High output voltage fluctuations for a FAN7554 based PSU

[zyuan]

Hi,

I am attempting to repair a 12V/4A PSU for my Buffalo Linkstation NAS drive.  The drive failed to wake up automatically at a scheduled time. I swapped the PSU with a known good one, and the Buffalo NAS worked just fine.  The initial  test with that PSU  under load looked okay with a multimeter, but on the Oscilloscope, it showed very high voltage fluctuation about 2V pp.  So, I broke the power brick open to check, and found that the main capacitor was leaking and one of its leg was broken off. Besides that, PCB looked very clean  and no burn marks. I replaced all capacitors on that power supply with  new ones, and checked the switching MOSFET, voltage regulator (TL431), optocoupler and diode on the output side, everything looks just fine, however, the output voltage fluctuation didn't seem improved.

I then realized that this PSU is using the Fairchid PWM chip FAN7554, working under the bust mode.  It turns off switching on the primary side when feedback voltage below 0.3V during the light load for saving energy. Such behavior causes the higher voltage fluctuations at no or light load. The circuit diagram of my PSU is almost identical with the example circuit in the Fairchild data sheet, See https://www.alldatasheet.com/datasheet-pdf/pdf/51202/FAIRCHILD/FAN7554.html, except having very beefy filter capacitors, e.g. main capacitor is rated at 100uF/400V and output capacitors are at 1000uF/25V and 470uf/25V. 

Understood of the limitation on FAN7554, however, the 2Vpp is still too much to me, although The PSU look like having a decent quality. I don't know the original specification of this PSU .  But I don't think such voltage fluctuation is good for my NAS drive.  I tested all components I can test, even ordered a new FAN7554, nothing helped to improve the voltage fluctuation for this PSU.

I would like to know how I can reduce the voltage fluctuation for the FAN7554 based SMPS.

Any help and comment are highly appreciated.

Thank you  in advance!

[ingalopez]

Hi there!

 Does the voltage stabilize if you connect aditional load to the PSU? Try some more load, to offset the chip "low power" mode.
Check the specs of the FAN 7554. (I guess you already did...), there may be some way to avoid that mode alltogheter.

[zyuan]

Hi, @ingalopez,

Thank you very much. I really appreciate your advise and tips here.

You are right that I probably should try to add a resistor between the feedback ping 2 of FAN7554 and phototransistor of the opotocoupler, to bring up the feedback voltage above 0.3V or somewhere near that, to keep the PWM controller ON for the most of time. It may take some time for me to test that, as I need to get a backup PSU first, in case I damage this one.

Thank you again.