Linear AC power sources: how do they work? - Page 1
Forum section: electronics / beginners

author: ifonlyeverything
author_rank: Regular Contributor
timestamp: April 21, 2025, 12:15:18 am
content: I found this very high level diagram from herehttps://pacificpower.com/wp-content/uploads/2023/12/Selecting_a_Linear_or_PWM_Power_Source.pdfFor a 120VAC RMS output and let's say a 200VDC bus voltage, how exactly do you get around having to dissipate 200V * several amps near zero crossing? Is the DC bus voltage varying with time or is there something else going on here?

author: BILLPOD
author_rank: Frequent Contributor
timestamp: April 21, 2025, 01:12:14 pm
content: Good Morning Ifonlyeverything, Here's a video you can watch, for starters:https://youtu.be/brB1sZyJPls

author: ifonlyeverything
author_rank: Regular Contributor
timestamp: April 21, 2025, 06:58:50 pm
content: Quote from: BILLPOD on April 21, 2025, 01:12:14 pmI understand how AC to DC linear regulators work. The usual trick here is to run a multi tap transformer and select the tap that is a greater than yet closest to your desired DC voltage set point. This isn't an option with a variable frequency AC to AC linear regulator. So how does it work?

author: UnijunctionTransistor
author_rank: Frequent Contributor
timestamp: April 21, 2025, 08:40:54 pm
content: The white paper you linked explains it properly: it is a class AB power amplifier.Much like an audio power amp on steroids.

author: ifonlyeverything
author_rank: Regular Contributor
timestamp: April 21, 2025, 08:56:40 pm
content: Quote from: UnijunctionTransistor on April 21, 2025, 08:40:54 pmThanks, will study class AB amplifiers. So there really is no trick here -- that for a 5A load it is dissipating ~1kW as a 120V AC RMS waveform approaches 0 volts? The heat sink must be absolutely massive.

author: edavid
author_rank: Super Contributor
timestamp: April 21, 2025, 08:58:23 pm
content: Quote from: ifonlyeverything on April 21, 2025, 12:15:18 amWhy do you think you need to? For a lab power supply, you can just use big heatsinks and fans.QuoteIt can, that's called class G or class H. It can be a good tradeoff between circuit complexity and cooling.It's a moot point since almost all of the market has gone to switching supplies.

author: bdunham7
author_rank: Super Contributor
timestamp: April 22, 2025, 02:21:25 am
content: Quote from: ifonlyeverything on April 21, 2025, 08:56:40 pmIf the power factor is near 1.0 (resistive load) then as the voltage approaches zero so does the current so the dissipation would be zero at that point. A typical fixed linear supply AB amplifier operated near but below clipping will dissipate roughly the same energy as it puts out or even a little less. But I don't think anyone today would build such a barbarically simple design for a high-powered device like that.https://www.psaudio.com/products/directstream-power-plant-20

author: David Hess
author_rank: Super Contributor
timestamp: April 22, 2025, 03:10:05 am
content: Quote from: ifonlyeverything on April 21, 2025, 12:15:18 amThere are a couple of options, but your example shows the brute force method of using a high power linear class-AB or class-B output stage. A switching class-D amplifier is more common today because of its greater efficiency, allowing higher power.A sometimes used alternative is to carry the AC input through to the output with the secondary of a low output voltage transformer wired in series. Now the class-AB amplifier drives the transformer primary to add and subtract from the AC input to produce a clean AC output. In this configuration, the class-AB amplifier can be much lower power, but the AC output range is constrained to be close to the AC input.

author: MrAl
author_rank: Super Contributor
timestamp: April 22, 2025, 03:04:15 pm
content: Quote from: ifonlyeverything on April 21, 2025, 12:15:18 amHello there,That diagram does not look detailed enough to say exactly what that particular design is doing to produce the AC output.There are a few ways of doing this.The first is a pure linear circuit, which could waste as much as 50 percent of the total input power. Not very good, but clean output. This would be just an audio amplifier really, but not a switching type just pure linear.The second would be to combine a linear back end with a switcher front end. The switcher output would follow the AC output closely keeping the voltage just high enough above the instantaneous output AC to produce enough output voltage regardless of what the phase was. You can think of it as a tracking DC to DC converter that can go negative as well as positive. The better DC power supplies use this idea too to keep efficiency high.Another would be the class D that has been mentioned already. That's just a switching audio amplifier. It develops an AC output by varying the pulse width over the angles of the sine wave.Yet another is an actual sine synthesized AC converter. One choice is DC input and the other choice is AC input. The AC output is a good approximation to a sine wave and can really put out some high power even 30 kilowatts. When I worked in the industry this is the kind I designed and tested. We were able to get very high efficiency around 90 percent (and that was very good at the time with the devices available at that time) and very good THD. This design is a like a DC to DC converter that can constantly vary the output into an AC sine wave almost pure. With a rectifier type input it can take an AC input and it already puts out a sine wave that is very clean. The PWM pattern is designed to mimic a sine wave and with just a little filtering can be very clean. This type is often referred to as a pure sine converter but it's actually a synthesized AC output which becomes very clean.There have been various other sub categories like a resonant converter, but more recently there has been interest in resonant switching, which attempts to get the transistors to switch at zero volts and zero current and thus reduce switching losses by a large amount.The cheaper version of this idea is the kind that put out just a single pulse for each half cycle. These do not put out a clean sine wave. Although the first harmonic is dominant there are other harmonics that also appear on the output and they usually do very little to reduce them other than set the pulse width as good as possible. It's impossible to get a clean sine with just two pulses per cycle, one positive and one negative. Since it still works with a lot of equipment, apparently, they still produce them and people use them for all sorts of products. I myself do not like them very much especially since my background was in the high-power high-quality sine output types, but I have tested one with a wall wart and it seemed to work.There is one drawback to any type of switching converter and that is that there is always some switching noise that gets into the equipment being powered one way or another. Even though this can be reduced dramatically, it is not eliminated entirely and this interferes with some sensitive circuits. In those cases, a pure linear converter would be required with a careful design.I think the most interesting part of a synthesized sine converter is the pattern and how to generate it. There are different methods each interesting in its own right. I studied this in depth and looked at many different patterns and how to generate them. Some ways are better than others, but they all work much better than the two pulse per cycle types.