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Part Number: PMP20199
I am using the PMP20199 reference circuit for my project.
In the output voltage measured across TP3 and TP4 there is a ripple of about 12V. It ranges from 57V to 45V. Also at the Secondary2 of T1 the voltage is varying from 3.6V to 0V. There is an off time of around 500ms. I think the UCC28700 is going into Undervoltage Lockout (UVLO) condition. All the values and parts are unchanged from the original reference circuit (PMP20199).
In the UCC28700 datasheet the recommended value for CVdd (C14) is 0.047uf to 1uf and according to my calculations RLC (R5) should be 2.89kOhms.
Please also let me know what will happen if I make the following changes to the circuit:
1. Make C14 = 1uF.
2. Change R5 = 2.87kOhms
3. Change value of R3 from 30.1k to 75k.
Please suggest how I can debug this board.
Thank you in advance.
It is helpful to look at the VDD waveform, a switching waveform (e.g. gate drive or VDS, or secondary winding voltage) and the output voltage at the same time.
This will point to what is causing the shutdown.
If VDD is dropping to the turn-off threshold when switching stops, then the primary biasing circuit needs to be adjusted. More VDD capacitance provides longer hold up, so dropping to 1uF might not help. However, the combination of 10uF with the 30 series resistance might be too high of a time constant to keep VDD charged at very light loads, so I could see that dropping either the 30 ohm or 10uF value could possibly have an effect.
Another shutdown mechanism is OVP. If the VDD is not near the turn-off threshold when switching stops, but the output is floating higher, this could be the source of the shutdown. Applying a small preload on the output can prevent the output voltage from floating higher at no load.
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In reply to Brian King:
I checked my circuit again and apparently I was using 30.1kohms in place of 30.1ohms for R3. I changed that to 33ohms as that was the closest value and it worked. The PWM controller has stopped resetting and is functioning as expected. Thanks for your help.
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