Flyback design with LYTSwitch LYT5226D, converter does not starts up once line input voltage is connected

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Hello,
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) 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

Hi emilzatopec
Thanks for using LYTswitch 5. I would like to check the following to fix the start-up issue.

1. Please check on the pCB layout the location of the Line sense resistor RL. THe line sense resistor RL should be connected as close as possible to the L pin of the LYTSwitch-5 device. This is to suppress the switching noise coupled to the L pin trace. As much as possible the trace where the line sense resistor is connected should be away from the drain. If relocating the line sense resistor is not possible due to layout constraint, try to add 1nF ceramic capacitor across the device L pin to source Ground. Note that the line sense is a sensitive terminal pin with OVP threshold of 116uA.

2. Please also check the ROC resistor- Sometimes the bias voltage is higher than expected due to leakage inductance. The OC pin current computed as (Vbias-2.25V)/ROC should be less than 127uA. You can try to increased the ROC reristor and verify if the unit can start-up at all line condition. Same with RL resistor it is ideal to connect the ROC resistor near the OC pin of the device to suppress switching noise.

3. For all your voltage measurement on the IC using the voltage probe. Please connect the ground to the source ground.

4. If the start-up issue still can't be solved after above suggestion. I would like to request your schematic with values , PI expert spreadsheet and the picture of your PCB layout.

THanks and have a good day

Hi Martok,
Thank you for your reply and for your tips to fix the problem.

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. I am so glad to match with your recommendation. Thank you very much!.
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. Ok, understood. I thought I had to do it in such a way but I wanted to confirm it anyway.

4. 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?.
I attach you the PI expert spreadsheet and the schematic and expect your early reply because I still need technical assistance.

Thank you all in advance,
Best Regards

Hi Emil,

1. Efficiency Issue
1. Try to use an E-cap with lower ESR for Output Capacitor. Output capacitor rms current is high due to high output current specs and with the driver operating on discontinuous mode.
2. Reduce the leakage inductance by interleaved primary winding and optimized your snubber values.
3. For 700mA output current I suggest to use schottky diode for a better efficiency.
4. If the efficiency still does't meet your specification then try to use larger transformer core to lower the copper loss.

2. Output regulation issue.
1. Components that affects the output current are the bias voltage, RoC , RFB , RDC1, RDC2 and RDC3.
2 Bias voltage must be consistent with respect to line. If not then it must be due to leakage inductance since your using Isolated transformer. You can try increasing the RBF from 10R to around 47R to limit the effect of leakage on the bias supply.
3. When optimiizing output current. First step is to adjust the RDC1 and RDC2 to meet 700mA at low line (85V) . Second step is to increase the value of RDC3 to increase the output current at high line condition.

Regards
Mark

Hi Martok,
After reading and following your tips I have tested the converter and have some comments I want to tell you.

Efficiency Issue.
1. I have checked that the output capactor is according to the LOW ESR required for my design and even though, I have increased the output capacitor until 1,4mF in order to reduce ripple voltage. The result was exactly the same and there were no impact in the efficiency of the converter.
2. I do not know the reason, but when you modify your original spreadsheet for updating some values in the "Feedback and Protection Parameters with Fine Tuning" the description of the transformer is modified too. Of course, my primary winding is interleaved so as to reduce leakage inductance. I attach you the original spreadsheet design before been modified to adjust output current (LYTSwitch-5 Isolated Flyback Design_85_305Vac_EE13_core_original).
I also have optimized my snnuber values and I have finally obtained an efficiency of 77% but I cannot improve the perfomance more.
3. I am using a schottky diode and I have proved with other ultrafast diodes as well with same results as before.
4. As for the transformer I am completely agree, but I am also worried for the output current regulation, which depends too much of the line input variations as I commented in my last post (... 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.)

Output Regulation.
1. The measurements I made in the bench by changing the Rdc1 and Rdc2 values to adjust output current were as expected, more or less. I mean, I adjusted output current to 685mA and 710mA.
The problem is that for achieving this value, the converter takes a few seconds (30-40 seconds) to get this current and once it is accomplished, continues varying and is never stopped. I mean, is moving from 678 or 695mA in little steps but as I have already said, never stops. This happens whichever line input voltage selected.

