TI buck converter oscillation under light load - Page 1
Forum section: electronics / projects

author: tom66
author_rank: Super Contributor
timestamp: February 15, 2016, 10:23:19 pm
content: I'm working on a design using a TI TPS54331 controller. I've used and laid out this chip before. This is the first time I'm experiencing a rather unusual issue. Under zero or light load (<150mA) when the input voltage is close to the output voltage the converter oscillates - the output has approximately 2Vp-p of ripple on it at around 30-40Hz. Under load, this oscillation disappears, and if the load is then removed, the converter functions normally and doesn't oscillate until the input voltage drops significantly (below 12V) which "resets" the cycle. If the input voltage is about 14.5V or so, the oscillation never happens, it's only in the region of 13.5V ~ 14.5V when it will oscillate when it first starts.Basic parameters: the input voltage is 13.5V ~ 17V DC, output voltage is 12V and nominal load ranges from 10mA ~ 500mA with non continuous load up to 2.4A.I've tweaked the compensation network to no avail. I've increased the load capacitance on it to over 100nF which destroys the transient response, and yet it still oscillates. In place of the 10uH inductor, I've tried 13.3uH (10uH + 3.3uH in series) and 6.6uH (3.3uH x 2.) Changing the inductor causes the point at which the converter oscillates to change (a higher inductance causes it to occur later) but it doesn't seem to give me much play (only a few hundred mV either way.) I've also tried doubling the output capacitance which changes the frequency of oscillation, but again doesn't fix the issue.Schematic and board layout attached. The compensation components are mounted underneath the IC and attached with vias very close to the COMP and GND pins. I have not yet placed the charge pump components (so they can be excluded for now as part of the issue.) I calculated the original resistors incorrectly by using the wrong reference voltage; that is fixed on this board so the output is nominal 12.15V when working.I should note this is a university project (my portion of it: high power motor / power controller PCB & software.) The board is 4-layer; power and ground layers connect the controller, although all local current flows only on the top and bottom planes (they are connected to the main power and ground planes at only one point.)

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: February 16, 2016, 03:07:17 am
content: Quote from: AcHmed99 on February 16, 2016, 01:00:52 amTom,You can check this easily. Do a change to your feedback divider so the output is set to 11V or even 10.5V and see if the input voltage where the oscillation occurs shifts to a lower input voltage. You can also monitor the voltage on the Boost capacitor.For this application a part like the LT8610A can go to very duty-cycles (>99%).Link:http://cds.linear.com/docs/en/datasheet/8610abfa.pdfis useful.Regards,Jay_Diddy_B

author: tom66
author_rank: Super Contributor
timestamp: February 16, 2016, 09:27:40 am
content: Thanks guys.I did reduce the output voltage and the oscillation point did go lower (lower input voltage required to make it start oscillating.) Similarly for increased output voltage. However where I noticed the biggest change was increasing the softstart capacitor significantly from 8.2nF to 220nF. The rise time is now ~100ms but the converter never oscillates under any condition that is within reason (Vin > 12.4V essentially), and will actually hold 12.2V output under light load with just 12.4V in (so the maximum duty cycle is very much underspecified.) "Fixed!" Well... The rise time is quite slow; I'd like to improve it, this works OK for now but may prove a problem in the future.I wonder if when the converter was powering on, the input supply may have dropped too far causing the converter to get trapped into some kind of oscillation loop. This might just indicate I need more capacitance on the input, or a 100nF in parallel with the two 10uFs.Also, I fitted the charge pump components and it still works quite well. I was able to blow my (expensive HRC) 500mA meter fuse by shorting the output after the diode. d'oh.

author: AlexDavidson
author_rank: Contributor
timestamp: February 16, 2016, 09:21:17 pm
content: Section 8.4.1 of the data sheet for TPS54331 mentions that it operates in pulse skipping mode at light loads to improve efficiency. That could be the cause of the 2v ripple you see under 150 mA.Apart from using a much larger inductor, you would need to use a part that does not have this feature, or one where it can be disabled. For a comparison of ripple waveforms under light load between a pulse skipping converter and one that isn’t, have a look at the data sheet for MCP16311/2:http://ww1.microchip.com/downloads/en/DeviceDoc/20005255B.pdf

author: tom66
author_rank: Super Contributor
timestamp: February 17, 2016, 01:35:56 am
content: Quote from: AlexDavidson on February 16, 2016, 09:21:17 pmI am aware of pulse skipping and the controller does do this under low loads once out of this oscillation state. However, 2Vp-p ripple would be unacceptable in any instance, even with a pulse skipping converter. This is clearly not by design...

author: Alex Trofimov
author_rank: Contributor
timestamp: February 24, 2016, 05:56:32 pm
content: If I would experience sometheing like that, I would first of all check the two things: is loop compensation is correct, and isn't output cap's ESR too low (for example, if you use ceramic capacitor) - some converters may be unstable with very low ERS caps.