Converter not starting up once line input voltage connect

In summary, the main problem is that the OVP voltage output protection is being triggered and is not letting the converter to run at full power. The added capacitor of 330pF/50 seems to have solved the problem.
  • #1
Joshua_S
3
1
Isolated_Flyback_Topology_LYTSwitch-5.png


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!
 

Attachments

  • Isolated_Flyback_Topology_LYTSwitch-5.png
    Isolated_Flyback_Topology_LYTSwitch-5.png
    22.4 KB · Views: 899
Engineering news on Phys.org
  • #2
Joshua_S said:
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.
 
Last edited:
  • #3
  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.
 
  • #4
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:
Isolated_Flyback_Topology_LYTSwitch-5_v4.png
 

Attachments

  • Isolated_Flyback_Topology_LYTSwitch-5_v4.png
    Isolated_Flyback_Topology_LYTSwitch-5_v4.png
    28.4 KB · Views: 559
Last edited by a moderator:
  • Like
Likes Tom.G
  • #5
Joshua_S said:
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.
 
  • Like
Likes Tom.G
  • #6
What is the voltage across both C8 and COUT at both low and high line voltage?
What is the value of RSN?
What are the part numbers for DFW, DB, DSN, COUT, CBP, CSN, CDC, and the transformer core?
What is the number of turns on each winding? What wire size?

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

1. Why is my converter not starting up when I connect the line input voltage?

There could be several reasons why your converter is not starting up. One possibility is that there is an issue with the power supply, such as a blown fuse or a loose connection. Another possibility is that there is a problem with the converter itself, such as a malfunctioning component or a faulty circuit.

2. Is there a specific sequence I need to follow when connecting the line input voltage to the converter?

It is important to follow the manufacturer's instructions when connecting the line input voltage to the converter. In most cases, you will need to first connect the line input voltage, then turn on the converter. If the converter still does not start up, try disconnecting and reconnecting the line input voltage in case there was a loose connection.

3. Can using the wrong type of line input voltage cause the converter to not start up?

Yes, using the wrong type of line input voltage can potentially damage the converter and prevent it from starting up. Make sure to check the specifications of your converter to ensure that you are using the correct type of line input voltage.

4. How can I troubleshoot and fix the issue with my converter not starting up once the line input voltage is connected?

If you are experiencing issues with your converter not starting up, first check the power supply and make sure all connections are secure. If the problem persists, consult the manufacturer's troubleshooting guide or contact their customer support for further assistance. In some cases, the converter may need to be repaired or replaced.

5. Are there any safety precautions I should take when troubleshooting a converter that is not starting up?

It is important to always follow proper safety precautions when troubleshooting any electrical equipment, including converters. Make sure to disconnect the power supply before attempting to troubleshoot or repair the converter. If you are unsure of how to safely troubleshoot the issue, it is best to consult a trained professional.

Similar threads

  • Electrical Engineering
Replies
14
Views
715
Replies
4
Views
1K
Replies
8
Views
911
Replies
31
Views
2K
  • Electrical Engineering
Replies
17
Views
3K
  • Electrical Engineering
Replies
34
Views
3K
Replies
6
Views
3K
  • Electrical Engineering
Replies
1
Views
1K
  • Electrical Engineering
Replies
1
Views
3K
Replies
2
Views
3K
Back
Top