discontinuous flyback question - Page 1
Forum section: electronics / projects

author: Colt45
author_rank: Contributor
timestamp: July 08, 2022, 06:42:02 am
content: I made a small flyback with UC3843, 12V in, 220V out at piddly current. Turns ratio maybe 6:1, 20 or 30uH primary, peak switch current is around 4A. This is at a little more than I'd ever expect to need, worst case... 30mA / 6.x watts load.In the past I don't remember the idle.. free resonant..? part trying to swing below zero? (and showing up as a dip on primary switch, too) What would cause that?top trace is the clock, next switch current sense resistor (after filter), and bottom is switch collector / primary low side

author: moffy
author_rank: Super Contributor
timestamp: July 08, 2022, 07:15:34 am
content: If you are using a MOSFET with parasitic diode then the diode looks to be clamping the negative swing. You have primary/secondary inductance interacting with a capacitor of some sort giving your LC oscillation.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 14, 2022, 03:49:47 pm
content: What's the rectifier diode?This happens at high enough voltage ratios, when there's little enough damping that the free ringdown simply touches GND. If the diode is ideal, then it starts at an initial voltage of Vpk-Vin, and swings to somewhat less than 2Vin-Vpk, due to loss during the 1/2 cycle getting there. It's also less because Coss rises at low voltage, slowing the ringdown waveform locally, around the negative peaks.If the diode is nonideal, then its own capacitance (also being larger near Vr ~ 0) slows the initial descent, and if it has recovery, then ringdown is delayed until recovery is complete, at which point some extra current I = (Vpk-Vin) t_rr / L (for I, Vpk, L referred consistently -- all with respect to primary side, or secondary side, as the case may be) is drawn, basically starting further along the curve (i.e. below the cos(0) peak) with higher amplitude.So a combination of those is going on here, probably.Does absolutely nothing of course; MOSFETs always have a body diode* so it just slaps into that, the excess current discharges into the supply, and it rings down with a peak amplitude of Vin (again, give or take Coss nonlinearity).*Or if you want to be precise: DMOS always does, and SOI FETs still have a diode-like characteristic for Vds < -Vgs(th) (for Vgs(off) = 0). Which is the case for most (all?) GaN FETs for example.In other matters, that turn-off ringing looks pretty mild; must be a fairly nice transformer?Tim

author: Picuino
author_rank: Super Contributor
timestamp: July 14, 2022, 06:16:28 pm
content: The oscillation is produced by the parasitic capacitances in parallel with the transformer inductance.Below zero volts is clipped by the mosfet diode.Can you post the circuit?Maybe a Snubber network (R-C in parallel with primary) helps to reduce these oscillations.https://www.ti.com/lit/ml/slup254/slup254.pdf

author: TimNJ
author_rank: Super Contributor
timestamp: July 14, 2022, 06:20:10 pm
content: If I understand your question correctly...Once the magnetizing inductance is de-energized, it free oscillates with Cds/Coss (as you show). The ringing/oscillation is centered around Vin (12V, in your case). Depends on what you've made before, but let's say you designed a 160V -> 12V converter before (as might be typical for 120Vac input). In this case, the Lm + Cds ringing will be centered around 160Vdc, so from a really basic perspective, 160V is (relatively speaking) much farther away from 0V, compared to 12V. Of course, there are other factors which influence the amplitude/depth of the oscillation itself (not the average DC level), which is related to, in some way, the damping/Q of that LC circuit.

author: Picuino
author_rank: Super Contributor
timestamp: July 14, 2022, 06:36:17 pm
content: From what you have commented and what is seen in the graphs:Vin = 12VV_primary_peak = 48V (32V over 12V of primary power supply)Transformer ratio = 1:6V_secondary_peak = -72V ... +216VV_ringing_amplitude_peak = 48V - 12V = 36VIf the mosfet diode were not present, it would go down to -24V (12V - 36V).Edit: I modify some values to bring them more in line with reality.

author: Colt45
author_rank: Contributor
timestamp: July 15, 2022, 12:28:03 am
content: The output rect is I think UF4007 or 8, into 2.2 or 4.7u electro // 0.1u film.Transformer is a dinky little EP13 core, 1 layer of ... #28? for primary, and 4 layers of #32? for secondary, stacked up S-S-P-S-S. I think it's TDK 'PC40', with Al like 50nh/t or so (the PN is supposed to be AL=100, but I don't have any ungapped halves).Efficiency seems to be about 80% at 40mA / 215V, which is about as far as it goes before getting into gnarly CCM. I should only need maybe 15 or 20mA in practice.I'll have to look at it 'as built' before I post the schematic, I had to mangle some parts onto it (snubber and rtct mostly, ohh and the compensation).It makes sense about the voltage ratio, Most of the SMPS I worked with before were 48V down to 5 or 12.

author: Colt45
author_rank: Contributor
timestamp: July 15, 2022, 04:59:58 am
content: Ok, this should be reasonably accurate rendition:The snubber is whatever with no real thought... surely didn't mean to put a whole 220nF there but that is apparently what I installed.Ct is actually 1220pF, just found another cap stuck underneath..

author: Picuino
author_rank: Super Contributor
timestamp: July 15, 2022, 07:31:53 am
content: https://www.ti.com/lit/ds/symlink/uc3843.pdfhttps://www.vishay.com/docs/86040/byv26.pdfhttps://tz.micro-semiconductor.hk/datasheet/a6-BUK9575-100A,127.pdf

author: Picuino
author_rank: Super Contributor
timestamp: July 15, 2022, 07:42:26 am
content: To find answers to the problem you can search the internet for "discontinuous flyback ringing"https://www.google.com/search?q=discontinuous+flyback+ringingYou can change the transformer ratio to 1:12 (adding turns to secundary) to reduce the amplitude of the oscillations.The new voltages will be:Vin = 12VTransformer ratio = 1:12V_primary_peak = 12 + 220/12 = 12V + 18.3V = 30.3VV_secondary_peak = -144V ... +220VDiode byv26d peak = 364VV_ringing_amplitude_peak = 18.3V

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 15, 2022, 03:46:07 pm
content: Quote from: Colt45 on July 15, 2022, 04:59:58 amHeh... phasing is wrong. (But scope obviously shows it's right so that's just a typo.)Tim