Sanity check - linear high frequency inverter?  - Page 1
Forum section: electronics / projects

author: Tjj226_Angel
author_rank: Contributor
timestamp: April 11, 2025, 08:09:04 pm
content: I have a situation where I want to power some circuits without a direct connection to mains voltage and I am thinking about using a battery pack with an inverter.If at all possible, I would also like to avoid using any sort of switching circuitry. My thought is it use the batteries to power a class AB amplifier that will output 36v rms. From there I can use a 36v to 120v step up transformer to give me my output.All standard stuff up to this point. Here is where I am wondering if I can get a bit tricky.Instead of running the AB amp at 50/60hz, I am wondering if it is possible to run it at 60-100Khz and use a AC to DC smps flyback transformer for my step up? Obviously the amplifier would have to have a significant bandwidth, among other technical challenges, but Im not entirely sure SMPS transformers can be used in this fashion.All transformers work on AC, so I don't see why not, but I feel like there is a "gotcha" somewhere.

author: DavidAlfa
author_rank: Super Contributor
timestamp: April 11, 2025, 08:37:20 pm
content: What electronics? Lots will run fine on HV DC.Otherwise, get an isolation transformer, or make a poor-man's one by using two transformers back-to-back.

author: TimFox
author_rank: Super Contributor
timestamp: April 11, 2025, 08:42:58 pm
content: Go to the electronic textbooks about transformers.You can certainly run supersonic frequencies through appropriate transformers, and you might want to resonate the transformer for use at a single frequency, to reduce the effect of leakage inductance.The nice thing about higher frequencies is that you can use much less iron due to a reduction in the required "magnetizing inductance" at higher frequencies.Many, many years ago I used a Spellman high-voltage supply that drove a resonated air-core transformer from a class-C amplifier (4x 6146 tubes), tuned to the driving frequency.The output was controlled by screen modulation through a 6Y6 pass tube.

author: Tjj226_Angel
author_rank: Contributor
timestamp: April 11, 2025, 09:17:25 pm
content: Quote from: TimFox on April 11, 2025, 08:42:58 pmYeup. Shrinking the transformer (and the filter components) would be the ultimate goal.Most high frequency transformers are designed for switching applications (square wave). I just want to be 100% sure that I can feed a SMPS transformer with AC (sinewave) instead. And if not, what kind of transformer should I be shopping for instead.

author: bdunham7
author_rank: Super Contributor
timestamp: April 11, 2025, 11:18:07 pm
content: I don't think you want aflybacktransformer specifically, but other types of SMPS transformers should work. What are you going to do with the output? Obviously your HF output can't serve as a direct substitute for mains voltage, so what are you powering with this? Will you rectify the output?

author: PCB.Wiz
author_rank: Super Contributor
timestamp: April 11, 2025, 11:20:16 pm
content: Quote from: Tjj226_Angel on April 11, 2025, 08:09:04 pmThe gotcha is if your 'some circuits' can work with a massive shift from mains cycles to 60-100kHz.What exactly do you hope to power from this? Do any of them spec for 60-100kHz AC input ?

author: Smokey
author_rank: Super Contributor
timestamp: April 11, 2025, 11:55:27 pm
content: What input and output currents are you talking about here. If you haven't worked with power dissipation in a ClassAB amplifier before, prepare to be shocked how inefficient that is.Don't try to be too clever here. Power conversion is one of those things that engineers have been working with for many many many years now. If it's not something that's commonly done there is probably a really good reason it's not done.

