UCC256301: Fail ! unable to work properly

Hi Eilision,

Sorry to hear about the trouble. What I would suggest is to take a look at the LO pin to determine if the LLC is attempting to switch every 1s. If yes, the device is detect a fault (likely OCP or OVP). My initial thought is that the bottom VCR capacitor is quite big and this is causing the initial switching frequency during startup to be a bit low which is resulting in OCP being detected.

If you are able to share waveforms of VCR, ISNS, BW and LO when the LLC attempts to switch, it would help determine the issue.

I also want to mention we have an upgraded version of UCC25630x called UCC25640x. It may be worthwhile to use this device instead.

Best Regards,

Ben Lough

Hi Ben

I checked the waveform of the RVCC pin and it restarted every second, presumably because the fault restarted.

I shorted the resistance of the ISNS pin to GND to rule out the cause of OCP.

I designed the capacitor of the VCR to be a bottom capacitor of 20nf. After the problem occurred, I immediately changed the value of the capacitor and reduced it to 6.8nf, but it still couldn’t solve the problem....

I only saved the voltage waveform of the VCC pin, and the rest needs to be retested to provide.

I bought your demo board UCC25630-1EVM-291, it can work normally, I can’t understand why I can’t output the voltage

Thanks a lot

Best Regards

Eilision Xie

Hi,

can you show any waveform to show the unstable issue?

I would suggest you probe vout, Lo of primary LLC low side driver, SS pin voltage and resonant current together when you found the unstable phenomenon.

thanks.

Hi Ben，

Sorry, due to improper operation during the test, the previous sample was damaged, I re-soldered a piece, I will give you the test waveform you need below, blue is the waveform of LO, red is the waveform of VCR, ISNS, BW respectively .

The first is the test waveform without load.

The picture below is the waveform of LO pin and VCR pin without load

The picture below is the waveform of LO pin and INSNS pin without load

The picture below is the waveform of LO pin and BW pin without load

Then the test waveform under the condition of 0.7A load

The picture below is the waveform of LO pin and VCR pin under load

The picture below is the waveform of LO pin and ISNS pin under load

The picture below is the waveform of LO pin and BW pin under load

Thanks a lot

Best Regards

Eilision Xie

Hi Eilision,

The amplitude of the gate pulses are slowly drooping as switching continues during the startup. It looks like VCC might be drooping too much and is causing the controller to shut off due to not enough VCC voltage. Could you try removing D6, use a bench supply to apply 15V across C22 and try to start up the LLC? This chould help confirm if this is the issue or not.

Best Regards,

Ben Lough

Hi Ben,

Your suspicion is that the output energy of the auxiliary winding is not enough?

I have done this kind of test before. I once connected a large electrolytic capacitor in parallel with the electrolytic capacitor of C28, so that when the high voltage starts, the large electrolytic capacitor will be charged enough to replace the constant current. Power source to supply power to VCC to confirm whether it is the auxiliary winding problem.

Of course, my experiment was unsuccessful. My friend thought my method was wrong, so I followed your suggestion, and the situation is still the same

.So I don’t think it’s the auxiliary winding problem for the time being

By the way, do you have special requirements for winding transformers?

Thanks a lot

Best Regards

Eilision Xie

Hi Eilision,

You cannot apply voltage directly to VCC as this would interfere with the HV startup function wanting to charge up VCC to 26V before switching begins and would fail to start up. But if you apply the external voltage to the anode of D11, it should be fine. Do you still see the droop in the LO amplitude when using the external DC supply at C22?

There is no special requirement for the aux winding. VCC voltage just needs to be kept in the recommended range

Best Regards,

Ben Lough

Hi Ben，

I am glad that I have output the voltage and current normally, I replaced the ucc256304 chip and MOS tube, thank you for your help

Thanks a lot

Best Regards

Eilision Xie