Half Bridge Series Resonant converter operation above resonant frequency

[sabu31]

Dear All,

I am developing a Half Bridge Series resonant inverter for 1kW rating. The inductance is 22uH and the capacitance value used is 0.94u. However, the power generated is much higher and I need to operate at higher frequency of 58kHz to get desired output power level.
As the frequency is lowered, the power delivery is increased. I want to know whether the devices are operating in soft switching mode when the operating frequency is higher than resonant frequency. The output voltage and current waveform is attached for your reference

Ch4: Output Current; Scale 20A/div
Ch3: Output Voltage: Scale 200V/div

 

[cupoftea]

...it "should" be soft switching when above the upper resonant frequency, yes.
As you know, you shoudl have protection to stop you going so low in fsw that you are in the bad capacitive mode...

Though many are saying if you use SiC FETs and SiC antipllel diodes then you wont get bad rev rec anyway.....or GaN FETs with A-pllel sic diode.

You can tell by looking at The fet gate drive...there should be no miller plateau.
Also, if you scope fet gate and switching node at same time (use diff probe), you can see what level the VDS is at when the fet gets turned on.

Also, if its not in softswitching, if you scope the VDS...you see a sudden change in gradient when the fet suddenly turns on and yanks the switching node up/down to the rail/gnd....IE instead of a smoother gradient when the sw node gets up the the rail/gnd before the fet suddenly switchs on. (soft switching)

Try AN-4151 by "fairchild" to get some goodies on the LLC or the SRC.

 

[Easy peasy]

Short answer = yes - you will know if you ever get into the hard switching areas ( below resonance ) as the mosfets will get very hot and then go bang !

having said that, you still need just the right amount of dead time above resonance to get perfect ZVS and soft switching

why are there no screen shots of the half bridge voltage transitions - zoomed in to 50nS / div ?

please add

--- Updated May 8, 2024 ---

there are a lot of classic newbie mistakes in your component sizing - if there was a complete schematic, with Vin. Vout, and transformer data - it would be a simple matter to give you the best component values for full power at your desired freq of operation ( also not given - but assumed to be 35kHz ).

 

[Easy peasy]

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