Flyback converter instability - Page 1
Forum section: electronics / projects

author: thaumatichthys
author_rank: Newbie
timestamp: August 08, 2024, 05:34:39 am
content: I am trying to make a nixie tube power supply using the LM5155 as a non isolated flyback.I should say that I don't really know what I am doing when it comes to designing power supplies, and I got the compensation resistor and capacitor values from here:https://github.com/jmwilson/nixie-flyback/blob/master/script/dcm_compensator.pySpecifications I am trying to get:9-15V in,170V out,90mA out,Fswitch 110kHz,DCM operationIt seems to regulate the voltage sort of okay, but the duty cycle is not constant and seems to oscillate. The oscilloscope picture below is with a 50k resistor load, and powered by a 9V battery.At first I thought this was a slope compensation thing, so I added a 1k external slope comp resistor, R10. This didn't really change anything.My other thought was maybe since I built the circuit on a 1 layer pcb, the lack of a proper ground plane was causing issues, although I don't know how I would test that yet.The transformer core is 2 * B66311G0500X127, with 8 turns of primary and around 80 turns of secondary, making about 4uH primary, and if I am not mistaken, should saturate at about 20 amps.Does anyone know what is going on?

author: moffy
author_rank: Super Contributor
timestamp: August 08, 2024, 06:18:37 am
content: My guess is a light load and not enough output capacitance producing some jitter.

author: fourtytwo42
author_rank: Super Contributor
timestamp: August 08, 2024, 06:48:22 am
content: Could be compensation, what is the value of C6 ?

author: thaumatichthys
author_rank: Newbie
timestamp: August 08, 2024, 07:27:30 am
content: I didn't install anything for C6.I have tried loading it with 8x100k resistors in parallel, and it appears to still be kind of okay? but not great still. Idk but it seems that it has a hard time starting up, the output voltage looks extremely terribly noisy for about 200ms, and then it seems to be fine after. But when I probe the mosfet gate, it seems to pulse about four times correctly, then it skips one, then repeats.Could it be that its going into CCM on startup? I'm not sure if that would have such a big effect thoughI put the waveforms of the startup and the gate drive + AC coupled output voltage

author: moffy
author_rank: Super Contributor
timestamp: August 08, 2024, 07:47:49 am
content: It skips a pulse everytime the output maximises, then it has to wait for the output voltage to drop, skipping pulses on light loads can be quite common.

author: thaumatichthys
author_rank: Newbie
timestamp: August 08, 2024, 08:11:18 am
content: oh interesting, I didn't see before that it actually does mention this on the datasheet, thanks for bringing this up

author: TimNJ
author_rank: Super Contributor
timestamp: August 08, 2024, 06:55:17 pm
content: Try increasing C2 and/or reducing R2. I am not sure what a suitable amount of output capacitance is at 160V/15W, but a 0.47uF MLCC seems pretty small to me. The super low ESR of the MLCC could also be tricky. Try a ~10uF electrolytic and see if that helps.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 08, 2024, 07:13:04 pm
content: L is too large or Fsw is too high -- this is a normal side effect of CCM for a peak current control, it's subharmonic oscillation or chaos.Slope compensation helps, but only to extend stable operation to somewhat lower ripple fraction (ratio of Ipp to Idc).Tim

author: thaumatichthys
author_rank: Newbie
timestamp: August 09, 2024, 03:06:50 am
content: I changed C1 to a 2.2uF MLCC to make it start much slower, and that removed some of the chaos at startup. I also replaced the output MLCC with a 10uF 200V electrolytic, but I don't know if that improved it.I think if I use a higher input voltage (>12V), the output starts to oscillate at about 3.7Hz. Anyone know why it does this?edit: I added a 10nF MLCC in in parallel with C2, and it seemed to stop the oscillations.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 09, 2024, 04:06:03 am
content: Low frequency is control oscillation -- tweak values of C2, R2, and maybe try small C6.In particular, C2 should be increased roughly in proportion to C4. Note that C4 ceramic is a bad idea as they lose substantial value under bias (look up the characteristic curves for the part, or of comparable types).A combination of ceramic or film, and electrolytic, gives large value and low ESR at high frequencies.Tim

author: youngda9
author_rank: Regular Contributor
timestamp: August 09, 2024, 02:07:59 pm
content: Do you have the ability to perform a Bode plot test on it?

author: youngda9
author_rank: Regular Contributor
timestamp: August 09, 2024, 02:10:21 pm
content: Your post with the load on it...three pulses ratchet up the output voltage. Then it shuts off for a cycle. Seems that it needs more load since the min duty-cycle isn't sufficient. You can also try looking at the current feedback signal & filtering. Load capacitance may need to be increased as well.