MOSFETs (active rectification) in place of diodes in linear PSU

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Hi everybody,

I was wandering about an idea. Among tube lovers, it is widely accepted that in audio gears tube rectifiers generally gives better results (better sound) than sand state rectifiers, even when coupled with massive CLC filtering.

Indeed, there are some valid technical reasons behind this. Tube rectifiers are not affected by the (direct and reverse) switching noise, plus their transfer characteristic is such as to drastically reduce the rectification noise magnitude and spectral bandwidth thanks to the "softer" and "smoother" currents that flows into the filter capacitors.

Unfortunately, tube rectifiers have all sorts of well known drawbacks, too.

So I was wondering: why not trying to "emulate" the behaviour of tube rectifiers replacing the simple diodes with some active solid state circuitry?

Thinking about it, it became obvious that using some active circuitry it should be possible to hunt for (and achieve) even better results than the mere "emulation" of a tube rectifier. Better results which of course can include some degree of embedded voltage (pre-)regulation, too.

Possibly (but not necessarily, given the different goals) this can be sort of like the synchronous rectifiers used in some SMPS.

Then something came to my mind... I remember a long time ago I have read somewhere about someone who have written an article on some magazine (maybe Glass Audio, or MJ, or some other one... or was it the JAES? can't remember) proposing to replace diodes with MOSFETs in audio PSUs. That is, possibly something similar to what I was wondering about (don't really know, I never got to read the article itself, only heard about it).

Now the questions for you guys are:

what do you think about the idea?

did anybody tried to do something like that?

does anybody know about the aforementioned article?

Thanks a lot!
 
Seems like there would be some painless ways of doing this with special taps, "extensions" more precisely, of the power trans secondary... something on the order of 10 Volts higher (per leg) to drive the gates. The trick being that you would have to avoid waking up the body diode of the FET.

You would need a clear picture of the conducting angle to make sure you had the right amount of gate drive over the range of input voltages and load currents.

I 'spose you could test this easily enough with a second trans with a 1.1 step up ratio...or so. Actually, you could phase 2 heater windings on the secondary to pull this off... just for testing.
 
Put a 10 to 100 ohm resistor in series with a 1N4007 diode and you'll have a nice tube rectifier.

Active rectification with MOSFETs is not an "improvement" because the body diode of the MOSFET is used, and it's much like a 1N4007 or a bit faster.

Also, diode behaviour is strongly mistified (usually by people that talks a lot about vague things like "noise" and "switching" but has never looked at the real thing with an oscilloscope). There is a thread showing actual current and voltage waveforms that I think it's worth reading :

http://www.diyaudio.com/forums/showthread.php?s=&threadid=66542&perpage=10&pagenumber=5

(The most interesting stuff starts at page 5 or 6).
 
Eva, you can do some tricks with Infinoen's BSP, BST IC's. One trick is to connect a powerswitch backwards and let the internal charge pumps make the mosfet conducting with the wrong polarity.

I'm not sure this will work as rectifier but it sure works as a super diode with no losses! We use it commercial products with good results.

This is a real smart idea I would never thought of myself :idea:

http://www.diyaudio.com/forums/showthread.php?s=&threadid=53690&highlight=
 
Hi poobah,

Seems like there would be some painless ways of doing this with special taps, "extensions" more precisely, of the power trans secondary... something on the order of 10 Volts higher (per leg) to drive the gates. The trick being that you would

mmmh... not sure if I got it right (could you be so kind to post some rough sketch?).

BTW, if I understood it correctly it looks a lot like (it is?) a self-driven synchronous rectifier... and indeed it sounds like an interesting idea to start with! :cool:

(I guess you'd need a center-tapped secondary... or do u think this can be arranged as a bridge, too?)

But then... there would be any real advantage here? If I understood it correctly, the MOSFETs are operated as switches, thus I guess that currents (and noise) will not be much different from what you would get from a conventional (diode) rectifier.

As I said, my goals are pretty different from what have pushed synchronous rectifiers into the SMPS world. I couldn't care less about efficiency (which is what SR are all about).

I do care mostly about reduction of rectification noise, and would welcome some degree of voltage regulation at the rectifier level (so that the following massive filtering will decouple it as much as possible from the signal path).

