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a question for FB phase shift converter operational waveform in application note (U-136A)

h scott
Prodigy 10 points
Other Parts Discussed in Thread: UC3875

Hello,

After read the application note(U-136A PHASE SHIFTED, ZERO VOLTAGE TRANSITION DESIGN CONSIDERATIONS and the UC3875 PWM CONTROLLER), I have some questions for the operational waveform at page 8 as below,

1.What do ID14, ID15,Va,Vb refer to? I think ID14 and ID15 just are ID1 and ID2? then

2.For the current waveform of ID14 and ID15, According to Ampere circuit law  I think the current exchange occur at t2,(i.e. when Ipri begin to decrease with slope Vin/Lleakage) and be end  at t4, furthermore, in Figure 12(left leg transition t2<t<t3) ,we can see D1 and D2 are conducting simultaneously. But in Figure 14, the current exchange begin at t3, Why?

Thanks and Best Regards,

 

 

PS. I want to attach a picture,but failed.