Hi Guys, first post here....
I was searching the internet and stumbled quite by accident upon this rather interesting thread.
This particular problem of feeding high frequency PWM into a toroidal mains transformer has interested me for quite some time.
A couple of years ago I was given a free (dead) commercial 1.5Kw grid tie inverter and reverse engineered it out of curiosity to try to learn the "trick" of exactly how to do this, which all turned out to be a rather interesting project.
This particular PWM output stage used a full bridge fed from about 200 volts dc nominal, PWM'd at 20 Khz into a 2:1 step up toroidal transformer to generate a 240 volt at 50 Hz sinewave output.
There was a 2.7mH choke in series with the primary, and a 5.0uF capacitor directly across the primary of the transformer.
Tests revealed the 5.0uF capacitor and transformer primary resonated at about very roughly 85Hz, with nothing connected to the transformer secondary.
This is all rather clever once it is realized how it's all supposed to work.
By resonating the primary at about roughly 1.5 times the sinewave operating frequency, direct high amplitude resonance at 50Hz is avoided.
The tendency to resonant energy buildup is damped by causing each half cycle to be out of phase with the next half cycle of the 50Hz exciting frequency.
That way you avoid massive destructive resonant buildup of voltage with zero load on the inverter output.
But its still close enough to 50Hz resonance for the Xl of the transformer primary to cancel out much of the Xc of the 5.0uF shunting high frequency filter capacitor.
So the whole system passes 50 Hz fairly efficiently without massive no load losses due to the rather large 5.0uF shunting capacitor.
Note that this is for a 200 volt dc system.
If running at lower voltage, you may need a lot more than 5uF to resonate the primary at a suitable frequency, as it will have fewer turns and a lot less inductance.
You are going to need a very good capacitor, one that has a very high ripple current rating, and a very low self inductance if it is going to both work properly and survive.
At the 20 Khz PWM frequency, the 5uF capacitor and 2.7mH choke act as a two pole low pass filter, as in a buck regulator, so little of the 20 Khz remains as ripple across the primary of the transformer.
Fr works out at around 1.37 Khz, so its about 14.6 times lower than the 20 Khz switching frequency. Almost four octaves at 12db per octave is pretty good attenuation.
The 2.7mH choke is an iron cored device, made from two tape wound C cores, and has to handle at least 15 Amps of low frequency current without core saturation.
The whole thing was very carefully thought out by someone, who knew exactly what they were doing. And it works very well with a completely clean looking output sinewave.
This seems to be the way to do it, although it still needs to all be properly engineered and thought through to suit the dc voltage and power level of your output transformer. Its far easier to do effectively and get good results at a higher dc operating voltages and lower currents.
Cheers, Tony.