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would it be possible design any psu without any electrolytic capacitors?
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Berni:

--- Quote from: jesuscf on March 02, 2020, 04:55:46 am ---Once I saw a 7500VDC @ 20kW power supply built with a three phase rectifier (made of many smallish diodes connected in series and parallel).  It didn't have any capacitors.

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Yep uses the trick described by Circlotron up there earlier https://www.eevblog.com/forum/chat/would-it-be-possible-design-any-psu-without-any-electrolytic-capacitors/msg2938666/#msg2938666

The catch is that you don't always have 3 phase power available to you. Some houses don't even have the street itself wired for 3 phase power.

Some trains also use DC power. Back before semiconductors the massive amounts of DC current needed ware delivered by mercury arc rectifier tubes in sizes larger than a refrigerator. Its basically a giant glorified neon bulb where the combination of mercury and carbon electrodes makes electron emission in one direction happen much easier, creating a diode. To make it multiphase simply multiple carbon electrodes are inserted making these things very common with 3 or 6 anodes (6 is for full wave rectification)
dietert1:
Usually the larger electrolytic capacitors last longer. If you replace the small ones up to 100 uF by ceramic multilayer caps or film caps, the device will probably last some decades. Recently i recapped a Keithley 148 nanovoltmeter made in 1965, so that's 55 years now. It had many 100 uF electrolytics (Sprague) that were all in good condition. One 22 uF cap was dry with an ESR of 68 Ohms. This is a low power instrument running cool inside, at least most of it.

The main enemy of electrolytic capacitors are close-by power resistors running at several hundred °C. I mean the durability of a power supply is a matter of thermal design. This is well known to those sad engineers who need to implement premature failure for commercial reasons.

Regards, Dieter
David Hess:
It is completely feasible to design power supplies and regulators which require no electrolytic capacitors except in the case of a single phase AC input where the minimum capacitance is determined by the required holdup time which results in large values of capacitance.  And even there, it can be done but with a lower level of practicality; for instance, an inductive input filter could be used instead.

Military, aerospace, and high reliability (long life) power supplies and regulators are sometimes designed to use only ceramic, film, and tantalum capacitors that lack inherent wear-out mechanisms.


--- Quote from: twospoons on February 27, 2020, 04:57:03 am ---first stage would be a boost PFC to 400V bus. That lets you pull current over most of a mains cycle.
I'm going to say 50Hz system, so the bus cap needs to hold enough energy to last for 0.01 seconds.  Its actually even shorter than that thanks to the PFC, but for arguments sake I'll use 0.01s
For 750W output, thats 7.5J in 0.01s.
So lets store 10J for some margin. 10J @400V is 125uF.   Thats doable => 5 x ~25uF 500V film caps. At a cost of roughly 5 x the cost of electros
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An active PFC stage is a great way to lower the required input bulk capacitance by increasing the conduction angle however using it this way compromises the power factor.
SiliconWizard:

--- Quote from: dietert1 on March 02, 2020, 04:38:33 pm ---The main enemy of electrolytic capacitors are close-by power resistors running at several hundred °C. I mean the durability of a power supply is a matter of thermal design. This is well known to those sad engineers who need to implement premature failure for commercial reasons.

--- End quote ---

Yup. Electrolytic caps are very sensitive to temperature, which is a particular problem in power supplies.

And yes, sometimes this is used on purpose. My Samsung LCD TV (well, the power supply board) was exactly designed like this. There were a handful of electrolytics that were mounted right against heatsinks. A couple of them went dry within a couple years. The fix was just a few bucks for new caps (which I tried to mount a little further away from the heatsinks), but this is irritating.
T3sl4co1l:

--- Quote from: David Hess on March 02, 2020, 04:55:09 pm ---An active PFC stage is a great way to lower the required input bulk capacitance by increasing the conduction angle however using it this way compromises the power factor.

--- End quote ---

You can get the best of both worlds using an active capacitor multiplier -- essentially, use a smaller capacitor at a higher ripple fraction, by application of a synchronous buck converter (controlled to act as, I think, a step-up transformer relative to a virtual midpoint).

In combination with poled ceramics (e.g. Ceralink), this enables record low volumes (e.g. Google Little Box inverter from some years ago).

Tim
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