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If I'm not mistaken the main goal with transformers is to keep the flux below the saturation flux. I think 2000 gauss is a good value for ferrite cores.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Daedronus wrote ...
What about this ?
If I'm not mistaken the main goal with transformers is to keep the flux below the saturation flux. I think 2000 gauss is a good value for ferrite cores.
Yes, but that is not the ultimate limiting factor in V/turn since you can keep the peak flux density the same as long as the frequency is increased proportionally to increase in V/turn.
The true limit is imposed by losses, especially eddy current losses which are proportional to square of V/turn (and to less extent hysteresis losses, which are proportional to frequency).
Registered Member #1321
Joined: Sat Feb 16 2008, 03:22AM
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When I look at the (simulated) current in a voltage multiplier circuit, I see that it's pulses. For a given load on the multiplier, as I decrease the duty cycle of the square wave drive - significantly - there's hardly any change in the output voltage across the load, accordingly.
This suggests to me that if you're going to drive a voltage multiplier with a HFHV ferrite transformer, it's probably better to drive the primary with a waveform having a crest factor higher than 1; IOW something other than a straight (approx. 50% dc) squarewave from a hard-switched bridge. (Will this be running open loop?) Doing this should allow you to use fewer primary (and thus secondary) turns for the same final output voltage while keeping the peak flux in the core under control because of the smaller volts-seconds.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
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Dr. Quark wrote ...
Yes. The N27 datasheet linked above gives the following core loss figures:
25kHz, 200mT: 155kW per cubic metre 100kHz, 200mT: 920kW per cubic metre
which is 0.155 and 0.92W per cubic centimetre. The core set has a volume of 518cc, so that adds up to quite a lot of core loss!
The cores are good for 2.5kW, so 80W losses @ 25kHz is around 3%. These will be oil cooled.
jpsmith123 wrote ...
When I look at the (simulated) current in a voltage multiplier circuit, I see that it's pulses. For a given load on the multiplier, as I decrease the duty cycle of the square wave drive - significantly - there's hardly any change in the output voltage across the load, accordingly.
This suggests to me that if you're going to drive a voltage multiplier with a HFHV ferrite transformer, it's probably better to drive the primary with a waveform having a crest factor higher than 1; IOW something other than a straight (approx. 50% dc) squarewave from a hard-switched bridge. (Will this be running open loop?) Doing this should allow you to use fewer primary (and thus secondary) turns for the same final output voltage while keeping the peak flux in the core under control because of the smaller volts-seconds.
I'm planning on incorporating a capacitor in series with the primary. I believe this will 'shape' the pulse, to more of a sine wave than a square wave, but I'm not certain about this. Is this the kind of thing you had in mind?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
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Yes, these voltage multiplier things run with discontinuous current. The "SLR"- series load resonant converter- is a good solution. It uses an inductor and capacitor in series with the transformer primary to tune the current pulses into half sinusoids of known frequency and well controlled amplitude. The usual way of driving it is with 50% duty cycle at half of this resonant frequency. This isn't maybe the most efficient way to drive it, but it's the easiest to control.
If you know what you are doing, the inductor can be built into the transformer's leakage inductance, but I've not seen any DIY builder pull this off yet.
If I'm not mistaken the main goal with transformers is to keep the flux below the saturation flux. I think 2000 gauss is a good value for ferrite cores.
Yes, but that is not the ultimate limiting factor in V/turn since you can keep the peak flux density the same as long as the frequency is increased proportionally to increase in V/turn.
The true limit is imposed by losses, especially eddy current losses which are proportional to square of V/turn (and to less extent hysteresis losses, which are proportional to frequency).
Marko
As long as you use a reasonable frequency, like 30-60Khz heat should not be a problem. Also, it should be easier to diagnose....if it gets hot you need a bigger core. But if you saturate the core it's probably much worse, so I personally first pick a reasonable frequency, that see if I can wind enough turns so I'm far from saturation.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Daedronus wrote ...
. As long as you use a reasonable frequency, like 30-60Khz heat should not be a problem. .
I can see that hysteresis losses will increase with frequency, and I can see that I can use smaller capacitances in the multiplier if the frequency is higher.
I'm aiming for 25kHz to start with, but will then see how far I can increase the frequency before it becomes inefficient.
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Joined: Sat Feb 16 2008, 03:22AM
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Well I tried to simulate a SLR type half-bridge using "CircuitMaker 2000", and under certain conditions I seem to be getting waveforms similar to what I see other people getting, but it seems to be rather "touchy" and load dependent and the power throughput isn't very good.
I'm going to try it using pspice and see what happens, as I don't always trust circuitmaker.
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Volts per turn question.
