UCC28C53: UCC28C53 replace UCC28C43 issue

Hi,

Where pin 7 connected in your design? Your circuit only shows a bar but no indication where connected.

Also please provide pin 7 hiccup waveform.

Hi Hong,

Pin7 connect to D6 (pin3).

Please see attached waveform. UCC28C53 can start up in 1min after power on, but it maybe always can't start up.

.UCC28C53 VCC waveform_20230808.xlsx

Hi,

The waveform shows VDD stays below 9V until 1 min later. It means your start up circuit somehow cannot make further charge up until later .

Lets first see if this IC unit has something wrong. You can use a brand new to replace the one you tested. If the new one can work then the old one may have something not good or damaged or partial damaged. Let’s first confirm if a new unit can work.

Hi Hong

I had tried several UCC28C53, they show the same issue.  

I think there is sonething different about start-up between UCC28C53 and UCC28C43.

 Reduce charging resistor R37/R38/R39/R40/R28, could it be better？

Hi,

Yes, reduce these resistor values should help to have higher startup current to charge up the VDD. But if UCC28C43 can start up with the resistor values, UCC28C53 should start as the datasheet shows UCC28C53 startup current not higher than UCC28C43. But you can try to reduce these resistor values to retest to see if the converter can startup without wait for 1 min. It might because that particular batch UCC28C43 have lower startup current. What are these resistor values and what is your input voltage you are using to start the converter?

Hi Hong

Resistor 390kohm, input voltage 200Vac~265Vac. 

Hi,

The startup current max 75uA so the IC should be ok to start. It looks something is not correct. How much total capacitance on VDD pin?

Also can you try ucc28c43 on this board to see if ok to start?

Hi Hong

 Total capacitance 80uF，and I increase to128uF as well. it still can't work.

ucc28c43 can start up normally, had tried 4 boards. 

Could it be the loop repone issue ?

Hi Hong

I change resistor to 260kohm, it can't start-up, too.

BTW, when the board stat-up in 1min, and repeat power off and on, it can start-up immediately.

Hi,

What are your feedback circuit component values, such as R76, R49, and R82, and the output voltage ?

Hi Hong,

R76=0ohm,R49=1.8kohm,R82=1kohm

Output voltage=13V

Hi Hong

Vout(UCC28C53 pin6) and Vcs(R62) waveform is attached. the board repeat to try start-up, but it fail.

Please help check.

UCC28C53 start-up issue waveform_20230809.xlsx

Hi,

Can you measure pin 1 waveform along with pin 6 and Vcc?

Hi Hong

Please see below waveform

4705.UCC28C53 start-up issue waveform_20230810.xlsx

Hi,

It looks the issue is due to COMP pin current different. The opto coupler collector current in your design can be roughly estimated as a bit higher than 3 to 3.5mA. UCC28C43 COMP current with such values, COMP pin voltage can be pulled down to close to zero. But UCC28C53 COMP needs a current over 4 to 4.5mA to pull down COMP to close to zero. So your existing opto coupler current not enough.

It looks your circuit has R45 and C50 not used. You may try short C50 and add a resistor to R45 place to see if the issue can be resolved or improved. The resistor value should be between 1.5k ohm and 2.5k ohm, you may start with 2k ohm to see if ok to start or start with much less time than before. If so, you can try different resistor values from 1.5k to 2.5k to achieve proper start up.

Hi Hong

Actually, R45NA=10kohm and C50NA=0.01uF now. Should I try your solution at this situation?

Could it change R49 and R82 to increase COMP current ?

Hi Hong

It seems improve the start-up issue, when increase resistor of Rcs(R62,R64,R65,R100,R101,R102). Could you analyze what's the root cause?

BTW, there's another problems,

1. When power off the board. UCC28C53 still work in hiccup mode, till the C5 energy run out.

2. Sometimes pin6 will be damaged in power off progress, Pin6 and Pin5 is short circuit (Impedance will be 0ohm, 30ohm or 70ohm). There several UCC28C53 broken in this case

Hi,

You can try to change R49 from 1.82k to some where 1k to 1.5k to see if there is improvement.

Increase Rcs will keep COMP at a lower voltage during startup from your waveform due to narrower pulse width from CS reaches threshold earlier. What is the converter load range? Depending on the load range there is still a chance to make COMP stay high then not able to operate normally.

You need to provide pin 6 waveform for analysis. 

Hi Hong,

I'd tried R49=1.5kohm, it can startup. But pin6 was damaged when power off.

This onvertor load range is 13V from 0.1A to 7A, pin6 waveform as below.

UCC28C53 start-up issue waveform_20230811.xlsx

Hi,

The waveform of pin 6 needs to be captured at power off. I guess one possible reason is the pin 6  spikes too high or something shorted pin 6.

Normal operation waveforms do not show there so we cannot find the reason then not sure how to fix.

You can use normal trigger or single trigger to capture the pin 6 waveform during power off. or use zoom in feature if your oscilloscope has such a feature.

Hi Hong

For the pin6 damaged issue with USS28C53, waveform as below

1. Power off waveform of Pin6.

Measure impedance of pin6 to pin5, it's 63ohm

2. Power on again, wavefor of Pin6, it can't start-up.

3. Change back to UCC28C43, it work normally at power on and off.

Hi Hong

Change R19 from 5.1ohm to 10ohm, UCC28C53 pin6 can be fine in power off stage. 

This test show UCC28C53 pin6 is more sensetive than UCC28C43 pin6, at sink current. But OUT pin current range is the same in there datasheet.

Hi Hong

Additional question, when turn down input voltage less than 150Vac, it can't start-up.

We would need wide input voltage from 85Vac~265Vac. What should I do?

Hi,

Can you show the COMP pin waveform at power on when Vin < 150Vac?

Hi Hong

1. 85Vac Low input voltage can't start-up COMP like this

2. 100Vac Low input voltage start-up COMP as below

Hi,

Can you try further reduce R49? R49 = 1.5k, right? what if reduce it to close to 1k?

Hi Hong

R49 =1kohm when measure the waveform 

Hi,

Where is the waveform for R49 = 1k?

Hi 

This is the waveform at R49=1kohm，already  

Hi,

can you please provide the waveforms of OUT, COMP, VCC, and Isense all with zoom in like the COMP zoomed in with R49? Please plot these waveforms in one picture if you can.