Stepping Down Voltage Most Effeciently (and various others questions) - Page 1
Forum section: electronics / beginners

author: TheDood
author_rank: Regular Contributor
timestamp: July 29, 2019, 02:43:50 am
content: TransformerX-rated CapacitorResistorPhase angle control(Any others not listed)Is a precision rectifier more effecient than a syncronous rectifier? Ie, what actually is the power dissipation of an ideal diode?? And is it designed to dissipate less power than an SR typically?In the case of an X-rated capacitor, is power dissipation equivalent to the [V^2 ÷ ESR], or [V^2 ÷ Capacitive resistance]?Also, what is the power dissipation when charging a capacitor in a DC cct?Why can't you PWM a full bridge with a power MOSFET? Is phase angle control more effecient? Zero point switching only has to turn components on twice per period, while PWM-ing a full bridge output would require a higher freq of switching compared to diac+triac, which in turn draws more power?How about a combination of different types of V regulation? ...(Phase angle control) + (SCR) ?(Precision Rectifier) + (Transformer) ?Interested in finding an effecient way to variably regulate voltage. Power dissipation generally goes up with lower load requirements and looking to learn and understand the ~% of power dissipated in various techniques of rectification and voltage regulation. I don't have a cct or schematic I'm currently working on, just looking to acquire knowledge to further my understanding. Thanks.

author: David Hess
author_rank: Super Contributor
timestamp: July 30, 2019, 08:42:54 pm
content: Quote from: TheDood on July 29, 2019, 02:43:50 amThey are effectively the same thing. Both replace a standard diode and have lower forward voltage drop yielding higher efficiency.QuoteMost of the loss comes from the ESR. Some comes from dielectric loss which can be significant at high voltages and high frequencies.QuoteSame as the above, ESR and dielectric loss, but the later will be insignificant albeit measurable in precision applications.QuoteYou can PWM a full bridge. Why wouldn't you be able to?QuotePhase control requires awfully big passive components for good efficiency.Resonate converter design applies just as much to transistors as it does to thyristors.QuoteThat is sometimes done but was more common in the past before power MOSFETs became better and less expensive. It still might be done in the highest power applications.The Tektronix 2215 series oscilloscope started out with thyristor phase control driving a linear regulator driving an inverter. Later the thyristor phase control was replaced with a MOSFET buck converter.QuoteEfficiency on switching converters drops at low load currents because fixed switching losses and quiescent current become a larger part of the total power loss. But that does not mean dissipated power is greater; it still drops.Switching losses can be lowered at low power by using multiple MOSFETs in parallel and switching fewer of them. Some PoL (Point of Load) regulators for high performance logic do this.Quiescent current losses can be lowered by burst mode operation or lower operating frequency at low output power.

author: TheDood
author_rank: Regular Contributor
timestamp: August 03, 2019, 05:02:57 pm
content: Does this look right (see attached)? The site gave a less than confident disclaimer.https://xtronics.com/wiki/Capacitors_and_ESR.html

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 04, 2019, 02:54:32 am
content: Note that, if it's truly theEquivalentSeries Resistance, DF is included.In general, ESR depends on frequency. Because well, it's an equivalent, eh?Its power is V^2/R but only the V dropped across the ESR, which you can't measure externally*. I^2*R is more practical.*I mean, you can, actually -- through the magic of complex numbers! But phase-sensitive voltmeters are unusual, and having an accurate enough phase null of the capacitive voltage is quite tricky indeed.I take it, the voltage you want to regulate, is AC mains, and the regulation shall be with respect to source and load variations (the full meaning of "regulation")?The simplest off-the-shelf (but probably second least efficient) option is: the ferroresonant transformer. Sola is the famous brand here, but there are others I think. A transformer is wired as an inductor coupled to a resonant tank; the coupling is through a ferromagnetic core, operated on the threshold of saturation. If voltage rises, saturation rises, reducing the transformer's gain. If load rises, saturation falls, increasing gain. One downside: saturation depends on flux (the product of voltage and time, or equivalently, the ratio of voltage and frequency), so while this works nicely on mains (that is amazingly consistently 50.000 / 60.000 Hz), it won't help all that much on a genset for example. These are inefficient because the core loss at saturation is quite high; a 1kVA transformer may dissipate 100W under most any condition, giving a maximum efficiency of 90%. Other benefits are the strong filtering of the resonant tank (removes EMI and surges), and the prevention of inrush currents (the output short-circuit current isn't much higher than the nominal rating).A variac with servo control is much more efficient, but slow, and, well, requires a servo controller, eh?Impedance dividers aren't really relevant, because there is no regulation as such. If you have a means of varying the impedances, you can achieve active regulation. This is feasible at radio frequencies (varactor diode), but harder to do at line frequencies. There is once again, the saturable reactor, or magnetic amplifier, which can be used with reasonable efficiency to create such a system. These were common back in the day, but are much heavier than modern alternatives like phase control. (Like the ferroresonant transformer, they are very robust, and can be made to filter and protect; a ferroresonant transformer is essentially a conveniently integrated example.)One of the "other" options you alluded to -- you can construct an AC/DC transformer with a buck or boost switching circuit. To handle AC, you need to use bidirectional switches, and the switching must be synchronous (you can't rely on catch diodes when switching off). The input and output filtering must use nonpolarized capacitors, and can only afford so many uF of them (without drawing too much reactive current at mains frequency). A typical case might be a 100kHz switching frequency, with the filters rolling off at say 10kHz, so the switching noise is well filtered while only requiring a few uF total. With MOSFETs, this can be made arbitrarily efficient, but some downsides are surge and inrush robustness, which semiconductors really just don't have, not by themselves. A typical solution would be, MOVs on the input and output, clamping surges to 1-1.5kV; switches using 1.2 or 1.7kV SiC MOSFETs; isolated gate drivers; and a current and voltage mode control, to limit inrush current to safe levels and achieve output voltage regulation.Tim

author: TheDood
author_rank: Regular Contributor
timestamp: August 04, 2019, 08:16:50 pm
content: Quote from: T3sl4co1l on August 04, 2019, 02:54:32 amThanks Tim, I'm not familiar with a MOV, could you enlighten me?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 05, 2019, 01:28:31 am
content: It's a nonlinear clamping device, with relatively poor accuracy, but quite high energy handling per dollar.They're used for absorbing surges from direct and induced lightning on mains wiring, and anything else long enough (or between facility grounds) to pick up significant lightning surge.https://www.littelfuse.com/~/media/electronics_technical/application_notes/varistors/littelfuse_varistors_basic_properties_terminology_and_theory_application_note.pdf(The big cylinder ones pictured, are used in stacks, to absorb direct lightning strikes to distribution power lines (nominal 5-40kV)!)Tim