Toridal Mains Transformer

Yes this what is normal. It sets a sensible magnetising current in the core which is the same at @ 230V.

Keep ? You haven't said anything about the secondaries yet.

Dunno. You haven't mentioned the secondary windings at all.

It'll cause 4 x the copper losses. P=I^2R remember.

You don't understand transformers that well.

Graham

PFC is done on the ac input side of a device, not the output side, so your PFC module wouldn't be 'following' the transformer. It is the power utility which requires PFC to reduce power losses and voltage waveform distortion etc on its network so that it is more efficient at delivering power.

And if you are using a toroidal isolation transformer between the AC supply and the DC load, why do you need PFC? This is usually only needed when the item being powered draws a non-sinusoidal current. Transformer isolated supplies draw sinusoidal currents from the mains and don't require PFC. Power units such as PC switch mode supplies draw non-sinusoidal currents and these days are required to use PFC.

That initally puzzled me too. But then he couldn't use a line freq toroid.

I think he really does have a PFC circuit on the output side. No reason why not.

Graham

I read in sci.electronics.design that Ross Herbert wrote (in ) about 'Toridal Mains Transformer', on Thu, 8 Sep 2005:

Oh, dear, no! How could they?

OK, not strictly correct I admit. Since the OP hasn't said whether he is using switching techniques to generate the 300V dc (after the isolation transformer) I assumed he was simply wanting a linear dc supply, otherwise why include a toroidal transformer. He may simply need it for the isolation factor but that seems unneccessary.

Linear supplies of the size generally found do not contribute very much at all in the way of power factor distortion and I haven't come across any linear power supply which incorporates PFC, but you might know of one. PFC is generally not necessary on linear supplies but it is required on switch mode supplies due to the very short duration current spikes needed to top up the input filter capacitor on these devices.

Ambiguous.

Do you mean: "How could they draw sinusoidal currents?"

Or do you mean: "How could they require PFC?"

IMHO, transformer isolated supplies DO draw sinusoidal currents and it is a complete mystery to me why the OP thinks he needs PFC for such modest power. Perhaps he is confused about the terminology and means simple voltage regulation or some such?

Also should have nothing to do with whether the transformer is toroidal or conventional rectangular, should it? Presuming that any commercially availble toroidal power tranny is properly designed and constructed to avoid saturation?

I'm thinking of conventional linear transformer, bridge, large filter cap type supples. What would cause non-sinusoidal current flow in such a circuit?

Waitaminut.. Of course not! At first, there is a sinusoid, as the caps charge, but pretty soon, they get up to a standard level. Then, you just get small current sawtooths as the supply voltage becomes greater than the charge voltage plus diode drop(s). Definitely not sinusoidal. The big thing is that it is in phase with the source voltage, so there is no lead/lag to deal with. Or, maybe I'm wrong... 8-)

Charlie

The current into a transformer is a factor (the turns ratio) times the current out of a transformer, with minor variation (magnetizing current, saturation peaks, inter winding capacitive3e currents, etc.) If a transformer feeds a capacitor input filter rectifier, the primary current will look a lot like the rectifier currents, only scaled up or down. If the secondary feeds a half wave rectifier (and the primary has a little DC offset added to prevent core saturation) the primary current will also be unidirectional.

Yes, different question, entirely.

One might have to be extra careful that the load is completely symmetrical when using a toroidal core, because they tend to saturate with less DC than a stacked E-I core does.

"Andy"

Is this a complete SMPS with PFC which delivers 130 volts DC ?

Is the DC output non-isolated ?

There are ** FOUR** windings, remember.

.......... Phil

"Ross Herbert"

The OP's module is in the right place.

From wall warts on upwards.

Ones employing toroidal transformers are the worst offenders in terms of poor PF.

........... Phil

I read in sci.electronics.design that Ross Herbert wrote (in ) about 'Toridal Mains Transformer', on Thu, 8 Sep 2005:

Only below about 50 W, where the resistive losses in the transformer can widen the conduction angle to over 65 degrees.

Any linear supply of reasonable efficiency also has short duration current spikes, certainly short enough to require PFC. Perhaps not as short as an SMPS run well below rated power, where the pulse width may be less than 20 degrees, creating wall-to-wall harmonic currents.

I read in sci.electronics.design that Richard Crowley wrote (in ) about 'Toridal Mains Transformer', on Thu, 8 Sep 2005:

Please try to remain humble. But try not to be wrong.

"Ross Herbert"

".... followed by a bought in PFC module which can handle 85-260VAC in."

Supplies with no isolation transformer eliminate a bit of series esistance - ie the copper winding resistance of the transformer.

A linear supply, using a toroidal or C-core tranny, has an IDENTICAL supply current waveform to a SMPS.

You simply have never investigated the matter.

............ Phil

I read in sci.electronics.design that Richard Crowley wrote (in ) about 'Toridal Mains Transformer', on Thu, 8 Sep 2005:

The diodes in the bridge conduct only while the AC input voltage exceeds the DC voltage on the filter cap, i.e. around the peak of the AC voltage waveform.

OK. But do you think whatever power factor is even significant for such a modest power supply? It would be different if we were talking about a big theatre with

I read in sci.electronics.design that Charlie Edmondson wrote (in ) about 'Toridal Mains Transformer', on Thu, 8 Sep 2005:

The pulse isn't symmetrical, because the DC voltage on the cap is lower at the beginning of the pulse than at the end. This result in a small leading phase angle of the fundamental current. But it's negligible compared with the effects of harmonic currents.

What you're referring to is the conduction angle of the recifiers and how that creates harmonic currents. It's not actuaaly about 'power factor' per se but it's often called that. ( except in the standards )

I can assure you that a larger linear PSU will have a harmonics spectrum nearly as bad as a typical SMPS without PFC.

Graham

You mean 75W don't you ? ;-)

Or has the IEC finally made up its mind ?

Graham