PMP40988: What is the purpose of bridge diodes

Hi experts,

I am a colleague of O.H. I have a comment and additional question on this thread. In the totem pole PFC topology, the input diode bridge is not required. However, in TI's reference design, the input diode bridge is probably added for the following reasons.

1. Start-up stability:
In the totem pole PFC, power supply may be required before switching begins at startup. At this time, the input diode bridge can act as an initial power supply path. This allows stable power to be supplied at startup and allows for a smooth transition to switching operation.

2. Protection during abnormal conditions:
In abnormal conditions, if the totem pole PFC switch fails or if some problem occurs in the control circuit, the diode bridge can provide a current path instead, preventing the power supply to the load from being completely cut off. This improves the reliability of the system.

3. Resistance to transient voltage fluctuations:
If a transient voltage fluctuation (such as a surge or spike) occurs on the input side, there is a risk that the switching element will not operate normally. In such cases, the diode bridge performs a transient protection function and can minimize damage to the circuit.

Question 1.
Am I correct in my understanding of points 1 to 3 above as to why a diode bridge is inserted? Please let me know if you have any additional comments or suggestions.

Question 2.
I would like to know more about cases in which the input diode bridge is omitted. If a high-efficiency design is the top priority, or if the circuit is used under the assumption that it will operate stably, is it possible to omit the input diode bridge? Please tell me what to pay attention to when omitting the diode bridge (such as measures to take in the event of an abnormality or transient situation)

Thanks,

Conor