Rereead post #10

I understand, but the poster of #10 says he believes that to be the case. It’s always nice to get other people’s opinions also.

 

Here’s a paper that includes an isolated two transformer topology instead of using a single transformer. It states that:

“In high step-up bridge converter seen in Fig. 20 two transformers are utilized to double the voltage conversion ratio [26]. Distributed magnet- ic components not only lower the power losses and thermal stresses of the converter but also reduce transformer turns- ratio.”

So it does appear people are realising some benefits from multi-transformer dcdc isolated converters.

 

Sorry - here is the paper I mentioned: https://pdfs.semanticscholar.org/3b16/8b56695558fda2dca2fefb424da0e331de14.pdf

 

If it needs to be smaller and lighter and feed a variableload, such as, perhaps, a high powered linear amplifier, such a unit has been designed and the design published in QST magazine, a publication of the ARRL. I don't recall the issue but if you are not a member then visit your local library with a decent camera and photograph the article pages. or use the copier. The high frequency transformer is a whole lot smaller and thus lighter. AND, as advice, and to copy what Motorola did with some of their medium level transmitters, one transformer with two secondaries, and separate rectifiers and filters for each, and then the supplies in series. That allows lower voltage rectifiers and capacitors and makes each section more servicable. It also reduces the insulation stress in each secondary.
So here I have presented two options, the high frequency switchmode supply is really a great way to get lots of power in a smaller and lighter box.

 

If it needs to be smaller and lighter and feed a variableload, such as, perhaps, a high powered linear amplifier, such a unit has been designed and the design published in QST magazine, a publication of the ARRL. I don't recall the issue but if you are not a member then visit your local library with a decent camera and photograph the article pages. or use the copier. The high frequency transformer is a whole lot smaller and thus lighter. AND, as advice, and to copy what Motorola did with some of their medium level transmitters, one transformer with two secondaries, and separate rectifiers and filters for each, and then the supplies in series. That allows lower voltage rectifiers and capacitors and makes each section more servicable. It also reduces the insulation stress in each secondary.
So here I have presented two options, the high frequency switchmode supply is really a great way to get lots of power in a smaller and lighter box.

Hi Bill!

Yes, planning on running the power supply at a high frequency, possibly using new wide band gap power devices as well to get high power density and efficiency. I will check out the publications that you outline and see if I can get a view of the power supply you mention.

My load is is pulsed at about 50% duty cycle.

Just to clarify with the MOTOROLA supply. What do you mean by the supplies in series? Do you mean connect the secondary transformer windings in series or is there actually two supplies, each supply a transformer and then you connect the two separate supplies in series after the filters and rectifiers?

At the moment, I have a single transformer with multiple secondary windings, connected in series after voltage multiplier circuits. This sounds like what you are saying? Please correct me if I have misunderstood.

 

The two cores will have a lower saturation current and must carry the same current as the original. Unless the saturation current of the orignial was larger than needed by a factor of 2, you lose.

Bob

Hi,

Yeah what you say is not the way it works.

Take any transformer with wire size let's say #12 with current say 10 amps. Assume it is designed correctly.
Now wind the same core with #10 wire and raise current to 15 amps. What changed?
The current increased, but alas the same core works for both designs.

The current is only of importance when there is a static DC current involved, which in transformers is not a consideration except in converters, and then it better be mitigated anyway beforehand.

This is why current is NOT in the transformer equation. It's all about volts, frequency, core area, number of turns, and NOTHING about AC current!

If we need a particular current, then we need a particular size wire, that's it, unless the wire size gets too large then we maay have to wind bifilar.

 

Hello Bob,

That’s true, didn’t think of that. Although the same current will flow through the primaries of the two transformers, does the halving of the voltage not have an effect? What about the series connection of the secondaries, or parallel connection or the primaries?

Hello,

It is only of concern with a DC current, which transformers usually dont have.
Excitation current is dependent on frequency, number of turns, core area, etc., but in the transformer equation it is absent. The only variable in that formula is voltage not current.
Now with DC current there is an adjustment, but in most AC transformer applications that's not a consideration.

