Series Resonant Converter using leakage inductance of transformer - Page 1
Forum section: electronics / general

author: Magnethicc
author_rank: Contributor
timestamp: May 02, 2023, 12:11:18 pm
content: Hi everyone,Does the Leakage of the transformer changes during the operation of an series resonant converter?I am not talking about transients but in the cycle - by - cycle.is there any reason the leakage inductance value will change from one switching cycle to another?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 02, 2023, 02:12:33 pm
content: Do you have any particular reason to suspect it would?What transformer design? Please show drawing and winding schedule.Magnetizing inductance can, due to core saturation; the inductance drops at current extremes. The change is (more or less) instantaneous, depending on current magnitude only; it is not stateful during a cycle, or between cycles.Typically very little if any leakage flux flows in the core, thus, not only is it free from saturation effects, but it's independent of core placement as well! This is most true for an interleaved or shell type winding, and less true for widely spaced windings.Arrangements such as sectoral windings on toroids (e.g. common mode chokes) have some effect from the core; the overall effect is both windings act in parallel (that is, as solenoids in the same direction), thus a large external field is created (flux leaking out of the assembly!), and the core acts like a wide rod core. (Imagine a rod core inductor, but the rod is very rectangular, but not actually a rectangle but a disc instead. So, instead of turns wrapping a long thin rod, it's turns wrapping sideways across a short wide rod.)Or the most obvious and explicit case: a split (banked) winding scheme, with magnetic shunts partially separating the windings. The cross-section of said shunts gives A_e for the leakage mode, and their air gap (to the limbs of the main core) is its l_e. Take a microwave oven transformer (the old iron-core kind, that is) for example: the shunts sit between two block windings, and aren't wide enough to handle the full flux of the core; they are also wrapped with insulation, which sets an air gap. Together, these determine the impedance and operating range: the output voltage have poor regulation, i.e. the voltage drops with load current, at a rate determined by the air gap. This works until the output voltage has dropped a certain amount, at which point the shunts begin to saturate, and load current increases: the V(I) output curve goes more constant-voltage at some point. This is perfectly fine for the application, as the magnetron acts like a zener diode -- it has a CV sort of characteristic, so needs some impedance limiting to drive it, but not over the full range (it'll never be a short circuit) so the shunts can be economized a bit.Tim

author: Magnethicc
author_rank: Contributor
timestamp: May 02, 2023, 06:19:03 pm
content: Transformer Design:PQ65/54Windings:Layer 1 - Primary - 2 turns of copper foil (32mm height x 0.8mm thickness)Layer 2 - Secondary - 1 turns of copper foil (same)Layer 3 - Primary - 2 turns of copper foil (same) (in series with Layer 1)Layer 4 - Secondary - 1 turn of copper foil (same) (in series with Layer 2)Turns ratio - 4 : 2The build-up of the transformer window kinda sucks tbh.I have reason to suspect so because when I view the primary current in the scope I see the resonant current but its shape changes as if the current "dances". I can see the tops of the sine current change shape. I will upload some scope shots tomorrow at work to let you all know what I see.