So, despite the efficiency issue, I asume that there is a problem with the regulation of the converter because it depends on the line variations too much as I have checked (varying input line with a sudden brief or wide burst results a clear light variation).

I am a bit lost right now, because I have built a transformer like the spreadsheet recommeds and I have proved the converter with the expected values for feedback and output current and as I have checked, it is not tight regulated.
Besides of that, I changed the RFB values according to AN-65.pdf, which describes that the tc = RfbxCfb shall be between 3ms-6ms and the regulation does not improves anyway. I only got new output current value, but as you told me in your last post Rfb impacts on the output current.

To conclude, Is this behaviour normal with this device? Is really so depending on the line input variations and how can I improve the output current regulation?, How can we proceed to improve that?
Another thing that is really driving me insane is the fact that the transformer built matches the spreadsheet design requirement and at the end, is becoming a problem for the efficiency.
Thanking you all in advance,

Best Regards
Emil

Hi Martok,
After reading and following your tips I have tested the converter and have some comments I want to tell you.

Efficiency Issue.
1. I have checked that the output capactor is according to the LOW ESR required for my design and even though, I have increased the output capacitor until 1,4mF in order to reduce ripple voltage. The result was exactly the same and there were no impact in the efficiency of the converter.
2. I do not know the reason, but when you modify your original spreadsheet for updating some values in the "Feedback and Protection Parameters with Fine Tuning" the description of the transformer is modified too. Of course, my primary winding is interleaved so as to reduce leakage inductance. I attach you the original spreadsheet design before been modified to adjust output current (LYTSwitch-5 Isolated Flyback Design_85_305Vac_EE13_core_original).
I also have optimized my snubber values and I have finally obtained an efficiency of 77% but I cannot improve the perfomance more.
3. I am using a schottky diode and I have tested with other ultrafast diodes as well with same results as before.
4. As for the transformer I am completely agree, but I am also worried for the output current regulation, which depends too much of the line input variations as I commented in my last post (... 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.)

Output Regulation.
1. The measurements I made in the bench by changing the Rdc1 and Rdc2 values to adjust output current were as expected, more or less. I mean, I adjusted output current to 685mA and 710mA.
The problem is that for achieving this value, the converter takes a few seconds (30-40 seconds) to get this current and once it is accomplished, continues varying and is never stopped. I mean, is moving from 678 or 695mA in little steps but as I have already said, never stops. This happens whichever line input voltage selected.

So, despite the efficiency issue, I assume that there is a problem with the regulation of the converter because it depends on the line variations too much as I have checked (varying input line with a sudden brief or wide burst results a clear light variation).

I am a bit lost right now, because I have built a transformer following the spreadsheet recommedations and I have tested the converter with the expected values for feedback and output current and as I have checked, it is not tight regulated.
Besides of that, I changed the RFB values according to AN65.pdf, which describes that the tc = RfbxCfb shall be between 3ms-6ms and the regulation does not improves anyway. I only got new output current value, but as you told me in your last post Rfb impacts on the output current.

To conclude, Is this behaviour normal with this device? Is really so depending on the line input variations?, How can I improve the output current regulation?, How can we proceed to improve that?
Another thing that is really driving me insane is the fact that the transformer built matches the spreadsheet design requirement and at the end, is becoming a problem for the efficiency.
Thanking you all in advance, and expecting a reply as soon as possible.

Best Regards
Emil

HI Emil,

1. 77 % Efficiency is really low for 8.4Watts Flyback design. I suggest you need to take a thermal scan on your unit to see what are the heating components and work from there. The expected efficiency should be >80%. Also Try optimized the transformer windings by increasing the Vin from 85V to 90V, also fix the secondary turns at 15 turns. You can see the Primary turns will be significantly lower by around 40 turns. This way the leakage inductance will also

2. For the output Current Regulation, May I know your Output current regulation limit?
- Output current should be within +/-5% of nominal from 90-265V. But could be above +/-5 % if the input voltage is 90-305V. Also can you send to me your PCB layout? thanks