Low frequency oscillation induced during CCPS discharge.

Thread Starter


Joined May 22, 2019
Hello all,

I am designing a CCPS and using a phase-shifted, series-resonant bridge as the switching topology. There is a DC block capacitor in series with the transformer primary winding.

When the target voltage is reached, the H-bridge switches are turned off until a command for the next pulse is receive from the control system. These can be seen in the attached photo.

However I am suffering from a substantial problem that occurs when the command for output capacitor discharge is received. The output rectifier diodes begin to conduct some of the discharge current and this seems to excite a low frequency resonance within the transformer. This frequency is equal to the resonance between the equivalent series capacitance and the combination of the transformer leakage and the magnetising inductance, at around 12kHz. The issue seems to get better with additional resistance in the discharge path, but of course it becomes difficult to implement resistors with such large pulse power discharge rating.

Is this a common issue faced in resonant converters when employed in capacitor discharge power supplies? Is there any possible fix for this to ensure zero transformer flux exists when the H-bridge is turned off before and after the load is discharged? It is quite important that these oscillations are reduced significantly or removed completely such that the transformer operating conditions are more favourable.

thanks in advance,