Hi team,
I have a request from my customer, please see info below.
"We are still seeing some unusual behavior with the UCC256403D Hybrid-Hysteresis LLC controller when the converter is loaded to about 3% of full load.
There will be random glitches that happen for one or two or several cycles that seem to come from nowhere after running normally just prior to the glitch. The most surprising signal is at the “VCR” pin7. In normal operation this voltage is a steady sawtooth that bounces up and down between the upper and lower thresholds set by the feedback error signal. But at random times the sawtooth will shoot all the way up to 7 volts and then recover. See scope photos attached. Channel 2 is mis-labeled, it is actually the voltage at U302pin3 with 10K ohms to PGND.
The feedback error signal is steady during the glitches; we put a 10K ohm resistor in series with the opto-transistor emitter to monitor its current. 2.5V=250uA.
The strangest thing is that the VCR signal sawtooth glitches always go to positive rail, never negative. Why is that? The equivalent block diagram in the UCC256403 spec sheet does not show any obvious reason, as the sawtooth should stay centered around the VCM voltage.
Are the sawtooth glitches the result of some other interference, or are the sawtooth glitches causing the LLC to stutter irregular cycle timing?
The glitching seems to happen right around the power level where the controller goes into Burst Mode. Not really sure if it is Burst Mode because there are not exactly 40 cycles, and there does not seem to be BMTl/BMTh hysteresis as set by programming option #5.
If we raise the BMT (lowering R314 ohms) then that can totally eliminate the glitches, but then BMT happens at a power level greater than we desire."
They have also sent a schematic, when this is being answered please reach out to me and I will send it internally. I am having trouble attaching all of the scope shots as well so I will send those internally too.
Thanks!
Lauren
