AC output switchmode power supply

[Parmin]

Hey guys,
I wonder if there are any AC output switchmode power supply?
Or is it even possible?

I am needing about 200+ units of 24VAC 200W output power supply.
I guess I can perhaps add a DC to AC converter to a DC output unit but this would make the design much more complex.

I understand that I can use a simple transformer for this, however,
the reason for switchmode is for it to be lightweight and they are usually able to input 90 to 265Vac.

Anyone can send me in the right direction?

[bigtoy]

It is possible. It's called an AC to AC Converter (google, wikipedia, etc). But not very common. For lower powers a transformer is typically used. For high powers oddly enough a motor driving a generator is sometimes used. If you look at a rectifier (AC to DC converter) it's typically 2 stages: a PFC stage followed by a DC-DC. The output of the PFC is DC (or an approximation thereof), so you can see the issue. I don't know where to find something off-the-shelf like what you're looking for, unless you accept either a transformer, or a rectifier followed by an inverter (basically a strange UPS variant without the batteries).

[kripton2035]

I was also searching for something like this, but to make an enough powerfull variac in a small size.
a mains AC to mains AC variable power supply. they are really expensive.
and I did not find any online schematic for this.

[Sideshow Bob]

What are your input voltage range and type. And what is your output frequency range? Not that important but is your AC output at 24 volt RMS or peak/peak to peak. Also what kind of accuracy do you need for the output

[pickit2]

I done a project a few years ago.
it was 240v ac to 120v ac
it had two 600v power FETS back to back
hand full of diodes and caps to make a dc supply to control the fet gates
I used a small transformer in the finished unit, to power a few other things.
the idea is the fets are used as a power resistor, the voltage controlled via the gates.

without getting out of this lock down I can't get in to works.
but maybe you could get a prototype working easily
concept drawing

[Ahmad_k]

When you say switchmode power supply that's mean you should feed the input with DC current, then use a stepdown or boost converter.

What you want is feeding an AC signal from 90to 240 and get AC signal 24V 200W ?

The question is: what kind of load are you using ? Sensitive to harmonics ? is it inductive load ?

Another approach to pickit2 design is to use TRIAC or dual SCR (Resistive load only)

[dennis78]

@pickit2

Which FET's you used?

[pickit2]

This works in proteus

If you use this try use a small dc power supply for the fet gates.
As it is you need to take care with high voltage that can harm you.

[dennis78]

@Parmin

Can you write some details as galvanic isolation, type of load, is output pure sine,...?







Posted on: June 01, 2020, 05:29:18 17:29 - Automerged

This works in proteus

If you use this try use a small dc power supply for the fet gates.
As it is you need to take care with high voltage that can harm you.

It is totall different topology from your first picture. First is impossible to work

Second design can work but for 200W, 24V-> 8A current, fets will burn for less than secs. Only way is pwm driving, but EMC for that design can be nightmare.

Easiest and more reliably way is standard DC psu, then inverter.

Best way is simple transformer good designed with quality core

[pickit2]

The devices used in the sim are only a token, to show it could work. it is not a finished project.

In works I have a good supply of 800 & 900 volt 60A Fets that are used in Microwave cookers.
we also have a supply for Induction Heaters. also high amps.

We have had a unit in the field using a similar design 12V 25A used 12hours a day for 18 years. 

[Sideshow Bob]

You can use PWM to create an AC form DC. This method is used by most modern power inverters. It will produce a good enough approximation to a sine wave in many cases. But if it is good enough for your load condition. I do not know. Anyway I found a free document here
https://digitalcommons.wpi.edu/cgi/viewcontent.cgi?article=1184&context=mqp-all

[Vineyards]

Hey guys,
I wonder if there are any AC output switchmode power supply?
Or is it even possible?

As far as I know it is possible. Transformers can be impractically heavy especially for mobile devices -a fact  that stood as a burden impeding further miniaturization of electronic devices for many decades. AC that we use today in our homes is merely 50/60Hz which requires a hefty size. The way SMTP achieves smaller sizes is through the increased increased efficiency that comes from higher frequencies involved. You can convert frequency either mechanically or electronically or try inverters. All of these existing solutions will usually result in bulkier and costlier implementations. Presently, SMTP stands out as the most logical option a: it is highly available, cheap, tried, tested and time proven.

If you can create a low cost, efficient AC to AC design for your device that will revolutionize the electronics world; you might as well forget about those 200 items and set out to promote that cutting edge power supply technology.

[Parmin]

Thank you for the replies.
Apologies for not replying sooner, live sort of catching up with me and I was bedridden in the past few days.

Unfortunately I have signed an NDA and thus cannot describe the actual use of the device.
The requirement is quite simple, an isolated AC power output of 18 to 24 VAC max at 200Watts.
The approximation to sine wave should be good enough to do the job.
The best I can describe it is that the load is somewhat inductive and intermittent 

A variac is not possible since full isolation from the mains is required on the output.

I have previously attempted to use simple uc controlled L294 to produce square wave from DC24V and get a somewhat usable result with a coupling AC capacitor on the output stage to attempt to round the wave a bit, but max power I can get from this is only about 100W.

What I really want is to purchase off the shelf power supply factory units with proper certifications rather than banging out a product that might cost more to pass the certification requirements. 
I have contacted a few PSU makers such as Meanwell, Recom etc. but none actually have AC output units.

[vern]

I checked and found some  DC to AC converters with 24V AC Output, but not 200W.
I would buy a 24V to 110V or 220V AC converter, there are plenty and really cheap.
Then simply add a matching transformer, 110 or 220V to 24V and voila. Its even better because the transformer will even improve the sinus because it filters the high frequency parts of the spectrum.
110V coverter is better because it produces a 60Hz sinus, this lets you use a smaller Transformer than 50Hz.
But it depends of course on your application which frequency you can choose.