This reference design does not use bulk capacitors at the rectifier output. It uses small ceramic capacitors. On the bright side, I like it because it eliminates the reliability issues of electrolytic caps with time. But the ripple should be big. How do I decide if I can get away with no using bulk input capacitors and if my circuit can tolerate the high ripple? Can flyback design like this inherently tolerate it?
3 Answers
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If you have a 3-phase power source with a neutral (as in your example) and a full-wave rectifier you don't need reservoir capacitors at all for a switching power supply that can function with a smallish ripple voltage. So a relatively tiny bypass capacitor is sufficient. Graphic from here:
If you have a single phase power source you need to keep the capacitor voltage high enough for the SMPS to operate even during the troughs of the input voltage, so you need a relatively large capacitor and a ceramic part may be impractical.
Although ceramic capacitors have less in the way of a defined lifetime, they are not necessarily more reliable especially when you try to make something relatively huge. They also have unpleasant voltage coefficients, temperature coefficients and aging that mean you can't take the claimed capacitance at face value. I've also seen a number of infant failures in large value (very expensive) ceramic capacitors.
answered Apr 7, 2020 at 16:28
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The keyword is 3-phase mains input. If you rectify 3-phase mains, there will always be enough DC voltage at the output of the rectifier, even without capacitors. The ripple will be big, and the small capacitors are for high frequency bypass.
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One key thing to note here is that this design uses a three-phase input into a three-phase rectifier. This has substantially less ripple already coming out of the rectifier than a single-phase rectifier; in fact, the output of a polyphase rectifier never goes to zero (while appropriate balanced polyphase input is supplied, anyway).
Take a look at this image:
(image source: https://commons.wikimedia.org/wiki/File:DC_voltage_profile_of_B6_three-phase_full-wave_rectifier.jpg)
Notice how the rectified voltage (solid red plot at the top) never goes below a bit more than the peak input voltage. Since this never goes to zero, no smoothing capacitors are needed at all, as long as you have enough headroom for your DC-DC converter or linear regulator to operate.
