Transformer-Rectifier-Filter Design

Discussion in 'The Projects Forum' started by apqo1, Dec 23, 2012.

  1. apqo1

    Thread Starter AAC Fanatic!

    Oct 5, 2008
    I'm working on the transformer-rectifier-filter stage of a distributed power supply system. It features two buck regulators at point-of-load, fed by a remotely mounted, common TRU. Reliability and longevity are more important than cost.

    Supply is 115VAC, 400Hz.

    Each buck regulator outputs 5.125V at 2.1A. Regulator board efficiency is ~80% as configured, so I've calculated ~27W total DC load.

    I've selected the Torotel C40PXNN 40VA-rated transformer and the Comchip GBU2510-G full-wave bridge rectifier.

    I would like to see Vin at the regulator boards of ~9-12VDC, after 15 feet of 22-ga 2-cond shielded wire.

    I'm pretty rusty at AC-related stuff, so I could use some help selecting the specific transformer model and capacitor(s). Here are my questions:

    1) What transformer secondary Vrms should I select? (Available options are ...8, 9, 10, 11, 12, 12.6, 13, 15, 16, 18, 20, 22, 24...)

    2) What capacitance value is appropriate?

    3) I know additional capacitance reduces ripple, but is there an associated penalty?

    4) What minimum capacitor working voltage is appropriate?

    5) What minimum capacitor ripple current rating is appropriate?

    6) I'm finding aluminum electrolytic capacitors with ESRs ranging from 9 to 60 mOhms. Do I need to be concerned with values at the upper end of this range?

    Many thanks!
    Last edited: Nov 25, 2013
  2. bountyhunter

    Well-Known Member

    Sep 7, 2009
    I'd use a higher vA rated transformer. Ther recommended derating for a FWB with capacitive filter is that the RMS transformer secondary rating should be about 1.8 X the DC load current, so VA rating should also be 1.8 times DC load power. You also have to allow for rectifier losses. The bucks will have non linear efficiency, which is to say losses increase at lower VIN typically. I'd do at least 50VA for transformer.
    Last edited: Dec 24, 2012
  3. bountyhunter

    Well-Known Member

    Sep 7, 2009
    Those are interactive variables. Smaller cap (more ripple) means you have to use higher transformer voltage.

    I think a rule of thumb is ripple can be maybe 10 - 15% rated output voltage, but may be higher if switchers tolerate the ripple well.

    Cap ESR is related to internal heating. That affects reliability and operating life. The switchers will be drawing high frequency peak currents so the filter caps will have to be able to supply that.

    You said:

    If this is a Hi-REL design, you should probably hire an experienced power supply designer to do it. There are tons of things to consider for this type of design..... like the fact aluminum electrolytics are INHERENTLY unreliable (short lived) and forbidden for use in most MIL or medical apps for HI rel. We were required to use MIL grade tantalums. Alums just don't belong in mission critical apps.

    The transformer spec:

    and the use of 400 Hz power made me assume this is a design for MIL or aircraft.
    Last edited: Dec 24, 2012