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Registered Member #2288
Joined: Wed Aug 12 2009, 10:42PM
Location: Cambridge, MA
Posts: 179
I'm building a high voltage DC power supply for extremely low current draw at 900V. I'm trying to figure out what voltage CCFL to get. I know they are rated in RMS, but what does the AC wave form look like? I haven't found anything online referencing the output type.
If it is the first waveform in the image attached then I would need a 650V module to get 900V rectified, but if it's the second waveform then I would need a 1300V module since only half of the wave (the positive half) would be going into my output. Does anyone know which one to go with?
Registered Member #311
Joined: Sun Mar 12 2006, 08:28PM
Location:
Posts: 253
CCFLs shouldn't be run on DC. They need a high starting voltage (1KV or so) and a current-limited lower running voltage. The normal topology is a Royer converter, with a series cap on the output, whhich generates the striking voltage by series resonance with the secondary, then acts as a ballast for current-limiting once struck.
Registered Member #2288
Joined: Wed Aug 12 2009, 10:42PM
Location: Cambridge, MA
Posts: 179
So a CCFL unit won't put out 900V stable at 500uA? Any suggestions as to where to look for a power supply that would do this? I'd rather not pay the money for a power transformer due to the $100+ price of this high voltage transformer.
Registered Member #1807
Joined: Tue Nov 11 2008, 07:36AM
Location: San Luis Obispo, California
Posts: 19
In order to have a stable output voltage w/ respect to line and load variations, you'll need a control loop of some kind. I agree that the Royer topology might be a good choice. A simple way to control the Royer converter output may be to PWM the input voltage applied to the Royer current-sourcing inductor. Basically, you slap a simple boost or buck converter onto the front of the Royer circuit. Here's an application note from Linear Technology that describes this approach quite well.
If you don't need extremely low noise outputs, then you can forgo the RC lowpass output filters.
Instead of a CCFL lamp, you will be rectifying and filtering the transformer output voltage. A voltage divider off of the output provides the feedback for the buck/boost converter. While the circuit won't put out much power, it can charge output caps to dangerous levels, so be careful.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
I think the original poster meant that he wants to make a CCFL inverter into a HV power supply. Not that he wants to make a power supply for running CCFL lamps off DC.
The peak/RMS ratio is the same for fullwave and halfwave: 1.41. But the ripple is greater with halfwave, and you also run the risk of saturating the transformer.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Steve McConner wrote ...
The peak/RMS ratio is the same for fullwave and halfwave: 1.41. But the ripple is greater with halfwave, and you also run the risk of saturating the transformer.
When rectifying AC, the peak on the rectifier is 2 times higher with halfwave than with fullwave for the same output voltage...
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CCFL Power Supply
