup\$
    – JYelton
    Aug 14, 2023 at 19:01
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Diodes are used at voltages where they are a small contributor to power loss.

At very small voltages (e.g. 6V 60Hz AC obtained from a transformer) one could conceivably use MOSFETs which lower the voltage drop, at the cost of complexity.

At moderate to high power levels (upwards of 1000 watts), thyristors (SCRs, triacs) are commonly used, because they can block in both directions and can be used to avoid surge currents into DC link capacitors.

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