UCC28634: Very Low output Current

Hi Rushab,

in you schematic, there is a mistake on aux-winding of the transformer The homonymous ends should be at Ground point.

please double confirm whether it is the root cause.

Hello David
Thank you for you're response
The schematic symbol for aux winding is wrong 
I cross-checked it with the original transformer
the homonymous ends are connected to ground 

Hi Rushab,

if the transformer homonymous can match the PCB layout, please double confim the CS signal waveform to see whether an over current protection is triggered.

Hello David 

Yes the Over Current is triggered
in the no-Load condition, the voltage at CS pin (with respect to GND of the chip) is around 350mV
as soon I Connect the Load the voltage Jumps to 3V 
also, there is a -35V on the CS pin, irrespective of the load is it normal?
Sharing all the Images from the scope

Hi Rushab,

Thanks for your interesting in TI's part.

Here are some comments for your reference:

1. The maximum allowed voltage for CS pin is -0.3V~1.5V . so if you captured  a -35V and +3V voltage on CS pin . The controller already damaged . or you didn't captured a right signal. CS pin waveform should likes a triangle shape with some spikes.

2. Auxiliary winding "dot-end" should be connected to rectifier diode rather than ground.

3. The SD pin can connect a 200kohm resistor if you don't want to use it.

4. You can capture MOSFET VDS , transformer primary winding current . VDD in the same waveform for analysis . all of protection are listed in datasheet page 43 table3 . You can check it one by one.

Thanks.

Hello,
Thanks for your reply
yes, the chip indeed was damaged
the problem still persists even after changing the chip
I thought it might be because of poor layout or noise 
so I removed the MOSFET and Mounted it on a secondary board and Connection is done Via Wires
I Have Added a 1nF and 1k Resistor on the cs pin as a filter yet no effect
by Observing the waveform on the secondary of the transformer, I can see the MOSFET turns on every 500mS and turns back off
so it is not able to deliver the current needed

Hi Rushab,

Could you please share some waveforms which you captured and some photos show how you to do the test ?

And can you capture the signals which I mentioned in last reply ?

Thanks.

Hi Jaden
I will share the waveform as soon as I have them

in the above you mentioned point 3

3. The SD pin can connect a 200kohm resistor if you don't want to use it.

can you clarify that the 200k resistor should be Pulled up, Pulled down or Floating

Thanks in advance

Hi, RUSHAB,

Sorry for confusing you. the resistor on SD pin should be connected to ground.

Thanks.

Hi Jaden
Thanks for your response I Connected a 200K resistor between SD and ground but it had no effect on the output current

I don't have any Isolation probe to Capture the VDS of MOSFET sorry about that 

However, I Disconnected the aux winding in the circuit and Captured the waveform
the following image is of the first 3 Pulses that the controller generates after every reset
What I Found was the waveform when MOSFET is ON is does not reach the value VIN x (NB/NP) = 345VDC x (8/50)=55.3
it is 40V in my observations also it is not flat either 
As mentioned in Figure 23 page 24 of Datasheet

Also notice a Spike on the Positive side of Waveform not sure the cause for it 

I guess the issue could be Improper Mosfet or the transformer core Saturation 
Attaching the Datasheet for MOSET here https://www.onsemi.com/pdf/datasheet/fcd4n60-d.pdf

Thanks 

Hi RUSHAB,

Firstly , make sure your transformer windings' polarity are correct , otherwise , there is a short through happens when mosfet turn on leading to a very narrow pulse . the waveform shows the first pulse not turn on . and 2nd and 3rd are very narrow .

Secondly , please add a RC filter on CS pin , just shown in EVM board design（R=1K .C=120pF） to filter the leading edge blanking spike . so you will not see a very narrow pulse .

UCC28634 driver capability is enough to turn on the mosfet you selected .