Question One:Does the leakage inductance of a transformer only affect the voltage between the DS terminals of the MOSFET when the MOSFET is turned off?

When the MOSFET is turned off, the leakage inductance can generate a voltage spike. I would like to ask, does this leakage inductance not affect the MOSFET when it is turned on?

Question Two: MOSFET Reverse Conduction Issue

Taking the circuit in the diagram as an example, when I apply a constant voltage source between the gate and source, so that Ugs > Uth, and then Uac is an AC source, with a waveform as shown on the right.

1. So, when Uac crosses zero, will the MOSFET's D->S channel turn off?

2. When Uac is in the negative half-cycle, what is the conduction situation of the MOSFET? Does it first conduct through the reverse-parallel diode and then through the S->D channel?

Previous Answers:

Previous Answer One: One high-conducting and one low-conducting, the second type, the VBAT current will still flow to the IO port, through the 100k.

Previous Answer Two: There are many problems with the short circuit of the secondary diode, such as when the secondary diode is shorted, it is equivalent to a positive excited transformer. At this time, the output voltage is generally very high, and the capacitor may not withstand the voltage and explode.

As for exploding the board, it is due to the lack of magnetic reset, which leads to magnetic saturation when conducting for the second time, which is equivalent to a short circuit from the MOS to the ground, and the board will definitely explode. So, if the secondary diode is shorted, and without load, or with a load of about 1W, the change in magnetic flux density is also very small, in which case the board generally will not explode.

If the secondary diode is not shorted in the problem, and the output terminal is shorted, then it is a flyback power supply, and there is no magnetic reset problem, so it will be protected.