Hello
I am designing LLC Half-Bridge converter.
Pout = 500W Out= 12V Vin=390V
Fr = 80kHz Lr = 30 uH, Cr = 120nF , Lp = 190uH
The converter is working always above resonant frequency.
The oscillographs you see are measured in 200W Pout.
As you can see the green channel is Lr current, red channel is switching node and yellow channel is capacitor voltage.
So first question is could that be a reason for low efficiency 93.7% ( transistor heated to 95 degrees Celsius and transformer is heated to 80 degrees Celsius, measured in max power )
Second question is why capacitor voltage is not dropping to zero, even when I make simulations in LTspice with the resonant frequency the voltage looks like in the shown picture from LTspice.
I understand that the RMS voltage across the capacitor is regulated by the feedback and is satisfying the formula Vcap = Vout*turns ratio = 12*16 = 192.
Thing that I though about: resonant frequency is going high probably because Lr value is decreasing with current.

 

Hello
I am designing LLC Half-Bridge converter.
Pout = 500W Out= 12V Vin=390V
Fr = 80kHz Lr = 30 uH, Cr = 120nF , Lp = 190uH
The converter is working always above resonant frequency.
The oscillographs you see are measured in 200W Pout.
As you can see the green channel is Lr current, red channel is switching node and yellow channel is capacitor voltage.
So first question is could that be a reason for low efficiency 93.7% ( transistor heated to 95 degrees Celsius and transformer is heated to 80 degrees Celsius, measured in max power )
Second question is why capacitor voltage is not dropping to zero, even when I make simulations in LTspice with the resonant frequency the voltage looks like in the shown picture from LTspice.
I understand that the RMS voltage across the capacitor is regulated by the feedback and is satisfying the formula Vcap = Vout*turns ratio = 12*16 = 192.
Thing that I though about: resonant frequency is going high probably because Lr value is decreasing with current.

It's meant to work above resonance to ensure that the load looks inductive and to get ZVS. Below resonance the load appears capacitive and ZVS is lost. At resonance it will probably make a lot of voltage and blow up.
What did you use for Lr?

 

It's meant to work above resonance to ensure that the load looks inductive and to get ZVS. Below resonance the load appears capacitive and ZVS is lost. At resonance it will probably make a lot of voltage and blow up.
What did you use for Lr?

That not quite true. Under resonance you can achieve ZVS and lower turn off losses as well. Anyways Lr is the leakage inductance of the transformer and it is part of the LLC resonant circuit.