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LLC converters (SMPS) do not have zero voltage switch OFF?

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Flyback

Well-Known Member
Hello,
I thought LLC converters were supposed to have zero voltage switch_ON and Switch_OFF of the FETs?

However, as this waveform view of VDS (red) and I(FET) (green) shows, it definitely doesn't have zero voltage switch off.
https://i44.tinypic.com/2w4zqqa.jpg
Why is this?
LTSPICE simulation and schematic attached

By the way, the secondary diodes are zero voltage switched ON and OFF.
-All this refers to operation with F(sw) at the upper resonant frequency, that is, the resonant frequency of the resonant capacitor and the resonant inductor.
 

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  • RESONANT LLC 80V 1A875.pdf
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  • RESONANT LLC 80V 1A875.asc
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I thought LLC converters were supposed to have zero voltage switch_ON and Switch_OFF of the FETs?
A example of ZVS on both edges is the horizontal switch in a TV set.
 
Horizontal switch:
>At the center of the picture, current is 0 and voltage is 0 and the transistor is on.
>Current ramps up to 6A, V is close to 0 because the transistor is on.
>The transistor is told to turn off. There is a large capacitor across the C-E of the transistor so the voltage can not rise fast. In the time the transistor turns off (current falls from 6 to 0A) the voltage only rises maybe only 10 volts. ZVS
>Now the switch is off and the C across the transistor and the Ls in the system resonate and cause a 1/2 sign wave of 1000 volts.
>At the end of the 1/2 sign the voltage is back to 0. The current drives into a diode across the transistor. (the voltage is 1 diode drop negative and current is in the diode)
>The transistor is turned on with a negative voltage on CE and 0 current. ZVS and ZCS
>Current ramps form -6A down to 0. {repeat the sequence }

With out ZVS it is possible to have 3A and 500V across the transistor for 100nS. Huge power loss.
 
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