Hi,

I am testing a 600W LLC half bridge converter and had a problem with oscillations on my output diodes. By removing the output filtering inductor the oscillations went away completly. I tried searching about the effect of an output inductor but could not find anything about that. Any help would be appreciated. Thanks in advance.

 

Hi,

I am testing a 600W LLC half bridge converter and had a problem with oscillations on my output diodes. By removing the output filtering inductor the oscillations went away completly. I tried searching about the effect of an output inductor but could not find anything about that. Any help would be appreciated. Thanks in advance.

Can you share your schematic and/or a simulation that shows the problem.

 

An LLC converter only has inductors on the primary side of the transformer. The secondary of the transformer connects straight to the capacitors via a rectifier.

 

An LLC converter only has inductors on the primary side of the transformer. The secondary of the transformer connects straight to the capacitors via a rectifier.

We don't really know what the TS has in the absence of a schematic. It would not be the first time an SMPS was mislabeled.

 

here is the shcematic and the voltage on the diodes.

 

What value is L1?
L2 is not generally used in an LLC converter
C10 and C11 will no be doing your circuit any good at all.

 

If you have the inductance and the DC resistance of the inductor along with the ESR of the capacitors you may be able to establish the lack of damping in the resonant circuit presented by the inductor and capacitors.

ETA: It also would be helpful to know your switching frequency.

 

Is that a no-load condition??? Was there a complaint about this supply?? Is the supply a commercial product, or one built from a circuit from "some source"? It certainly does look like some serious ringing. AND I have not seen a DC filter inductor immediately following the diodes in any power supply EVER!! switching or not!!

 

Is that a no-load condition??? Was there a complaint about this supply?? Is the supply a commercial product, or one built from a circuit from "some source"? It certainly does look like some serious ringing. AND I have not seen a DC filter inductor immediately following the diodes in any power supply EVER!! switching or not!!

A forward converter or asymmetric half-bridge would have that output stage. Not an LLC because the impedance seen by the power transistors is already inductive.

 

WHY is the TS testing the supply??Is there a problem with that waveform??

 

Hi everyone,
Im sorry for not giving enough info about this. I have figured the problem out so I thought I might as well post it here just in case someone else has a similar problem. This circuit is a 600W battery charger for a project of mine. The input is provided from an autotransformer and the test load was a 1kW 100ohm rheostat (no inductive loads). Also the switching frequency range is 40kHz~80kHz. The value of the resonance inductor L1 is 75uH.

So apparently an output inductor changes the resoonance frequency of the circuit which ment we were having oscillations on the secondary causing that waveform on the diodes. The resonance frequency can be calculated in a way to allow for a filtering inductor but as @Ian0 stated it isn't usally used in an LLC half bridge.

Thanks for your help I hope this thread might help someone else.

 

Some possibly good news is that most battery charging applications do not depend on, nor benefit from, a smooth source voltage. The attached battery management circuit may require filtered power, but batteries usually charge very well with even quite noisy power.
Filter inductor resonance has been a power supply concern and potential problem ever since inductors were used for DC supply filtering.