UCC256404: Ucc256404 not start with full load

Hi Andrej,

Probably controller might be hitting OCP during power up. Could you try reducing the ISNS resistance value?

Regards

Manikanta P

I reduce Risns from 133 to 900 R. System starts to load 1A, but only once.

Here is TP20 point voltage  (voltage in ISNS) after start with load 1A:

After switch power off and on again, system not starts, it try starts every second, and p20 Voltage is:

There are same voltage levels of TP20.

But, I found next: When I discharge manually 375V input with rezistor (it is from UCC 28064EVM-0040), and power up, then system starts again. Problem now is that input DC 375 V is not discharged after power down, and when it§s not discharged, UCC256404 not starts.

This is measurement In pin BLK when system starts:

And this is pin BLK after power restarts and system not starts.:

Hi,

you mentioned that power up is okay at 0.5A. Could you share the waveforms of Vout, SW,ISNS, VCR with the modified board? What other components have you changed other than the transformer? Could you share the modified schematic?

Are you powering up LLC directly from DC input or using PFC EVM?

Regards

Manikanta P

Here are voltage waveforms, 

for all measurements is input AC 130 V,but tested from 65 to 240V. 

1. no load (10k resistor in output)

output voltage 86V

BLK:

VCR:

BW:

LL/SS:

ISNS:

2. first start (pfc from UCC 28064EVM-0040 discharged), system correctly starts:

load 80 Ohm (1 A)

output voltage 86V

BLK:

VCR:

BW:

LL/SS:

ISNS:

3. AC power switch off and on, (PFC not discharged), not starts

same load - 80Ohm (1A)

output 76V

BLK: - strange, info ib bottim in point 4.

VCR:

BW:

LL/SS:

ISNS:

4. When AC restarted, and measuring BLK - oscilloscope probe is connected to TP16, system starts correctly with load 1A:

BLK:

With probe connected to TP16, system starts every time from ac power on/off. No pfc discharge needed.

Next measurements:

In excel calculation is BLK sense resitor calculated to 45.89 k. In eval board is R37 - BLK sense resistor 41,2 k. Above measurements are with resistor R37 - 41.2 k.

I try to change R37 to 45.8k, without effect. Next change R37 to 39k, without effects. After AC power off/on, it is same measurements as in 3.

In original UCC25640EVM-20 is only changed Risns from 133 Ohm to 90 Ohm. (In excel calculation is recomended 75 Ohm).

No other changes. 

Of course, the transformer is custom but wound on the original core and the feedback is adjusted to 80V. Added Resonant Lr coil  netween TP6 and pin 2 of transformer.

Attached last excel file with calculation.

UCC25640x Design vypocet_80V_4A_2nd.xlsx

Hi Andrej,

I dont see output voltage waveform in your captures. You shouldn't use the same feedback structure as the EVM since Shunt regulator abs max voltage is less than your output voltage.

You should connect additional Zener's to reduce the Voltage across the shunt regulator

Refer the feedback structure in this schematic: PMP40379 reference design | TI.com

Regards

Manikanta P

I have feedback as in attached file.

feedback_80.pdf

This is measurement with all signals, FB and Vout included. (above measurements had set oscilo to AC, and signal wafeforms has not been clear too.

Here are new measurement with signal set to DC. Vout probe is with 1:10 .

First measurement is after power AC first start, second after AC restarts.

measuring_ucc256404_1A_load.pdf

Hi Andrej,

As I mentioned in my previous response, you should modify the feedback structure so that the voltage across the shunt regulator doesnt exceed its max voltage.

Regards

Manikanta P

Hello. Still noi effect. I create feedback by this your scheme: https://www.ti.com/tool/PMP40379. It is for +-42V, I need 80V, then I not chnge it, it shold set output to 84V, when I connect to output and output ground.

When no load, system starts, output 84V.

When load is 1A, then feedback is:

And optocoupler input (diode anode) is the same but max amplitude is 11V:

Hi Andrej,

Could you share the modified feedback structure with me ?

Regards

Manikanta P

feedback_zener.pdf

Zener diode connected to FB shold be 8.2V, becouse RVCC in UCC256404 is 13V, but I thing that, this is not the main problem in my case now.

Here are waves un some pins, started withoud load (only 10kOhm). Any starting load causes that system not starts. But from transformer is chirping and is hot.

BLK:

VCR:

BW:

LL/SS:

ISNS:

FB:

Vout (1:10):

Hi Andrej,

You need to connect a resistor in series with Zeners as shown below.

Also, replace C207 with 0.01uF and R295 with 33k in your schematic.

Regards

Manikanta P

I did changes, with R295, C207 and added 10Ohm resistor by scheme.

System not start with load higher ten 0,5A . When starting with no load, I can increase load up to 2A.

Problem is that with this feedback is transformer chirping, and resonance Lr and Cr  is hot.

In attached files are complete measuremnts with loads from 0 to 2A step 0,5 A.

mesurement_ucc256404_zenerfb.pdf

Hi Andrej,

I wont be able to debug from the waveforms you shared since you are using only single channel.

It would be easier if you capture the waveforms using 4 channels (HS, ISNS, VCR, Vout).

I would suggest increasing the LL/SS cap value to increase the switching frequency during startup.

Regards

Manikanta P

Here are 4 channels, Vout is extra, it is not on same ground. LL/SS cap is now not changed, I will do it today.

mesurement_ucc256404_zener_fb_combined.pdf

After change of C36 (Css) to 100nF

mesurement_ucc256404_zener_fb_combined_C35.100nf_Load1A.pdf

Hi Andrej,

Could you increase the soft start cap further to 1uF.

Also, Could you let me know which fault is being triggered?

Refer the bring up guidelines of the app note to find out the fault: UCC25640x LLC Resonant Controller Features Brief Overview and Bring up Guideline

Regards

Manikanta P

Increasing start capacitor helps - now I can start with load 2 Amps. But still have problem start in cycles - switching off power to PFC EVM and switch on again, it not starts. After manually load PFC EVM output with 10k resistor and start power to PFC EVM againt, it starts.

here is starting sequence after power off/on:

yellow - BLK

blue - LL/SS

red - VCR

green - ISNS

and the same when starts after power switch off and after some minutes switch power on.

Andrej,

Which PFC EVM are you referring to? 

I feel this kind of communication is getting less effective. May I suggest you to contact our local FAE and channel to us through emails?

Thanks

At the beginning of the conversation I wrote that PFC is used UCC 28064EVM-0040.

But I think that main problems has been solved, I can start with full load, and there is not problem to change load in run.

I need help with the second problem which I describe above too. With using feedback like has been recommended above, resonant coil and capacitors temperature is increasing. With load 2A resonant coil increases in 15 minutes from 20 to 80 °C and resonant capacitors from 20 to 60 °C .

Output voltage is slowôy rising too, from 77V to 79V.

With using sinple feedback Type 2, there has not been any problem with temperature increasing.  

Is there any recomendation for Resonant coil using? 

OK, I see the PFC information is buried in the early conversations. As this thread gets longer, it is hard for someone else to catch all the details, which is why I suggest to contact our local FAEs.

I'd suggest to check the switching waveforms to see if the peak current is hitting the transformer saturation region. Maybe the feedback dictate burst mode instead of normal mode, which resulted in higher switching current. I don't know your transformer spec in the previous conversation so I cannot recommend what to change.