Converter not starting up once line input voltage connect

[Joshua_S]

Hello guys,

I have designed and built a prototype with the aid of PI Expert Designer tool according to the following characteristics:
- Vinput = 85-305Vac
- Voutput / Ioutput = 12Vdc / +700mAdc

I also attach you the schematic of the converter, which was taken from DER-528 application note (14 W Non-Dimmable, High Efficiency (>86%), Power Factor Corrected Isolated Flyback LED Driver Using LYTSwitchTM-5 LYT5226D, this one http://www.kynix.com/Parts/80393/LYT5226D.html and the datasheet here https://led-driver.power.com/system/files_force/product-docs/lytswitch-5_family_datasheet.pdf) as a starting point to design my own converter.

The problem is the converter is not started up once the line input voltage is connected.

In fact, between 30Vac-60Vac the converter tries to start up and the output leds are blinking but not lightning continuosly which in returns means the converter is not really started up and operating well.
For this voltage range, output voltage is near the output led (Vf 12 aproximately) and the auxiliary winding is triying to reach the high level, which is set up at 20Vdc, but never reaches it because of blinking.
So, I assume the converter is not started up because of some OVP input or output protection, but after reviewing the RL and Roc resistances designed with the PI Expert, the value is correct.
The OVP voltage output protection is set up in 14,9Vdc and I have tested that this level is never reached at all.

I cannot understand what is going wrong in my design, I have tried some tests but nothing changes this behaviour.
I tried to remove Cdc and leave alone Rdc1 and Rdc2, but it does not change the status of the converter.
The most disturbing fact is that when I check the voltage in the Line Sense (L) pin with a tester probes, the converter starts up and operates correctly (12V-670mA but only for 60-140Vac aproximately) and when I remove the probes the converter switches off again.
Concerning this measurement, I actually do not know where I should have to connect the tester probe for the return point. It should be connected to Source (S) pin or to -Vdc line reference?.
Whichever the place chosen for connecting the return probe, the converter starts up and the voltage measurement is less than 1,8Vdc. I also realized that when this voltage is near 2Vdc the converter is switched off.

In summary, could you please explain to me this abnormal operating condition? I cannot see the mismatching point between spreadsheet calculations from PIExpert and the prototype I designed.
Need technical support.

Best regards!

 

[Rive]

The most disturbing fact is that when I check the voltage in the Line Sense (L) pin with a tester probes, the converter starts up and operates correctly (12V-670mA but only for 60-140Vac aproximately)

The original design contains two pieces of 2MOhm resistors, yours contain only one piece of 4.xxMOhm resistor.
The usual value for 'maximal voltage' on 1206 SMD resistors is around 150V.

Ps.: also, MOhm resistances are in the range when measurement will modify the current/voltage circuitry.

 

[Svein]

1. I am a bit confused by the two connected LEDs on the output. Since the voltage drop across each LED is about 1.5V, the output voltage cannot go higher than about 3V before the output is current limited...
2. In any case, if the converter starts up and works for half a second or so, the IC will use the feedback voltage as the main current supply.

 

[Joshua_S]

Thank you for all your replies.

1. To start with, the PCB size is quite tight, so there is no much space to place the components. But, I think that there is no problem with the layout associated with the net of Line sense resistor RL. The distance between this net and the drain net is 0.8mm and the location of the RL resistor is as close as it is possible.
Anyway and considering the fact of this sensitive terminal (116uA) I realized that some OVP protection was being triggered. So, I added a capacitor of 330pF/50 to this RL to source and then the converter started up properly. That's great!

2. As for Roc resistor, it is placed near the OC pin and there is no much to add to me to this point. Just the fact that I have increased its original design value after some tests.

3. New issue: (Adjust Io and Efficiency)
After the start up issue, I have tried to adjust the output current value to 700mA. The problem I have now, is that the "Feedback and Protection Parameters with Fine Tuning" does not matches the expected value after testing them on bench. I mean, I have measured the output current and typed it at the IO_ACTUAL field of the spreadsheet and then I have tested the converter with the suggested value and I do not get the expected value. What it is wrong here? Why does not match it?. I also noticed that varying input voltage the output current changes around 45mA (i.e: 645-600mA) and it is not tight regulated as I expected.

I also have problems with the efficiency because I expected a value of 0,85 and I only get 0,70. According to the spreadsheet the Lp of the transformer is 693uH which I think it is too much. That is why there are a lot of turns in the primary winding. I think that this Lp value can produce some losses which in turns become a low efficiency.

To summarize, how can we proceed to increase the efficiency and get an accurate output current regulation?

Thanks for you time!

I would attach you the PI expert schematic:

 

[Rive]

But, I think that there is no problem with the layout associated with the net of Line sense resistor RL.

I did not say it that is a layout problem. I said it is a (supposed) violation of component maximum rating.

 

[Tom.G]

What is the voltage across both C8 and C_OUT at both low and high line voltage?
What is the value of R_SN?
What are the part numbers for D_FW, D_B, D_SN, C_OUT, C_BP, C_SN, C_DC, and the transformer core?
What is the number of turns on each winding? What wire size?

Also, for safety I recommend that C_YC have a higher voltage rating, at least 450V.