TOPSwitch-JX start-up problem
Hi,
I'm designing a power supply with the TOP267 for 14V 2.5A output. Schematic as attached.
I've followed the recommended startup procedure; disable the undervoltage lockout and using a variac to slowly increase the voltage from zero.
I have replaced the TL431 with a 15V zener, it managed to startup once but giving 12V output at no load. I didn't try to apply load but the unit has stopped working since.
I have reduced the output capacitor, C3 to 330uF. I have also changed the value of components around the feedback circuitry. R8 is 7.32k, R9 is 866R, C11 15nF, R12 1K, D7 removed and C12 is 33nF. This is the recommended value by PIExpert.
The waveform of voltage at C-pin and the output voltage are as attached.
V(c-pin) oscillates between 4.8 and 5.8V, and Vout has sawtooth waveform of 7V peak. It has failed to reach regulation.
At input voltage of < 40V, the oscillation and buzzing is faster ( about 1 second interval) but above that, the oscillation started to slow down to 2-3 seconds interval.
What can I do to fix this?
Thanks.
Comments
How can I check the trasformer phasing without opening it up? Rapid Transformer made it from our PIExpert file.
Look at the waveforms as I described.
Hi,
Yes, the transformer has correct phasing. What else can I check?
Thanks!
Pls. capture the waveforms of TP10 and Vb at the same time.
Hi,
Since the two has different reference, should I join pin 4 and 8 of the transformer?
No but just ground each scope probe at the respective grounds of the signals you are looking at.
This assumes you have a proper AC isolation transformer powering your circuit. A proper offline power supply test setup will have one.
Hi, Waveform is as attached. Input AC voltage is 95Vac. I noticed that the buzzing speed slows down when I attached the probe to measure Vb.
Hi, it's working now. When I removed the UV resistor for 1st time startup, I left V-pin floating and not joint to PGND. Thanks.
The device is in auto-restart - it's not receiving feedback into the Control pin in a timely manner.
Check your transformer phasing. The voltage on the Drain pin should be out of phase with the secondary winding, and the aux winding is in phase with the secondary.