the ucc28700 output voltage is AC ? the effective value is AC 5V？

the output peak voltage is about 40v we can see in the second photo , the DRAIN peak voltage of q1 is about 300v and it is a pulsating dc voltage . the drain voltage frequency is 50HZ of the Q1.

Dear Sir,

Thanks for using TI's solution in your design.
I think the polarity of secondary side rectifier diode D3 is wrong.
In the flyback, when primary side switch turn on, the transformer will store energy in transformer's air gap. When primary side switch turn off, the secondary side diode will conduct to deliver the energy to output.
Also I didn't see the polarity of auxiliary winding, but the IC can work, so I guess the polarity is correct.

I hope after you change the polarity of D3 will help you solve the issue.
If you still have any issues, please feel free to let me know.

Best Regards
Kevin

Hello KEvin:

                        i  did a experiment ,  the polarity of d3 is right, the next photo is the volatge of VOUT.  

I change the polarity of D3 ,I find that the VOLTAGE of vout are the same.

Can you please describe how you measure the output voltage Vout?

Also, can you measure the MOSFET Vds, Vgs, and the device Vcs at the same time so we can review these waveforms.

Hello hong :

                this voltage  waveform is measure on resistor R8 whch i marked a circle in the  sch,

                    

                     

                             

Please provide the waveforms of the MOSFET Vds, Vgs, and the device Vcs at the same time so we can review these waveforms. These waveforms can help analysis why Vout is like that.

I see you opened additional strings. I will work to support through this string and will close the other string to avoid confusion.

Please provide the waveforms of the primary MOSFET Vds, Vgs, and the device Vcs at the same time so we can review these waveforms. These waveforms can help analysis why Vout is like that.

On your design need, certainly we can continue to debug to make it work. But there is another option for you to consider. We have an existing design PMP15002 for 5V/3A. PMP15002 design files and test results all can be downloaded from ti.com. I strongly recommend that you have a look on PMP15002 design and I believe you can save a lot of time if PMP15002 can work for you. If you need suppor on PMP15002 please let me know.

Hello hong :

yeh ,we can communation in this string. i am sorry for my delay response. i do not quite understand what you mean is ? is that allowed to measure the primary mosfet waveform of VD VG to the VS? if we do that would it damaged THE oscilloscope? maybe your mean is to measure the waveform of VD and vg to earth?

I want to see the waveforms of Q1 drain to GND (pin 5 of U1), Q1 gate to GND, and Vcs (U1, pin 4 to GND), on your oscilloscope at same time.