Boost Converter Oscillation Bursts During Load Step - Page 1
Forum section: electronics / projects

author: tinfever
author_rank: Regular Contributor
timestamp: February 18, 2025, 02:39:42 am
content: I’m designing a two-phase boost converter and I’m struggling with an oscillation issue I haven’t been able to completely solve. This converter is designed to boost from 18-34 V input to 60 V 16 A with only intermittent loading. It uses an LM5171 two-phase controller operating at 600 kHz.Initially, the issue presented as oscillations on the output voltage at around 20kHz that occurred when the converter was cold (you could hear the frequency changing as it warmed up until they went away) or under load. After trying almost everything I could think of, I managed to mostly resolve the issue by adding common-mode and differential mode filter caps (100pF and 10nF respectively) to the current shunt sense traces for the inner current control loop. However, I just discovered that there are still a few oscillations happening during a load-step and I’m running out of ideas on how to fix this. Doubling the added filter caps now made the issue worse, with about 5 oscillation cycles happening during a load step instead just 1-2.Given that adding the current shunt sense trace filter caps almost entirely fixed the issue, I suspect either I’m getting noise coupling into my current sense traces due to my layout, or there is noise coupling into the analog ground that I split from the power ground but made a large loop when doing so. Any other ideas? Is it possible for the inner current control loop to be marginally stable in a way that doesn’t show up in the bode plot of the voltage loop? If the issue is the inductance of my analog ground trace, could adding more filter caps to the current sense lines be causing more noise coupling into the analog ground and that's why adding more made the issue worse?Things I tried previously and the associated result:Adding input capacitance without damping. Oscillations became worse and were more rounded / sine wave shaped.Adding input capacitance with damping (either 100uF electrolytic or 47uF aluminum polymer with series resistor). Oscillations now only occurred at a higher load.Powering board from battery pack instead of bench PSU. Found that different input wire lengths change the oscillation frequency.Using a large resistor for load instead of electronic load. No change.Bode Plot Stability Analysis. Phase margin, gain margin, and crossover frequency all seem normal. 70+ degrees of phase marginChanging the current loop compensation from 10pF // (36.5k + 150pF) to 10pF // (36.5k + 10nF). This helped somewhat and made it more difficult to reproduce the issue.Changing the voltage loop compensation from 220pF // (4.02k + 22nF) to 220pF // (4.02k + 44nF). No change.Other things I’ve noticed:The voltage on the input caps seems to show the same oscillation but at a greater magnitude than the output voltage. I think oscillating current may be causing fluctuations in input voltage, which causes fluctuations in output voltage?A schematic is attached. Ignore where it says +75V. It's only configured for 60V output right now. I've also attached images of the board layout and rendering. It is a 6-layer 2oz copper board.Stackup:Layer 1 - Signals and Power PoursLayer 2 - GNDLayer 3 - Signals, gate drive traces, and Power PoursLayer 4 - GND, gate drive returnLayer 5 - Power PoursLayer 6 - GNDThe scope shots show a 0 to 3A load step at two different time scales.

author: tinfever
author_rank: Regular Contributor
timestamp: February 18, 2025, 02:41:16 am
content: More images. The one showing the oscillation here shows what the issue looked like before adding the current sense trace filter caps that partially fixed the issue.

author: mtwieg
author_rank: Frequent Contributor
timestamp: February 18, 2025, 04:37:54 am
content: I think you forgot to explain what signals are being shown in the scope captures.Have you looked into whether the oscillation is shared by the two phases? If you disable each phase (and halve the load current), does the behavior change? If it still happens with one phase, then debugging it will probably be easier like that.The fact that it seems to change depending on source impedance points to a generic loop stability issue, but I don't expect that to manifest in sawtooth waveforms which start and stop abruptly. Need to get a grip on what all parts of the system are doing during the oscillation (the COMPx pin(s), inductor current(s), output voltage, and FET driver(s)).

author: tinfever
author_rank: Regular Contributor
timestamp: February 18, 2025, 07:17:58 am
content: Quote from: mtwieg on February 18, 2025, 04:37:54 amThank you. The scope captures in the first post are showing a 0 to 3A load step where CH2 is the output voltage, CH3 is the switch node, and CH4 is the output current. The two captures are of the same waveform, just at different time scales. The capture in the second post is has Vin on CH1, Vout on CH2, and the switch node on CH3. I meant to actually upload the one I just attached though, which also has I_in on CH4.On this board because I tied the SS/DEM1 and 2 pins together, I can't just enable one phase without modification. I've could desolder and lift pin 9 for SS/DEM2 on controller and then only use EN1 to only run phase 1 though.I'll test with only one phase tomorrow and measure the COMPx pins too. Lifting the inductor and adding a wire for a current clamp would be pretty invasive but I can do that too if needed.

