How to calculate peak current value - flyback secondary - Page 1
Forum section: electronics / beginners

author: MacIntoshCZ
author_rank: Regular Contributor
timestamp: September 09, 2024, 08:43:35 pm
content: I have create simulation in Ltspice. If you look at waveform it starts aprox at 9mA and it rise by 9mA every other cycle. I get that it stores more energy than it need.I am just interested on why it rise its peak value every time -> How can i determine this peak current value? DC resistance is same for all cases and since its flyback its voltage rise up until current starts to flow.Thanks!

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 09, 2024, 08:49:50 pm
content: What do you mean doubles? It's not rising geometrically, but arithmetically i.e. constant addition. (Actually it is geometric, with the "constant addition" dropping cycle on cycle, but the time constant of that drop might be longer than you care about, i.e. the L/R time constant of the transformer.)C2 voltage is evidently quite low. Given how small L2 is, and the duty cycle, it will take some time to charge up. This is a CCM flyback.Tim

author: Benta
author_rank: Super Contributor
timestamp: September 09, 2024, 10:51:26 pm
content: "Given how small L2 is".To me, 1 mH is rather big at 50 kHz.But of course the current keeps increasing. It has nowhere else to go than into the cap, and the energy stored in the transformer core just keeps increasing. This simulation only works with an ideal transformer, otherwise it would have crashed early.

author: Konkedout
author_rank: Frequent Contributor
timestamp: September 10, 2024, 02:15:36 am
content: Based on the square root of the inductance ratio, your transformer turns ratio is 28:1. With 300V input you will have 8400V reverse across the diode. This analysis is simplified and probably optimistic. This is probably working with an ideal diode in your simulation.In real & practical terms this makes no sense. I doubt that even a silicon carbide schottky diode can handle that voltage, and any other diode which can do so will be much too slow to work in a continuous conduction mode flyback.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 10, 2024, 02:24:11 am
content: Large* yes

author: MacIntoshCZ
author_rank: Regular Contributor
timestamp: September 10, 2024, 05:25:10 am
content: Quote from: T3sl4co1l on September 09, 2024, 08:49:50 pmYou were to quick in reply, i have also noticed my mistake.Quote from: Benta on September 09, 2024, 10:51:26 pmI am interested in equation which will gave me those peak values. Equation which will give me "9mA" then "18mA" and so on according to energy in flux or others parameters i am missing, i dont understand why exactly "9mA" etc... Ive only setup 10m ohms series resistance in secondary winding. Cap itself in simulation has no ESR i guess.

author: MacIntoshCZ
author_rank: Regular Contributor
timestamp: September 10, 2024, 05:28:52 am
content: Maybe it will be helpful to add simulation file. Its just learning scenario for me to observe flyback behavior...

author: PGPG
author_rank: Frequent Contributor
timestamp: September 10, 2024, 07:49:53 am
content: Quote from: MacIntoshCZ on September 10, 2024, 05:25:10 amHave you noticed that step by step in your simulation:- step increase in current is about the same,- after that current fall is higher and higher.If you make your simulation longer state will stabilize.