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SMPS Always Trying to Restart

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dmerry03:
Hello,

I am currently working on a galvanically isolated 15V Switch Mode Power Supply. I am having an issue where the IC I am using is constantly trying to restart after about 1 second. This is demonstrated in the oscilloscope shot below.
[attachimg=1]

Here is an image of my schematic:
[attachimg=2]

It is not shown, but there is a inrush current limiter and a bridge diode to the left, where the "HV" Net Label is.

I am at a loss for what is going on. I do not have extensive experience with these power supplies, but I have tried the following:
1. I removed the D5.b diode and C7.b to determine whether there was a short on the output. The restarting behavior persisted.
2. I used a thermal camera to locate shorts anywhere on the circuit. To no avail.
3. I putting a large Cap (330uF) in parallel with EC11.b, thinking maybe there was a high current draw when the LN9T11 began switching the primary, but this did not help.

Thanks in advance for your help.

jmelson:
The design is supposed to trickle current into EC11 until the chip has enough voltage to start, and then D9 is supposed to keep it above the undervoltage lockout setpoint.  It sounds like D9 is not able to keep the chip powered up.  When  not powering a load, the current in the transformer may not develop enough voltage in the winding connected to D9.  Possibly, you have the winding connected to D9 backwards.  Flyback supplies are sensitive to having that winding phased correctly.
Jon

magic:
What are those scope traces showing?

TimNJ:
Do you have a full datasheet for this IC? Only can find an abridged one. English preferable.

Also, what is the yellow waveform? VDD pin? Quite a low VDD voltage, which is why I'd like to know the VDD under-voltage threshold and start threshold is. Did you build the transformer yourself? If the coupling of the auxiliary winding (to secondary winding) is poor, it may not be able to provide much current once the IC switches over from charging via HV.

dmerry03:

--- Quote from: jmelson on October 26, 2021, 04:31:47 pm ---The design is supposed to trickle current into EC11 until the chip has enough voltage to start, and then D9 is supposed to keep it above the undervoltage lockout setpoint.  It sounds like D9 is not able to keep the chip powered up.  When  not powering a load, the current in the transformer may not develop enough voltage in the winding connected to D9.  Possibly, you have the winding connected to D9 backwards.  Flyback supplies are sensitive to having that winding phased correctly.
Jon

--- End quote ---

Thanks Jon. I had this SMPS designed by company in China, then I integrated the design into my product and localized the BOM (aside from the custom transformer). They have been telling me to check that winding as well, but I am certain I integrated it into my design correctly. Maybe you could tell me if I am wrong . . .

Their PCB Layout
[attachimg=1]

My PCB Layout
[attachimg=2]

So if I attach a more substantial load, would that feedback winding develop a higher voltage?

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