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Old 8th Apr 2020, 3:46 pm   #1
astral highway
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Default Ferrite/ magnetics expert... what's happening here?

Pic shows what should be a square wave, with obvious defects.

My gate drive transformer, on type 77 material, is into a load of 18nF on one of its windings. I can't accurately test the inductance of the GDT but it appears to be about 2.76mH

The pulse should be a perfect square wave, frequency is about 200KHz.

I can see there's an initial very fast rise time, then a slight overshoot, then the core is biased in the opposite direction, but problems have begun from the start and this is far from being a square wave.

Overshoot in itself is not a problem as currently no gate resistor is in place. It's the overall shape that needs to be tight and a conventional square wave.

I note the parasitic oscillation addition at the end of the waveform, as it decays to zero.

Only I can't think of probable causes of this. The gate drive circuit is very capable indeed and not underpowered. The windings on the core are tight and leakage inductance is minimal.

I'd appreciate any technical reasons for this waveform and its saturation behaviour if relevant.


Thank you!
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Old 8th Apr 2020, 5:03 pm   #2
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Default Re: Ferrite/ magnetics expert... what's happening here?

I think it might help if we knew the circuit, the layout, the waveform at the primary and the display units.
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Old 8th Apr 2020, 5:35 pm   #3
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Default Re: Ferrite/ magnetics expert... what's happening here?

The 'parasitic oscillation' looks like shock-excited ringing (I reserve the term 'parasitic oscillation' for a situation where there is an active device, unwanted feedback at an unexpected frequency, and oscillation occurs. But that might be just me.)

Definitely agree with GMB, 'scope scaling, circuit, and layout photo much needed!

I'm intrigued! Looking forward to more info!
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Old 8th Apr 2020, 5:40 pm   #4
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Default Re: Ferrite/ magnetics expert... what's happening here?

The square pulse was set at 150uS. This is also the time between the displayed vertical rising edge and the end of the ramped waveform before the decaying sinusoidal wave at the end. In other words, the entire up, down, up, down up, part of the waveform before the decaying sinus wave is 150uS.

The circuit is pretty complex, with quite a lot of logic on board, and beyond the scope of this thread, but it and its layout are beyond any suspicion. It will rely on current feedback in its final application but is being switched by an external signal for the time being.

I can't measure the unloaded primary waveform without destroying the driver IC's.

What can you say about the circuit without reference to the circuit, only as it likely relates to the magnetising/ changes in magnetic flux in the ferrite and the generic causes of these sorts of behaviours and spurious waveforms?

Kalee/Peter - Cross post!

Ah, thanks for calling out that misnomer. 'Shock-excited ringing,' it is, then. That part of the waveform doesn't bother me so much as the ups and downs of the rest of it.

If you're able to share any hunches about the dereliction of the waveform without looking at the circuit specifics, what would you say? Assume a conventional gate driving topology with two well-decoupled gate driver IC's with plenty of oomph behind them.
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Last edited by astral highway; 8th Apr 2020 at 5:51 pm. Reason: Cross-posting!
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Old 8th Apr 2020, 5:53 pm   #5
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Default Re: Ferrite/ magnetics expert... what's happening here?

Not a hope of understanding anything without comprehensive data.

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Old 8th Apr 2020, 5:58 pm   #6
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Default Re: Ferrite/ magnetics expert... what's happening here?

Hey folks, I'm not in a position to expand this thread with all the data it should have, so I'm going to ask for it to be closed and I'll start it again in a couple of weeks when my partner's holiday is finished.

Mods, please close thread.
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Old 8th Apr 2020, 7:38 pm   #7
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Default Re: Ferrite/ magnetics expert... what's happening here?

Look forward to then, Astral!
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