UCC256403: Running the UCC256403 open loop for initial testing

Good day Manikanta,

Thank you, I understand this now better.

My purpose is to slowly increase the input voltage to the converter (in open loop) from 0 to 390V whilst measuring all the switching waveforms to see whether they are correct.

Once I have the unit working correctly in open loop, I will replace the top VCR capacitor and remove the resistor on the FB pin.

So, from what I understand, at 0 input voltage the slew rate of the HS pin will be 0 and the slew rate will gradually increase as I increase the input voltage to 390V.

As mentioned in my first post, the switching frequency measures 6.4kHz at 0 input voltage.

My problem is that the main transformer will not be able to work at 6.4kHz at low input voltage and I am hoping now that the frequecy will jump to the programmed 90kHz at a fairly low input voltage to prevent my main transformer from saturating.

What can I do to bypass the slew rate detection so that the UCC256403 switches at near 50% duty cycle at 90kHz at 0 input voltage to the converter?

Or is this not possible?

It seems to me that my transformer is indeed saturating as I increase the input voltage because of the initial low switching frequency.

Thank you.

Hi Manikanta,

I have connected the controller to the gates of the  MOSFETs and I have the problem mentioned in my previous post.

In my previous post you will notice that I then actually started to increase Vin in an attempt to have the slew rate to rise.

Unfortunately the frequency is not rising.

I can clearly see that the ISNS pin is being activated.

Regards,

Stephen.

Could you share the waveforms and schematics and your design calculator?

I would like to see these waveforms: RVCC, HS, LO, ISNS, VCR.

Also, could you let me know VCR lower cap value and FB pin resistor values and BLK pin resistor values,LL/SS cap value that's been connected in order to operate the converter in the open loop?

Regards

Manikanta P

Hi Manikanta,

Unfortunately I am not at work now, but here is what I can give you now:

UCC256403 Design 25 Jan 2023.xlsx

The VCR lower capacitor value is 8.2nF and the VCR upper cap value (which is presently not fitted) is 68pF.

Resistor on FB-pin is 82k.

The BLK-pin is presently connected to +15V by a resistor divider to enable switching and is at approximately 4.3V

Resistor from RVCC to LL/SS is 56k and 

the resistor from LL/SS to ground is 22k.

Anything that can help I will be so pleased.

I know the UCC256403 will work in the end.

Thank you so much.

Regards,

Stephen.

Hi Manikanta,

Here are the circuits:

AAA.pdf

BBB.pdf

Unfortunately, due to the narrow duty cycles on the gates of my FETs, I have just blown them.

Vin was only at 100V when this happened.

Please see if you can find anything wrong on the circuits.

Thank you.

Regards,

Stephen.

Hi Stephen,

Could you share the waveforms with me if you had captured them before you blew them up? It would help me for debugging. Also, Let me know if you want me setup a webex meeting to discuss the issue.

Regards

Manikanta P

Hi Manikanta,

Here is the very narrow gate waveform of the low side MOSFET with Vin = 0:

You will see from the photo that the frequency is 6.34kHz

The high side MOSFET's gate waveform, compared to the waveform above, is much narrower and is just a turn-on spike.

As soon as I increase Vin to approximately 12V (still far from 390V), I get the following gate waveform for the low side MOSFET:

The frequency is now 328kHz. There is also a severe ringing on the waveform before the FET turns on.

The high side MOSFET's gate however stays a narrow turn-on spike.

I even applied an external PNP turn-off BJT on the gates of both FETs, but the ringing in the photo above does not go away. 

Unfortunately these are the photos I have.

My last resort is to replace the UCC256403 with a new ic.

If this does not help, I may abandon the UCC256403 - unless you can convince me to continue with it!

Waiting urgently on you.

Regards,

Stephen.

Hi Manikanta,

Please note that I disconnected the secondaries of my main transformer to ensure that the synchronous rectification circuitry was not the cause of the problem.

Regards,

Stephen.

Hi Stephen,

You will have to apply higher input voltage (~300V) than 12V. Then only the slew rate will be detected at the HS pin. otherwise, you won't be able to see the correct pulses. Also, as the current magnitude is lower (as there is no load connected and lower input voltage), ZCS protection will kick in due to which frequency will be changed. 

Lets do this way:

1. Populate the secondary diode rectifiers and connect no load

2. Depopulate upper LL/SS pin resistor which would make the BMTL as 0.2V 

3. Remove the FB pin resistor so that feedback will take care of the frequency

4. Apply Rated input voltage.

And reshare the waveforms: RVCC, HS, LO, ISNS, VCR, Vout

Regards

Manikanta P

Good day Manikanta,

I am going back to the UCC25600.

As I said before in a previous thread on E2E, I have had 3 very successful designs so far with the UCC25600.

The beauty of the UCC25600 is its simplicity and the fact that one can see the waveforms even at zero Vin voltage.

I also have full control with the UCC25600 and can limit my minimum frequency.

The short circuit protection of the UCC25600 also works extremely well.

There is no other manufacturer in the world who can boast to have a similar controller as the UCC25600.

Best regards,

Stephen.