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LLC converter magnetising current reversal during dead time

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Selvam_Perumal

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Half bridge LLC resonant converter for below spec
Input voltage :300V to 350V
output voltage:50V @ 40A
Transformer details:
Llkg primary:0.81uH
Lprimary:7uH
Resonant capacitor:37.6nF
Resonant Frequency:912Khz
While loading the current to 10A at 300V input,the insufficient ZVS is observed by looking at SW node which is due to the fast slope of resonant current after turn off(50ns) which is not intended.
Due to this,the next switch is hard switched and increasing the losses in it which is higher than intended and failing over a time.
What would be the reason for current reversal during dead time(135ns).
FET Cds:287pF

Attached waveform:
Yellow:Resonant current
Blue:SW node
Red:Vcr node
 

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  • Ir vs BG vs SW vs Vcr 300v 10A 2.bmp
    489.5 KB · Views: 100

Looking at your scope image, it looks as though you have a proper sinewave shaped by the series LC (or LLC). However when current flow does not coincide with other waveforms in the circuit, then it could be lag/lead effect caused by inductors and capacitors.

Thus if current continues to flow after device is supposed to shut off, it is because L value is too large. "Inductor current lags voltage."

If current starts before device is supposed to turn on, then C value is too large. "Capacitor current leads voltage" (characteristically when waveform is sinewave).

Try playing with values, and you'll see the sine crossing zero earlier or later, in relation to gating signals.
 

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