author: mtwieg
author_rank: Frequent Contributor
timestamp: February 18, 2025, 12:44:54 pm
content: Quote from: tinfever on February 18, 2025, 07:17:58 amThanks for the clarification. Output current isn't going to be very useful (especially if it's after the output bypass caps), input current is much better. Looks like there's some aggressive lowpass filtering on that trace (can't see any fsw ripple), might be best to remove that so we see the "real" waveform. Or maybe you're measuring the current via the IMON pin(s)? I'm guessing you're also showing both phases combined, which further reduces ripple.QuoteI see, SS/DEM pins tied together means disabling one effectively disables the other. I guess you could also try disabling a phase via the power circuitry (removing the FETs on one side, or removing the gate series resistors and adding gate pulldown resistors instead). But before disabling either I would check whether they are properly sharing current at all times (including on very short timescales, based on the expected bandwidth of your average current mode control loops).One observations from your waveform: during the oscillation, Vout seems to drop about 2V in 40us. Based on your schematic, the overall output capacitance is >100uF, which means the output current has to be around 2/40e-6*100e-6 = 5A. But you say this happens for a load current of just 3A? That doesn't add up. I'm guessing most of those output caps are class II MLCCs so their effective capacitance is lower, maybe that explains it. If not, then maybe the converter is actually sinking current from the output during the oscillation. Might even be some cross conduction involved...

author: youngda9
author_rank: Regular Contributor
timestamp: February 18, 2025, 03:01:19 pm
content: Seems to me that the design is very sensitive to input impedance. Looking at your schematic, for the amount of output power this design puts out (close to 1kW) as a boost converter, which demands high peak input current draw, you have very little on-board capacitance (57uF). You should probably have >200uF at that node. Try adding more bulk Capacitance.The power supply that you're using to supply this as well may not be up to the task. Have you tried a more robust power supply? Also make sure your input power cables are beefy and short, for low impedance (resistance and inductance).Looking at your Bode plot...you could increase your bandwidth closer to 10KHz. Try changing R63 to 36.5K (100mW dissipated) and R62 to 1K to increase the bandwidth.The current sense feedback: R17 is a very small value...have you tried increasing this for a higher signal level? I see that you copied the current sense feedback values from Figure 7-8 in the datasheet. I don't know what C49-C50 are doing, especially since R18-R19 are 1ohm.. I would remove these. A 1ohm resistor into a .01uF capacitor isn't a good setup..that is a 1us time constant which is much too large for a 600KHz switching converter...this will make your input current sensing very flat and not look much like a ramp that the PWM is looking for. This is a major reason why you're having load transient issues. I would change to 100ohm and 330pF as a starting value. You want to filter out the HF switching transition spike without suppressing the amplitude of the ramp signal.

author: tinfever
author_rank: Regular Contributor
timestamp: February 22, 2025, 01:53:48 am
content: Thank you all for the feedback. The suggestion by mtwieg to check the inductor current was a good one. I change the CFG resistor so the IMON outputs indicate the inductor current, and then reconfigured the components so each IMON output goes to a 40.2k resistor to ground, so the IMON outputs are no longer combined or low pass filtered either. The IMON signal for PH1 looks like the expected triangle wave. The IMON for PH2 looks very bizarre, essentially a sine wave. The attached scope shot shows this with CH1 (Yellow) = IMON2, CH2 (Green) = Vout, CH3 (Blue) = SW2, CH4 (Red) = IMON1. This is at no-load.I removed all of the previous filtering capacitors I had added on the current shunt signals, just in case I'd accidentally added a larger capacitor than expected, but removing these had no effect on the odd IMON2 signal. I measured the IMON2 signal directly on the pin of the controller and it was the same, so I think this is actually the signal coming from the controller. I don't know how this could be real though. The switch node on phase 2 looks completely normal and is 180 degrees out of phase with phase 1, so I'm quite sure that both phases are switching as expected.I think next I need to bite the bullet, lift the inductor, and add a wire loop for a current probe so I can confirm if this current waveform is real or not. Any other ideas?QuoteAt 75V DC Bias, those 10uF caps are rated for about 1.7uF each. The effective capacitance on the output should be ~17uF from the ceramics + 30 uF from the two aluminum polymer caps.Quote from: youngda9 on February 18, 2025, 03:01:19 pmI tried adding a 100uF electrolytic to the input previously with no impact. This issue happens with the board directly powered from an 8-cell LiPo battery pack with about 4 inches of wire length, so I don't think the issue is the input power supply.I'm not sure I follow the reasoning about 1 Ohm into a 10nF being too large of a time constant. I think the differential-mode cut off frequency should be 1/(2*pi*2*1R*10nF) = 7.96 MHz. The common-mode cutoff should be 1/(2*pi*1R*100pF) = 1.59 GHz ...so you're right that that part isn't doing anything at all.