help ! - inverter driving LC resonant circuit interesting behaviour

hello dear forum,

my ( ultrasonic generator ) inverter driving a LC resonant circuit behaves very weird

can you please explain and give advice to me



the behaviour

() when there is 12 x ultrasonic transducers ( 28 Khz ) connected to the output trafo with total capacitance of 55 nF and 560 uH matching inductance there is 3.5 A input power at the mains - and the IRFP460 mosfets case temperature is 42 degree Celcius - the peak Voltage at the transducer terminals is 1000 V and it is pure sinus

() when I connect 18 x ultrasonic transducers to the output trafo with total 95 nF and 310 uH matching inductance there is 3.5 A input power at the mains - and the mosfet case temperature is 120 degree Celcius - the peak voltage at the transducer terminals is 450 V only

I adjusted inverter frequency to 28 KHz which is close to resonant frequency of the load at 2 cases above

1 ) why is this strange behaviour ?

2 ) why is the mosfet is cold at first and hot at second trial ?

please advice

thank you

In the second case you would expect the load to be 2/3 of the first so the current will be higher, you can see this as the voltage has fallen. This could lead to core saturation of the transformer and excessive current through the non existent primary inductance.
Frank

thank you

how can I understand if the trafo saturates ?

can I detect saturation with osciloscope ? how ?

if you operate at or close to resonance (or worse below) the mosfets will hard switch and get hot, always operate above resonance so that there is a bit of current in the fets when they switch off...!!!