As shown above, I designed a forward circuit on your main power design software.
The above waveform in the above figure is the voltage and current waveform of the rectifier tube D1, red is voltage and blue is current.
The below waveform in the above figure is the voltage and current waveform of the freewheeling tube D2, red is the voltage and blue is the current.
In the first stage, the green time period, the MOS tube is turned on, the rectifier D1 flows current, and the freewheeling tube D2 is turned off;
In the second stage, during the blue time period, the primary Nd winding is reset, the rectifier D1 is turned off, and the freewheeling tube D2 is turned on.
The problem is coming:
As shown in the simulation,
In the third stage, during the orange time period, the rectifier D1 is forward-conducting, but no current flows, the free-wheeling tube D2 is turned on, and all secondary current flows through the freewheeling tube D2.
Why is the rectifier D1 turned forward in this period, and the secondary winding Ns is reset, like a wire, but D1 does not divide the secondary current with D2? But the secondary current all flows through D2?
I simulated it with simplis and found that during the third time period after the magnetic reset is completed, that is, the orange time period, the rectifier D1 will have a small current flowing. There is no current flowing through in your TI software simulation.
Excuse me, where does this small current come from? Is the excitation current of the secondary freewheeling current to the transformer secondary Ns?
Blue is the primary MOS tube DS voltage waveform.
Green is the current waveform of the secondary freewheel D2.
Red is the current waveform of the secondary rectifier D1.
I don't know if I understand is right?
In the third period after the magnetic reset, the orange time period. The primary winding Np and the secondary winding Ns of the transformer are similar to short-circuited coils. At this time, the current released by the secondary filter inductor mainly flows through the freewheeling tube D2, and a small part of the current flows through the transformer secondary Ns and the rectifier D1. Due to this lost current, a small voltage is generated at the secondary Ns terminal. After the freewheeling tube D2 is turned on, the voltage drop at both ends is clamped at 0.7V, so only a small voltage can be generated on the secondary Ns, and only a little current flows through the rectifier D1, because if Ns is given If the current of the D1 branch is large, the voltage drop on Ns will become larger, and D1 will not be turned on.
