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Old 17th March 2008, 09:14 PM   #11
Eva is offline Eva  Spain
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CFL require complex drive for good performance, including preheating, ignition and fault detection. Consider a microcontroller.
I use to feel like the small child in The Emperor's New Clothes tale
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Old 17th March 2008, 10:01 PM   #12
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Does it mean something is wrong if the transformer dissipates heat with no load on secondary? Nothing else gets warm but the toroid transformer. I'm still pretty new to SMPS design and theory.

*I'm just about to test the CFL with it, I'm just concerned about the transformer temp and/or saturation.

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Old 17th March 2008, 11:43 PM   #13
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You could reduce the flux excursion to reduce core loss How high is it now?

Is it a CFL = Compact Fluorescent or do you mean CCFL, Cold Cathode Fluorescent?

edit: You said 120V or thereabouts, I assume you do mean compact fluorescent.
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Old 25th March 2008, 01:45 AM   #14
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So far, so good, it runs the bulb, but the heaters stay on.....I didn't leave the bulb on long, so I didn't burn out the heaters. Also, I could adjust the brightness with the dead time adjust.

I also used a 25W 150 ohm resistor for current limit, and it got really hot So this thing has got some power.

I'll lower the frequency and see what happens.

I'm getting about 220V AC from this thing, the transistors stay cold.

The transformer is warm, load or not. Probably normal.
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Old 25th March 2008, 02:11 AM   #15
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Originally posted by EWorkshop1708
Does it mean something is wrong if the transformer dissipates heat with no load on secondary? Nothing else gets warm but the toroid transformer.
Nope, the volume power dissipation (in mw/cm^3) of the transformer is determined by material and flux density, not how much power you are drawing from the secondary. (that is limited by 'window area' all else being equal - but completely filling the window will smother the core and not allow it to dissipate heat well) To make the transformer dissipate less heat, add more primary turns to lower the flux density. This of course, deteriorates the ability of the core to shed heat, which is a tradeoff. Smaller cores can usually hand more flux density because there is not as much inner material to get real hot, they dissipate heat well.

A typical safe place to run a ferrite core is dB = 1500 gauss.

and for a push-pull converter, dB = [(V - 1)(0.8T/2)x10^8]/[Ae*N]

V = input dc voltage
T = switching period = 1/fs (seconds)
Ae = cross-sectional core area (cm^2)
N = primary turns

(taken from Pressman, Abraham. "Switching Power Supply Design," 2nd Edition, McGraw-Hill, 1998)

another thing to do if you have no current-mode control is to add small resistors to the drain of each driving mosfet. This will help prevent core runaway. If the core become saturated in one direction, the resistor will drop more voltage because more current is being drawn, therefore dropping the effective applied primary voltage, reducing the saturation.
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Old 28th March 2008, 01:04 AM   #16
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Very interesting......

Thank you for the information and help.

So it's doing fine, I just have to make some adjustments.
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Old 13th April 2008, 10:46 PM   #17
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Update, I will be making a few changes.

I got the 100W bulb to glow, but it glowed best at high frequency, but I had to play with cap/resistor values. However, ony the ends would glow, only one time nearly the whole bulb glowed. My Mosfets got really hot, and the transformer was even hotter.

Seemed to work, then one of my 75N06's shot a 3 inch flame out of it and burned the board! I was running this from a 14.4V NiCd battery, I didn't realize it was powerful enough to burn a 75A transistor

It's a bit hard to get it right, and drive a CFL directly. So instead I may just use this supply to get High Voltage DC instead.

After reading the UBA2021 Data sheet for CFL driver, I may instead just use this SMPS to get a high DC voltage, and use the HV DC to run the UBA2021 and MOSFETS. I have some parts from some broken 120V ballasts I can use. That way I can use the proper circuit to drive the CFL, and all I have to do is give it power. Less complicated that way.

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