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Old 20th February 2013, 11:26 AM   #1
Elvee is offline Elvee  Belgium
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Default Flexible supply idea

Here is a new supply concept that will probably appeal to tube lovers: not only does it allow the generation of any arbitrary high voltage supply without a dedicated HV transformer, but as a bonus it also allows the use of a DC source easily, and without additional components.

This means that making a tube gear portable is no more a problem.

For a fixed output voltage, the voltage potentiometer can be replaced by a simple resistive divider

Additional details can be found here:
High Voltage Variable Supply on a Shoestring
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Last edited by Elvee; 20th February 2013 at 06:47 PM. Reason: Typo
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Old 20th February 2013, 05:53 PM   #2
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Excellent work!

It's always fun to use up some junk-box transformers.

How does it scale up? Can we use a 50V 250VA transformer and get 350V 180W?

The switch would need to handle more power - but the FETs to do that are cheap enough.

What sets the limits on switching frequency?
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Old 20th February 2013, 07:10 PM   #3
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by Rod Coleman View Post
Excellent work!

It's always fun to use up some junk-box transformers.

How does it scale up? Can we use a 50V 250VA transformer and get 350V 180W?
Quite easily, on the condition it is not a toroidal.



Quote:
What sets the limits on switching frequency?
I didn't test the concept at frequencies greater than ~25KHz.
The problems are basically the same as with any switcher: switching losses increase with frequency, and also since it is the leakage inductance that is exploited, the field dispersion is relatively important, meaning that losses caused by eddy currents in the bulk of the copper wire can become significant.
Each transition also causes a quantum of loss in the part of the iron not shielded by the primary.
For all these reasons, I think it is preferable to stay just at ultrasonic frequencies. With some derating, it is certainly possible to go significantly higher.

The lower frequency is set by the leakage inductance: when the frequency is lowered, the ΔI during the cycle becomes progressively higher, until the discontinuous mode is reached. It is perfectly possible to work in discontinuous or critical mode, and it even simplifies the control because no slope compensation is required for the current mode converter, but it is not very efficient: the large peak currents lead to a relatively large rms current compared to the average output current.

I will give some hints about the dimensioning in the Power Supplies thread. I have already used the concept under several forms, and I have some experience I can share. Lots of aspects, particularly with high boost ratios still need to be explored. The general outlook is very promising though.
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Old 22nd February 2013, 08:35 AM   #4
Elvee is offline Elvee  Belgium
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Quote:
Originally Posted by Rod Coleman View Post
Excellent work!

It's always fun to use up some junk-box transformers.

How does it scale up? Can we use a 50V 250VA transformer and get 350V 180W?
I have used these figures to illustrate a design change example, see here:
High Voltage Variable Supply on a Shoestring
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Old 2nd April 2013, 05:10 PM   #5
Merlinb is online now Merlinb  United Kingdom
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This is highly ingenious... It appears you are using the transformer's leakage inductance as the usual inductor for a boost converter, yes? Does the control circuit not need to be synchronous with the mains supply?
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Old 11th April 2013, 04:53 PM   #6
Elvee is offline Elvee  Belgium
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Sorry for the late response, I wasn't following this thread anymore.

The circuit doesn't need to be mains-synchronous, because the switching frequency is much higher than 50/60Hz: it doesn't really matter whether there are 200 or 201 switching periods during one mains half cycle, nor their phase when it is terminated: statistically, it averages to the same value.

If the switching frequency was a few hundred hertz, it would begin to matter, but such a low frequency is not only unpractical because it would be audible, but also incompatible with a normally built transformer (one without magnetic shunt)
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