Quote:
Originally Posted by kalee20
That doesn't seem right to me!
The 'dead time' with a mechanical vibrator, allows the transformer's primary inductance to gracefully resonate with a capacitor across the primary, so that by the time the vibrator's 'new' contacts have made, the voltage across them is already very low because a half-cycle of oscillation has taken place. So they join with minimal sparking. When the contacts separate, there is also minimal sparking because the primary capacitor prevents a sudden rise of voltage as they separate.
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I agree with these remarks and we are on the same page here. exactly.
However, there is more involved when there is no timing gap that allows this process.
When one of the mosfets goes into hard conduction, it switches one side of the primary winding across a fixed voltage (which is the power supply) at that moment, from the alternating current perspective, one half of the winding is effectively shorted out. This neutralizes all of the inductance in the other half of the primary, that is all of it that is magnetically linked.
The other half of the primary inductance proper electrically vanishes, leaving only the leakage inductance. This leakage inductance resonates with the winding capacitances, and the external tuning capacitances to create a very high Q circuit. As a result, shortly after one mosfet goes into conduction, a large voltage transient appears on the drain of the contralateral mosfet. The negative going component of it gets clamped off by the D-S parasitic diode in the Moffett.
Vibrator transformers were also not specifically designed for low leakage inductance between the two halves of the primary either, making it worse than usual.
Have a look at the waveforms it the article on the pages I cited.
Here is the real catch 22:
Most designers of vibrator substitutes think they have done a good job, because when they are finished they put the scope on the drain or collector and look at the waveform and it looks like a respectable square wave.
However, on a usual scope display, set to view about 1/2 to a few cycles of oscillation, it is very difficult to see these brief transients. They need to be expanded out and viewed on a good scope with a delay timebase. Have a look at the oscillograms in the article and you will see what I mean.
Hugo.