 

From Wikipedia:

Saturation puts a practical limit on the maximum magnetic fields achievable in ferromagnetic-core electromagnets and transformers of around 2 T, which puts a limit on the minimum size of their cores.

This is one reason why high power motors, generators, and utility transformers are physically large; to conduct the large amounts of magnetic fluxnecessary for high power production, they must have large magnetic cores.

Splitting a tranformer core will limit the current it can handle due to saturation.

Bob

 

From Wikipedia:

Splitting a tranformer core will limit the current it can handle due to saturation.

Bob

Hello,

Yes, but that pertains to DC current. AC current is not a consideration for the core size. It's indirect, more directly based on the AC voltage.
In other words, knowing the required AC voltage leads to a good design.
Can you even design a transformer knowing the AC current? I dont think so.

I tried to give a comparative example.
Case 1:
You design the primary of a transformer with a core 1 inch square to handle 120vac 1 amps. The 120vac is used to select the number of turns, the wire diameter is used to accommodate the current it needs to pass.

Case 2:
Now we realize that we need 120vac at 2 amps. Do we change the core size?
If there is enough room in the window we dont have to change the core size, only the wire diameter.
If there is not enough room in the window we might change the core to one with a larger window, but we DONT change the stack height
because the stack height was determined by the operating voltage of 120vac which did not change.

As part of my job back in the 80's i designed transformers for sine wave converters and other power line driven applications. I designed transformers that range in size from about 1x1 inches to about 24x18 inches or so. The biggest ones you could not pick up off the floor they were so heavy. The smaller ones were mostly for DC converters or for bias transformers used in high power converters ranging in power from about maybe 1 watt up to about 30 kilowatts.

 

Hi Bill!

Yes, planning on running the power supply at a high frequency, possibly using new wide band gap power devices as well to get high power density and efficiency. I will check out the publications that you outline and see if I can get a view of the power supply you mention.

My load is is pulsed at about 50% duty cycle.

Just to clarify with the MOTOROLA supply. What do you mean by the supplies in series? Do you mean connect the secondary transformer windings in series or is there actually two supplies, each supply a transformer and then you connect the two separate supplies in series after the filters and rectifiers?

At the moment, I have a single transformer with multiple secondary windings, connected in series after voltage multiplier circuits. This sounds like what you are saying? Please correct me if I have misunderstood.

The Motorola power supply that I mentioned has multiple high voltage windings, each separate winding feeding it's own rectifier bridge and filter capacitors, so yes, there are individual supplies then connected in series.Simply putting windings in series provides no benefit at all, while the described arrangement allows the use of less expensive components. And being that the supplies were used for public safety radios they must have been very reliable.

 

The Motorola power supply that I mentioned has multiple high voltage windings, each separate winding feeding it's own rectifier bridge and filter capacitors, so yes, there are individual supplies then connected in series.Simply putting windings in series provides no benefit at all, while the described arrangement allows the use of less expensive components. And being that the supplies were used for public safety radios they must have been very reliable.

But there is still only single transformer, with multiple windings. Is each winding really considered a single “supply” or is the supply formed when they are all connected in series after the filters and the rectifiers? Do you happen to have access to any schematic for this supply, or know where I can find one (or what the supply is called so I can research it myself)

Thanks for your input.

 

But there is still only single transformer, with multiple windings. Is each winding really considered a single “supply” or is the supply formed when they are all connected in series after the filters and the rectifiers? Do you happen to have access to any schematic for this supply, or know where I can find one (or what the supply is called so I can research it myself)

Thanks for your input.

In the case of the Motorola Radio power supply the splitting was done for cost and reliability, and also to make the assembly easier to service. And they were functionally separate supplies connected in series.

Now I need to address a separate issue, which is about current and voltage. A transformer supplies POWER, volts times amps, and the core material works to direct and concentrate the magnetic flux that transfers that power.The more power the more core is needed. Different materials have different efficiency levels and capabilities at different frequencies. That is an inescapable reality. In addition, different core designs optimize different variables, and so that also needs to be considered. Unfortunately my transformer expertise is limited to knowing all of the things that I need to specify when requesting a transformer. The audio transformers that I created wer optimized for performance , and so size, cost, and efficiency did not matter.