I am using high frequency transformer with EE 55 Ferrite core in battery charger in half bridge topology. I am repeatedly facing MOSFET failure in charger. Initially i believed that shoot through was caused due to gate drive circuit.
But yesterday i tested my transformer immediately after charger failure. And found that the transformer inductance rised to 4 times of normal inductance. My switching frequency in 25kHz.
After testing the same transformer at 135° C, the inductance increased 4 times similar to what i observed. What could be the reason behind this..?
Please suggest what could be the reason and how can i overcome that.?
The battery charging current is 15 A, while primary current is around 7 A.
At present, current density is around 3 A/mm2.
Hello Joshi,
I guess that the current ratings of your transformer are insufficient. Therefore, the flow of currents increases the temperature of the transformer. Consequently, the ferromanetic strength of the core is lowered and the core tends to be paramagnetic. At this point, the inductance should significantly decrease not increased as you stated!!!!
The decrease of the transformer inductance increases the primary current flow which might be the cause of the failure of the MOSFET due to overtemperature. Please re-measure the inductance again at a high temperature.
Ferrite should not go beyond 100C. After that there is a dramatic permeability drop which would cause the tranformer to disappear! 3A/mm2 would not cause such a rise in temperature; check B (0.25-0.3 T)
and if you have a transfomer your control loop must limit peak current cycle by cycle.
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