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Hi, I was reading a datasheet for DC1744A which is a demo board of LT3799 and this is the rectifying circuit of the board in their datasheet.
I am not sure the purpose of RL parallel circuit before the bridge rectifier
and also why they used the choke inductor (L3) after the rectifier not before. (if it is just a choke inductor of course)
The circuit drives a transformer with high frequency FET switch.
Could someone help me please?
Best Answer
Either the FCC or some regulatory committee requires that any EMI generated by a switching power supply be filtered so that it does not back-feed into the AC power lines. Hence the reason why L1 and L2 have substantial inductance. 3.3 mH is a lot of inductance, a brick-wall filter to block HF noise.
The parallel resistors help prevent the L1 and L2 cores from DC and LF saturation, in which they would behave like a short circuit. Remember they also have to handle the 50/60 HZ current passing through them to feed the power supply. To a 3.3 mH inductor 50/60 HZ passes right through them almost as easy as DC current.
As for L3 it offers the first barrier to the extreme switching noise produced by a SMPS type supply. I am used to seeing common mode inductors at this location but a single ended filter is good enough, since L1 and L2 filter out any remaining noise.
Not shown in the image are the large value capacitors by the MOSFET switch and transformer.
NOTE: I should point out that this design is atypical of SMPS design. No common mode filters and large 3.3 mH inductors with rather small value capacitors suggest the designer was just throwing parts together, or had SEVERE EXTERNAL noise to contend with. I do NOT recommend this design for SMPS supplies.