Quote:
Originally Posted by Argus25
Have a look at the waveforms it the article on the pages I cited.
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I had done. Hugo thinking is as meticulous as Hugo handiwork. Rarely equalled, even more seldom bettered. Homework was therefore done before querying!
Quote:
Originally Posted by Argus25
...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.
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Yes, the leakage inductance is all that is left. The nice elegant resonance, with the resonating capacitor, is due to the magnetising inductance of the transformer and I'm exactly with you there.
The leakage inductance is all that is left when one of the vibrator contacts is 'made' or the equivalent mosfet is 'on'. The contralateral MOSFET, however, just before being it was turned 'off' is carrying load current as well as magnetising current, and this is flowing in the leakage inductance. When the device turns 'off' (or contacts separate), you get a big voltage overshoot which peaks when all the energy stored as current in leakage inductance is transferred to voltage across circuit capacitances. The ringing frequency is fairly high, because leakage inductance is much less than magnetising inductance, so things are easily missed, till the MOSFET dies.
As you say, you also get a similar ringing when the other transistor turns 'on' because the coupled turns of the half-primary are whacked up to double the supply voltage, leaving the uncoupled bit of leakage inductance with a step voltage change imposed across it - so it oscillates again.
Quote:
Originally Posted by Argus25
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.
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A 'scope with delayed timebase is one of my 'must-have' instruments. Examining switching edges has formed a big part of my life for many years!
Your article deals with fettling an existing vibrator for symmetric operation; separating of opening and closing times for the twin pairs of contacts in a synchronous vibrator, etc. So having emphasized the importance of this, I'd certainly say that a vibrator replacement should guarantee equivalent precision - hence recommending the use of an SMPS control chip (running at an unusually low frequency!) for stability, equality of drive to each side, and easy adjustment of dead time.