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Old 2nd December 2010, 11:13 PM   #11
fredos is offline fredos  Canada
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I lost some watts at idle, but got lot better effiency at (equivalent) 0% and 100% modulation, when load is connected to +/- Rail. I know that I short 1 turn to rail (less diode drop), but this seem to force freewhelling current trough the diode and not the body diode. A major advantage at high power. I drop supply to +/- 60V ( 2 turn tap) today and place a modulator (open loop) to check at different duty cycle. It seem that at all duty cycle, less than 5% of freewhelling and recovery current pass trought body diode. More I got close to 0% and 100%, less idle power is lost in the external diode. At idle (50%) I got about 1A flowing trough diode, nothing stressing, about 0.6W of loss (50% duty cycle). When I reach 30A of load current, I got about 15.5A flowing trough external diode (20W dissipation), nothing stressing too. Still have less than 1A trough body diode at all modulation. It's hard to compare with and without this trick, since body diode in the APT5010 are very slow ( 250ns at 25 celcius and 500ns at 100 celcius!). For sure at idle it's worst than any 200V mosfet, but at high volatge and high current it seem very better.

Wave are clean with no overshoot or rigging ( with snubber of course!), turn off behavior of both diode and mosfet seem good to me..
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Old 3rd December 2010, 03:37 PM   #12
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I agree with darkfenriz - its a short circuit...
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Old 4th December 2010, 02:43 PM   #13
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By the way: has anyone tried to store the energy of switching-on event and body diode conduction and recuprate it afterwards?
Recently I came across the so-called "flyback reset ZCS lossless snubber" or something like that, which introduces a one-way inductor to soften the switch-on/body diode conduction period and in the other direction recuperates the energy back into rails in a flyback style.
Any experiences, ideas, thoughts??

Adam
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Old 6th December 2010, 09:19 AM   #14
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When I tried it in 1988 I also had the impression that it causes a short-circuit (FETs got very hot) but the switching signal looked impressively clean OTHO - although the circuit was built on breadboard.

The Peavey circuit uses a series resistor. I assume it is there to reduce the circulating current.

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Charles

Last edited by phase_accurate; 6th December 2010 at 09:21 AM. Reason: typo
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Old 14th November 2012, 05:50 AM   #15
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Well this works, even i checked it.
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Old 16th November 2012, 03:27 AM   #16
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You'll want that to be something like 1 turn out of 50 (depends on supply voltage, essentially Vf(diode) / Vsupply), which may be a steep ratio for most coils that don't need that much inductance. Too low a ratio and it doesn't do jack; too high and you short out some fraction of the transistors' saturation region.

Personally, I'd rather shunt the MOSFETs with good diodes. A purpose-made silicon junction diode has lower voltage drop and much greater speed than a MOSFET body diode, so it turns on sooner, carries more current, and both together recover faster as a result. Schottky is also acceptable at *most* audio voltages. ("Super schottkies" are available up to 300V, though they have voltage drops comparable to, or greater than, junction diodes, the advantage being zero recovery and thus higher speed. SiC schottkies have significantly greater voltage drop and internal resistance, and are only useful in high voltage, very high frequency switching applications where recovery cannot be tolerated.)



If you use a core with multiple return paths (E types, or even those four prong things if you can ever find them), you can use a flux unbalancing winding to allow fractional turns around a single limb. The method acts like a vernier on the windings.

Example: suppose you have 10 turns on a core to achieve the correct inductance. Obviously you can't make a 1/50th ratio the normal way. If you loop a separate winding around the two limbs, one turn each, so that the flux in each must be equal, then you can make half-turn windings successfully. But 1/2 out of 10 is only 1:20, and we need 1:50 in this example, so we need 1/5th of a turn. You can wind 4 turns on one limb and 1 turn on the other, to enforce a 4:1 split (1/5 and 4/5 of the total), but this reduces the A_e of the core by almost half (40% -- the 1-turn leg saturates much more easily). If you do a 2:3 split, A_e only drops by 20% (probably within design allowance). Now, if you pass a turn in the positive direction around the "3" leg and negative around the "2" leg, you get (3-2) / (3+2) = 1/5th of a turn, which can be connected in series with the main winding for a 10:0.2 ratio.

This is presented in more detail in an old Unitrode appnote, including winding techniques to minimize leakage inductance.

Tim
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Last edited by Sch3mat1c; 16th November 2012 at 03:31 AM.
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Old 16th November 2012, 06:58 AM   #17
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Here is another way
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