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Power Supply Smoothing without Capacitor?

Power Supply Smoothing without Capacitor?

Power Supply Smoothing without Capacitor?

I posed this question to some of my electronics engineering colleagues and they couldn't really offer a good answer. Can anyone theorize what might be a possible way for smoothing a rectified AC waveform to DC without the use a capacitor?? Basically how can this be achieved with the use of conventional electronics components without a capacitor?

RE: Power Supply Smoothing without Capacitor?

Some regulators could probably pull it off by using an accompanying inductor. I know I've seen LED drivers that boasted "no capacitors needed".

Keith Cress
kcress - http://www.flaminsystems.com

RE: Power Supply Smoothing without Capacitor?

The two classes of storage components are capacitors and inductors so you're limited to one or the other of them. I guess you could argue batteries also qualify?

RE: Power Supply Smoothing without Capacitor?

The Cuk DC-DC converter uses input/output inductive coupling to kill output ripple, although there are capacitive elements within the converter: https://en.wikipedia.org/wiki/%C4%86uk_converter.

Otherwise, there is no there, there. Even assuming you could come up with a flattened output voltage without storage elements, the output would droop under load, since it's the storage elements that not only reduce ripple, but also redistribute the available power throughout the AC cycle so that the output voltage doesn't droop under load.

I can do absolutely anything. I'm an expert!
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers

RE: Power Supply Smoothing without Capacitor?

Once upon a time, a colleague was designing a power supply. 115 VAC 3-Phase 400 Hz to DC (with DC-DC converters). His analysis indicated that the 3-phase power - full wave rectified - wouldn't require filter capacitors before the DC-DC converters. I told that I'd never seen it done without capacitors. But it went ahead to prototype. Didn't work. Capacitors were required, and added.

Motor-Generators (AC in, DC out) don't need capacitors.

RE: Power Supply Smoothing without Capacitor?

Feed a bunch of frequncies from a bunch of frequency doubling circuits and use diodes to combine them. I think it is doable but a lot of work to get Around using a smoothing cap.

RE: Power Supply Smoothing without Capacitor?

For a more realistic application the question might be: "How can I reduce the size of the filter capacitor in a AC/DC power converter?" Some in the industry use active filtering to smooth the ripple voltage of an output capacitor or a DC link cap. Instead of using a huge aluminum electrolytic to passively filter the ripple, a buck/boost circuit is used to charge and discharge a much smaller cap to a large voltage and back to the rail voltage at 2X line line freq. CE+T famously did this recently to win the google littlebox challenge. They showed a photo of one of their small SMT caps which took the place of a huge electrolytic.

Darrell Hambley P.E.
SENTEK Engineering, LLC

RE: Power Supply Smoothing without Capacitor?

On Hamburger's point... motherboards with the larger processors use multi-phase power supply chips to provide a smoother rail with a reduction in smoothing cap size. 8- and 16-phase supplies are pretty common these days.

Dan - Owner

RE: Power Supply Smoothing without Capacitor?

That is correct MG2000. But in such applications, the AC/DC part is already done with and a nice DC is available to the switcher. So, the multiphase technology doesn't really help in an AC/DC PSU.

VE1BLL has an interesting point in saying that a motor-generator doesn't need a capacitor. It is correct, in a way, but not correct if you need a low DC source impedance. You still need capacitors to take care of switching transients.

Another interesting point is that the equivalent diagram for a (PM) DC motor is the same as for a capacitor. And the same differential equations apply.

Gunnar Englund
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Power Supply Smoothing without Capacitor?

This is a very open ended question with no specification on the AC source or DC power required.

You get a reasonably low ripple DC output without filter capacitors that is perfectly suitable for many uses when you run 3-phase through a phase shifting transformer to a 18-pulse or 24-pulse rectifier.

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