Tool/software:
We have designed one 750W EV charger for electric 2 wheeler. The output voltage & current are 58 V,12 A. The DC-DC stage is an half bridge LLC resonant converter. I have attached the schematics as below
We are observing random intermittent spikes as follows which is causing spikes across the Mosfets & UCC24624 IC(VD1 & VD2) Pins during the starting when there is no load
Hello Biswajit,
I see the blue voltage spikes during the abnormal sequencing of the pulses at start-up.
Note, even though there may be no external load on the LLC output, the charging-up of the output capacitors is still a significant load on the LLC system until Vout comes into regulation.
The blue voltage spike occurs when the high current in SR Q5 is abruptly interrupted by the LLC primary turning on the Q7 side.
In this case, I see that the UCC24624 is turning on the SR Fets when their respective Vds goes below 0V, as expected.
What is unexpected is that the polarity of the LLC transformer suddenly switches to the opposite at a faster frequency than normal.
My feeling is that the UCC24624 is doing what is expected to do from the signals that it "sees". I think the source of the problem is that something is interfering with smooth operation of the LLC controller and its switching, which indirectly affects the SR controller.
Please note the center area where the blue voltage does not fully drop below GND, so Q5 is not triggered by the SR and no voltage spike occurs. 
I feel that the abnormal sequencing is not caused by the SRs, but is applied to the SRs by the LLC operation.
Please investigate this behavior on the primary side to see what may be causing that.
Regards,
Ulrich
Hi Ulrich,
As per our discussion, with an isolated DC supply to LLC, we have captured UCC256403 ICs BLK, HO, LO, ISNS, and VCR pin voltages at no load condition. The waveforms are shown below.
Here, we observed that the gate pulse of the high-side HO pin has voltage spikes at turn-on and turn-off. Simultaneously, voltage spikes are also observed on the BLK, ISNS, and VCR signals, at the same time.
After observing voltage spikes at the BLK pin, we have added a filter cap at the BLK pin, but the same spikes are still observed.
Hi Ulrich,
At every HO switching transient, there is a voltage spike at (UCC256403) BLK, VCR, ISNS. Then, at no load, why is there random switching from UCC24524?
From the figure below, Q7 & Q5 are gate pulses of synchronous MOSFETs driving with UCC24624.
Why is there random switching from UCC24624, even though there were voltage spikes at every switching transition, UCC256403 ICs (BLK, VCR, ISNS, and HO)?
