Hello,
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
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