TNY Switch strong drain current ringing at start up dangerous?

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Hello,
I have a flyback design with TNY279GN.
At start up there is heavy ringing on I drain and I drain gets negative, as you can see in the scope screenshots attached.
It's clear that it should be improved, but actually there is no EMI problem.
My question is, is this negative ringing dangerous for the MOSFET? Could the MOSFET get destroyed because of that?

best regards
Hans Günter

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Id_start_up1.jpg130.71 KB
Id_start_up2.jpg146.87 KB
Id_Uds_start_up1.jpg164.1 KB
Id_Uds_start_up2.jpg170.23 KB

Hi Hans,

Can you please a schematic and indicate where the current probe is inserted?

From your Id start-up image, the negative current occurs at the mosfet off time interval. This negative current may come from your primary snubber circuit. You need to ensure that the current measurement only measures the current into the drain pin.

Hello PI-Jango,
thanks for your reply.
I have lifted the drain pin of TNY279 and added a short wire from pin to the pcb.
The current was then measured with a current probe.

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Current_Probe.JPG19.96 KB

Hello PI-Jango,

again, is this negative ringing dangerous for the MOSFET?

best regards
Hans Günter

Hello,

has anyone else an issue like that? If so, what is your experience with the drain current ringing?

best regards

Hello PowerIntegrations,

is there any document available, that shows the impact of this ringing to the MOSFET?

I look forward to a feedback.

Hans Günter

Hello PI-Jango,
could your find out some information?
I'm looking forward to your answer.
regards Hans Günter

Hi Hans,

The MOSFET's body diode will conduct the negative current and clamp the negative voltage seen across Vds. The dead time between 5-10ms in "Id_start_up1.jpg" seems too much. Can you send a full schematic? And can you send another startup waveform but with no load? Thanks

Hi PI-Jango,
is there a limit for the current the body diode can conduct?
Please find attached the no load startup and the schematic.
best regards
Hans Günter

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ID_startup_no_load.JPG83.07 KB
schematic.JPG92.03 KB

Hi Hans,
The negative current spike in Ids is due to the large leakage energy dissipated during CCM (at startup). You may also see this at low line operation, depending on your transformer design. You ca n minimize this spike by reducing the leakage inductance, but should be ok as long as Vds does not go below -0.3V (page 12, TNY-III datasheet).