Measures to prevent tube explosion of fast recovery diode — SUNMATE

After the fast recovery diode breaks down, it is equivalent to that the output capacitor is connected in parallel to the secondary of the transformer. This breakdown is essentially different from the output short circuit. After the rectifier breaks down, it is equivalent to short circuiting the secondary side of the transformer. The most intuitive thing is that the inductance of the primary side is gone (except for the very small leakage inductance). At this time, the VCC of the IC still exists.

Therefore, the primary side MOS will blow up within one cycle. When the input voltage is relatively high, the higher the VCC residual voltage, the longer the VCC maintenance time, and the greater the short-circuit current when the secondary side is short circuited. When the input voltage is low, the VCC voltage is relatively lower, and the VCC falls below UVLO faster.

This is secondary. The most important reason is that the current flowing through the MOS decreases at this time, so it is not necessary to blow up. When the output is short circuited, the output capacitor is short circuited, and the inductance at the primary side of the transformer still exists. At this time, the IC will pull the duty to a larger position, which will cause the primary side current of the transformer to rise to trigger iSense protection in one to several cycles. Therefore, if the design is appropriate, it will not blow up.