Hi,
here is the schematic that I drew for the part of the PSU i'm trying to troubleshoot: (click to open larger image)
![](http://jfsimon.net/forum/corsair/schem2.jpg)
so (not unexpectedly) this is very similar to the reference schematic provided by Fairchild in the FSQ0165R datasheet that I attached above.
I have a very large ripple at the output, so large that it's not even ripple anymore, it's more like a sawtooth waveform, at a low frequency (around 300 to 600 Hz) compared to the switching frequency (around 57 kHz). I didn't take new scope screenshots, but it's the same shape as previously attached, and the amplitude of the sawtooth can be even more ridiculous, more than 3 V at full load. I'm pretty sure that the IC shuts down several hundred times a second, which makes audible noise, because one of its protections triggers. But I don't know which one... According to the datasheet (see above), it has Overload Protection (OLP), Over-Voltage Protection (OVP), Abnormal Over-Current Protection (AOCP), Internal Thermal Shutdown (TSD).
I've tested a vast majority of the components on the board *** and couldn't find a culprit, so either I'm not looking hard enough, or my measurements are not trustworthy, or both...
I should mention two more things:
- this is voltage dependent. I lifted pin 2 of the IC and brought power directly to it with a variable bench power supply. When the supply voltage of the IC is lower than normal (around 9 to 10 volts instead of 15), the switching starts correctly and the output is regulated correctly at 5 volts. Then I tested the current capacity with a variable load, and the supply could deliver 2.5 A @5 V as specified. So could this be the OVP triggering ? The IC is monitoring the voltage on the "sync" line and triggers OVP at 6 volts. I carefully looked at the waveform on this pin and couldn't see it going above 6 V, but I'm not sure, my scope doesn't have storage and my eyes aren't fast enough to record transients...
When I crank up the supply voltage, the ripple is rapidly increasing and the current capability of the supply decreases. at around 11.5 V I can still draw 2.5 A, but there is a strong audible noise (which is not present when Vcc is lower). At 15 V, the PSU shuts down at 1 A.
- this is also load dependent. When the PSU is started with a low voltage supplied to the control IC as described above, I've noted that:
- at no load, the low frequency ripple disappears, I have a smooth 5 V (with a small amount of 57 kHz ripple which is fine)
- when I crank up the current, the low frequency ripple (and sound) comes back, but its amplitude and frequency are increasing with load.
- at some point when the load is "enough" (that "enough" depends on the Vcc of the control IC), then the low frequency ripple disappears and the output is clean again.
I'd be very glad if someone could point me in the right direction, or give any advice that could help me to find what's going on! Maybe tell what component you would suspect first, or maybe give me tips to find out whether it's really one of the protection modes of the IC that triggers or not.
Thank you again !
*** : tested OK and/or swapped with new ones to see if it changed anything, with no luck (but will happily test again if you smell something dodgy!)
U101
U102
C122, C123, C104
D121, D103, D102
D101, ZD102
+ all resistors