Help with Isolated Flyback Switching - Page 1
Forum section: electronics / general

author: viggstable
author_rank: Contributor
timestamp: July 09, 2019, 07:22:59 pm
content: Attached is a picture of Vsw-Gnd of my isolated flyback converter. Im using the LT3574 and im trying to understand why the oscillations at the end of each switch period have such a large peak to peak swing compared with what the data sheet shows. Does this have to do with the Resonance of the primary side of the transformer and the capacitance of the Lt3574 switch?Sent from my iPhone using Tapatalk

author: ahbushnell
author_rank: Frequent Contributor
timestamp: July 09, 2019, 11:08:27 pm
content: Schematic???Specs on the system???

author: xavier60
author_rank: Super Contributor
timestamp: July 10, 2019, 12:28:32 am
content: Quote from: viggstable on July 09, 2019, 07:22:59 pmIt is due to resonance of the primary side of the transformer and all stray capacitances while operating in DCM and lightly loaded.What are you comparing to in the data PDF?Extra; The flyback pulse looks unusually well clamped with almost no overshoot.

author: viggstable
author_rank: Contributor
timestamp: July 10, 2019, 12:33:20 am
content: Vin= 24VVout= 12V x 3 isolated railIout(max)=50mA / RailSent from my iPhone using Tapatalk

author: viggstable
author_rank: Contributor
timestamp: July 10, 2019, 12:50:22 am
content: Quote from: xavier60 on July 10, 2019, 12:28:32 amYes, i was comparing my Measurement to what the Datasheet is showing (attached below). I was thinking that the switch to ground measurement looked very unusual.While the power supply is not failing i would like to ensure the design is released without any issues. This is the first time ive used an isolated Flyback Converter controller for one of my designs at work.Typically in the past i would have used 3 individual 1/4W supplies but they take up too much space and its not really as efficient as using a single supply

author: xavier60
author_rank: Super Contributor
timestamp: July 10, 2019, 02:44:46 am
content: Just a quick reply for now. The 140ns pulse is the the energy in the primary's leakage inductance being snubbed.Followed by brief ringing of the leakage inductance. For the rest of the trace, the core's energy is being discharged into the secondary side load.It doesn't show the point of total discharge followed by primary inductance ringing.

author: MagicSmoker
author_rank: Super Contributor
timestamp: July 10, 2019, 11:37:11 am
content: Quote from: viggstable on July 09, 2019, 07:22:59 pmOvershoot and ringing immediately after the switch turns off is from the resonant circuit formed between the lumped parasitic capacitances present at the switching node (dominated by the drain-source capacitance) and theleakageinductance of the transformer (that is, the inductance remaining across any one winding with all other windings shorted). This ringing should be relatively low in amplitude but high in frequency and therefore can be easily damped with an RC network across the switch or the transformer primary (latter is preferred).Larger amplitude but lower frequency ringing later on in the waveform occurs when the secondary diodes stop conducting and is the result of the resonant network formed between the lumped parasitic capacitances and themagnetizinginductance (that is, the inductance measured with all other windings open). It is very difficult to damp this oscillation because the resonant frequency tends to be quite close to the switching frequency, but it is often the thing that causes a flyback to miss EMC emissions compliance testing. Operating the flyback in boundary conduction mode (which necessarily involves variable frequency operation), using conventional PN output diodes (they have a lower junction capacitance than equivalent rated Schottkys), a switch with a higher voltage but lower current rating (again, less junction capacitance), an IGBT instead of a MOSFET, a BJT instead of either an IGBT or a MOSFET, and many other schemes that start to get more complex are all possible solutions.One solution that is especially popular at lower power levels - say, under 50W or so - is to use an RCD clamp across the primary of the transformer but intentionally use a slow "mains frequency" rectifier, like a 1N4007. The long recovery time basically shorts out the oscillations. It's a bit of a kludge in my opinion, but you can't really argue with its effectiveness or low cost.EDIT - corrected "especially power" to "especially popular"...

author: xavier60
author_rank: Super Contributor
timestamp: July 10, 2019, 11:53:35 am
content: The snubber also wastes some of the useful energy stored in the mutual inductance.Allowing the energy stored in the leakage inductance to develop a higher pulse voltage shortens the duration of the pulse, reducing the time that energy is also being drained from the mutual inductance.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 10, 2019, 12:54:04 pm
content: Easy, it's showing this part.The ringdown is the inductor and the diode + transistor capacitance. The initial value is Vpk, and the ringing proceeds from there.It is usually a low enough frequency not to be much of a concern with EMI, but you can dampen it with Cs = 3*(Cjo + Coss) and R = sqrt(L / (Cjo+Coss)).Tim

author: ocset
author_rank: Super Contributor
timestamp: July 14, 2019, 01:08:52 pm
content: Yes the ringing that you see is , as others have discussed, the normal resonance between “capacitance at the drain node” and the L(PRI)….i wouldn’t worry about that too much…in fact, I wouldn’t worry about it at all…..its good to see you’re in DCM as it avoids the bad snap-off of the secondary diode(s)…….(instead the sec diode just naturally turns off). This helps get rid of reverse recovery current…but then again, you have sec shottkys so wouldn’t really suffer that anyway…and your power is so lowI can see from your ringing, that your vin is approx. 24v or so.So your clamp capacitor voltage is around 26v....beware are you overvoltageing your c1?You look to be in deep DCM, and you could go a bit less into DCM…but then I appreciate that sometimes you have an offtheshelf core whatever that you pick that’s cheap and you just have to put up with deep dcm and do so because your power is so low.I hope at startup you dont overvoltage that schottky in the rcd clamp…because schottkys don’t like overvoltage spikes….also depending on your leakage inductance, your sec shcottkys may need rc snubbers to prevent overvoltage spikes on them.Also, i dont see any feedback regulation?...this would need to be isolated feedback.Are you going to use one of the sec coils for feedback?

author: viggstable
author_rank: Contributor
timestamp: July 14, 2019, 04:31:45 pm
content: Quote from: treez on July 14, 2019, 01:08:52 pmThank you All for Responses, thanks for pointing out the capacitor voltage i hadn’t realized. Also the input is a battery bus voltage which i tapped off of.Right now the main issue i am seeing with the supply is a Large Ripple Current through F1... It is a consequence of the boards primary application ....The FLYBACK supplies these:3 optos , 2 -555 timers, and 2- Gatedriver ICHowever the main application is to act as a Relay(high fet switches) between a 24V Lithion battery and a Fuel Cell which charges the battery. The Second Function is to Pulse the Stack (aka Ground Pulse the Fuel Cell).So when ever we pulse the stack (100ms every 15sec) there is a larger in-rush of current coming into the primary side of the supply. This was a major problem because the supply kept blowing, i needed to add a current limiting resistor on the inputto get rid of this current.to answer your other question...No, i am not using and feedback regulation, perhaps it maybe worth using the spare coil on the transformer to do so (according to what is shown in the app notes of the data-sheet of the lt3574)