To fully implement my idea, the active devices (be that MOSFETs, BJTs or whatever happen to be the most appropriate ones for the task) should be operated in their linear region (dissipating fairly amounts of power) at least in some part of the cycle.

In the device control network there must be some R-C to limit the current slew-rate (that is, to limit the bandwith of the current pulses), as well as some voltage reference (and comparators, etc) to gradually reduce and eventually cut-off the charging current when the voltage across the filter cap approaches the set value (which of course must be fairly less than what you would get from a "normal" rectifier).

This way I _hope_ to get something /better/ than a silicon diode with a series resistor! ;)

...which BTW will work quite well if you can afford to have a high enough R value in the circuit; I did that using an integrated bridge -> _470R_ -> 100uF (-> 30H -> 330uF) on some 380V/20mA PSU for the pre/driver section of my last "baby" (a 6C33 SET), and it is as noiseless as it could be... but try doing the same on the 200V/500mA+ PSU for the output tubes!

With this respect, I have to admit I do care a bit about efficiency... it should not end up producing more heat than the room heating system! :clown:
 
Paulo,

I don't know if I would bother with all this work for rectification. See what SY & EVA say above. Regular diodes can work well. It is a curious idea. Also, I haven't totally thought this thru... just a crude idea and a VERY crude drawing!

I think this would work in bridge or center tapped. Play with the idea... I would use sand...
 

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SY said:


Yes. I've done precisely that and when spectra were taken, the 1N4007 with series resistor showed less noise on the rail than either a GZ34 or a diode-connected 572B when the resistor was adjusted to give an equal voltage output.

sure... see my previous post. :)

Problem is, with a fixed resistance in series with a diode you will not only limit the slew-rate of the pulse, but the max peak current as well. To be effective, this trick require to waste a sensible fraction of the power on the resistor, and will soon become impractical :hot: as the required PSU output current increase.

The active rectifier/regulator I'm proposing would dynamically adjust its series impedance during the cycle to limit the charging current slew-rate without necessarily limiting the max peak current.
On average (I hope!) it should be a bit more power efficient (though possibly a bit more noisy) than an "equivalent" D+R.

And for sure it would be much more efficient than a D+R followed by a conventional linear voltage regulator... in fact, I guess that (being partly "switching" by its very nature) it should also be more efficient than any conventional linear voltage regulator alone.

Of course, if it's followed by any serious (CLC and/or CRC) filter, it will provide no regulation against fast load fluctuations (well, unless some more tricks are used to also sense the overall PSU output voltage - that is, a feedback loop taken from the last capacitor of the chain...).

But this is a very desired feature indeed! I personally believe that any voltage regulator directly interacting with the audio circuit destroys the sound quality. :smash:


poobah said:

I don't know if I would bother with all this work for rectification. See what SY &

see above... it would not be just for rectification!

Moreover I have to say that when it comes to hi-quality audio, all of the PSU -including rectifiers- deserve the maximum attention.

Having done a few experiments with different (sand) rectifiers applied to a couple of amplifiers (a SET and a sand), I still can't believe how _MUCH_ these can affect the overall sound, even when they are followed by a ( by all means! :D ) massive CLC filter (330uF+30H+560uF+470uF+47uF) in one case or when they are used in a hi-NFB SS design which is supposed to (does!) have a huge PSRR. :eek:


BTW: now that the idea should be clearer, are there any ideas/suggestion for possible implementation, anyone?


P.S.: Thanks for the drawing.
 
peranders:

This switch IC trick is nice. However, what I was trying to point out is that a MOSFET connected to a transformer as a synchronous rectifier has to be turned off just before the input voltage tries to fall below the output voltage. This forces the body diode to conduct for a brief period of time and to show its reverse recovery characteristic, thus there is no EMI improvement over a normal diode (although there is an obvious dissipation improvement).

I'm employing synchronous rectification in my 15V 120A PSU so I know this behaviour quite well. At full load I'm turning my MOSFETs off approx. 500ns before they are asked to stop conducting and I'm turning them back on approx. 800ns after they are asked to start conducting back. Body diode behaviour is crucial during these glitches (particularly due to reverse recovery) altough additional conduction losses are negligible considering that the switching period lasts approx. 16us.
 
