I have designed an LLC converter with the UCC25600. My input voltage is 400V from a PFC stage. The desired output voltage is 400V. My transformer is an ETD49 core of 3C94 material. I have 35 primary turns and 70 secondary turns. My minimum operating frequency is 62 kHz. The desired power level is 1100W
Lr = 19uH
Lm = 180uH. I have tried Lm = 100uH as well.
Cr = 132nF
My problem is this: With a light load of 10W my converter does not maintain regulation. I have characterized the gain of the converter and found that the lowest gain I can achieve is 1.22. This gain is found at about 75 kHz. The max gain I have seen is 1.42. This happens at a frequency of 62 kHz. Increasing the frequency of the converter past 75 kHz causes the gain to go up rather than down as it should.
Under a larger load of 300W, the converter maintains regulation as desired.
Things I have tried: I added a copper shield between the primary and secondary and grounded it to the common on the primary side. I also tried winding the secondary of the transformer in a different way to reduce its self capacitance. These two fixes had only a very small effect and did not fix the problem.
I suspect that my converter has problems because of its high output voltage. This causes the self capacitance of the secondary and the capacitance of the secondary rectifier diodes to be more of a problem than for most LLC converters that use a step down transformer.
Any suggestions on improving the no load regulation of the converter would be appreciated.
Lr = 19uH
Lm = 180uH. I have tried Lm = 100uH as well.
Cr = 132nF
My problem is this: With a light load of 10W my converter does not maintain regulation. I have characterized the gain of the converter and found that the lowest gain I can achieve is 1.22. This gain is found at about 75 kHz. The max gain I have seen is 1.42. This happens at a frequency of 62 kHz. Increasing the frequency of the converter past 75 kHz causes the gain to go up rather than down as it should.
Under a larger load of 300W, the converter maintains regulation as desired.
Things I have tried: I added a copper shield between the primary and secondary and grounded it to the common on the primary side. I also tried winding the secondary of the transformer in a different way to reduce its self capacitance. These two fixes had only a very small effect and did not fix the problem.
I suspect that my converter has problems because of its high output voltage. This causes the self capacitance of the secondary and the capacitance of the secondary rectifier diodes to be more of a problem than for most LLC converters that use a step down transformer.
Any suggestions on improving the no load regulation of the converter would be appreciated.
I would check ratio of lr to lm and also the Cs value, you forgot to mention the Q value you are using.Check the appnote for suggested values.Also the winding of the transformer will make a difference.
Most SMPS have problems with light loads.
PC's always have a load so the problem doesn't occur.
I found with my designs I needed resistors on the output to maintain a small load.
PC's always have a load so the problem doesn't occur.
I found with my designs I needed resistors on the output to maintain a small load.
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