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i am glad you resolved the issue.  So, what was the problem?  Many other users may have this same issue and you could be of great assistance by sharing your experiences.

I also have a similar problem with my 120V to 5V converter.  I am using the UCC28600 and I see the VCC pin oscillate in voltage as it approaches the 13V turn on threshold.  What was you solution?  Thanks!

Check the SS pin.  For start up, the SS voltage must rise above FB (FB immediately rises to 5V at start up) to indicate "SS Done".  If it's getting discharged then the device is detecting a fault such as line overvoltage, output overvoltage, over current, REF_Okay not okay, or over temperature.  Assuminig the switch is not on long enough to generate an output over voltage, and he device has not hit thermal limit, the most likely suspects are the CS, line voltage, or Reference. 

One thing that must be corrected on the schematic that you sent is there is no decoupling capacitor on VDD.  VDD must have a 0.1uF ceramic capacitor, placed with short traces between GND and VDD, this provides essential filtering for the internal REF-Okay signal. 

FB is senstive to noise and using a 330pF to 390pF capacitor on it (instead of the 100pF that you have) may prove helpful.  Do not use more than 390pF on FB though as the timing with the internal 20k pull up may compromise operation. 

Check the resistor values that you have on OVP.  If OVPThe values are despendent upon the minimum valley vlotage  on the bulk voltage input.  The bulk input voltage is dependent upon the bulk capacitor value.  If you are using half of the input capacitance as the referenced design, your bulk voltage is sagging considerably more and modifications to the OVP components, power limit resistor, current sense resistor, and may be even the primary inductance, may need to be made.

Hi Allen,

Please check your feedback circuit, ususlly something in the feedback ciruit is wrong, then it causes oscillation.

in my case, it is simple, the TL431 got blown somehow in a wierd way, once replaced it, I never have had any problems in many PCBs.

 Hope it helps,

Thanks to both of you (Lisa and Harrison) for your helpful comments; I will try some of the solutions that Lisa suggested.  I will also look more closely at the feedback and SS pins.

Here is a more complete description of what I am currently seeing:

Upon application of AC power, I see the VDD voltage, on pin 6 of the UCC28600 device, slowly rise to about 13 VDC and then start to oscillate between 9 Volts and 13 Volts. I don’t get any output signal to the MOSFET switch and therefor, there is no bias voltage generated by the transformer to maintain the VDD voltage. Simulations of my circuit, using the Tina-Ti spice simulator, don’t show this anomaly.  I have a 22uF ceramic capacitor placed very closely to the VDD pin (actually measures 22uF in circuit); however, the oscillating voltage that I see on the VDD pin has the predicted slow rise time, from the 2M series resistance, and then I see a very quick drop from 13V to 9V - this repeats forever.  I did try using a 75 volt DC supply and I lowered the 2M VDD series resistor low enough to allow the device to turn on.  This did allow the power supply to start to function - maybe this points to my need for larger filter capacitors afte the bridge rectifier.  The quick drop in VDD voltage still is confusing; I wouldn't have expected this.

I will hopefully find a solution to this in the next few days and post my results to the forum.  Thanks again for your comments. :)

Allen

Oh, and BTW, I am using the TI design example PMP5192 as a base design and I reduced the filter capacitor values from 4.7uF to 2.2uF to save room on the PCB.

Allen

Reducing the input capacitors will require modifications to the OVP, CS, and PL resistor values as these values are depenent upon the minimum bulk valley voltage. 

Thanks for updating us with your results! 

Allen, try reducing that filter capacitor on CS.  I honestly believe that it should be 100pF, not 1000pF...that's way too much filtering.  Also,does the unit start up under no-load conditions? 

Hello Lisa,

Thanks for your suggestions!  I will try reducing the capacitor on the CS pin to 100pF and see if that makes a difference.  The UCC28600 device doesn't start under any kind of load; I tried using no-load and using a 15 Ohm load.  I rechecked the PCB layout several times and it is okay; also, the component values all measure correctly.  I did try reducing the 2M series resistor on the Vdd pin to 100K Ohms, this time using a 75V DC supply and 120 VAC separately as inputs, and the device looked like it was working, but only when there was no load; the SS pin was still low in this state.  I guess this indicates some sort of error condition.   I will do some more measurements; I hope this all doesn't sound too scatter-brained at this point! :)  I'm sure I will find the answer with your assistance. - Allen

Okay it is now working!!  Thanks for your assistance Lisa.  I made the following changes:

1.  The capacitor on the CS pin was changed from 1000pF to 100pF

2.  I added a 0.1uF ceramic capacitor directly on the power pins of the UCC28600

3.  I changed the capacitor on the FB pin from 100pF to 390pF.

4.  I added a 22uF aluminum electrolytic capacitor across the 22uF ceramic capacitor feeding the VCC pin of the UCC28600.

5.  My big mistake here, I changed the zener shunt regulator from my incorrect TL431A to the correct TLV431A; different VREF threshold between these devices.

The real difference that made the device work was adding the 22uF Electrolytic capacitor across the ceramic capacitor.  This must have changed the ESR of the capacitor combination enough to eliminate some ringing that was affecting the UCC28600.  I will now experiment with capacitor values to get a correct value for my circuit layout.

The device works well at no load and with a 350mA load.

Thanks to all for your help.

Allen

Glad to hear that the problem was resolved.

I am just curious as I think the real difference should be the incorrect  TL431, adding 22uF aluminium cap at the input should not make the difference as the ESR of the ceremic is already low enough and the 22uF capacitance should be enough for this power rating as well.

After you fixed the TL431 issue, did you try to remove the 22uF aluminum cap?

just feel strange , but you know better for sure...

have a great day.

Yes, I agree that is it a strange problem.  I did try the circuit again after fixing the TL431 issue and it still didn't work.  The UCC28600 was indicating that it was seeing an error condition of some kind by sending the SS line low when the SS voltage reached about 1 V; the device would not start and would just oscillate in this startup mode.  I think the problem was that the ceramic capacitor had too low of an ESR for my particular layout.  I have seen ringing occur in my other power supply designs when I use a capacitor with too low of an ESR (such as a ceramic capacitor); the inductance of the circuit traces combine with the low capacitor ESR to create an overvoltage condition due to ringing of the quick voltage transients.  I have been able to solve this by adding a higher ESR capacitor in parallel with the ceramic to provide the current surge ability with ringing suppression from the higher ESR capacitor.  I hope this explains my intentions.  I am thinking that the ringing on the VDD line was causing some internal problems that shut down the device; but this is just a guess.

Well, you know, anything is possible.  I was moving fairly quickly through my tests yesterday.  Maybe I should try the 22uF ceramic again in case I missed something and the circuit was really working with just the TL431 change, but I don't think so.

- Allen