MOSFETs (active rectification) in place of diodes in linear PSU - Page 2 - diyAudio
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Old 12th January 2006, 07:50 AM   #11
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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

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".
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Old 12th January 2006, 08:11 AM   #12
mirlo is offline mirlo  United States
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you can avoid body diode conduction by series connecting two mosfets back to back so that the body diodes oppose each other, tieing the gates together.
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Old 12th January 2006, 01:30 PM   #13
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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.
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Old 12th January 2006, 02:10 PM   #14
poobah is offline poobah  United States
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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.
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Old 12th January 2006, 03:59 PM   #15
UnixMan is offline UnixMan  Europe
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Quote:
Originally posted by poobah

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!


Quote:

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... );

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

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


Quote:

However, if you are genuinely interested in the mental exercise, I will post a drawing and an explanantion.
...sure I am!!!
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Old 12th January 2006, 04:32 PM   #16
poobah is offline poobah  United States
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Here is the schematic:
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File Type: gif resonant.gif (4.4 KB, 328 views)
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Old 12th January 2006, 04:50 PM   #17
poobah is offline poobah  United States
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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 -


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Old 12th January 2006, 04:56 PM   #18
poobah is offline poobah  United States
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Oh,

By the way, I would use a buck derived smps before I would do all this work...

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Old 12th January 2006, 05:02 PM   #19
UnixMan is offline UnixMan  Europe
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Quote:
Originally posted by poobah
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! ).

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 ), 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?
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Old 12th January 2006, 05:27 PM   #20
poobah is offline poobah  United States
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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!

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