Talk:Synchronous rectification

Untitled
Correct me if I am wrong, but something is very fishy about this article. The conduction path is Source to Drain (for an N-Type FET), and the diode is anti-parallel to this direction and points Source to Drain. I believe this is inherent to MOSFET construction; I do not think you can have a MOSFET without this diode. Conversely, in a P-Type FET the current flows from Drain to Source and the diode points from Source to Drain. If the current in the diode went the same direction as the FET (P- or N-Type) they would be impossible to turn off, and I KNOW they can be turned off.


 * You can have a MOSFET without this "diode", but what you get instead is a parasitic BJT with a floating base. The structure is necessarily there, but the connection isn't necessarily as a diode from source to drain; the diode's anode (in N-channel MOSFETs) or cathode (P-channel) can be connected to anything, but the cathode (N-channel) or anode (P-channel) is necessarily formed by the combined source and drain electrodes.  The MOSFET article has the nice diagrams for understanding these parasitic structures. Bernd Jendrissek (talk) 19:05, 24 November 2010 (UTC)

I was looking for this article to learn about Synchronous Rectification, which I think might be synonymous to Active Rectification. You see, Passive Rectification is the use of passive diodes in a Full-Bridge Rectifier, or Diode Bridge to go from AC to DC; it's passive because it takes no tertiary input and simply relies on 4 individual P-N junctions to switch the current flow from an AC source in such a way as to provide a DC output. Using "normal" diodes there is 0.7V loss in each diode (typically) and there are two diodes in use at any time for rectification contributing to a 1.4V loss. This loss is further manifested as a power loss in the P-N junction which does have some finite resistance and will heat up, and while the diode will maintain its forward voltage drop very well, the more current you need the more power you lose, and the less efficient the converter becomes (P=I*V). This can be improved slightly by trading the "normal" diodes out for Schottky Diodes, which can have forward voltage drops even lower than the stated 0.3V in this article.

I don't know what is truly meant by Synchronous Rectification, but International Rectifier has defined Active Rectification. They have developed an Active Integrated Rectifier Regulator (AIRR) for the automotive industry. In the press release announcing such they define active rectification as "the replacement of passive diodes with active FETs to dramatically reduce power losses."


 * I'm skeptical about letting companies' PR blurbs define the meaning of terms. The "Active" in AIRR to me seems to refer to the regulator, not the rectifier.


 * Personally, I would prefer the term Synchronous Rectification for the concept of using active components switched in synchrony (but not necessarily in phase! see other comments re: timing of control signals), whether those are MOSFETs, electromechanical commutators. If it were up to me I'd use "active rectification" for a diode in an opamp feedback loop, by analogy with active filters.  It'd be a rectifier, but "activated".  And for what it's worth, the term "synchronous rectification" wins in a googlefight.  I'm pretty sure The art of electronics also refers to synchronous rectification, although IIRC that was in the context of RF detection. Bernd Jendrissek (talk) 19:05, 24 November 2010 (UTC)

From Buck Converter technology I know the Synchronous Buck Converter to be a sub-set of the Buck Converter that replaces the free-wheeling diode with a FET, and some call this Active Rectification, although I am uncertain as to what it being rectified here, I think it should be termed Synchronous Flyback or Synchronous Switching, but from my understanding there is no rectification.

I can only conclude that Active Rectification and Synchronous Rectification must be synonymous, since the act of replacing a diode with a FET seems to make it necessary to use the term synchronous - probably because you have to externally control the FET, and it must be synchronized to the rest of the circuit's operation and timed properly for correct operation. In a similar manner it can also be called active rectification since you must actively control the switches instead of allowing them to go on their on as in the implementation of a passive rectifier using only diodes, without a tertiary input, and without any other control circuitry or even power.

I propose we change this article to redirect to an article titled "Active Rectification," which has not yet been created, and place the information I have proposed above in that article.

MORE
You can not simply place a diode in parallel with a MOSFET and have it turn the transistor on when it is forward biased as the article suggests. In fact, most MOSFETS have an intrinsic body diode already built into them.

I think this article needs to be scrapped altogether. Although the term "synchronous rectification" is used fairly widely, it is highly misleading. Active Rectification, or Active Switching in the particular case mentioned in this article, is much more accurate, since it is rarely synchronized in time at all, but usually waits for some matching condition. In systems with transient loads, the timing will continuously vary. In systems with constant and stable loads, the switching is still not done with any reference to time whatsoever, therefore, synchronous is very misleading. —Preceding unsigned comment added by 131.204.47.232 (talk) 21:26, 27 May 2009 (UTC)


 * "Synchronous" is perfectly okay; what you don't have is tight phase lock as the term is normally understood. You certainly can't have a control signal that slips cycles with respect to the input power.  If your input power completes 4320023 cycles in a day, so will your control signal - certainly not 4321729 or 4319821 cycles!  Bernd Jendrissek (talk) 19:05, 24 November 2010 (UTC)

Synchronous Rectification is very old, and originally referred to mechanical means of rectifying AC into DC. For example, a synchronous AC motor could spin a switch that looked much like an automotive distributor in order to rectify very high voltages. I've got old textbooks around that refer to this stuff. ISC PB (talk) 18:46, 2 July 2009 (UTC)

one more opinion
Yup, the article is technically wrong (by omitting the need for active control of the MOSFET gate timing) and much too narrow (by omitting the original non-semiconductor meaning of Synchronous Rectifier). Proposal: Start with a disambiguation statement. Branch 1: Traditional electromechanical devices w/ synchronous motor and commutator, vibrator-based PS, etc. Branch 2: Application of synchronously switched MOSFETS for improved efficiency rectification of, say, automobile alternator output. Branch 3: For switch-mode-power-supply app (as described in present article), explain why "synch. rect." is a misnomer, though unfortunately popular. Refer reader to Active Rectification article (and move most of the present content of SR article to AR). -RKF

Merge request
Perhaps someone can propose a merger??? A synchronous rectifier is a mechanical device and involves timing, hence the name synchronous. A more accurate name is Active Rectifier since it is actively controlled with digital and/or analog circuitry.


 * Support merging synchronous rectification into active rectification (or the other way if people prefer). Presently, the synchronous rectifier article is completely wrong in its definition.  The diode is incidental; the synchronously controlled switch is key, whether BJP, MOSFET, or mechanical.  Dicklyon (talk) 00:11, 6 December 2010 (UTC)

The request was old, and clearly supported and necessary, so I went ahead and did it. Dicklyon (talk) 00:56, 6 December 2010 (UTC)