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SMPS input capacitor and pi filter selection values

balasubramani r
Expert 1170 points
Other Parts Discussed in Thread: UCC28064, UCC256301, TIDA-010015

Hi There,

I have designed power supply based on Ucc28064 and Ucc256301 ic based in TIDA-010015 model. It is 600W power supply and need some input on choosing these filter components. But i want to get feedback based on the component values choosed in the circuit. Corresponding values are for you reference. 

Input- 85 to 265VAC

PFC circuit switching frequency minimum -40KHz

Efficiency expected- 94.5%

PF expected- 0.99

input current-7.6A

Please advise the selected components are good with respect to EMI/EMC . This components are used here for common mode and differential mode filter as per my knowledge. 

C6 formula used

Cf= half of c6= 1uF

Any advise/suggestion would be appreciated. 

  • 0
    TI__Guru 72635 points
    Hello,

    I reviewed your schematic you have a differential mode filter (FL1,C4 and C5) and a common mode filter (FL2, C3 and C9). I would suggest not adding the differential mode filter formed by C6, L2 and C7.

    In regards to your filter scheme typically the common filter would be first, followed by the differential mode filter. I would suggestions adding Y caps across both sides of the common mode filter.

    After the bridge rectifier you could add a small X capacitor of 0.33 nF. You just need to rate it and handle the current of the downstream PFC pre-regulator. I was looking for an application note on this and could not find one. However, equations 22 and 23 from the data sheet give the peak inductor current and inductor RMS currents. www.ti.com/.../ucc28064a.pdf

    You can estimate that the AC portion of the sum of these two currents will be filtered by the X cap. Please not that because of interleaving the inductor ripple current seen by the input capacitor should be cut in half. You should multiply by 1.5 to design for margin.

    Irms_input_cap roughly = ((ILRMSX0.5)^2- (Pout/Efficiency)^2)^0.5) *1.5

    Regards,

    Mike
  • 0 in reply to Mike O'
    Expert 1170 points

    Hi Mike,

    Thanks for your interest here.Please have a look for the changes suggested . 

    As im following reference design's differential mode filter (L2,C6,C7) as it, But here im tweaking some values like increasing power rating to 100W more. So the current ripple may expect more compare to the reference design (500W). 

    My question is how the Pi filter designed on what basic we need to do the calculation . Since using above calculation i couldnt meet the reference value provided in the reference design. Does SMPS have any specific formula for this filter considering EMI/EMC standard. As many of the power supply having 0.47UF as a fixed one in the pi filter and other values changes based on the current rating .

    Do comment what is the thumb rule for designing PI filter in the power supply.   

  • 0 in reply to balasubramani r
    TI__Guru 72635 points
    Hello,

    You should be able to scale the design by 100W and will most likely have to change the transformer, and inductors and capacitances based on higher peak current and RMS currents. The UCC256301 and UCC28064 excel design tools should help with these calculations and selecting the components. You can find these in the product folder under the tools section.

    The common and differential mode filters. You could start with common made and differential mode chokes that are rated for the new RMS currents. Once you have those you may have to fine tune C2,C8,C3 and C9 to pass EMI for common mode noise based on EMI results. This is a double pole LC filter that will attenuate noise at – 40dB per decade. The double pole occurs at fp = 1/(2*3.14*(L*C)^0.5). The differential mode choke I would set the double pole at 1/10 the switching frequency to start with.

    You could also check the website of the manufacturer of the common mode and differential mode chokes. They should have applications note on how to select and adjust the filter capacitors to meet EMI.

    Regards,

    Mike