Electronics > Projects, Designs, and Technical Stuff

simple llc converter prototype resistive load ok, but oscillation with rectifier

(1/3) > >>

julian1:
Consider this simple low-voltage DC-DC full-bridge llc converter prototype. using a mcu peripheral timer driving two half bridge gate drivers. cap is 1uF, resonant inductor is 3.3uH, transformer windings are 18uH.
 
When driving a purely resistive load (halogen bulb) directly from the secondary up to 30W, the traces appear good. light blue + yellow traces are fet gates, The pink trace is one node of the primary, and the dark blue trace is the secondary trace which is gnd referenced by the scope gnd clip.
 
but after connecting the output section - consisting of rectifier + reservoir caps + low pass filtering - an oscillation problem appears on the primary (and secondary).
 
- the oscillation issue, exists regardless of whether the output is loaded (eg halogen bulb) or open.
- problem exists - with 10uF/50V MLCC. or 220uF aluminium electrolytics for resevoir caps
- problem exists when inductor of low-pass LC filters are shorted.

The rectifier diodes are fast schottky 4A SMC MBRS4201.

The resevoir caps are unfortunately bodged through-hole (mlcc, or electrolytic) - and have long leads - so lead inductance is suspect. But I still find it odd that this might affect the trace of the primary winding.

Any thoughts?

TimNJ:
Well done on the waveforms. Primary side waveforms are indicative of operation above the resonant frequency of the LLC tank. Basically the transformer is still energized when the next switching cycle is forced. This results in hard commutation of the output rectifiers because the conducting diode abruptly gets reversed biased while it is still delivering current to the load. Thus, you get ringing due to non ZCS condition.

Not that I understand the control algorithms well (or at all), but how are you controlling (i.e. what is dictating) the switching frequency?

julian1:

--- Quote from: TimNJ on May 24, 2022, 02:26:49 am ---Basically the transformer is still energized when the next switching cycle is forced. This results in hard commutation of the output rectifiers because the conducting diode abruptly gets reversed biased while it is still delivering current to the load.

--- End quote ---

Ok that sounds reasonable and makes sense.

I believed it was possible to stay in the inductive region (above resonant freq around 35kHz) freely, for control over power transfer, with the only tradeoff being efficiency due to no ZVS.

And it appears to work ok (everything stays cool, no oscillations), so long as there not a rectifier on the secondary.

In fact the one case/condition I have not tested - is operation right at resonant frequency/full power - due to limitations in source/load . So I should probably test that, and see if the rectifier behaves.

The design at the moment is running open loop. So I just play-around with setting freqency, and deadtime, over usart. 

I did try running at resonant frequency, with high (eg. 90% deadtime), but the waveform was awful. But that was with hard switching and no free-wheeling one of the fets (like psfb).

strawberry:
when run above resonance it is no longer resonant LLC but regular hard switching converter(where it operates at no load/standby)
need fast ZCS detector to keep as close as possible to resonance (unreliable detection will result in ringing and transistor failure)

uer166:
It was my understanding that you always run an LLC primary well above resonance (in the inductive region), and do control via frequency. The idea is the higher frequency, the farther away you are from Fres -> less gain, and vice versa.

Navigation

[0] Message Index

[#] Next page