LT3751 flyback converter doesnt work in LTspice when leakage inductance added..why?
We are trying to design a Boundary Conduction Mode Flyback with an LT3751 controller.
We wish to charge a 4uF output capacitor up to 300V within 5ms. Vin = 12V.
The attached is the LTspice simulation of the LT3751 based flyback. For some reason it doesn?t work when the coupling in the flyback coupled inductor is made less than one. Eg, if we make K=0.995, then the flyback doesn?t work properly. Why is this?
The attached simulation works absolutely fine when the coupling factor, k, is put as one. Why doesn?t it work when we add the leakage inductance into the transformer?
(On page 20 to 21 of the LT3751 datasheet, it says some strange things about mitigating the leakage inductance in flyback converters. It only mentions using an RC snubber across the primary to mitigate leakage, whereas a flyback with several Amps of peak primary current would always need an RCD clamp. It also says that the leakage inductance could make the drain node ring below ground and thence damage the NFET. ?This is nonsense, after all, the NFET has an anti-parallel diode inside it. The datasheet states that a simple RC snubber should be placed across the primary to mitigate the leakage inductance voltage spike?..but says that the time constant of the RC used should be no more than 1us. That?s far too low for any flyback with more than a few Amps of primary current. Do you think that the LT3751 is a bit of a dodgy part?)
LT3751 datasheet:
http://cds.linear.com/docs/en/datasheet/3751fc.pdf
To quote a forum rule:
Apparently the controller isn't prepared to deal with the oscillating waveforms you get in your flyback circuit when enabling leakage inductance. The waveforms are not specifically related to LT3751 operation, but it seems to react on it in an unwanted way.
If you can't get rid of the oscillation by tuning the transformer circuit, you should ask LTC support if they know a way to change the controller behavior in this regard.
I believe the issue needs deeper investigation and is a bit beyond the level of a usual forum thread. And the LT3751 related part is really specific.
Thanks, by “it doesn’t work”, I mean that as soon as leakage inductance is added to the simulation , there is severe staircasing of the flyback primary current at startup, and the current goes up to 55A instead of being limited to 18A, as it should be by the source sense resistor.
when running the simulation with k=1, it becomes obvious how badly its operating when k=0.995, say (k being the transformer c oupling coefficient)