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MOSFET Gate starts to oscillate at Miller Plateau.

vignesh4488

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Hi,

My circuit is actually used for current sensing. I have provided the overall circuit for reference.

I am facing a gate oscillation issue at 24Mhz when the MOSFET begins to conduct at threshold.

I have attached the waveform and schematic for your reference.

Green -Gate Of MOSFET Orange- Vds

MOSFET used: UF3SC120040B7S
Input voltage: 35V

Please provide your suggestions regarding the above problem.

Regards,

Vignesh.

Circuit.png






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Hi,

the schematic may be from a simulation ... I don´t expect it to oscillate.
I rather expect the real circuit to oscillate.
--> Please calrify.

If so, I xpect a bad PCB layout / bad wiring / bad part selection to be the root cuase for the oscillation.

Klaus
 
Hi,

I have probed gate and drain of MOSFET.

Layout: Source of MOSFET is routed longer followed by one 0.47uF capacitor.

Actual my circuit is to work on 350V. When I have turned on MOSFET with 350V, D-S are shorted.

So, I have started debugging with 35Vdc but still oscillation in D-S and G-S.

I have also gone through some research paper on online they have mentioned oscillation because of source inductance. In CSR has inductance of 5nH therefore I have shorted that tested and same problem exists.

Another one question, How to calculate PCB trace inductance and any idea on that?

Regards,
Vignesh.
 
1nH/mm is roughly right but coupling is anyone's guess. And that's the big deal, where is your feedback, your gain and your phase shift? About the only a priori "knowns" are rough Cgx and rough gain vs OP for one path.
 
Layout: Source of MOSFET is routed longer followed by one 0.47uF capacitor.
you already posted megabytes of scope pictures ... why not a photo of your circuit - including the scope connections? (100kBytes should be sufficient.)

Klaus
 


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