I'll think this type of rectification is good _only_ when you need much current at low voltages and high efficiencies. Otherwise, is it worth the trouble :scratch:

The polarity protection I pointed out is brilliant! With BTS660 you have 50-60 A super diode with no losses! Amazing! At 20 A you'll get 2.8 Watts and a voltage drop of 0.4 V. Compare this to a regular diode! Bu the nicest thing is when you have even small currents say 10 A. The other good thing is that the package is TO220, pretty small to be a 60 A "diode".
 
Employing these integrated switches for low current applications may be disadvantageous because the internal charge pump and control logic draw some current by itself.

Consider a 50mA load. It would cause approx. 20mW dissipation in a small schottky diode, but one of these switches operated at 12V and drawing 3mA on his own would be already wasting 36mW (at the expense of added functionality, though). Much higher average load currents are required to make it worth the effort, particularly in battery-operated applications.
 
Paolo,

I have derived a topology that will achieve ALL of your requests. It borrows heavily from resonant SMPS design. I have not derived a control strategy or transfer function... and this would be most difficult. Nor do I think it would be practical to implement; I do not agree with your assesment that post supply regulation is so bad for the sound... a capacitor can fix that.

However, if you are genuinely interested in the mental exercise, I will post a drawing and an explanantion.
 
poobah said:

I have derived a topology that will achieve ALL of your requests. It borrows heavily from resonant SMPS design.

Wow, that's a great news! :happy1:



I do not agree with your assesment that post supply regulation is so bad for the sound... a capacitor can fix that.

so here is were we somewhat disagree... I think that some "serious" CLC or CRC low-pass filtering (with Ft < 20Hz) is needed if you want to completely "decouple" the audio circuit from the regulator, and that would be no less than a good ripple filter... ;)

Of course, how much this can really matters to the sound is:

a) very "system" dependent... e.g. it's likely that results would be different for a feedbackless SET rather than a fully balanced hi-NFB SS circuit (I have to admit that for the sake of briefness I have been quite a bit too drastic on my previous statement... :rolleyes: );

b) as for anything related to "sound quality", much of a subjective issue...

...thus arguing on that would be completely pointless. :)



However, if you are genuinely interested in the mental exercise, I will post a drawing and an explanantion.

...sure I am!!! :yes:
 
OK,

I have shown only one side of the circuit... it must be doubled for full wave.

Conditions: Steady state, Inductor is "dry" (no current), both switches are open, we start at bottom of sine wave (theta = 0).

1. Switch 1 is turned ON sometime AFTER the primary voltage is greater than the voltage of C1. Because the inductor has zero current, the current will build in a SOFT manner. Volatge on C1 will build because of charging.

2. When the voltage at the primary passes the peak of the sine wave, it will fall until Vprimary = VC1, at this point; S1 is opened and S2 closes.

3. Current in the inductor will decay to zero, The tail of the decay will be SOFT.

4. When the current in L1 equals zero switch 2 is Opened.

Wait and then; back to step 1:

** C1 and L1 must be chosen to have a resonant frequency... maybe about 10 or 20 times more than 120 Hz.

** L1 must must run "dry" between cycles.

You are on your own to do the State-Space math -


:D
 
poobah said:
Here is the schematic:

Oho!

This may be somewhat like what I had in mind... or it may be not (well, I guess it all depends on the L/C values and on what you put in the control unit! :D ).

Anyhow, I was not thinking about a resonant circuit, and in my mind the "switches" were connected to a C-input filter rather than to some LC.

Moreover, to keep noise as low as possible I was not thinking about operating the devices like "pure switches" but rather operating them in a "hybrid" fashion (with a bit of fantasy we could name it "soft-switching" mode :clown: ), i.e. letting them be in their linear region for part of the cycle (somewhat like in a "class C" amplifier).

BTW: I don't know much about all the many SMPS techniques... what are the principle of operation and main (dis)advantages of a resonant SMPS?
 
Well... only something to consider... mental exercise; but you DID ask.

As far as resonant switchmode is concerned: I have not built any. I read the documentation for curiousity's sake. There are 4 factosr that are key: inductor currents are brought to zero, capacitor voltage are brought to zero (OR zero dv/dt), Switches are opened or closed at points of zero current or zero voltage. These factors decrease switching losses. ON SEMI & others have many write-ups and there are some devices available for some topologies. Control and math is complicated... not impossible; but NOT easy!

Cheers,
 
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