author: Tjj226_Angel
author_rank: Contributor
timestamp: April 12, 2025, 12:08:05 am
content: Quote from: bdunham7 on April 11, 2025, 11:18:07 pmI have been making my own test equipment, amplifiers, new regulator circuits, and now I am playing with image sensors more and more.The problem I have been having is that there is a ton of random hash on my mains. I have used isolation transformers, common mode chokes, and so on. They all make a big difference, but the don't solve the problem. I have made a few battery powered benchtop supplies that completely eliminate the problem for circuits that need 50 watts or less. I want to make a big one that can start powering larger projects say 200-300 watts.I can simply use a 500VA toroid transformer to output 120v rms and call it a day, but that would mean that all of my circuits would require their own 120v isolation transformer. Not a big deal, but if I could use a smaller high frequency transformer, it would certainly save a lot of money and heavy lifting.My idea would be to have the box output 36v rms ac voltage through a molex cable or something simple. Then each circuit I make would require an appropriate matching transformer to step up or step down the 36Vrms into whatever I need and then rectify the voltage and pass it through a regulator like any other linear power supply.My thought is to have a kind of linear switch mode power supply if that makes sense. A non switching power supply but something that still uses the smaller components of a SMPS.

author: Tjj226_Angel
author_rank: Contributor
timestamp: April 12, 2025, 12:17:07 am
content: Quote from: Smokey on April 11, 2025, 11:55:27 pmOh no. I am more than aware that what I am doing will generate maybe 100 watts of heat. Good news is that I live in a climate where it will be snowing AGAIN next week

author: PCB.Wiz
author_rank: Super Contributor
timestamp: April 12, 2025, 02:24:20 am
content: Quote from: Tjj226_Angel on April 11, 2025, 08:09:04 pm...Quote from: Tjj226_Angel on April 12, 2025, 12:08:05 amThese posts self-contradict.A high frequency signal from a transformer that you then rectify,isa switching power supply. The diodes are switching current.Note that not all mains units can tolerate a DC feed voltage. Anything with a real AC Iron transformer in it, will hate that.

author: Andy Chee
author_rank: Super Contributor
timestamp: April 12, 2025, 03:18:44 am
content: Quote from: Tjj226_Angel on April 11, 2025, 08:09:04 pmThe gotcha is the diodes in your 120V bridge rectifier. They will generate switching noise.

author: David Hess
author_rank: Super Contributor
timestamp: April 12, 2025, 10:23:04 am
content: Quote from: Tjj226_Angel on April 11, 2025, 09:17:25 pmWhat you are proposing will work fine. Only the power capability, voltage ratings, and magnetizing inductance matter. The inductance needs to be high enough to keep the transformer out of saturation. Power capability is limited by loss in the windings and core.Flyback transformers are gapped for energy storage, so are not ideal in your application, but will still work. Inverter transformers pass energy through without storing it, so are smaller for the same power level. However this difference also means that inverter transformers, like AC mains transformers, are sensitive to flux balance, so there should be some provision to control this like AC coupling with a capacitor or current drive. Inverter transformers are sometimes gapped like a flyback transformer so that they can tolerate some imbalance.The transformers used in transformer isolated switching regulators, except for flyback converters, have the properties you need. Gate drive transformers are also suitable but are designed for low power levels.

author: TimFox
author_rank: Super Contributor
timestamp: April 12, 2025, 02:48:58 pm
content: The distinction I see here is between square-wave (or asymmetrical) transformer drive and sinusoidal drive (which allows resonating the transformer) before rectification.Also, with sinusoidal rectification, it is possible to use a choke-input filter to increase the conduction angle of the rectifier (with reduction of the DC output voltage).

author: David Hess
author_rank: Super Contributor
timestamp: April 12, 2025, 04:08:36 pm
content: Quote from: TimFox on April 12, 2025, 02:48:58 pmSquare wave drive works fine to drive a transformer into resonance. On the secondary it is difficult to tell the difference. Seems like it would lead to poor secondary regulation, but I have never tried it with a real load.A blocking oscillator using a single transistor, or a two transistor Royer oscillator can drive a transformer for power conversion. Both can be configured to drive the transformer at resonance by adding a capacitor, but then they are not blocking oscillators and all of the examples I know used regulation from the secondary back to the primary to control the output.The only modern designs I found online are resonant Royer converter which include a feedback loop to regulate the output. Check out Linear Technology application note 118 for a pile of examples.