LM5180: Timing of MOSFET

Hi

 there is 3 working mode for LM5180 which describe in datasheet section 8.3.2. it has FFM, DCM and BCM.

for DCM  , the mosfet turn on by internal clock.

in BCM, the mosfet turn on after certain delay after the transformer current go to 0

in FFM, the mosfet turn on by internal clock, but internal clock will change by the COMP which can adjust the output voltage by frequency.

Thanks

Hi Daniel,

I have forwarded your response to the customer and here is his feedback.

Thank you so much for your information. I think in my case the LM5180 works in the BCM mode. As you said the MOSFET turns on after certain delay after the transformer current goes to zero, but I have two questions about this:

1) how the LM5180 know when the transformer will go to zero? It will detect the MOSFET current, but after the MOSFET is turned off, the current flowing through the MOSFET is zero. Then, the LM5180 cannot directly measure the transformer current.

2) You said the MOSFET turns on after a certain delay after the transformer current goes to zero. Is this delay constant or not?

The reason I want to know these is that the LM5180 does not work well, and I found that sometimes it is turned on just after it is turned off for 1-2us, and at that moment the transformer current has not go to zero. After it is turned on, the transformer current will hit the limit and overcurrent protection will be triggered.

Regards,

Danilo

HI

1.  when the current of diode go to 0, the SW voltage will falling down due to the parasitic capacitor resonant with inductance of transformer. so that it can be sensed by chip 

2. it is constant, and you can even ignore it because it is very short , it is not impact the BCM performance 

Thanks