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In the Forum: Melquiades Amplifier
In the Thread: 6 Channel Version of Super Melquiades
Post Subject: Diode HandoffPosted by hagtech on: 1/7/2008
Looks to me like the horizontal and vertical steps in the waveform are due to the diode switching.  One diode turns off while the other turns on.  Storage time on the diodes is normally short.  But it depends on the test setup!  They are tested from positive current to negative current - how long do they take to turn off?  Probably 50ns or 100ns.

However, we do not have that test circuit here.  We do not turn off or reverse the current.  The load is a large choke, acting like a flywheel in that it is continuously pulling current, regardless of voltages on the diode.  So even with input sinusoid going negative, the diode still conducts forward.  During zero crossover, one diode should hand off current to the other.  Well, the conducting diode has to turn off first.  It does not turn off properly as there is nothing to sweep the majority carriers out of the junction (it is NOT reverse biased!).  And so it turns off slowly.  Meanwhile, you get an autoformer effect between secondaries, thereby screwing up the sinusoid.  Look at the voltage on the other side of the diode, it will have a discontinuity at zero volts.  Add in the leakage inductances from the secondary, and you get this unusual glitch.  The vertical step is when the diode finally turns off and the other one begins to conduct.  I think if the leakage of the secondary were lower, the diodes switch faster.

This won't happen with vacuum rectifiers.  Only with solid state diodes.  The question is, how to turn off a diode while still forward conducting? 

Meanwhile, the LC will filter this glitch out and it won't be a problem on the audio side.  It will, however, get reflected back onto your ac mains, where it can find its way into something else.  A snubber on the transformer might help.

jh

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