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Part Number: UCC28630
Tool/software: WEBENCH® Design Tools
We have developed the AC-DC Adapter circuit using UCC28630DR for 160V to 260V input AC supply working upto 100 watts. We faced the following problem during testing of the circuit.
The AC input voltage was set to 170V AC and unit was turned ON, we were able to get DC voltage.Capacitor C2 output around 280V DC, this is fed to primary points of transformer. The supply point VDD of IC was13V initially when we switch on the circuit and voltage varies in between 13V to 8V later. But no voltage was observed across secondary and auxiliary points of transformer. At this stage R8 resistor valve was 6.49 Kilo ohms (Rbld). We decreased R8 resistor valve to 3.75 Kilo ohms (Rbld) but still no voltage was observed across secondary and auxiliary points of transformer. VDD of IC was13V initially when we switch on the circuit voltage varies in between 13V to 8V. We request you to advice us to solve the issue.
In reply to Ulrich Goerke:
Wave forms observed at different pins of UCC28630 IC is attached.Please review.Wave forms of UCC28630.rar
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In reply to jayaprakash khodhanpur64:
Thank you for the waveforms; they are a good start to debugging this.
From “VDD 2.jpg” we can see that VDD builds up from 5V to ~15V (actually 14.75V), then drops rapidly to 8V and then sags more slowly down to 5V and repeats. Sometimes it is a little different, and I’ll get to that shortly. The main point here is that the slow sag is about 1 second long which corresponds to the “tRESET(long)” parameter in Table 6 (page 52 of the datasheet). This indicates that the fault is an auto-restart fault, not a latching fault.
A latch-off fault would show VDD stuck oscillating between 10V and 8V (“VDD(latch-hi)” and “VDD(latch-lo)” in Table 6) indefinitely until AC power is removed.
Sometimes VDD builds up from 5V to 15V and then sags slowly to about 12V, then drops down to 5V rapidly and restarts. This is a different case, and I believe it is related to the “tRESET(short)” parameter in Table 6, where an AC undervoltage condition or X-cap discharge event triggers a 0.5sec off time. This provides a clue where to look for abnormal behavior.
In the UCC28630 column of Table 3, only “VDD UV”, “AC brownout”, and “Overload timer” are non-latching faults. In Table 4, only “DRV-open” and “HV-short-to-VDD” are hiccupping faults (non-latching).
Well, we can eliminate “VDD UV” and “Overload Timer” faults. This leaves “AC brownout”, “DRV-open”, and “HV-VDD short” as possibilities.
All three of these are some-what interrelated. You can check “DRV-open” by looking for gate pulses at the gate of Q1 (not on the IC pin itself). The DRV signal is involved with VSENSE through R5, which affects the ability to sense the bulk voltage during the FET on-time (indirectly measuring the AC line).
There may be more than one problem in the system, and start-up may fail for different faults at different times. So it is important to check them all out.
You will have to use a much faster sweep speed, such as 10us/div or so to see the switching pulses. Also, it will be helpful to look at more than one signal at a time if you have more than one probe. Key waveforms are DRV, Vaux, and CS. Place the zero-reference indicators on a reticule dotted line, please, to make it easier to read the voltage levels. Also, please indicate the test conditions (input Vac, output load, etc.) for each waveform.
Also, please tell me what the part numbers are for D6 and D7. The wrong diodes here can impair proper operation.
TI recommends Diode D6 value to be 1N5711WS-7-F. we have also used the same part.
TI recommends Diode D7 value to be SK220ATR. we have used DFLS1200-7 (SK220ATR not available. TI web bench recommenbed alternate part as DFLS1200-7 )
Thank you for the diode information. These parts are suitable for the job, so we still need to analyze the switching waveforms for more clues.
Several days have passed since my last reply to you. Hopefully, you have identified and solve the problem. I need to close this thread. If there is still a start-up issue, you may re-open this thread. Otherwise, if necessary you can start a new posting.
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